by todd caldecott lesson vi: nervous system, part one€¦ · 2. integrative: analyzing sensory...

Anatomy and Physiology: A Phytotherapeutic Perspective Lesson Six By Todd Caldecott

©2003 by Todd Caldecott and the Wild Rose College of Natural Healing

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Lesson VI: Nervous system, part one As we learned in Lesson I, the various functions of the body are kept within normal physiological parameters by homeostatic mechanisms. Generally speaking, there are two physiological systems that share in the responsibility of maintaining homeostasis: the endocrine system and the nervous system. Whereas the endocrine system functions to maintain control by releasing substances called hormones that travel through the blood to target specific receptors, the nervous system exerts its influence by the transmission of electrical impulses that travel through nervous tissue. The nervous system has three basic functions: 1. sensory: sensing certain changes (stimuli) within the body (e.g. a change in the pH of the blood or stretch receptors in the gut) and without (such as the sensation of a raindrop or the sun shining on one’s arm) 2. integrative: analyzing sensory information, stores information and makes decisions regarding behaviour 3. response: responding to stimuli by initiating muscular contraction or glandular secretions

I. Nervous tissue O r g a n i z a t i o n o f n e r v o u s s y s t e m The nervous system is divided into two principle divisions, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS is comprised of the brain and spinal cord and its function is to integrate and correlate all the incoming sensory information. Medical science considers the CNS to be repository of the mind, emotion and memory. The CNS is connected to sensory receptors, muscles and glands in the peripheral nervous system. The PNS consists of cranial nerves that arise from the brain and spinal nerves that emerge from the spinal cord. The input component of PNS consists of nerve cells called sensory or afferent neurons, which conduct nerve impulses from sensory receptors from various parts of body to the CNS. The output component of the CNS consists of nerve cells called motor or efferent neurons, originating in the CNS and conducting nerve impulse from the CNS to muscles and glands.



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The PNS can be further subdivided into the somatic (SNS) and the autonomic (ANS) nervous systems. The SNS consists of sensory neurons that convey information from cutaneous and special sense receptors (smell, taste, vision, touch, equilibrium and hearing) primarily from the head, body wall and extremities to the CNS. These impulses are then integrated by the CNS and conducted by motor neurons from the CNS to skeletal muscles only. The SNS is (mostly) consciously controlled and thus voluntary. Skeletal muscles are directed by a single nerve that travels from the CNS to the skeletal muscle without interruption. Therefore, stimulation in the SNS is an “all or none” phenomenon. The ANS consists of sensory neurons that convey information from receptors found primarily in the viscera to the CNS, which are then integrated and conducted to motor neurons in smooth muscle, cardiac muscle and glands. The ANS is (mostly) unconsciously controlled and thus involuntary. Effectors are directed by the ANS through a network of nerves that arise from the CNS. The activation, therefore, of autonomic effectors is modified by the connection between these networks (i.e. by the synapses). The ANS consists of two further subdivisions, the sympathetic and parasympathetic divisions. The viscera receive instructions from both the sympathetic and parasympathetic divisions, which have opposing actions. The parasympathetic division is the energy conservation-restorative system, regulating those activities that conserve and restore body energy during times of rest and recovery. Parasympathetic responses include salivation, lacrimation, urination, defecation and a decrease in heart rate. The sympathetic division, on the other hand, prepares the body for emergency situations, concerned primarily with processes involving the expenditure of energy. Activation of the sympathetic aspect of the nervous generally results in motion a set of physiological responses called the fight or flight response. H i s t o l o g y o f n e r v o u s t i s s u e Within nervous tissue there are two principle kinds of cells, neuroglia and neurons. Neuroglia support, nurture and protect the neurons, are generally smaller than neurons but outnumber them by 5 to 50 times. Neuroglia are able to multiply and divide in mature nervous system. Certain kinds of neuroglia produce the myelin sheath, a multi-layered lipid and protein covering that electrically insulates



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the axon of the neuron and increases the speed of nervous conduction. There are four types of neuroglia found in the CNS, including astrocytes, oligodendrocytes, microglia and ependymal cells. Each has a specific function, from protecting neurons and forming the BBB, to digesting foreign pathogens and cellular debris. There are two types of neuroglia within the PNS, neurolemmocytes (Schwann cells), which produce the myelin sheath around PNS neurons, and satellite cells, which support neurons in ganglia of the PNS. Neurons are comprised of three parts, a cell body, a dendrite and an axon. The cell body contains a nucleus surrounded by cytoplasm and is responsible for the metabolism of the neuron. Dendrites are a kind of process that extends outward from the cell body, often tree-shaped and unmyelinated, conducting nervous impulses toward the cell body. Axons are another kind of process that extends outward from the cell body that may or may not be myelinated, conducting nervous impulses away from the cell body. The junction between the cell body and the axon is a cone-shaped elevation called the axon hillock, and attaches to the first portion of the axon called the initial segment. Except in sensory neurons, nerve impulses arise at this junction, called the trigger zone, and are then conducted along the axon to another neuron, muscle fiber or gland cell. At the terminal end of the axon the tip swells into a bulb-shaped structure called the synaptic end-bulb, and contains synaptic vesicles that contain neurotransmitters. The functional contact between two neurons is called a synapse. The synapse between a motor neuron and a muscle fiber is a neuromuscular junction, whereas the synapse between a neuron and glandular cells is called a neuroglandular junction. A nerve fiber is a general term for any neuronal process (axon or dendrite), and a nerve is a bundle of many nerve fibers that course along the same path in the PNS, such as the ulnar nerve in the arm or the sciatic nerve in the thigh. Most nerves include bundles of both sensory and motor neurons and are surrounded by connective tissue coats. Nerve cell bodies in the PNS are generally clustered together to form ganglia. A tract is a bundle of nerve fibers, without connective tissue elements, found in the CNS. A tract may interconnect different regions of the



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brain or extend long distances up and down the spinal column and connect with specific regions of the brain. Neurons display a great diversity in size and shape. The cell bodies can range in diameter from 5 um up to 135 um, and go from having no axons to axons which may extend for a up to a metre or more. Nerve impulses travel at speeds ranging from 0.5 to 130 metres per second (up to 468 km/h). C l a s s i f i c a t i o n o f n e u r o n s Neurons can be classified according to their structure or according to their function. The structural classification is based upon the number of processes extending from the cell body. A neuron may be: unipolar, having just one process that is a fusion of a dendrite and an axon; bipolar, having one dendrite and one axon; or multipolar, having several dendrites and a single axon. Unipolar neurons are often specialized to monitor environmental changes, sometimes aided by receptors and impulses that arise at the first neurofibral node. Bipolar neurons are typically found in the retina of the eye, the inner ear and within the olfactory region of the brain. Multipolar neurons are most commonly found in the brain and spinal cord. The functional classification of neurons is based upon the direction in which neurons transmit impulses. Afferent neurons transmit sensory impulses from receptors in the skin, sense organs, muscles, joints and viscera to the spinal cord and brain. Efferent neurons convey motor nerve impulses from the brain and spinal cord to effectors, which may be either muscles or glands. A third type of neuron are association neurons, which carry nerve impulses from one neuron to another, and make up 90% of all neurons in the body. Contained within the spinal cord and brain there is a differentiation of neuronal tissue based on colour, called white and gray matter. White matter is made up of aggregations of myelinated processes from several neurons. Gray matter, on the other hand, is made up nerve cell bodies, dendrites, axon terminals or bundles of unmyelinated axons and neuroglia. Within the spinal cord the white matter surrounds the gray matter, whereas in the brain the gray matter is a thin outer shell around the brain that form the two cerebral hemispheres. Most nerve cells



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in the PNS and all tracts in the CNS are myelinated (white matter). N e u r o p h y s i o l o g y Communication between neurons is dependent upon two basic properties of their plasma membranes, the resting membrane potential (RMP), and ion channels. The RMP occurs because of the small build-up of negative charges just inside the membrane and an equal build up of positive charges on the outside of the membrane. The separation of positive and negative charges creates an electrical voltage, measured in millivolts (mV). The greater the difference in the charge across the membrane the larger the membrane potential, or voltage. Neurons typically display an RMP of –40 to –90 mV, the minus sign indicating that the inside is negative in relation to the outside of the cell. A cell that exhibits a membrane potential is said to be polarized. Ion channels open and close, allowing the transmembrane flow of ions to regulate the membrane potential. There are two types of ion channels, leakage channels, which are always open, and gated channels, which only open and close in response to some sort of stimuli. This stimuli could be changes to the RMP, the binding of ligands such as neurotransmitters and hormones, mechanical pressure, vibration and even light. Electrical charges in the body are mostly carried by ions. A positive ion is a cation, such as Na+ (sodium) and K+

(potassium). A negative ion is an anion, such as Cl-

(chloride). The electrochemical dynamic of K+ and Na+ and their movement through the ion channels regulates the membrane potential. Within the cell there is more K+ and without there is more Na+. When the ion channels open and allow Na+ to flow inwards there is a depolarization of the membrane, which is to say, the outer surface that was previously positive charged becomes negatively charged, and the inside which was negative becomes positive. When this happens, the RMP changes from –70mV to +30mV. Repolarization occurs when the plasma membrane opens up the K+ channels allowing K+ to flow inward, and Na+/ K+ active transport pumps push Na+ outward replacing it with K+, restoring the RMP to –70mV. This whole process takes about 1 msec in a typical neuron. The refractory period is the time during which a second potential cannot be activated, anywhere from 0.4 msec to 4 msec.



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An action potential or nervous impulse is the depolarization of adjacent areas of the plasma membrane along an axon, much like a domino effect. In unmyelinated axons there is a step by step depolarization of adjacent areas of the plasma membrane, called continuous conduction. In myelinated axons the plasma membrane is insulated against ionic currents, but at intervals along the axon that interrupt the myelin sheath, called nodes of Ranvier, there are concentrated clusters of voltage gated Na+ channels. At these locations membrane depolarization occurs, and the nervous impulse is propagated along a myelinated axon by the ionic current that flows through the axoplasm from one node to the next. This apparent jumping of the nervous impulse from one node to the next is called saltatory conduction (saltare (F) = to leap). T r a n s m i s s i o n a t s y n a p s e s A synapse is the area between two neurons, or between a neuron and an effector, filled with extracellular fluid. The neuron that sends the signal is the presynaptic neuron and the neuron receiving the message is the postsynaptic neuron. There are two kinds of synapses, electrical and chemical. Electrical impulses are sent through gap junctions, minute fluid-filled tunnels between the plasma membranes of each neuron. Electrical synaptic transmissions are faster than chemical synapses, and can provide two-way communication between cells, synchronizing the activity of a group of neurons or effectors. Chemical synaptic transmission requires the presynaptic neuron to convert the electrical signal into a chemical one, called a neurotransmitter. Depolarization at the terminal end of the axon opens chemically gated Ca2+ channels, allowing Ca2+ to flow into the neuron because of its higher concentration in the extracellular fluid. This increase in intracellular Ca2+ promotes the exocytosis of synaptic vesicles that contain neurotransmitters. These vesicles merge with the plasma membrane of the presynaptic neuron and release neurotransmitters into the synaptic cleft. These neurotransmitters then diffuse across the plasma membrane of the postsynaptic neuron or effector, to continue or inhibit the nervous impulse. This process of chemical transmission creates a slight delay in communication and is only one-way.



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The advantage that chemical transmission offers is that these neurotransmitters may be excitatory, causing the depolarization of postsynaptic neuron, or inhibitory, causing the hyperpolarization of the postsynaptic neuron. Excitatory neurotransmitters cause a depolarization by opening chemically gated ion channels, allowing K+ and Ca2+ to exit, but not in sufficient quantities to mediate the effects of inward flowing Na+, which promotes an excitatory post synaptic potential (EPSP). Inhibitory neurotransmitters that cause the hyperpolarization of the postsynaptic neuron by opening chemically gated Cl- or K+

channels initiate a inhibitory postsynaptic potential (IPSP). The inward flow of Cl-is mediated by the negativity of the neuron and has a minimal affect upon the RMP. The outward flow of K+ serves to hyperpolarize the neuron, further increasing the negativity of the plasma membrane. The removal of the neurotransmitter from the postsynaptic neuron, and therefore the inhibition of further neural activity, is a process of enzymatic degradation (e.g. acetylcholinesterase). This removal of the neurotransmitter can also take place through the reuptake of the transmitter by the presynaptic neuron. Many drugs can cause the neurotransmitter to remain in the synaptic cleft for a longer period of time, by either interfering with enzymatic degradation or with presynaptic uptake. An example is the activity of cocaine that inhibits the presynaptic reuptake of dopamine, causing dopamine to linger longer in the synaptic cleft, stimulating brain centers. Exogenous agents that facilitate the activity of a neurotransmitter are agonists, whereas exogenous agents that inhibit the activity of a neurotransmitter are antagonists. N e u r a l c i r c u i t s The CNS contains billions of neurons organized into complicated patterns, called neuronal pools. A neuronal pool may contain thousands or even millions of neurons. Neuronal pools are arranged in circuits, and are of four basic types: divergence circuits, convergence circuits, reverberating circuits and parallel-after-discharge circuits. Divergence circuits are a single presynaptic neuron that synapses with many postsynaptic neurons. This causes a single nerve impulse to stimulate an increasingly large number of cells within the circuit, such as a single motor neuron in the brain that stimulates many other motor neurons in the spinal cord.



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Convergence circuits are comprised of several presynaptic neurons that synapse with a single post synaptic neuron, such as a single motor neuron that synapses with skeletal muscle fibers at neuromuscular junctions, receiving input from several pathways that originate in different parts of the brain. Reverberating circuits are constructed in such a way that when the presynaptic cell is stimulated it will cause the postsynaptic cell to transmit a series of impulses. These impulse stimulate succeeding neurons, but axonal branches the later neurons send the impulse back to the earlier neurons. This output may last from a few minutes to a few hours, and is turned off by inhibitory neurons. Examples of reverberating circuits include breathing, coordinated activities and short term memory. Epilepsy is thought by some researchers to be a dysfunction of the reverberating circuits. Parallel after discharge circuits are a single presynaptic cell that stimulates a group of neurons, each of which synapses with a common postsynaptic cell. The differing number of synapses imposes varying synaptic delays so that the last neuron exhibits multiple excitatory and inhibitory postsynaptic potentials. Parallel after discharge circuits are believed to be used for precise thinking, such as mathematical calculations

II. The brain The brain is made up of about 100 billion neurons and weighs about 1.3 kg. It is vaguely mushroom-shaped, and can be divided into four principle parts: the brain stem (the stalk of the mushroom), the diencephalon (above the brain stem), the cerebrum (the cap of the mushroom), and the cerebellum (behind the brain stem). The brain stem is continuous with the spinal cord consists of the medulla oblongata, pons, reticular formation and midbrain. The diencephalon consists of the thalamus and hypothalamus. The cerebrum spreads over the diencephalon, occupying most of the cranium and contains the cerebral cortex, limbic system and basal ganglia. The cerebellum consists of the anterior, posterior and flocculonodular lobes.



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The brain is protected externally from injury by the cranial bones (the skull) and the cranial meninges. Both the brain and the spinal cord are bathed and protected against chemical and physical injury by a nourishing fluid called the cerebrospinal fluid (CSF). This fluid continuously circulates around the brain, in and through its ventricles and down the spinal cord. The approximately 80-150 ml of CSF serves as a shock-absorbing buffer that protects the brain and spinal column from being thrashed against the bony walls of the cranium and vertebral column. Essentially, the brain and spinal column float within this fluid. The CSF is a delicate balance of nutrients which provides an optimal environment for neuronal functioning, and even slight alterations in its composition can affect nervous transmission. The CSF is also a medium of exchange between the nutrients needed, and the wastes produced, by the brain.

B l o o d s u p p l y t o t h e b r a i n The brain is well supplied with oxygen and nutrients, and consumes about 20% of the total serum O2 at rest, and up to 25% of the total serum glucose. Any interruption in oxygen supply may cause unconsciousness, and a four minute impairment of O2 to brain cells may cause permanent damage. Although the brain uses up to 1/4 of the total serum glucose, storage of this nutrient in the brain is minimal, and low serum glucose may cause mental confusion, dizziness, convulsions and a loss of consciousness. The brain is protected by a barrier called the blood brain barrier (BBB) that prevents the entry of most substances into the brain. The BBB however, is completely permeable to glucose, O2, CO2, water, most lipid soluble substances such as alcohol, caffeine, nicotine, heroin and anesthetics, and partially soluble to substances such as creatinine, urea and most ions (including Na+, K+ and Cl-). Other substances such as proteins and antibiotics are inhibited from passing through the BBB, and thus the direct treatment of the brain is difficult. B r a i n s t e m The brain stem is comprised of the medulla, pons, reticular formation and the midbrain. The medulla develops from the embryonic myelencephalon and is continuous with the spinal cord, forming the inferior part of the brain stem. The



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medulla contains all of the ascending and descending tracts to and from the brain, and most of the tracts cross over from one side to the other within the medulla. The largest motor tracts of the medulla bulge outward to form the pyramids, and where they cross is called the decussation of pyramids. One of the functions of the medulla is to relay motor and sensory impulses between other parts of the brain and spinal cord. With the reticular formation, the medulla assists in the maintenance of consciousness and arousal (along with other parts of the brain). The medulla’s vital reflex centers regulate the heart beat, breathing (with the pons) and blood vessel diameter. The medulla also contains reflex centers that coordinate swallowing, vomiting, coughing, sneezing and hiccoughing. The medulla contains the nuclei of origin for cranial nerves VIII (hearing and equilibrium), IX (swallowing salivation and taste), X (the vagus nerve, relaying impulses to and from the thoracic region and viscera), XI (head and shoulder movements) and XII (tongue movements). The pons lies directly above the medulla and serves as a bridge, relaying impulses between the spinal cord and the higher brain. Along with the medulla, the pons maintains the pneumotaxic and apneustic area to control respiration. The pons also contains the nuclei of origin for cranial nerves V (chewing and facial sensation), VI (eyeball movements), VII (taste, salivation and facial expression) and VIII (equilibrium). The reticular formation are small areas of gray matter interspersed among fibers of white matter, extending into the diencephalon and the spinal cord. It has both sensory and motor functions, receiving input from higher parts of the brain that control skeletal muscles, regulates muscle tone and alerts the cortex of incoming sensory signals. The reticular formation also contains the reticular activating system (RAS), maintaining consciousness and awakening from sleep, activated by sensory stimuli (eyes, ears and skin) and body position. The midbrain, or mesencephalon, lies directly above the pons and relays motor impulses from the cerebral cortex to the pons and spinal cord and relays sensory impulses from the spinal cord to the thalamus. The ventral portion of midbrain is composed of the cerebral peduncles, containing nerve tracts that connect the upper parts of the brain with



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the lower parts. Posterior to the cerebral peduncles is the substansia nigra, which controls unconscious muscle movement. The dorsal portion of the midbrain is called the tectum, and is comprised of four parts. The superior colliculi is located in the dorsal regions of the midbrain, and serves as a reflex center for movements of the eyes, head and neck in response to sensory stimuli. The inferior colliculi is located below the superior colliculi, serving as a reflex center for movements of the head and trunk in response to auditory stimuli. The left and right red nuclei function with the basal ganglia and cerebellum to co-ordinate muscular movement. The red nuclei derives its name from the rich blood supply it receives and the iron-containing pigment in its neuronal cell bodies that cause it to be red. The midbrain also contains nuclei of origin for cranial nerves III (eyeball movement and changes in pupil and lens shape) and IV (eyeball movement). D i e n c e p h a l o n The diencephalon is comprised of the thalamus and the hypothalamus. The thalamus is an oval structure above the midbrain, constituting 80% of the total mass of the diencephalon. It consists of a pair of oval masses of mostly gray matter organized into nuclei that form the lateral walls of the third ventricle, joined together by a bridge of gray matter called the intermediate mass. Certain nuclei within the thalamus relay all sensory input to the cerebral cortex, including hearing (medial geniculate), vision (lateral geniculate), taste and somatic sensations such as touch, pressure, vibration, heat, cold and pain (ventral posterior nuclei). Other nuclei within the thalamus are synapses in the somatic motor system, the ventral lateral and ventral anterior nuclei mediating voluntary motor actions and arousal, and the anterior nucleus functioning in some aspects of emotion and memory. The hypothalamus is located below thalamus, and can be divided into the mammillary, tuberal, supraoptic and preoptic regions. The mammillary region is the posterior portion of hypothalamus, consisting of the mammillary bodies and the posterior hypothalamus. The mammillary bodies serve as relay stations in reflexes related to smell and controls feeding reflexes, and the posterior hypothalamus controls blood pressure, pupillary dilation and shivering. The tuberal region is located in the middle portion of hypothalamus, and consists of the dorsomedial, perifornical, ventromedial and arcuate nuclei. On the



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ventral surface of the tuberal region is the tuber cinereum, a mass of gray matter that connects to the infundibulum, a stalk-like structure that connects the hypothalamus to the pituitary gland. The medial eminence of the tuber cinereum encircles the site where the infundibulum becomes the stalk of the pituitary gland and contain neurons which synthesize the hypothalamic regulating hormones, regulating hormonal secretions of the anterior pituitary gland (i.e. hGH, prolactin, ACTH, MSH, TSH, FSH and LH). The supraoptic region lies above the optic chasm (point of crossing of the optic nerves), anterior to the tuberal region, and contains nerve fibers that extend from the paraventricular and supraoptic nuclei to form the supraopticohypophyseal tract, extending into the infundibulum to the posterior pituitary, transporting ADH and oxytocin. The preoptic region, anterior to the supraoptic region, regulates autonomic activities in association with the rest of the hypothalamus. Sensory input into the hypothalamus comes from afferent pathways in the somatic and visceral sense organs, and it’s the task of the hypothalamus to coordinate responses to these stimuli. The major functions of the hypothalamus include: 1) the control and integration of the activities of the ANS (e.g. smooth and cardiac muscle contraction, glandular secretion, heart rate, movement of food through the GIT, contraction of the urinary bladder) 2) rage and aggression responses 3) the regulation of body temperature 4) the regulation of food intake through the inhibitory activity of the satiety center 5) the regulation of thirst, stimulated by the rising osmotic pressure in the extracellular fluid 6) assisting in the coordination of arousal and sleep patterns. C e r e b r u m The cerebrum is located superior to the diencephalon, and although it is the largest portion of the brain, relatively little is known about its function. The surface of the cerebrum is composed of gray matter called the cerebral cortex, and lying under the cerebral cortex is the cerebral white matter. During embryonic development the gray matter enlarges much faster than the underlying white matter, and as a result the cortical region rolls and folds upon itself. These folds are called gyri or convolutions, the deep



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grooves between the gyri are called fissures, and the shallower grooves are called sulci. The most prominent fissure is called the longitudinal fissure, and divides the cerebrum into the left and right hemispheres. These hemispheres are connected internally by white matter that forms a large bundle of transverse fibers called the corpus callosum. The cerebral hemispheres are further divided into four lobes, named after the bones that cover them, and are the frontal, parietal, temporal and occipital lobes. The central sulcus separates the frontal lobe from the parietal lobe. A major gyrus, called the precentral gyrus, is located immediately anterior the central sulcus and is the primary motor area of the cerebral cortex. The postcentral gyrus is located immediately posterior to the central sulcus and contains the primary somatosensory area of the cerebrum. The lateral cerebral sulci divide the frontal lobe from the temporal lobes. The parietooccipital sulcus separates the parietal lobe from the occipital lobe. A fifth region is located inside the cerebrum and cannot be seen externally, called the insula or Isle of Reil. The white matter of the cerebrum consists of myelinated axons that extend in three principle directions, and are called the association, commissural and projection fibers. The association fibers connect and transmit nerve impulses between gyri in the same hemisphere. The commissural fibers transmit impulses from gyri in one hemisphere to the corresponding opposite gyri in the other hemisphere. Three important groups of commissural fibers include the corpus callosum, the anterior commissure and the posterior commissure. Projection fibers form ascending and descending tracts that transmit impulses to and from the cerebrum to other parts of the brain and spinal cord. The cerebral cortex contains three types of functional areas: sensory, motor and association. Sensory areas are located in the posterior half of the hemispheres, and can be divided into two types of areas, primary and secondary. Primary sensory areas have a direct connection with peripheral sensory receptors, whereas secondary sensory and sensory association areas lie adjacent to the primary sensory areas, receiving input from the primary sensory areas and from other parts of the brain. The secondary areas participate in the interpretation of sensory experience



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into meaningful patterns of recognition and awareness. Motor areas control muscular movement, and flow primarily from the anterior portions of each hemisphere. Association areas of the cerebral cortex consist of association tracts that receive and analyze signals from multiple regions of the cortex, as well as from other parts of the brain, and are comprised of the parieto-occipitotemporal, prefrontal and limbic association areas. The parieto-occipitotemporal association area is responsible for the continuous analysis of the spatial coordinates of all parts of the body, as well as the surroundings of the body. It assists the brain in controlling body movements and analyzing incoming sensory signals. Perhaps the most important area within the parieto-occipitotemporal association area is Wernicke’s area, a confluence of somatic, visual and auditory secondary and association areas, located in the posterior portion of the superior temporal lobe. Wernicke’s area is highly developed in the dominant hemisphere of the brain, and plays the single greatest role of any part of the cerebral cortex in the higher levels of brain function called intelligence. Areas adjacent to Wernicke’s area assist in reading comprehension and the capacity to name objects. The prefrontal association area functions in close association with the motor cortex in planning complex patterns and sequences of motor movement. It contains Broca’s area in the frontal cortex, which provides the neural circuitry for word formation, where the plans and motor patterns for the expression of individual words and short phrases are initiated. The limbic association area is connected to the limbic system, and is concerned with behaviour, motivation and emotion. Lateralization of the cerebrum Brain lateralization refers to subtle anatomical and obvious physiological differences between the left and right hemispheres. The left hemisphere controls the right side of the body, written and spoken language, numerical skills, scientific skills and reasoning. The right hemisphere controls left side of the body, as well as musical and artistic awareness, space and pattern perception, insight, imagination and the generation of mental images to compare spatial relationships. About 95% of the population display a left brain dominance, in which the Wernicke’s area is as much as 50% larger than in the right



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hemisphere at birth. Thus, the left hemisphere gets a head start in its development because of the tendency to direct one’s attention to the better developed hemisphere. Broca’s area is often larger in the dominant hemisphere as well. The remaining 5% of the population either develops both sides simultaneously, or more rarely, the right side alone is more highly developed. B a s a l g a n g l i a The basal ganglia are several groups of nuclei in each cerebral hemisphere that coordinate gross automatic muscle movement and regulate muscle tone. The largest nucleus of the basal ganglia is the corpus stratum, consisting of the caudate and lenticular nuclei. The lenticular nucleus is further subdivided into the putamen (laterally) and the globus pallidus (medially). The subthalamus and substansia nigra, although not classified as parts of the basal ganglia, interconnect with the basal ganglia and feedback to the thalamus. Although the circuitry of the basal ganglia is very complex, two major circuits, called the putamen and caudate circuit, make up the major functional aspects of the basal ganglia. The putamen circuit is responsible for executing learned, complex patterns of motor activity, such as writing letters of the alphabet and other skilled movements. It receives input mainly from those parts of the brain adjacent to the motor cortex, but not from the primary motor cortex itself. The putamen circuit then outputs to the primary motor cortex. When the putamen circuit is damaged, the cortical system of motor control can no longer provide these patterns and the writing is crude, as if one were learning for the first time. Lesions of the globus pallidus leads to spontaneous writhing movements of the limbs, neck or face, called athetosis. Lesions in the subthalamus lead to flailing movements of an entire limb, called hemiballismus. Multiple lesions in the putamen lead to flicking movements of the limbs, neck and head, called chorea. Lesions of the substansia nigra lead to paralysis agitans (e.g. Parkinson’s disease). The caudate circuit of the basal ganglia plays a major role in the cognitive control of motor activity. It receives input from all lobes of the cortex, including information from the associative areas, and outputs to the areas adjacent to the motor cortex, to accessory motor areas concerned with patterns of movement instead of individual muscle movements.



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The interplay of neurotransmitters within the basal ganglia include a dopamine pathway from the substansia nigra to the caudate nucleus and putamen, a GABA pathway from the caudate nucleus and putamen to the globus pallidus and substansia nigra, and an acetylcholine pathway from the cortex to the putamen and caudate nucleus. Both dopamine and GABA function as inhibitory agents, lending stability to motor control systems, while acetylcholine is excitatory to motor control. The basal ganglia also receives multiple, general pathways from the brain stem that secrete serotonin, enkephalin and norepinepherine. L i m b i c s y s t e m The limbic system is a ring of structures below the basal regions of the cerebrum that surround the brain stem, and is not so much an individual structure as aspects of several brain structures that control emotional behaviour and motivational drives. The major structure of the limbic system is the hypothalamus, which serves as the main communication link between the other structures of the limbic system and the rest of the brain. Other major structures include the limbic lobe (parahippocampal and cingulate gyri, the hippocampus), the dentate gyrus, the amygdala, the septal nuclei, the anterior nuclei of the thalamus and the olfactory bulb. Aside from the vegetative and endocrine function of the hypothalamus, stimulation or lesions of the hypothalamus can have profound effects on behaviour. The stimulation of the lateral hypothalamus increases general activity levels, sometimes leading to rage and aggression, as well as hunger and thirst. Lesions of the lateral hypothalamus can lead to extreme passivity, a lack of desire for food or drink, with a generalized loss of motivation. Stimulation of the ventromedial nucleus can cause the opposite reaction to stimulation of the lateral hypothalamus, including tranquility and satiety. Lesions of the ventromedial nucleus are opposite to those of the lateral hypothalamus, increasing the desire to eat and drink, increasing activity levels and feelings of extreme rage and aggression. Stimulation of the thin zone of the periventricular nucleus can lead to fear and punishment reactions. Stimulation of areas within the anterior and posterior regions of the hypothalamus can stimulate sexual activity. The limbic system is concerned with the affective nature of sensory experience, the determination of whether these



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sensations are pleasant or unpleasant. Researchers have identified certain areas in the limbic system that they associate with sensations of reward, punishment, learning and rage. Experiments have been conducted on monkeys to illustrate the specific functions of certain areas in the limbic system. These experiments make use of electrodes that are placed successively in different areas of the monkey’s brain, and a lever that activates the stimulation of these electrodes, placed in the cage. The monkey, by pushing down of the lever, is able to control the stimulation. If by stimulating a certain area of the brain the monkey gains a sense of reward or pleasure, it will continue pressing down of the lever, even if delectable food items are placed in the cage to entice them away from their activity. The areas that are been associated with pleasure and reward are the lateral and ventromedial nuclei areas of the hypothalamus, but in the case of the lateral nuclei, excessive stimulation seems to produce feelings of rage and aggression. Other areas of secondary importance in pleasure and reward include the amygdala, and certain areas of the thalamus and basal ganglia. If the area stimulated by the electrode causes a sense of fear or pain, the animal will cease pressing down on the lever. Prolonged stimulation of these centers for 24 hours or more can cause severe sickness and even death. The stimulation of these regions also seems to inhibit the reward centers. The most potent areas associated with punishment are the central gray areas surrounding the aqueduct of Sylvius in the midbrain, and the periventricular areas of the hypothalamus and thalamus. Other, less potent areas include regions in the amygdala and the hippocampus. Strong stimulation of punishment centers in the periventricular nucleus and reward centers in the lateral nuclei of the hypothalamus can lead to a rage pattern, such as defensive posturing, hissing, spitting and growling, piloerection, wide-open eyes and pupil dilation. Stimulation of the ventromedial nucleus of the hypothalamus, as well as the amygdala, hippocampus and anterior portions of the limbic cortex help to suppresses the rage pattern. Damage to these structures can make the animal or human more susceptible to bouts of rage. A m y g d a l a The amygdala is located immediately beneath the cortex of the medial anterior pole of each temporal lobe, receiving information from all portions of the limbic cortex as well as other parts of the brain, but especially from the auditory



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and visual association centers. In lower animals the amygdala is “hardwired” with the olfactory bulb, and while this is still true for humans to a certain extent, the amygdala has many other functions not associated with olfactory stimuli. Electrostimulation of certain areas within the amygdala can cause many of the same reactions as stimulation of the hypothalamus, such as changes in arterial pressure and heart rate, gastrointestinal motility, defecation and urination, pupillary dilation, piloerection, the secretion of anterior pituitary hormones, and rage, reward and punishment reactions. Stimulation of other areas in the amygdala can cause sexual activities including erection, ejaculation, ovulation, uterine activity and premature labour. The amygdala seems to function on a semiconscious level, integrating thought and emotion with specifics of the environment, allowing for appropriate behaviours on specific occasions H i p p o c a m p u s The hippocampus is located posterior to the amygdala, below the temporal lobe and stretches along upward and inward to form the ventral surface of the inferior horn of the lateral ventricle. Like the amygdala, the hippocampus is a channel through which incoming sensory information can lead to the appropriate behavioral reactions. Weak stimulation of the hippocampi causes psychomotor epileptic seizures that extend in duration beyond the period of stimulation. Specifically, the hippocampus seems to play a role in learning. In lower animals the hippocampus is part of the olfactory cortex. Olfactory stimuli is important for the animal to make decisions regarding the edibility of a given food, whether a certain smell indicates danger, whether a certain smell suggests sexual activity and many other functions. Thus, in the evolution of mammals, the hippocampus took on the role of decision making, determining the relative importance of certain stimuli, and the brain began to rely upon it for this function. If the hippocampus decides that a given stimuli is important, then the brain will store it as memory. Removal of the hippocampi in epileptic patients has resulted in a unique condition called anterograde amnesia, in which the person has full memory prior the surgery, but after, cannot commit newly learned activities to long term memory. These patients can only remember what happens throughout their day for a few minutes, after which the memory is completely lost. This dysfunction of memory however, only refers to symbolic or verbal forms of information, and



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persons afflicted with lesions to the hippocampi can continue with reflexive learning, such as physical skills learned through repetition, as in sports. Therefore, the hippocampus is responsible for the consolidation of long term memory. T h o u g h t , c o n s c i o u s n e s s a n d m e m o r y Thoughts, consciousness and memory seem to be the preserve of the higher functions of the brain. A thought is said to result from a pattern of stimulation generated by many parts of the nervous system, determined and coloured by the limbic system, thalamus and reticular activating system as being pleasurable or painful, and given discrete characteristics by the cerebral cortex. Consciousness is the continuous stream of sequential thoughts. Memory is classified into three types, immediate, short term and long term. Immediate memory lasts for seconds, and is the capacity to remember highly specific but limited amounts of information (such as a telephone number), only as long as the person actively thinks about it. There are several theories as to how immediate memory functions, and one theory is that it is the activation of vibrating neural circuits that continue to vibrate only continues for as long as the person exerts a conscious effort to remember. Short term memory is the capacity to remember information for days and even weeks, but unless the information becomes consolidated, the memory is lost. Long term memories last for years and are believed to be immediate or short term memories that initiate structural and chemical changes at synapses that enhance or suppress impulse conduction. This consolidation of information in long term memory is believed to require at least a 5 – 10 minutes to be minimally functional, and up to an hour to be completely retained. The brain engages in a rehearsal process, continually going over the information until consolidation has taken place. An electroencephalogram (EEG) can be used to determine the nature of the electric potentials generated by the brain, called brain waves, measured in cycles per second (hertz, Hz). To record an electroencephalogram 16 to 30 electrodes are attached to the scalp, connected to wires which are routed to an amplifier and an electroencephalograph. The various brain waves are then displayed as wavy lines on a moving sheet of graph paper by the electroencephalograph. The electrical discharge of a single neuron is insufficient to be recorded through the



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skull, so brain waves are indicative of several thousands and even millions of neurons firing simultaneously. Thus, the strength of the brain wave is dependent upon the synchronous firing of neurons, not on the total level of electrical activity in the brain. The are four types of brain waves, alpha, beta, theta and delta. Alpha waves (8 – 13 Hz) occur most intensely in the occipital region of the brain, and are present in the resting wakened state when the eyes are closed, but disappears during sleep. Beta waves (14 – 30 Hz) are recorded primarily in the parietal and frontal regions of the brain, and appear when the nervous system is active, during periods of sensory input and mental activity. These waves may occur in a waking state or during REM sleep. Theta waves (4 – 7 Hz) are found in the parietal and temporal regions and occur normally in children and in adults experiencing stress, frustration or disappointment. Theta waves occur in many brain disorders. Delta waves (1 – 5 Hz) arise from the cerebral cortex and occur in infants and in adults during deep sleep. In an awake adult, delta waves indicate a serious organic brain disease. An EEG reading can show considerable variability in the nature and number of spikes on the graph paper. An EEG of petit mal epilepsy for example, is typified by a spike and dome pattern. C e r e b e l l u m The cerebellum is located inferior to the occipital lobes of the cerebrum and posterior to the medulla and pons. It is separated from the cerebrum by the transverse fissure and by an extension of the cranial dura mater. It is shaped like a butterfly and the wings are representative of the left and right hemispheres. In turn, each hemisphere can be divided into an anterior and posterior lobe. The surface of the cerebellum is called the cerebellar cortex and is comprised of gray matter, arranged in a series of slender, parallel ridges called folia. Beneath the gray matter are white matter tracts called arbor vitae because they resemble the branches of a tree. Within the heart of this tree are the cerebellar nuclei, comprised of gray matter that give rise to the nerve fibers that convey information to the brain and spinal cord. The cerebellum controls skeletal muscle contractions required for skilled movements, coordination, posture and balance. It has no direct ability to cause muscle contraction however, but functions by sequencing motor activities and monitoring and adjusting motor activities elicited by other



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parts of the brain. It compares actual movements recorded by the peripheral sensory feedback system with movements intended by the motor system, and if the two signals do not compare favorably, then corrective impulses are sent back to the motor system to increase or decrease activation of the specific muscles. C r a n i a l n e r v e s There are 12 pairs of cranial nerves that arise from the brain and pass through the various foramina of the cranial bones. Each pair of cranial nerves are designated by a roman numeral, indicating the order, anterior to posterior, in which the nerves arise from the brain. A name is further appended with the numeral, giving indication of the nerves distribution or function. Some of these nerves have motor functions and others are sensory, but most have both sensory and motor functions. The cranial nerves are listed as followed, with a brief mention of whether they are sensory or motor (or mixed), what areas of the body they innervate, and their basic functions.

Cranial Nerve Type Area Function I. Olfactory sensory olfactory bulb smell II. Optic sensory retina vision III. Oculomotor mixed upper eyelid,

eyeball Sensory: proprioception Motor: eye movement

IV. Trochlear mixed superior muscle of eye

Sensory: proprioception, Motor: eye movement

V. Trigeminal mixed eye, face, mouth, tongue

Sensory: touch, pain, temperature, proprioception Motor: chewing

VI. Abducens mixed lateral muscle of eye

Sensory: proprioception Motor: movement

VII. Facial mixed tongue, face, glands

Sensory: proprioception, taste Motor: facial expression, saliva, tears

VIII. Vestibulocochlear mixed ear Sensory: hearing Motor: adjust function of hair cells in ear

IX. Glossopharyngeal mixed tongue Sensory: touch, pain, temperature Motor: swallowing, salivation

X. Vagus mixed throat, viscera Sensory: touch, pain, temperature, breathing Motor: swallowing,



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coughing, heart rate, digestion

XI. Accessory mixed neck, shoulders Sensory: proprioception Motor: swallowing, movement

XII. Hypoglossal mixed tongue Sensory: proprioception Motor: swallowing, speech

The spinal cord The spinal cord and spinal nerves initiate some of the fastest nervous reactions in the body. Think, for example, when you step upon something sharp like a pushpin, and before you are even conscious of the pain you retract your foot. This phenomenon, called a reflex, is a special feature of the spinal cord, an automatic response on the part of integrative neurons in the spinal cord that doesn’t require the activities of the brain. Besides displaying this feature, the spinal cord also serves as the main highway through which sensory nerves travel to the brain and motor neurons travel from the brain. Together with the brain, the spinal cord makes up the CNS. S p i n a l c o r d a n a t o m y The spinal cord is contained within the vertebral canal of the vertebral column, serving as the primary form of protection against spinal cord injury. Surrounding the bundles of nerves that make up the spinal cord is the spinal meninges, a connective tissue covering that protects the spinal cord and covers up the spinal nerves up to the point of exit from the vertebral column. There are three layers to the spinal meninges: an internal layer called the pia mater, a medial layer called the arachnoid and a superficial layer called the dura mater. Between the pia mater and the arachnoid layer is the subarachnoid space, through which the cerebrospinal fluid circulates. Triangular extensions of the pia mater form the denticulate ligaments, which project laterally on either side of the spinal cord, fusing with the dura mater. The denticulate ligaments protect the spinal cord against shock and displacement. The spinal cord is roughly cylindrical in shape, extending from the inferior portions of the brain to the border of the second lumbar vertebra. There are two enlargements of the spinal cord, a superior cervical enlargement and an inferior



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lumbar enlargement. Nerves to and from the upper extremities arise in the former (C4-T1), and nerves to and from the lower extremities arise in the latter (T9-T12). Although it is a continuous structure, when viewed externally the spinal cord appears to be segmented due to the presence of the 31 pairs of spinal nerves that arise from it. Each pair of spinal nerves arises from a spinal segment. There are two grooves that divide the spinal column into right and left halves. The anterior median fissure is a deep groove on the anterior (ventral) side of the spinal cord, and the posterior median fissure is a shallower groove that divides the posterior (dorsal) side of the spinal cord. The spinal cord is comprised of both white and gray matter, and in cross section, the gray matter is arranges in the shape of the letter ‘H’. The gray matter consists of the cell bodies of neurons and neuroglia, and unmyelinated axons and the dendrites of association and motor neurons. The white matter consists of bundles of myelinated axons of motor and sensory nerves. Making up the cross-bar of the ‘H’ in the gray matter is the gray commissure, and in the center of the gray commissure is a small space called the central canal that is filled with cerebrospinal fluid. This central canal is continuous with cavities found in the brain. The two portions of the ‘H’ that are located anteriorly are called the anterior gray horns, and the two portions of the H that are located posteriorly are called the posterior gray horns. Slight swellings in the regions of the H that make up the sides are called the lateral gray horns. The H-shaped gray matter divides the white matter into four distinct regions: the posterior white column, the two lateral white columns, and the anterior column. The anterior white column is separated by the anterior median fissure, but is connected in an area just anterior to the gray commissure, called the anterior white commissure. S p i n a l c o r d p h y s i o l o g y : s e n s o r y a n d m o t o r t r a c t s Tracts in the white matter of the spinal column conduct nervous impulses to and from the brain: sensory or ascending tracts transmit impulse from the peripheral nervous system to the brain; and motor or descending tracts transmit impulse from the brain to the peripheral nervous system. The names given to the various tracts indicate its location of origin and the direction that the nervous impulse



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is traveling. The anterior spinothalamic tract, for example, is located in the anterior white column, begin in the spinal cord and ends in the thalamus of the brain. Both sensory and motor information travel in several different tracts contained within the white matter of the spinal column. Sensory tracts can be roughly divided into two main tracts: the spinothalamic tract and the posterior column tracts. The spinothalamic tract conveys impulses for sensing pain, temperature, crude (poorly localized) touch and deep pressure. The posterior column tracts carry nerve impulses for proprioception (awareness of gross physical movement), discriminative (highly localized) touch, pressure and vibration. Motor impulses travel down the spinal column in two main types of tracts: pyramidal and extrapyramidal. Pyramidal tracts arise from the cerebral cortex of the brain to control precise, voluntary muscle activities. Extrapyramidal tracts arise from areas of the brain inferior to the cerebral cortex to convey autonomic movements, coordination, posture and muscle tone. S p i n a l c o r d p h y s i o l o g y : r e f l e x e s Apart from its function as a highway for sensory and motor tracts, the spinal cord serves as an integrating center for spinal reflexes. These reflexes are predictable, fast autonomic responses to changes to homeostasis. There are two basic forms of reflexes: somatic and visceral. Somatic reflexes involve the contraction of skeletal muscles, and are generally voluntary. Visceral reflexes control reflexes that are generally involuntary, such as the contraction of the heart, smooth muscle and glands. Unlike the white matter of the spinal column, the gray matter has its own capacity for receiving and integrating a nervous impulse from a sensory spinal nerve, and then sending the response the response through a motor spinal nerve. There are two separate points of attachment called roots that connect each spinal nerve to a segment of the spinal cord. The dorsal root contains sensory nerve fibers that conduct an impulse from the periphery of the body to the gray matter of the spinal column. The dorsal root ganglion is a swelling that contains the cell bodies of the sensory neurons (the term ganglion refers to a cluster of cell bodies outside the CNS). The ventral root ganglion contains motor neuron axons that conduct impulse from the



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gray matter of the spinal cord to the periphery of the body. There are no ganglia for ventral spinal nerves as the cell bodies are contained within the gray matter of the spinal cord. Whereas the cell bodies of motor neurons that supply a skeletal muscle are located in the anterior gray horn, motor neurons that supply smooth muscle, a gland or cardiac muscle is located in the lateral gray horn. R e f l e x a r c s The pathway that allows a sensory motor neuron to synapse with a motor neuron is called a reflex arc. One or more integrative neurons are located between the sensory and motor neurons to receive analyze and pass along an appropriate response to a stimulus. There are five major aspects to a reflex: 1. Receptor- a dendrite or an associated sensory structure responds to a stimulus by generating an action potential 2. Sensory neuron- nerve impulses are conducted from the receptor to the axon terminals of the sensory neuron in the CNS 3. Integration center- contained within the CNS, it is one or more association neurons that relay the impulse to other association neurons, as well as to the motor neuron. The simplest kind of integrating center is a single association neuron, called a monosynaptic reflex arc. If the integrating center contains more than two neurons it is called a polysynaptic reflex arc. 4. Motor neuron- the impulse generated by the integration center then travels along a motor neuron to the responding tissue. 5. Effector- receives and responds to the impulse, called a reflex. There are several different kinds of reflex arcs in the body, and vary in complexity. The stretch reflex is a monosynaptic reflex that results in the contraction of a muscle when it is stretched. Such reflexes are necessary to maintain the tone a specific muscle or muscle group, and since the stimulus for the reflex is the stretching of the muscle, it also prevents injury. As muscles are arranged in agonist and antagonist pairs, there is a polysynaptic reflex arc to the antagonist muscle or muscle group. This involves a branch of the sensory axon synapsing with an inhibitory association neuron in the integrating center. This inhibiting neuron then synapses with the motor neuron that innervates the antagonist muscle or muscle group, allowing them to relax while the agonists contract. This latter form of neural circuit is called reciprocal innervation. Another similar form of reflex arc is the tendon reflex. Whereas the stretch reflex operates a feedback mechanism to control muscle length, the tendon reflex operates a feedback mechanism that controls muscle tension.



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Another form of reflex is the flexor reflex, occurring in response to a painful or harmful stimulus, such as stepping on a tack. When this kind of stimulus happens, the sensory neuron conducts the impulse from the stimulated receptor to the spinal cord. Thus results in the firing of an impulse in an association neuron to the motor neuron to the affected body part so it can be retracted or moved out of harms way. One aspect of this reflex is identical to other types of reflexes, with the stimulation of the flexor (agonist) muscle and the inhibition of the extension (antagonist) muscle. All three of these reflexes are ipsilateral: the sensory and motor neurons enter and exit on the same side of the spinal column. When someone steps on a tack however, it doesn’t just involve the removal of the injured limb, but requires that the balance of the body must be shifted, requiring the contraction of muscle in other parts of the body. This is called a crossed extensor reflex. Thus, the incoming pain stimulus crosses to the opposite side of the spinal column, in areas at, just above and just below the latitude of entry. When the tack is stepped on, the injured foot is raised, and the muscles of the other leg are flexed accordingly to accommodate this shift in weight and maintain balance. Unlike the ipsilateral arrangement of spinal nerves in stretch, tendon and flexor reflexes, this crossing over of impulses is called a contralateral reflex. S p i n a l n e r v e s There are 31 pairs of spinal nerves, named and numbered according to the region of the spinal cord from which they emerge. There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 lumbar nerves, 5 sacral nerves and one pair of coccygeal nerves. At the cervical regions, spinal nerves are arranged so that their point of entry and exit through the vertebral foramina is lateral to the spinal segment. In the lower portions of the spinal column however, the vertebral foramina are located lower than at the location of fusion between the spinal nerve and spinal cord. Thus spinal nerves in these areas can be seen to be descending in a downward direction, the angle of descent increasing in an inferior fashion. Just beyond the point of exit out of its vertebral foramen, a spinal nerve divides into several branches, called rami. The ventral rami, except for the rami of T2 to T12, do not



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then go on to directly innervate the tissues of the body, but form networks on both the right and left sides of the body. These networks, comprised of groups of adjacent ventral rami, form plexuses. The principal plexuses are the cervical, brachial, lumbar and sacral plexuses. Emerging from each plexus are nerves that bear the name of the regions they innervate or the course they take through the body. Each nerve, in turn, may have several branches that are named for the specific structures they innervate. The ventral rami of spinal nerves T2 to T12 do not form plexuses and are known as the intercostal nerves, innervating the structures within the intercostal spaces (i.e. the axilla, intercostal muscles, skin of thorax etc.). D e r m a t o m e s The skin is supplied with somatic sensory nerve impulses that feed directly into the spinal cord, and each spinal serve innervates a specific and continuous region of the skin. All the spinal nerves except for C1 supply branches to the skin. A dermatome is a region of skin that is supplied by one pair of spinal nerves. Since a specific spinal cord segment innervates each dermatome, it is possible to determine the location any spinal nerve dysfunction by assessing for a response to various stimuli within that dermatome.

by todd caldecott lesson vi: nervous system, part one€¦ · 2. integrative: analyzing sensory...

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