Nervous System Organization

Nervous System Nervous System OrganizationOrganization

Dr. Carmen E. Dr. Carmen E. RexachRexachPhysiologyPhysiology

MtSACMtSAC Biology Dept. Biology Dept.

Organization of nervous system

CNS PNS

Sensory(afferent)

Motor(efferent)

autonomicsomatic

sympatheticparasympathetic

Single neuronpathway

Two neuronpathway

CNS

• Functions– Integrates information from PNS– Processes information– Cognition, learning, memory– Plans and executes voluntary

movements• Components

– Brain• Cerebrum• Diencephalon• Midbrain and Hindbrain

– Spinal cord• Ascending and descending tracts

Brain

• Cerebrum• Cerebral cortex

– frontal, parietal, temporal, occipital lobes• Cerebral lateralization

– Decussation of pyramids– split brain procedures of the corpus callosum in epilepsy

Major landmarks of CNS

Cerebrum• Only structure of the telencephalon.• Largest portion of brain (80% mass).• Responsible for higher mental

functions.• Corpus callosum:

– Major tract of axons that functionally interconnects right and left cerebral hemispheres.

Cerebral cortex• Convolutions

– Elevated folds: gyri– Depressed groves: sulci

• Frontal lobe– Anterior portion of each cerebral hemisphere.– Precentral gyri

• Contains upper motor neurons.• Involved in motor control.

• Body regions with the greatest amount of motor innervation are represented by largest areas of motor cortex.

Cerebral Cortex• Parietal lobe:

– Primary area responsible for perception of somatesthetic sensation.

– Body regions with highest densities of receptors are represented by largest areas of sensory cortex.

• Temporal lobe:– Contain auditory centers that receive sensory

fibers from cochlea.– Interpretation and association of auditory and

visual information.

Cerebral Cortex

• Occipital Lobe: – Primary area responsible for vision and

coordination of eye movements.• Insula:

– Implicated in memory encoding.– Integration of sensory information with

visceral responses.– Coordinated cardiovascular response to

stress.

Basal ganglia• Masses of gray matter

composed of neuronal cell bodies located deep within white matter.

• nuclei around thalamus that help plan voluntary movement– Corpus striatum = largest

part of basal ganglia• caudate nucleus• putamen• globus pallidus

Basal ganglia diseases

• Parkinson’s– Cause: lesions in

substantia nigra– Results: loss of

dopaminergicneurotransmitters

– Symptoms: tremor, rigidity, bradykinesia

Basal ganglia diseases

• Huntington’s chorea– Cause: genetic

disorder causing loss of striatopallidal and striatonigral neurons

– Result: loss of GABA (inhibitory)

– Symptoms: progressive dementia and bizarre involuntary movements

Cerebral Lateralization• Cerebral dominance:

– Specialization of one hemisphere.

• Left hemisphere: – More adept in language

and analytical abilities.– Damage:

• Severe speech problems.

• Right hemisphere: – Most adept at

visuospatial tasks.– Damage:

• Difficulty finding way around house.

Emotion and Motivation• Important in the neural basis of emotional

states are hypothalamus and limbic system.• Limbic system:

– Group of forebrain nuclei and fiber tracts that form a ring around the brain stem.• Center for basic emotional drives.

• Closed circuit (Papez circuit):– Fornix connects hippocampus to

hypothalamus, which projects to the thalamus which sends fibers back to limbic system.

Limbic system: functions• Controls emotional behavior, such as:

– aggression– fear– feeding– sex– goal directed behavior

• Papez circuit– Emotions and their expression governed by a

circuit of four structures interconnected by nerve fibers, not by a single structure

– Four structures: hypothalamus, anterior thalamic nucleus, cingulate gyrus, and hippocampus

Memory

• Several different systems of information storage

• declarative memory– ability to remember facts– short and long term memory– medial temporal lobe consolidates short term

into long term• protein synthesis consolidates memory• other structural changes in neurons and synapses

– formation of new synapses– growth of dendritic spines

Neuronal Stem Cells in Learning and Memory

• Neural stem cells:– Cells that both renew themselves through

mitosis and produce differentiated neurons and neuroglia.

• Hippocampus has been shown to contain stem cells (required for long-term memory).

• Neurogenesis = Production of new neurons• Indirect evidence that links neurogenesis

in hippocampus with learning and memory.

Diencephalon• Thalamus & epithalamus

(pineal gland)– relay center for sensory

information– alertness and arousal

from sleep• Hypothalamus &

pituitary gland– hunger, thirst centers– body temperature

regulation– visceral responses to

emotional state

Midbrain– Corpora quadrigemina:

• Superior colliculi:– Involved in visual reflexes.

• Inferior colliculi:– Relay centers for auditory information.

– Cerebral peduncles:• Composed of ascending and descending fiber

tracts.– Substantia nigra:

• Required for motor coordination.– Red nucleus:

• Maintains connections with cerebrum and cerebellum.

– Involved in motor coordination.

Hindbrain• Metencephalon

• Pons– apneustic and pneumotactic centers

• Cerebellum– coordination

• Myelencephalon• medulla oblongata

Myelencephalon:Medulla oblongata

• Pyramids• regulation of breathing

– respiratory center• regulation of cardiovascular responses

– vasomotor center -- enervation of blood vessels

– cardiac control center• RAS

– reticular activation system– nonspecific arousal of the cerebral cortex

Spinal cord tracts• Ascending

– sensory from proprioceptors, cutaneous, visual receptors

– decussation of pyramids• Descending

– Corticospinal– Extrapyramidal

Corticospinal (pyramidal) tract

• No synapse from cortex to spinal cord

• Most of the nuclei in the precentral gyrus

• Action: fine muscle control

• Lateral– decussates in the medulla– 80-90% of the tract

• Anterior– decussates in the spine

Extrapyramidal tract• Many synapses = more difficult to

diagnose location of stroke• Action: Gross motor control and

involuntary muscle excitation• Originates in the midbrain and brainstem• back circuits up to cortex and nuclei• Influence: trunk, neck, upper part of

limbs• Ex) Reticulospinal tract

– major descending pathway

Cranial and Spinal Nerves

• Cranial nerves:– 2 pairs arise from neuron cell bodies in

forebrain.– 10 pairs arise from the midbrain and hindbrain.– Most are mixed nerves containing both sensory

and motor fibers. • Spinal nerves:

– 31 pairs grouped into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and l coccygeal.

– Mixed nerve that separates near the attachment of the nerve to spinal cord.

• Produces 2 roots to each nerve.– Dorsal root composed of sensory fibers.– Ventral root composed of motor fibers.

Electroencephalograms (EEG)

• Records electrical activity of neurons = brain waves• Determined by # of neurons firing together• Four frequency classes

– Alpha waves (8-13 Hz)• “idling” brain = relaxed, calm, wakeful

– Beta waves (14-25 Hz)• Higher frequency, not regular• Concentrating on something

– Theta waves (4-7 Hz)• Irregular, common in children

– Delta waves (4 Hz or <)• High amplitude• During deep sleep or anesthesia• Indicator of brain damage in awake adults

EEG

• Change with age, stimuli, brain disease

• Aids in diagnosis and localization of lesions, tumors, infarcts, epileptic lesions

• Absence of brain waves = brain death

Peripheral nervous system

• Sensory receptors• Somatic motor neurons• Autonomic motor neurons

Sensory (afferent) • Sense environmental stimuli and

transduce signal• Transmit information to CNS

Somatic (efferent)• Innervate skeletal

muscle fibers• One neuron

pathway• Nerve cell bodies in

CNS• Axons leave either

through ventral root or cranial nerve

Autonomic nervous system• Innervates organs not usually under

voluntary control• Two neuron pathways

– Pre-ganglionic neurons in CNS – Post-ganglionic neurons in PNS

• Sympathetic = fight or flight • Parasympathetic = rest or repose• Synapse on autonomic effectors

– Cardiac muscle– Smooth muscle– Glands

Characteristics of Autonomic Neurons

• Preganglionic autonomic fibers originate in midbrain, hindbrain, and upper thoracic to 4th sacral levels of the spinal cord.

• Autonomic ganglia are located in the head, neck, and abdomen.

• Presynaptic neuron is myelinated and postsynaptic neuron is unmyelinated.

• Autonomic nerves release NT that may be stimulatory or inhibitory.

Autonomic ganglia• Located in head, neck, abdomen• Sympathetic chain ganglia

CNS PNS

preganglionic postganglionic

Sympathetic division• Synapse close to the CNS & far away from

effector organ• Travel within spinal nerves• Mass activation due to convergence &

divergence• Sympathoadrenal system: converge on

adrenal medulla

preganglionicpostganglionic

AChNE

Sympathetic Division

Parasympathetic division

• Terminal ganglia -- close to effector organ• Fibers outside of spinal nerves (usually)• No stimulation to: cutaneous blood vessels,

blood vessels in skeletal muscle, sweat glands, arrector pili muscles

ACh

ACh

preganglionicpostganglionic

ParasympatheticDivision

Cranial nerves and Parasympathetic Division

• 4 of the 12 pairs of cranial nerves (III, VII, X, XI) contain preganglionic parasympathetic fibers.

• III, VII, XI synapse in ganglia located in the head.• X synapses in terminal ganglia located in widespread

regions of the body.• Vagus (X):

– Innervates heart, lungs esophagus, stomach, pancreas, liver, small intestine and upper half of the large intestine.

• Preganglionic fibers from the sacral level innervate the lower half of large intestine, the rectum, urinary and reproductive systems.

Functions of the ANS• Sympathetic = fight or flight

– Increased HR– Bronchiole dilation– Increase blood glucose, etc.

• Parasympathetic = rest and repose– Decrease HR– Incr digestion– Dilation visceral bv

Neurotransmitters• Adrenergic: release norepinephrine

(NE)– Adrenal medulla

• 85% epinephrine• 15% norepinephrine

• Cholinergic: release acetylcholine (ACh)

catecholamines

Adrenergic receptors• Beta adrenergic receptors:

– Produce their effects by stimulating production of cAMP.

– NE binds to receptor.– G-protein dissociates into α subunit or βγ−

complex.– Depending upon tissue, either α subunit or βγ−

complex produces the effects.• Alpha subunit activates adenylate cyclase, producing

cAMP.– cAMP activates protein kinase, opening ion channels.

Adrenergic receptors

• Alpha1 adrenergic receptors:– Produce their effects by the production of

Ca2+

– Epi binds to receptor.– Ca2+ binds to calmodulin– Calmodulin activates protein kinase, modifying

enzyme action• Alpha2 adrenergic receptors:

– Located on presynaptic terminal• Decreases release of NE

– Negative feedback control– Located on postsynaptic membrane

• When activated, produces vasoconstriction

Response to adrenergic stimulation

• Excitatory and inhibitory effects.• Responses due to different membrane

receptor proteins.1) α1 : constricts visceral smooth muscles.2) α2 : contraction of smooth muscle. 3) β1 : increases HR and force of contraction.4) β2 : relaxes bronchial smooth muscles.5) β3: adipose tissue, function unknown.

Cholinergic stimulation• All somatic motor neurons, all preganglionic

and most postganglionic parasympathetic neurons are cholinergic.– Release ACh as NT.– Somatic motor neurons and all preganglionic

autonomic neurons are excitatory.– Postganglionic axons, may be excitatory or

inhibitory.• Two types of receptors

– Muscarinic receptors– Nicotinic receptors