THE NERVOUS SYSTEM

Introduction

Neurons: masses of nerve cells Nerves Structural and functional

units of nervous system Specialized to react to

physical and chemical changes

Neuroglical Cells: provide neurons with physiological requirements Functions like connective

tissue

Introduction

Nerve Impulse: information transmitted in form of electrochemical signal Travels along nerve fiber

Functions: sensory, integrative, motor Central Nervous System:

brain and spinal cord Peripheral Nervous System:

nerves

Sensory Functions

sensory receptors at the ends of peripheral neurons Detect changes outside the body

Light and sound intensities Temperature and oxygen concentration

Convert sensory information into nerve impulses Transmitted from peripheral nerves to CNS Integrative function: at CNS, signals are

integrated sensation, adding to memory, or produce thoughts

for perceptions

Sensory Functions

Motor Functions: after making conscious or subconscious decisions, act upon perceptions Effectors: signal from CNS

travels to PNS to respond Muscles and glands that

respond to nerve impulses Action counteracts the

change detected Maintain homeostasis

Neuron Structure

Cell Body: granular cytoplasm, cell membrane, and organelles Neurofibrils: network of fine threads which

extend into nerve fibers Chromatophilic substance (Nissl Bodies):

Scattered in cytoplasm Similar to rough ER protein synthesis

Nucleus: center of cell body Mature neurons don’t divide

Neuron Structure

Nerve Fibers: extend from cell body Dendrites: many

Shorter and highly branched Main receptive surface for communication

Axons: only one Conducts nerve impulse away from the body At end, fine extensions that contact

the receptive surfaces of other cells

Axon

Schwann Cell: enclosed axons Wind tightly around axon

Myelin: membrane layers of lipoproteins Myelin sheath: layers with more lipids Smaller axons lack a myelin sheath

Neurilemma: surrounds myelin sheath Nodes of Ranvier: narrow gaps in the myelin sheath

TYPES OF NERVE CELLS

Classification of Neurons by Structure

Bipolar Neurons: has only two nerve fibers Arise from each end 1 axon and 1 dendrite Eyes, nose, and ears

Classification of Neurons by Structure

Unipolar Neuron: single fiber that extends from cell body that divides into two branches Dendrite: connects to peripheral body part Axon: enters the brain or spinal cord Ganglia: aggregation of unipolar cells

Located outside brain and spinal cord

Classification of Neurons by Structure

Multipolar Neurons: many nerve fibers Axon: only one Dendrite: remaining fibers Neurons in brain and spinal cord

Classification of Neurons by Function

Sensory: afferent Unipolar or bipolar Carry impulses from PNS to CNS Specialized receptor ends at tips of dendrites Receptor cells: dendrites in skin or sensory

organs Changes inside or outside the body Single travels to brain or spinal cord

Classification of Neurons by Function

Interneurons: internuncial or association neuron multipolar Lie within brain or spinal

cord Link other neurons Transmit impulses from

one part of the brain or spinal cord to another Direct incoming sensory

impulses for appropriate processing

Classification of Neurons by Function

Motor Neurons: efferent neurons Multipolar Carry impulses out of the brain or spinal cord

to effectors Stimulate muscle to contract and glands to

release hormone

Classification of Neuroglical Cells

Fill spaces, support neurons, provide structural frameworks, produce myelin, and carryon phagocytosis

Outnumber neurons Schwann Cells Nicroglial Cells: scattered in

CNS Support neurons phagocytize bacterial cells and

cellular debris

Classification of Neuroglial Cells Oligodendrocytes: occur in rows along

nerve fibers Form myelin within the brain and spinal

cord Do not form sheaths

Classification of Neuroglical Cells Astrocytes: between neurons and blood

vessels Structural support, join parts, regulate

concentrations of nutrients and ions within tissue

Form scar tissue in CNS

Classification of Neuroglical Cells Ependymal Cells

form an epithelial like membrane that covers specialized brain parts

Forms the inner lining that enclose spaces within the brain and spinal cord

NERVE IMPULSES

Cell Membrane Potential

Surface of cell membrane is usually charge Polarized

Unequal distribution of positive and negative ions between sides of the membrane

Important in conducting nerve impulses

Distribution of Ion

Determined by pores or channels in the membrane

Some channels are always open Others can be closed Channels can be selective

Only allow certain ions to pass K pass easier than Na,

and less so than Ca

Resting Potential

Potential difference: difference in electrical charge between two regions Inside of cell: slightly negative Outside of cell: slightly positive

Resting Potential: potential difference in a nerve cell

Potential Changes

Nerve cell are excitable Respond to changes in their surroundings Temperature, light, or pressure Signals from other neurons

Changes in resting potential Depolarizing: inside of membrane becomes less

negative

Resting Potential

Resting potential is graded Amount of change in potential is

proportional to the intensity of stimulation Summation: accumulation of stimulation

Threshold potential Action Potential: 1/1000th of a second

Na diffuses freely inward Membrane becomes depolarized

K diffuses out of cell inside becomes negative repolarized

Nerve Impulse

When an action potential occurs in one region, it causes the bioelectric current to flow to the adjacent portions of the membrane

This stimulates the adjacent membrane Wave of action potentials down a nerve

fiber

Impulse Conduction

Unmyelinated fiber conducts an impulse over its entire surface

Myelinated surface prevents ion flow from the insulated membrane If the sheath were continuous, no impulse travel Nerve impulse jumps from node of Ranvier to next

node Contain Na and K channels

Impulse Conduction

Impulse is much faster than on an unmyelinated nerve

Speed of nerve impulse is proportional to the fiber diameter Skeletal Muscle fiber: 120

meters per second Sensory Fiber: 0.5 meters

per second Nerve stimulation is all-or-

none response Amount of stimulation

does not vary degree of response

The Synapse

Junction between two neurons Not a direction connection Gap: synaptic cleft

Synaptic Transmission: signal runs from dendrite of a neuron to the axon At axon, neurotransmitter

are released Bind with receptor on the

next neuron

Excitatory and Inhibitory Actions Excitatory: increases Na permeability

Trigger nerve impulses Inhibitory: decrease Na permeability

Lessens chance of nerve impulse

Neurotransmitters

~50 types have been identified Some neurons only release one type

where others release two or three Acetylcholine: stimulates muscles Monoamines: epinephrine,

norepinephrine, dopamine, serotonin Amino acids and peptides

Neurotransmitters

Synthesized in the cytoplasm of the synaptic knob

Stored in synaptic vesicles When action potential is reached, Ca ions

are transported inward Neurotransmitters are released Afterwards, some are reabsorbed while

others are broken down

Impulse Processing

Neuronal Pools: receive impulses, processes them, and conducts impulses away Facilitation: neuron receives a subthreshold

stimuli and becomes more excitable Convergence: two or more impulses from

different sources on a single neuron Additive effect on neurons

Divergence: impulses may pass into several output fibers Amplifies impulses

NERVE PATHWAYS, MENINGES, AND SPINAL CORD

Nerve Pathway

Routes nerve impulses follow through nervous system Reflex arc: involuntary actions

Begins with a receptor at the end of a sensory fiber

Leads to interneurons in CNS, or reflex center Communicate with motor neurons to effectors

Reflex Behavior; automatic subconscious responses to stimuli Heart rate, breathing, blood pressure, and

digestion Swallowing, sneezing, coughing, and vomiting

Reflex Behavior

Knee-jerk reflex: patellar tendon reflex sensory neuron communicating directly to a

motor neuron Helps maintain upright posture

Reflex Behavior

Withdrawal Reflex: touching a finger to something painful Motor neurons in the flexor muscles are

simulated Arm jerks back Signal is also sent to the

brain to signal pain Protective reflex

Meninges

Membranes located between the bone and soft tissues the nervous system

Protect the brain and spinal cord

Meninges: 3 layers

Dura mater: outermost layer Tough, white fibrous connective tissue Contains many blood vessels and nerves

Meninges: 3 layers

Arachnoid mater: thin, weblike membrane Lacks blood vessels Located b/w dura and pia maters

Meninges: 3 layers

Pia mater: very thin layer Contains many

nerves and blood vessels

Nourish the cells of the brain and spinal cord

Spinal Cord

Slender nerve column that passes downward from the brain into the vertebral canal

Spinal Cord

Structure of Spinal Cord

31segments Each gives rise to a pair of spinal nerves

Branch to various parts of the body Cervical Enlargement: thickening in the neck

Nerves to upper limbs Lumbar Enlargement: thickening in lower

back Nerves to lower limbs

Functions of the Spinal Cord

Conducting nerve impulses Ascending tract: nerve tract

or pathway that carries sensory information to the brain

Descending tracts: conduct motor impulses form the brain to the muscles and glands

Spinal reflexes: knee jerk and withdrawal Reflex arc passes through

spinal cord

BRAIN

Brain

Subdivides Cerebrum Cerebellum Brain Stem

Cerebrum

Two cerebral hemispheres Connected by corpus

callosum Cerebral Cortex: thin

layer of grey matter near the surface

White matter consists of myelinated nerve fibers that connect neurons within the nervous system and communicate with other body parts

Functions of Cerebrum

Higher brain function Sensory, motor, and association areas One hemisphere usually dominates for

certain intellectual functions

Diencephalon

Thalamus: central relay station for incoming sensory impulses

Hypothalamus: maintains homeostasis Limbic System: emotions and modifies

behaviors

Brain Stem

Midbrain, pons, and medulla oblongata

Midbrain: reflex centers Head and eye movement

Pons: transmits impulses b/w cerebrum and other parts of nervous system Rate and depth of breathing

Medulla Oblongata: all ascending and descending impulses Several vital and nonvital reflex centers

Brain Stem

Cerebellum: two hemispheres Reflex center for integrating sensory

information coordination of skeletal muscle movements Maintenance of equilibrium

PERIPHERAL NERVOUS SYSTEM AND AUTONOMIC NERVOUS SYSTEM

PNS

Cranial and spinal nerves Bunch form the brain

and spinal cord to all body parts

Somatic and autonomic systems

Cranial Nerves

12 pairs of cranial nerves connect the brain to parts of the head, neck, and trunk

Some sensory or primarily motor

Mostly mixed Some are somatic,

others autonomic

Spinal Nerves

31-pairs originate from the spinal cord Mixed nerves provide 2-way communication b/w

spinal cord and upper and lower limbs, neck, and trunk

Grouped according to the levels from which they arise

Emerges from a dorsal and ventral root Divides into branches beyond its foramen Most combine to form plexuses

Nerve fibers are sorted and recombined so that those associated with a particular region reach it together

Autonomic Nervous System

Functions without conscious effort Regulates visceral activities maintaining

homeostasis Autonomic functions are reflexes

controlled from nerve centers in the brain and spinal cord Sympathetic and parasympathetic Parasympathetic is most active

Autonomic Nerve Fibers

Motor fibers Sympathetic fibers leave the spinal cord

and synapse in paravertebral ganglia Parasympathetic fibers begin n the brain

stem and sacral region of the spinal cord and synapse in ganglia near viscera

Autonomic Neurotransmitters Differences in neurotransmitters

different effects of the autonomic divisions

Sympathetic and parasympathetic preganglionic fibers secrete acetylcholine

Parasympathetic postganglionic fibers secrete acetylcholine

Sympathetic postganglionic fibers secrete norepinephrine

Control of autonomic activity

Somewhat independent Control centers in medulla oblongata &

hypothalamus Utilize autonomic nerve pathways Limbic system and cerebral cortex control the

autonomic system during emotional distress