Chapter 7 The Nervous System

Biology 112

Tri-County Technical College

Pendleton, SC

Structural Organization Structural Classification which includes ALL

nervous system organs has two subdivisions: CNS-consists of brain and spinal cord PNS(peripheral)-is part outside CNS consisting

mainly of nerves that from CNS Spinal nerves carry info to and from spinal cord Cranial nerves carry info to and from brain

Functional Organization Concerned ONLY with PNS structures Sensory/Afferent division conducts

impulses to CNS Sensory fibers from skin, skeletal muscles,

and joints are called somatic sensory (afferent) fibers

SFs from visceral organs called visceral sensory fibers (visceral afferents)

Sensory division keeps CNS informed of events going on outside/inside body

Functional Organization, cont.

Motor/Efferent division carries impulses from CNS to effector organs, muscles, and glands Impulses activate muscles/glands They EFFECT (bring about) a motor

response MOTOR division has two subdivisions

SOMATIC NERVOUS SYSTEM AUTONOMIC NERVOUS SYSTEM

Autonomic Nervous System ANS is motor subdivision of PNS that controls body

activities automatically (involuntary) Composed of special group of neurons that regulate

cardiac muscle, smooth muscle, and glands ANS also called involuntary nervous system ANS has two distinct subdivisions Sympathetic and Parasympathetic

Serve same organs but cause essentially opposite effects Counterbalance each other to keep body systems running

smoothly

Organization of Nervous System

ANS, continued Preganglionic axons of sympathetic div. release

acetylcholine; Postganglionic fibers release noreeinephrine and/or epinephrine (adrenergic fibers)

Pre- and Postganglionic axons of parasympathetic division release acetylcholine (cholinergic fibers)

Sympathetic part mobilizes body during extreme situations (fear, exercise, rage) E for exercise/emergency/embarrassment

Parasympathetic part allows body to “unwind” and conserve energy D for digesting/defecation/diuresis

Somatic Motor Division

Somatic system allows conscious/voluntary control of skeletal muscles

Often called “voluntary nervous system” Cell bodies of motor neurons are inside

CNS and their axons (in spinal nerves) extend all way to skeletal muscles served

Skeletal muscles are effectors of somatic nervous system

Neurotransmitter is acetylcholine

Cells of Nerve Tissue

Two principal types of cells Neurons and supporting cells

Supporting cells of CNS lumped together as neuroglia (nerve glue)

Neuroglia (Glia) support, insulate, and protect delicate neurons

There are different types of “supporting cells” in the CNS

Supporting Cells, cont. Astrocytes account for nearly ½ of neural tissue Brace/anchor neurons to blood capillaries Form living barrier between capillaries and

neurons & play role in making exchanges between the two

Help protect neurons from harmful stuff in blood Help control chemical environment by picking up

excess ions and recapturing released neurotransmitters

Supporting Cells, cont.

Microglia are phagocytes that dispose of debris-dead brain cells, bacteria, etc.

Ependymal Cells line cavities of brain and spinal cord beating of their cilia help circulate cerebrospinal

fluid that fills those cavities and forms protective cushion around CNS

Oligodendrocytes wrap their flat extensions around nerve fibers producing fatty insulating coverings called myelin sheaths

FYI

Neuroglia are NOT able to transmit nerve impulses and they NEVER lose their ability to divide

Most brain tumors are GLIOMAS or tumors formed by glial cells

He ain’t heavy…he’s my brother

Supporting cells in PNS come in two major varieties

Schwann cells form myelin sheaths around nerve fibers that are found in PNS

Satellite cells act as protective, cushioning cells

Supporting cells, Visual

You’re getting on my nerve.. Neurons are highly specialized to carry impulses

(transmit messages) All neurons have CELL BODY (SOMA) and one

or more slender processes extending from cell body

Dendrites conduct electrical impulses TOWARD the cell body

Axons conduct electrical impulses AWAY from cell body

May be 100s of dendrites but only 1 axon Axons may branch at terminal end forming 100s

to 1000s of axonal terminals

Neuron, Visual

Neuron Part Functions

Cell body = nucleus and metabolic center Dendrites = impulses to the cell body Axon = impulses away from cell body to

either another neuron or the effector Most long neurons covered with whitish,

fatty material called MYELIN Myelin protects and insulates fiber and > the

transmission rate of nerve impulses

Parts, cont.

Axons outside CNS insulated by Schwann cells wrap tightly around axon jelly-roll style wrapping done, tight coil of wrapped membranes

called myelin sheath encloses axon most of Schwann cell cytoplasm ends up just beneath

outermost part of its plasma membrane this part of Schwann cell, external to myelin sheath is

called the neurilemma

Parts, cont.

Myelin sheath formed by many individual Schwann cells Gaps or indentations called NODES OF RANVIER

occur at regular intervals Myelin sheaths in CNS are formed by the

oligodendrocytes CNS myelin sheath lacks a neurilemma

Neurilemma plays vital role in fiber regeneration of injured fiber

Regeneration of damaged fibers largely lacking in CNS

Schwann cells, Visual

Nuclei, Ganglia, and more…

In CNS, cell bodies found in clusters called nuclei This well protected location essential to well-

being of nervous tissue Small collections of cell bodies called

ganglia are found outside CNS in the PNS Bundles of nerve fibers running through

CNS are called tracts Bundles of nerve fibers running though

PNS are called nerves

Nuclei and Ganglia, cont.

Terms white matter and gray matter refer to myelinated versus unmyelinated reions of the CNS

As a general rule, white matter consists of dense collections of myelinated fibers (tracts)

Gray matter contains mostly unmyelinated fibers and cell bodies

Classification of Neurons

Can be classified according to HOW they function or according to their STRUCTURE

FUNCTIONAL classification based on direction impulse traveling relative to CNS

Sensory (afferent) neurons carry impulses from sensory receptors (internal organs/skin) to CNS Cell bodies of sensory neurons always found in

ganglion outside the CNS Inform about what is happening in/out of body

Classification, cont.

Motor neurons (efferent) carry impulses away from CNS to viscera/muscles/glands Cell bodies of motor neurons always located

in CNS Association (interneurons) neurons

connect sensory and motor neurons in a neural pathway Cell bodies of association neurons always

located in CNS

Functional Classification, Visual

Structural Classification

Based on number of processes extending from cell body

Multipolar neuron has several processes all motor and association neurons are

multipolar most common structural type

Bipolar neuron has 2 processes—dendrite and axon Rare in adults; found only in some special

sense organs (eye, ear) where act as sensory receptor cells

Structural Classification, cont.

Unipolar neuron has single process emerging from cell body

Very short and divides into proximal (central) and distal (peripheral) fibers

Unique in that only small branches at end of peripheral process are dendrites

Remainder of peripheral process and central process function as axon

In unipolar neuron, axon conducts impulse both toward and away from cell body

Sensory neurons found in PNS ganglia are unipolar

Structural Classification, Visual

Resting State…I wish

Plasma membrane of resting (inactive) neuron is polarized

fewer positive charges on inner surface of membrane that on its outer face in tissue fluid

Major positive ions on inside are K+

Major positive ions on outside are Na+

As long as inside remains more negative as compared to outside, neuron will stay inactive (resting)

Resting potential of neuron is about –70 millivolts

Action, cameras, and more…

Action potential = nerve impulse Stimulus changes permeability of patch of

membrane and sodium ions diffuse rapidly into cell

Changes polarity of membrane at that location Inside becomes more +, outside more –

Event called depolarization If stimulus strong enough (at or >

threshold) action potential is initiated

Action Potential, cont.

Depolarization of first membrane patch causes permeability changes in adjacent membrane and event is repeated

Membrane potential goes from –70 mv to +30 mv

Action potential propagates rapidly along entire length of membrane

Don’t want to waste space on this slide..so, IS THIS FUN, OR WHAT????

Action Potential, Visual

Repolarization

After patch of membrane depolarizes, it repolarizes

Membrane permeability changes and K+ ions diffuse OUT of cell

Restores negative charge inside cell and positive charge outside

Sodium-potassium pump used to restore ionic conditions of resting neuron

Resting potential of –70 mv restored Until repolarization occurs, neuron CANNOT

conduct another impulse

Reflex Defined and More… Reflex is rapid, predictable and involuntary

response to a stimulus Reflexes occur over neural pathway called reflex

arc SOMATIC = reflexes that stimulate skeletal

muscles Dendrite of sensory neuron carries impulse to CNS Processing of info may/may not occur in CNS Axon of motor neuron carries impulse to effector pull hand away from hot object = somatic reflex

Reflex, cont. Autonomic reflexes regulate activity of smooth

muscles, heart, and glands saliva secretion and size of eye pupils two examples

of autonomic reflexes ALL reflex arcs have minimum of 5 elements Sensory receptor which reacts to stimulus Afferent neuron Integration center located in CNS Efferent neuron Effector organ (muscle or gland to be stimulated) Two-neuron (patellar knee-jerk) most simple Three-neuron (flexor/withdrawal) more complex

Somatic reflexes

Patellar knee-jerk somatic reflex receptors in patellar tendon effectors are upper leg muscles that cause leg

extension Withdrawal somatic reflex

receptors located in epidermis/dermis touching a hot object or finger stick effectors are the appropriate muscles can be used as a diagnostic tool Whenever reflexes are exaggerated, distorted,

or absent, nervous system disorders indicated

Reflexes, Visual

Cerebrum Functions

Speech Memory Logic Emotional Responses Consciousness Interpretation of sensation Voluntary movements

Basal Nuclei (ganglion) Modify instructions from cerebrum to skeletal

muscles Problems with basal nuclei lead to inability to

carry out movements in normal way Huntington’s disease=inability to control

muscles; individual exhibits abrupt, jerky, almost continuous movements helped by drugs that block dopamine’s effect

Parkinson’s disease=trouble initiating movement or in getting muscles going

Persistent hand tremor with thumb and index finger making continuous circles with one another Due to deficit of neurotransmitter dopamine

Functions of the Thalamus

Thalamus is part of diencephalon (innerbrain) that sits atop brain stem & enclosed by cerebral hemispheres

Encloses third ventricle of the brain Relay station for sensory impulses > cerebrum Provides “crude” recognition of whether

sensation about to have is pleasant or unpleasant Actual interpretation occurs in neurons of

cerebral cortex

Functions of the Hypothalamus

Part of the diencephalon & important autonomic nervous system center

Plays role in regulation of body temp, water balance, and metabolism

Center for many drives and emotions Important part of limbic system (emotional-visceral

brain) Thirst, appetite, sex, pain, and pleasure centers

located in hypothalamus Regulates pituitary gland and produces two

hormones: ADH and oxytocin

Functions of the Midbrain

Midbrain is small part of the brain stem Anteriorly composed of two bulging fiber

tracts called cerebral peduncles that convey > and < impulses

Dorsally composed of four rounded protrusions called corpora quadrigemina which are reflex centers involved with vision and hearing

Functions of the Pons

PONS is rounded structure that protrudes just below midbrain

Functions as “bridge” for ascending and descending impulses through this area

Contains important nuclei involved with BREATHING

Functions of Medulla Oblongata

Most inferior part of the brain stem Merges into spinal cord (no obvious

change in structure) Like pons, is important fiber tract area Contains many nuclei that regulate vital

visceral activities Heart rate, blood pressure, breathing,

swallowing, and vomiting

Functions of the Cerebellum

Has two hemispheres & convoluted surface Outer cortex of gray matter and inner region of

white matter Provides precise timing for skeletal muscle

activity, controls balance and equilibrium It’s activity = smooth and coordinated body

movements Continually monitors brain’s intentions with

action body performance by monitoring body position and amount of tension in various body parts

Protection for the CNS

CNS protected by enclosure within bone (skull and vertebral column); by watery cushion (cerebrospinal fluid) and by enclosure within membranes (meninges)

Also protected from harmful substances in blood by the blood-brain barrier

Three connective tissue membranes covering and protecting CNS structures are called meninges

CNS protection, cont.

Dura mater is leathery outermost layer double layer membrane where surround brain periosteal layer attached to inner surface of brain meningeal layer forms outermost covering of brain

and continues as dura mater of spinal cord Arachnoid mater is weblike middle layer

Its threadlike extensions span subarachnoid space to attach it to innermost membrane, the pia mater

Pia mater (gentle mother) clings tightly to surface of brain and spinal cord by following every fold

CNS protection, cont.

Meningitis is inflammation of the meninges and is serious threat to brain because bacterial/viral meningitis may spread to nervous tissue of CNS

Meningitis usually diagnosed by taking sample of CSF from subarachnoid space

Brain inflammation called encephalitis

Meninges, Visual

Cerebrospinal Fluid

CSF continually formed from blood by choroid plexuses (clusters of capillaries that hang from roof in each brain ventricle

Forms watery cushion in and around brain and spinal cord

Moves from lateral hemispheres 3rd ventriclecerebral aqueduct of midbrain 4th ventricle dorsalpons/medulla

Some fluid reaching 4th ventricle continues down central spinal canal of spinal cord

CSF, cont. MOST CSF circulates into subarachnoid space

through three openings in wall of 4th ventricle CSF returned to blood through subarachnoid space CSF contains water, glucose, proteins, and sodium

chloride ions Lumbar spinal tap used to collect sample of CSF

for testing Something affects CSF drainage—begins to collect

and exert pressure on brain Results in hydrocephalus (water on the brain)

Risks and treatments

CSF, Visual

Selected Traumatic Brain Injuries

Head injuries leading cause of accidental death in US

Brain injury at site of bloweffect of ricocheting brain hitting opposite end of skull

Concussion occurs when brain injury slight dizzy, brief unconsciousness/no permanent brain

damage Contusion result of marked tissue damage

brain stem contusion results in coma (hours to lifetime)

Injuries, cont.

Cerebral edema is swelling of brain due to inflammatory response to injury

Intracranial hemmorhage is bleeding from ruptured vessels

----------------------------------------------------- CARDIOVASCULAR ACCIDENTS

commonly called “strokes” 3rd leading cause of death in US CVAs occur when blood circulation to brain area

blocked by blood clot/ruptured vessel Vital brain tissue begins to die

CVAs, cont.

After CVA, often possible to determine area of brain damage by patient’s symptoms Left-side paralysis=right motor cortex

damaged Aphasias common result of damage to left

cerebral hemisphere where language areas located

Motor aphasia involves damage to “Broca’s area” and loss of ability to speak

CVAs, cont.

Sensory aphasia—person loses ability to understand written or spoken language

Temporary brain ischemic (transient ichemic attack or TIA) is uncompleted stroke

Lasts from 5 to 50 minutes Numbness, temporary paralysis, and impaired

speech Symptoms NOT permanent but RED FLAG

warning of serious impending CVAs

Alzheimer’s Disease

Degenerative disease of brain that results in dementia (mental deterioration)

May begin in middle age with memory loss (recent events), becomes moody, irritable, confused, and sometimes violent…then hallucinations

Abnormal protein deposits (plagues) and twisted fibers appear within neurons with local brain atrophy

Influx of calcium into brain implicated

Spinal Cord

Cylindrical spinal cord is continuation of brain stem

Provides 2-way conduction pathway to & from brain

It is the major reflex center (spinal reflexes completed at this level)

Enclosed within vertebral column and extends from foramen magnum of skull to 1st or 2nd lumbar vertebra (just below ribs)

Spinal Cord, cont.

Meningeal coverings do NOT end at L2 but extend well beyond end of spinal cord in vertebral column

NOT possible to damage cord below L2 so good place to draw CVS

Collection of spinal nerves at inferior end called cauda equina (horse’s tail)

31 pairs of spinal nerves arise from the cord

Spinal Cord, Visual

Gray matter of spinal cord

Gray matter of spinal cord looks like “H” in xs Surrounds central canal of cord which contains

CVS 2 Dorsal (posterior) and 2 Ventral (anterior)

horns Dorsal horns contain association neurons so is

sensory in nature Ventral horns contain cell bodies of motor

neurons of somatic (voluntary) nervous system

Gray matter, cont.

Damage of dorsal root results in loss of sensation from body area served

Damage to ventral root results in flaccid paralysis of muscles served nerve impulses DO NOT reach muscles

affected so no voluntary movement possible muscles begin to atrophy (waste away)

because they are no longer being stimulated

White matter of the spinal cord

Composed of myelinated fiber tracts Organized into columns (regions) by irregular

shape of gray matter Posterior, lateral, and anterior columns Each column contains number of fiber tracts

made up of axons with same destination and function

Descending tracts are motor (efferent) Ascending tracts are sensory (afferent) Posterior column has ONLY ascending tracts Anterior and lateral columns contain both

ascending and descending tracts

Spinal Cord XS, Visual

Spinal Transection

Spinal cord transected (cut crosswise) or crushed, spastic paralysis results

Muscles stay healthy—still stimulated by spinal reflex arcs and movement of muscles occurs However, all movements are involuntary

Cord carries both sensory and motor impulses, loss of feeling or sensory input occurs in body areas below point of cord destruction

Quadriplegic=injury high enough in cord and affects all four limbs

Paraplegic=injury low enough that ONLY legs are affected

Anatomy of Typical Nerve Nerve is bundle of neuron fibers found outside

CNS Within nerve, neuron fibers (processes) are

wrapped in protective connective tissue Each fiber surrounded by endoneurium

groups of fibers surrounded by perineurium to form fiber bundles or FASCICLESall fascicles bound together by tough fibrous sheath called epineurium to form cordlike nerve

Nerves carrying both sensory/motor fibers called MIXED NERVES

ALL SPINAL NERVES ARE MIXED NERVES

Nerve Anatomy, Visual

Cranial Nerves

12 PAIRS of cranial nerves that serve head and neck

Only one pair, the VAGUS, extends to thoracic and abdominal cavities

Most are mixed except for optic, olfactory, and vestibulocochlear nerves which are purely sensory

“Oh, oh, oh, to touch and feel very good velvet, ah”

Cranial Nerves, Visual

Spinal Nerves

31 Pairs of spinal nerves formed by combination of ventral & dorsal roots of spinal cord

Named for region of cord from which they arise

Cervical (8); Thoracic (12); Lumbar (5); Sacral (5); Coccygeal (1)

Almost immediately after being formed, each divides into dorsal & ventral RAMI

Getting on who’s nerves

Spinal Nerves, cont.

RAMI contain both sensory & motor fibers Smaller dorsal rami serve skin & muscles

of posterior trunk Ventral rami form complex networks of

nerves called plexuses which serve motor and sensory needs of limbs

Ventral rami of T1-T12 form intercostal nerves which supply muscles between ribs/skin and muscles of anterior and lateral trunk

Plexuses

Ventral rami form nerve complexes that serve sensory/motor needs of limbs

Formed by ventral rami of spinal nerves OTHER than T1-T12

Cervical (C1-C5) Brachial (C5-C8; T1) Lumbar (L1-L4) Sacral (L4-L5; S1-S4) Major nerves of body emanate from a plexus

Four Major Nerves of Body

Phrenic = breathing; damage results in respiratory paralysis and death if not treated promptly

Radial = triceps/extensor muscles of forearm; damage results in wristdrop—the inability to extend hand at wrist

Ulnar = wrist and many hand muscles; damage results in clawhand—inability to spread fingers apart

Sciatic = largest nerve in body; splits to form fibular/tibial nerves; damage results in sciatica (pain along peripheral distribution of nerve; inability to extend hip and flex knee

Autonomic Nervous System

ANS is motor subdivision of PNS that controls body activities automatically

Composed of special group of neurons that regulate cardiac muscle, smooth muscles (walls of visceral organs/blood vessels) and glands

Relative stability of homeostasis depends on workings of ANS

ANS called “involuntary nervous system” Sympathetic and Parasympathetic Divisions

Comparing…and more

Somatic motor division cell bodies are INSIDE CNS and axons extend to skeletal muscles they serve

ANS has chain of two motor neurons FIRST is in the brain or spinal cord Its axon (preganglionic axon) leaves CNS

to synapse with second motor neuron in ganglion outside CNS

Axon of second neuron (postganglionic) extends to organ it serves

Parasympathetic Division Most active when body at rest Often called “resting and digesting system” Concerned with normal digestion and

elimination of urine and feces; and with conserving energy by decreasing demands on CV system

Acetylcholine is neurotransmitter PRE- and POSTGANGLIONIC

Sympathetic Division Acetylcholine as PREganglionic; norepinephrine

and epinephrine as POSTganglionic Called “fight or flight” system Increases heart rate, BP, blood glucose levels,

dilates bronchioles of lungs, dilation of skeletal blood vessels and withdrawal of blood from digestive organs

**ONLY sympathetic division controls blood vessels

Nervous System, Visual