chapter 7 the nervous system biology 112 tri-county technical college pendleton, sc
TRANSCRIPT
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