Anatomy & Physiology Ch 7 Nervous System
Lets break is down!
CNS
PNS
PNS
MOTOR EFFERENT
DIVISION
SENSORY
AFFERENT DIVISION
↑
Somatic
Sensory
(skin, joints,
skeletal muscle)
↑
Visceral
Sensory
(visceral organs)
↓
Somatic
Nervous System
*voluntary
Skeletal Muscle
↓
Autonomic
Nervous System
*involuntary
Heart, glands,
Smooth Muscle
↙
↘
Sympathetic –vs--Parasympathetic
Division
Division
(fight or flight)
(craniosacral)
NERVOUS TISSUE
↙
↘
Support cells
Neurons
↓
CNS Support cells (Neouroglia)
Neuroglia-“nerve glue,” many types, generally support, insulate and protect, individually
called glia or glial cells. Glia are not able to transmit nerve impulses, never lose their ability to
divide, most brain tumors are Gliomas-tumors formed by glial cells.
Types of Neuroglia Astrocytes- Star shaped cells with many projections that cling to neurons, bracing them
and anchoring them to capillaries.
o Form a living barrier making exchanges between the two-helps protect not
allowing certain chemicals to pass from blood to neuron
o In the brain absorbs leaked K+ and recaptures released neurotransmitters.
 Microglia- spiderlike phagocytes
o Dispose of dead brain cells and bacteria
 Ependymal cells- ciliated cuboidal cells that line the CNS cavity
o Cilia beat to circulate cerebrospinal fluid
o Forms protective cushion around CNS
 Oligodendrocytes- wrap flat sheet like extensions around nerve fibers, up to 60 at a
time.
o Insulating covers called myelin sheaths
o These myelin sheath wrapped fibers are referred to as “white matter”
PNS Support Cells Schwann Cells- form myelin sheaths around PNS nerve axons by wrapping tightly
around and around the nerve fiber until all of the cytoplasm is pushed to the outer
portion of the cell called the –Neurilemma
 Satellite cells- protective cushioning cells
NEURONS (nerve cells )-specialized to transmit messages , all have cell body (control center)
and one or more thin processes that extend from the cell body. Several types exist.
Structure of a Neuron Cell body- is the broadened star shaped end which holds the nucleus. It carries out most
of the metabolic functions of neuron. It contains the nucleolus and most other
organelles EXCEPT centrioles because most neurons are amitotic. Specialized terms
include the Nissl substance (rougher) and neurofibrils (intermediate filaments)
 Processes- (fibers) vary in length from microscopic to 3-4 feet in a human leg.
2 types of processes
o Dendrites- convey incoming messages toward the cell body, hundreds of
dendrites may lead to the cell body.
o Axons- transmit messages away from the cell body, axons arise from a conelike
extension of the cell body called the axon hillock, there is only one axon, axons
may have collateral branch, all axons branch profusely at the ends and form
hundreds of axon terminals.
 Myelin Sheaths- a whitish fatty material with a waxy appearance, it protects and
insulates the fibers and increases the transmission rate of the impulses
o PNS sheaths are formed by Schwann cells which require many end to end to
cover the neuron. The neurilemma of the Schwann cell plays an important role in
regeneration. The gaps between the Schwann cells are called nodes of Ranvier.
****pictured and discussed previously
o CNS sheaths are formed from oligodendrocites which each may wrap several
neurons. ****pictured and discussed previously
Classifications of Neurons
Functional Classification-based upon the direction the impulse is traveling
 Sensory/Afferent Neurons move from sensory receptors to CNS. Cell bodies of sensory
neurons always found in ganglion outside of CNS. Dendrite endings of sensory receptors
associated with specialized receptors for sense organs (vision, hearing, taste, smell,
equilibrium)
o Cutaneous sense organs in skin detect temperatures and pain (bare endings)
touch (Meissners corpuscle) deep pressure (Pacinian corpuscle)
o Proprioceptors- detect tendon stretch and tension, and muscle activities to
maintain balance and normal posture.
 Motor/Efferent Neurons- carry impulses from CNS to muscles, glands, or viscera. The
cell bodies are always located inside the CNS.
 Interneurons/Association Neurons- connect sensory and motor neurons in “neural
pathways,” cell bodies always located inside CNS.
Structural Classification- based upon the number of processes extending from the cell body.
 Mulitpolar Neurons- (several processes) all Motor and Association neurons are
multipolar. Most common. Axon and multiple dendrites.
 Bipolar Neurons- 2 processes one axon and one dendrite, sensory processing cells, Rare
in adults (eyes, nose)
 Unipolar neurons- single short process which branches into a central and peripheral
process(axon) **unique only the end of the peripheral process has dendrites, the rest of
the axon conducts impulses both to and from the cell body, sensory neurons in the PNS
ganglia are unipolar.
Physiology of Nerve Impulses (Nut Shell Version)
Neurons have two functions
-Irritability- ability to respond to stimulus and convert it to nerve impulses.
-Conductivity- ability to transmit the impulse to other neurons muscles or glands.
Resting Neurons are said to be Polarized when there are fewer positive ions on the inner face
of the membrane than the outer face. (K+ inside and Na+ outside)
Initiation of an Action Potential (unmyelinated neuron)
-When adequately stimulated by either a receptor or a Neurotransmitter from another neuron
plasma membrane permeability changes very briefly.
-Na+ channels open and Na+ diffuses into the cell, now inside is more positive, called
Depolarization.
-this is called a Graded Potential, if strong enough activates a long distance signal called
Action Potential or Nerve Impulse.
-After transmission, permeability changes again becoming impermeable to Na+ but permeable
to K+, restores electrical conditions of membrane called Repolarized. Na+ K+ pump helps to
restore original concentrations.
Transmitting the signal to the next neuron:
-called conductivity
-When the action potential reaches the axon terminal, vescicles containing neurotransmitters
fuse with axonal membrane releasing neurotransmitter. It diffuses across to receptors on the
next neuron. If enough is absorbed the whole series of events begins again in each subsequent
neuron.
-Remember transmission of an impulse is an electrochemical event.
**Faster impulse propagation occurs in myelinated fibers where the impulse jumps from node
to node called Saltatory Conduction.
REFLEXES
-Reflexes are rapid, predicable, involuntary responses to stimuli. Much of what the body must
do everyday is programmed as reflexes. They occur over pathways called reflex arcs.
-Reflex arcs- have 5 main parts –sensory receptor-> sensory neuron-> integration center(CNS)> motor neuron -> effector (usually a muscle or gland)
-Reflex evaluation during an exam indicates nervous system disorders before obvious
symptoms occur.
-2 types of reflexes
-> Somatic reflexes- deal with stimulating skeletal muscle
-> Autonomic reflexes- regulate smooth muscle glands and heart.
THE CENTRAL NERVOUS SYSTEM
**The CNS is divided a zillion different ways according to what you are looking for. We will
only look at a few.
 Developmentally
-CNS appears as a neural tube which expands anteriorly by the fourth week becoming the
brain and spinal cord.
-It becomes enlarged in four regions forming chambers called the ventricles.
>lateral ventricle
>3rd ventricle
>Cerebral aquaduct
>4th ventricle
 Structurally
-CNS is divided into 4 major regions
Cerebral Hemispheres, Diencephalon, Cerebellum, Brain Stem
o Cerebral Hemispheres (Cerebrum)-large superior portion of brain which conceals
the Dienchephalon and most of the brain stem. Deeply textured with elevations
called gyri and grooves called sulci. Larger deeper grooves called fissureslongitudinal fissure seperates the 2 hemispheres. The sulci divide the
hemispheres into lobes which are usually named after cranial bones.
 Each Cerebral hemisphere has 3 regions
 Cortex- superficial gray matter
 White matter- internal to cortex
 Basal Nuclei- islands of gray matter within the white matter
o Cerebral Cortex- well studied and mapped into regions called the sensory and
motor homunculus. Responsible for speech, memory, logical and emotional
response, consciousness, interpretation of sensation, voluntary movement.
 The primary somatic sensory area interprets the sensory impulses from
external stimuli in the parietal lobe.
 Special senses are interpreted in the visual (occipital) and auditory
(temporal) lobes
 The primary motor area allows conscious control over skeletal muscles
(frontal lobe)
 Specialized areas such as Brocas area allow control of speech in a physical
way located in only one hemisphere
 The corticospinal or pyramidal tract leads to the spinal cord and is
composed of axons from the motor neurons.
 The cerebral cortex gray matter houses the cell bodies of the neurons just
mentioned.
 Cerebral White matter- composed of fiber tracts carrying impulses to,
from, or within the cortex.
 The corpus callosum is a giant fiber tract (called commisures) that connect
each side of the cerebral hemispheres.
 Basal Nuclei or Basal Ganglia-islands of gray matter embedded within the
white matter. Help regulate voluntary motor activities.
o Diencephalon or interbrain-divided into three regions-thalmus, hypothalamus,
eipthalamus
 Thalamus- (3rd ventricle) crude recognition, pleasant vs unpleasant
 Hypothalamus-(autonomic nervous system) regulate body temp, water
balance metabolism (limbic system) emotional visceral brain thirst,
appetite, sex, pain and pleasure- regulates pituitary glands and produces
hormones
 Epithalamus-(3rd ventricle) houses pineal body of endocrine system and
choroid plexus which forms cerebrospinal fluid.
o Brain Stem- a pathway for ascending and descending tracts, many small gray
matter nuclei produce autonomic behaviors necessary for survival, associated w
cranial nerves controlling breathing and blood pressure. 3 main regions- Mid
Brain, Pons, Medulla Oblongata
 Midbrain- composed mainly of 2 large fiber tracts the cerebral peduncles
which convey ascending and descending messages. Also reflex centers for
vision and hearing
 Pons- “bridge” mostly fibertracts, control of breathing
 Medulla Oblongata- merges with spinal cord-import fiber tract area
controls heart rate, blood pressure, breathing swallowing, vomiting,
regulate visceral organs
 ***Reticular Formation- neurons helping with control of visceral organs.
Reticular Activities System (RAS) consciousness and sleep cycles-Damage
causes Coma
o Cerebellum- has two hemispheres, outer cortex of gray matter inner white
matter, timing for skeletal muscle activity, balance, equilibrium-including inner
ear and eyes (our Auto-pilot)
PROTECTIONS OF THE CNS
-Due to the fragile nature of the irreplaceable central nervous tissue it has several
built in protections –Bone, membranes, watery cushion and blood brain barrier.
 Meninges (Membranes)- 3 connective tissue membranes
o Dura mater is a double membrane that surrounds the brain
 First layer is attached to inner surface of the skull forming the periosteum
 Second layer is the meningeal layer (which is 3 layers its self) forming the
outermost covering of the brain continues as the Dura mater of the spinal
cord- the layers are fused except in 3 areas called Dural venous sinuses that
collect venous blood. In several places the inner Dural membrane extends
inward to form a fold that attaches the brain to the cranial cavity.
 First Meningeal layer is the outer portion of the Meningeal layer
 Second Arachnoid mater- webby layer- with extensions inward to
the dura mater called arachnoid villi and also into Subarachnoid
space- filled with cerebrospinal fluid
 Third Pia mater- innermost- clings to every fold
 Cerebrospinal Fluid (CFS)-formed from blood by choroid plexuses . Choroid plexuses are
clusters of capillaries found in each ventricle. Forms a watery cushion. Inside the brain
CSF continually circulates. CSF returns to the blood in the cural venous sinuses through
arachnoid villi. About 150 ml (1/2 cup). Changes in CSF composition signs of meningitis
or brain pathologies.
 Blood Brain Barrier- If the brain were exposed to chemical changes uncontrolled neural
activites may result . neurons are kept separate from blood brain born substances by
least permeable capillaries of the body bound together by tight junctions. Only water
soluble substance which may pass include water, glucose, essential amino acids. Non
essential Amino Acids, K+ are not permitted and actively pumped out. Astrocytes
contribute to this barrier. Barrier is virtually useless against fats, respiratory gases and
fat soluble molecules.
SPINAL CORD-17 inches long, glistening white, continuation of brain stem. 2 way conduction pathway,
major reflex center enclosed within vertebral column, extends from foramen magnum to first
or second lumbar. Cushioned and protected by meninges extending well beyond the end of
spinal cord in the vertebral canal. Meningeal sac inferior to that point spot for removing CSF
for testing.
-31 pair spinal nerves arise from cord and exit from vertebral column to serve area close by. Its
about size of thumb most its length but enlarged in cervical and lumbar. Collection of spinal
nerves at inferior end cauda equine.
-Gray matter of spinal cord looks like a butterfly having both dorsal and ventral projections
called posterior and anterior horns respectively. Neurons with specific functions can be
located in the gray matter.
-Dorsal horns contain interneurons. Cell bodies of sensory neurons called Dorsal Root Ganglion
if damaged sensation from the body area served will be lost. Ventral horns- cell bodies of
motor neurons of SNS who axons extend out of the Ventral root. Dorsal and ventral roots fuse
to form Spinal Nerves.
- White matter on each side is divided into 3 regions-Dorsal, Lateral and Ventral Columns
made up of axons with same destination and function. Tracts conducting sensory impulses to
the brain are sensory Afferent tracts. Tracts carrying impulses away from the brain to skeletal
muscle are motor or Efferent tracts. (SNS)
PERIPHERAL NERVOUS SYSTEM (PNS)
-Consists of nerves and scattered groups of cell bodies (ganglia) outside the CNS.
-PNS is divided into to main systems the Somatic Nervous System and the Autonomic Nervous
system.
FIRST THE NERVES
-a Nerve is a bundle of neuron fibers outside the CNS within the nerve are neuron fibers
wrapped in connective tissue coverings in the same patterns that muscle fibers use.
 Each axon (including its myelin sheath) is wrapped in an Endoneurium. Groups of
fibers are wrapped in Perineurium and called fascicles. Last al the fascicles are
wrapped together in the Epineurium to create a Nerve.
-Nerves can be mixed nerves carrying both sensory and motor neurons. Spinal nerves are
mixed. They may be Sensory/Afferent nerves or just Motor/Efferent
Cranial Nerves
-primarily serve the head and neck. They are numbered in order manes reveal the most
important structures they control. Most cranial nerves are mixed but the Optic, Olfactory, and
Vestibularcochlear are purely sensory.
Spinal Nerves and Nerve Plexuses
-31 pair formed by ventral and dorsal roots of spinal cord named for region from which they
arise. Spinal nerves are divided into dorsal and ventral rami the rami contain both motor and
sensory fibers. So damage to spinal nerve or either of its rami results in both loss of sensation
and flaccid paralysis of the area of the body served. Smaller dorsal rami serve the skin and
muscles of posterior body trunk. Ventral rami of All other spinal nerves for complex networks
of nerves called Plexuses which serve motor and sensory needs of the limbs.
http://antranik.org/peripheral-nervous-system-spinal-nerves-and-plexuses/
NOW FOR THE DIVISIONS OF THE PNS
Somatic Nervous system vs Autonomic Nervous system
Somatic (SNS)The Somatic system is the efferent motor system which initiates the skeletal muscle
responses. Cell bodies of the Somatic motor neurons are inside the CNS and their axons (in
spinal nerves) extend all the way to the muscles they serve.
Autonomic Nervous System-truly connection point between CNS and PNS start in one and end in the other
-Chain of 2 motor neurons 1st in each pair is in the brain or spinal cord -> its axon the
preganglionic axon leaves the CNS to synapse with the postganglionic axon outside the CNS to
the organ it serves. ANS- 2 subdivisions- Sympathetic and Parasympathetic-both serve same
organs but cause opposite effects.
-Sympathetic –mobilizes body during extreme situations (fear, exercise, rage)
-Parasympathetic- unwind conserve energy
-Organs served by the ANS receive fibers from both division EXECPT, most blood vessels, skin,
some glands, and adrenal medulla which only receive sympathetic fibers. When they do have
both divisions serving the same organ they are antagonistic mostly because they release
different postganglionic neurotransmitters. Parasympathetic ( cholinergic ) fibers release Ach
Sympathetic (adrenergic ) fibers release norepinephrine.
Are you ready for the subdivisions for the ANS?????? OMG SUBDIVISIONS!!!
The Autonomic is divided into two division which work against each other. Here is how I see it.
Parasympathetic (RELAX WE’VE GOT THIS) Sympathetic ( Aint nobody got time fo dat)
HURRRY HURRY HURRY!!!!!!!!!!!!!!!!!!!!!!!
PARASYMPATHETIC DIVISION
STRUCTURALLY-
-preganglionic neurons of this division located in cranial nerves 3,7,9,10 serving head and neck
region. Vagus nerve (10) being most important and S2-S4 level of Spinal Cord serving pelvic
organs often called craniosacral division.
FUNCTIONALLY
-Often called the resting and digesting system most active when at rest. Promotes normal
digestion, elimination, conserving energy, decreasing demands of cardiovascular system.
-Heart, BP, respiratory rates regulated, skin is warm, pupils constricted.
-It’s the housekeeping system “calm down RELAX”
SYMPATHETIC DIVISION
STRUCTURALLY-often called the thoracolumbar division
-preganglionic neurons in gay matter of spinal cord from T1-L2 leave the cord in the ventral
root enter the spinal nerve then pass through one of the paths that lead to the Sympathetic
trunk (ganglion, chain)
-the second neuron in the sympathetic chain may take a variety of paths serving the skin,
abdominal and pelvic organs, or visceral organs)
FUNCTIONALLY-Often called fight or flight system it increases hear rate, BP, Glucose levels, dilates bronchioles
in lungs and many others
-withdrawals blood from digestive organs to focus on hear brain, muscles. Works at full speed
when emotionally or physically stressed- activates- adrenal gland which pumps out
epinephrine or norepinephrine whose effects last until the liver can destroy the hormonesrequires time to “calm down”
DEVELOPMENTAL ASPECTS WE WILL DO TOGETHER. NOW I HAVE HAND CRAMPS!!!!!!!!!!
AND YOU WILL HELP ME FIX THE TYPOS!!!!!!!!!!!!!!!!!!! OUT