Chapter 7 The Nervous System
You are driving down the freeway, and a horn blares on your right. You swerve to
your left. Charlie leaves a note on the kitchen table: "See you later-have the stuff
ready at 6." You know that the "stuff" is chili with taco chips. You are dozing, and
your infant son makes a soft cry. Instantly, you awaken. What do all these events
have in common? They are all everyday examples of the functioning of your
nervous system, which has your body cells humming with activity nearly all the
time.
The nervous system is the master controlling and communicating system of the
body. Every thought, action, and emotion reflects its activity. Its signaling device, or means of
communicating with body cells, is electrical impulses, which are rapid and specific and cause almost
immediate responses.
Organization of the Nervous System
 List the general functions of the nervous system.
o Master controlling and communicating system of the body
o Three overlapping functions
 Uses million of sensory receptors to monitor changes occurring both inside and
outside the body – changes are called stimuli and the gathered information is called
sensory input
 It processes and interprets the sensory input and makes decisions about what
should be done at each moment, a process called integration
 It then effects a response by activating muscles or glands – the response is called
motor output
 Explain the structural and functional classifications of the nervous system.
o Structural classification
 Central nervous system (CNS) – brain and spinal cord
 Peripheral nervous system (PNS) – part of nervous system outside the CNS
 Spinal nerves – carry impulses to and from the spinal cord
 Cranial nerves – carry impulses to and from the brain
o Functional classification
 Sensory or afferent division – nerve fibers that convey impulses to the CNS from
sensory receptors
 Somatic sensory fibers – deliver impulses from the skin, skeletal muscles,
and joints
 Visceral sensory fibers (AKA visceral afferents) – transmit impulses from
visceral organs (internal organs)
 Motor or efferent division – carries impulses from the CNS to effector organs, the
muscles and glands to bring about a motor response
 Somatic nervous system – conscious, voluntary control of most (not all)
skeletal muscles – skeletal muscle reflexes such as stretch reflex are initiated
involuntarily – often called voluntary nervous system
 Autonomic nervous system – automatic, involuntary activities such as
smooth and cardiac muscles and glands – often called involuntary nervous
system
 Sympathetic – mobilizes the body during extreme situations such as
fear, exercise, and rage – the fight-or-flight response
Parasympathetic – allows us to unwind and conserve energy – the
resting-and-digesting system
 Define central nervous system and peripheral nervous system and list the major parts of each.
o See above objective
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Nervous Tissue: Structure and Function
 State the function of neurons and neuroglia.
o Neurons – aka nerve cells – highly specialized to transmit messages in the form of nerve
impulses from one part of the body to another – all types have two common features:
 All have a cell body, which contains the nucleus and is the metabolic center of the
cell
 One or more slender processes extending from the cell body
o Neuroglia – supporting cells – includes many types of cells that generally support, insulate,
and protect the neurons – are not able to transmit nerve impulses – never lose their ability
to divide
 Astrocytes – star-shaped cells whose numerous projections have swollen ends that
cling to neurons, bracing them and anchoring them to their nutrient supply lines, the
capillaries – form a living barrier between capillaries and the neurons and make
exchanges between the two to help protect the neurons from harmful substances in
the blood
 Microglia – spiderlike phagocytes that dispose of debris such as dead cells and
bacteria
 Ependymal cells – glial cells line cavities o the brain and spinal cord – beat cilia to
help circulate cerebrospinal fluid forming a protective cushion around the CNS
 Oligodendrocytes – glia that wrap their flat extensions tightly around the nerve
fibers, producing fatty insulating covering called myelin sheaths
 Describe the general structure of a neuron and name its important anatomical regions.
o Cell body – metabolic center of the cell with the usual organelles except centrioles, rough
ER (Nissl substance), neurofibrils, intermediate filaments for maintaining cell shape
 Cell bodies found in the CNS are in clusters called nuclei
 Small collections of cell bodies called ganglia are found in a few sites outside the CNS
in the PNS
 Bundles of nerve fibers (neuron processes) running through the CNS are
called tracts whereas in the PNS they are called nerves
o Arm-like processes or fibers – vary in length
 Dendrites – convey incoming messages toward the cell body – may be present in
the hundreds depending on neuron type
 Axons – convey nerve impulses away from the cell body – each neuron only has
one, which arises from a cone-like region of the cell body called the axon hillock
 Each axon may have a collateral branch along its length but all axons
branch profusely at their terminal end forming hundreds of axon terminals
each with hundreds of tiny vesicles that contain chemicals called
neurotransmitters
 Each axonal terminal is separated from the next neuron by a tiny gap called the
synaptic cleft – a functional junction is called a synapse
 Myelin – whitish, fatty material with a waxy appearance that covers most long nerve
fibers – protects and insulates the fibers and increases transmission rate of nerve
impulses
Axons outside the CNS are myelinated by Schwann cells, support cells that
wrap themselves tightly around the axon in a jelly-roll fashion resulting in a
tight coiled of wrapped membranes called the myelin sheath encloses the
axon
 The external part of the Schwann cell is called the neurilemma
 Gaps at regular intervals in the myelin sheath that result from the
individual Schwann cells are called nodes of Ranvier
Describe the composition of gray matter and white matter.
o Gray matter – contains mostly unmyelinated fibers and cell bodies
o White matter – consists of dense collections of myelinated fibers (tracts)
o Both refer to regions of the CNS
List the two major functional properties of neurons.
o Irritability – the ability to respond to a stimulus and convert it into a nerve impulse
o Conductivity – the ability to transmit the impulse to other neurons, muscles, or glands
Classify neurons according to structure and function.
o Functional classification – groups neurons according to the direction the nerve impulse is
traveling relative to the CNS
 Neurons carrying impulses from sensory receptors to the CNS are sensory, or
afferent, neurons – the cell bodies of sensory neurons are always found in ganglion
outside the CNS – keep us informed about what is happening both inside and outside
the body
 Neurons carrying impulses from the CNS to the viscera and /or muscles and glands
are motor, or efferent, neurons – cell bodies are always located in the CNS
 Association neurons or interneurons connect the motor and sensory neurons in
neural pathways – cell bodies are always located in the CNS
o Structural classification – based on the number of processes extending from the cell
body
 Multipolar neuron – several processes extending from the cell body
 Bipolar neurons – two processes – an axon and a dendrite – rare in adults and
found only in special senses organs such as eyes and ears
 Unipolar neurons – have a single process emerging from the cell body but it is
short and divides almost immediately into proximal (central) and distal (peripheral)
fibers
List the types of general sensory receptors and describe their functions.
o Cutaneous sense organs – skin – pain receptors are least specialized and the most
numerous because pain warns us of body damage – strong stimulation by searing heat,
extreme cold, or excessive pressure is also detected as pain
o Proprioceptor – detect the amount of stretch, or tension, in skeletal muscles, their
tendons, and joints – they send information to the brain to the body can maintain balance
and normal posture
Describe the events that lead to the generation of a nerve impulse and its conduction from one
neuron to another. – for an unmyelinated nerve
o Read figure 7.9
o Cell membrane is polarized, at rest – fewer positive ions sitting on the inner face of the
neuron’s plasma membrane than there are on its outer face in the tissue fluid that
surrounds it – positive ions inside the cell are potassium (K+) and the positive ions outside
the cell are sodium (Na+)
o A stimulus excites the neuron to become active and generate an impulse – could be from a
sensory receptor (light, sound, taste, pain, pressure) or neurotransmitters released by other
neurons – this causes the sodium gates to open in the membrane
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Depolarization – there is an inward rush of sodium ions that changes the polarity of the
neuron’s membrane – the inside is now more positive and the outside less positive –
activates the cell to initiate and transmit an action potential (nerve impulse) – an all-ornone-response
o Repolarization – almost immediately after sodium ions rush into the neuron, the
membrane becomes impermeable to sodium ions and permeable to potassium ions
allowing potassium ions to diffuse out of the neuron restoring the electrical conditions at the
membrane to the polarized, or resting, state – until this occurs, a neuron cannot conduct
another impulse
o After repolarization – the initial concentrations of sodium and potassium ions inside and
outside are restored by the activation of the sodium-potassium pump that uses ATP to
move the ions back into place
o In nerves that have myelin sheaths, the nerve impulse must jump from node to node
along the length of the never because no current can flow across the axonal membrane
where there is fatty myelin insulation – this is a much faster impulse propagation called
salutatory conduction
 Define reflex arc and list its elements.
o Reflexes – rapid, predictable, and involuntary responses to stimuli that occur over neural
pathways called reflex arcs – classified as either autonomic or somatic reflexes
 Autonomic – regulate the activity of smooth muscles, the heart, and glands –
digestion, elimination, blood pressure, sweating, pupillary (eye), salivary secretions
 Somatic – include all reflexes that stimulate the skeletal muscles such as when you
pull your hand away from a hot object
o Have a minimum of five elements
 Sensory receptor – reacts to stimulus
 Effector organ – muscle or gland eventually stimulated
 Afferent and efferent neurons to connect the two
 The synapse between the afferent and efferent neurons represents the central
element – the CNS integration center
o
Central Nervous System
 Identify and indicate the functions of the major regions of the cerebral hemispheres, diencephalon,
brain stem, and cerebellum on a human brain model or diagram.
o See diagram page 215
o Cerebral hemispheres – speech, memory, logical and emotional responses,
consciousness, interpretation of sensation, and voluntary movements
o Diencephalon
 Thalamus – relay station for sensory impulses passing upward to the sensory cortex
– gives us a crude idea of whether the sensation is pleasant or unpleasant
 Hypothalamus – regulates body temperature, water balance, and metabolism, also
the center for many drives and emotions making it part of the limbic system
(emotional-visceral brain)
 Epithalamus – contains the pineal body (part of the endocrine system) and the
choroid plexus (forms the cerebrospinal fluid)
o Brain stem – pathway for ascending and descending tracts, gray matter which control vital
activities such as breathing and blood pressure
 Midbrain – parts are involved as reflex centers involved with vision and hearing
 Pons – control of breathing
Medulla oblongata – regulate vital visceral activities such as heart rate, blood
pressure, breathing, swallowing, and vomiting
 Reticular formation – involved in motor control of the visceral organs with the
reticular activating system (RAS) involved in consciousness and the awake/sleep
cycles – damage to this area can result in permanent unconsciousness (coma)
o Cerebellum – provides precise timing for skeletal muscle activity and controls our balance
and equilibrium – automatic pilot that compares the brain’s intentions with actual body
performance by monitoring body position and amount of tension in various body parts
Name the three meningeal layers and state their functions.
o Dura matter – outermost layer – tough/hard double-layered membrane that surrounds the
brain
 Periosteal layer – inner layer attached to the inner surface of the skull forming the
Periosteum
 Meningeal layer – outer layer that forms the outer-most covering of the brain and
continues as the dura mater of the spinal cord
o Arachnoid matter – middle cobweb-like layer whose threadlike extensions span the
subarachnoid space to attach it to the innermost membrane, the pia mater
o Pia matter – clings tightly to the surface of the brain and spinal cord, following every fold
Discuss the formation and function of cerebrospinal fluid and the blood-brain barrier.
o Cerebrospinal fluid – watery broth similar to blood plasma from which it forms but it
contains less protein, more vitamin C, and different ion concentrations – continually formed
from blood by the choroid plexuses (clusters of capillaries) and is constantly flowing until it
returns to the blood in the dural sinuses through the arachnoid villi
o Blood-brain barrier – keeps the brain neurons separate from bloodborne substances –
composed of the least permeable capillaries found in the body and of the water-soluble
substances, allows only water glucose, and essential amino acids to pass through easily
keeping out both nonessential amino acids and potassium, which are actively pumped out of
the brain – metabolic wastes (urea, toxins, proteins, and most drugs) are also prevented
from entering the brain – virtually useless against fats, respiratory gases, and other fatsoluble molecules, which is why alcohol, nicotine, and anesthetics can affect the brain
Compare the signs of a CVA with those of Alzheimer's disease; of a contusion with those of a
concussion.
o Cerebrovascular accidents (CVA) – strokes – occur when blood circulation to a brain
area is blocked, by a blood clot or a ruptured blood vessel leading to death of vital brain
tissue – it is possible to determine the area of brain damage by observing the patient’s
symptoms such as visual impairment, paralysis, and aphasias (speech problems)
o Alzheimer’s disease – a degenerative brain disease in which abnormal protein deposits
and other structural changes appear resulting in slow, progressive loss of memory and
motor control plus increasing dementia
o Contusion – is the result of marked tissue destruction – may result in coma lasting for
hours or longer
o Concussion – occurs when brain injury is slight – patient is dizzy or may lose
consciousness briefly but no permanent brain damage occurs
Define EEG and explain how it evaluates neural functioning.
o Electroencephalogram – recording of the electrical impulses by neurons of the brain
taken by placing electrodes at various points on the scalp and connecting these to a
recording device – the patters of electrical activity of the neurons are called brain waves –
each person’s pattern is unique
 Interference with the function of the cerebral cortex is suggested by brain waves that
are too fast or too slow – unconsciousness occurs at both extremes, sleep and coma
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result in slower than normal brain waves while fright, epileptic seizures, drug
overdoses result in abnormally fast brain waves – the absence of brain waves is
evidence of clinical death
 List two important functions of the spinal cord.
o Provides a two-way conduction pathway to and from the brain
o It is a major reflex center – the spinal reflexes are completed at this level
 Describe spinal cord structure.
o Spinal cord structure
 ~17 inches long continuation of the brain stem enclosed within the vertebral column
that ends just below the ribs - ~ the size of a thumb for most of its length and
enlarged in the cervical and lumbar regions where nerves serving the upper and
lower limbs arise and leave the cord
 Cushioned and protected by meninges, which do not end at L2 but extend beyond the
end of the spinal cord in the vertebral canal
 31 pairs of spinal nerves arise from the cord and exit from the vertebral column to
serve the body area close by
 Collection of spinal nerves at the inferior end of the vertebral canal is called the
cauda equine because it looks like a horse’s tail
o Gray matter of the spinal cord – the two posterior projections are the posterior, or
dorsal, horns; the two anterior projections are the anterior, or ventral, horns – the gray
matter surrounds the central canal of the cord, which contains CSF
o White matter of the spinal cord – composed of myelinated fiber tracts – divided into
three regions – posterior, lateral, anterior columns – each column contains a number of
fiber tracts made up of axons with the same destination and function
Peripheral Nervous System
 Describe the general structure of a nerve.
o A bundle of neuron fibers found outside the CNS – within a nerve, neurons fibers, or
processes, are wrapped in protective connective tissue coverings
o Each fiber is surrounded by a delicate connective tissue sheath, an endoneurium
o Groups of fibers are found by a coarser connective tissue wrapping, the perineuruim to
form fiber bundles, or fascicles
o Fascicles are bound together by a tough fibrous sheath, the epineurium, to form the
cordlike nerve
 Identify the cranial nerves by number and by name, and list the major functions of each.
o See table 7.1 on page 231 and 232
o 12 pairs of cranial nerves are numbered in order, and in most cases, their names reveal the
most important structures they control
 Describe the origin and fiber composition of (a) ventral and dorsal roots, (b) the spinal nerve
proper, and (c) ventral and dorsal rami.
o Ventral root – of the spinal cord – where the cell bodies of the sensory neurons, whose
fibers enter the cord by the dorsal root, and are found in an enlarged area called the dorsal
root ganglion
o Dorsal root – of the spinal cord – where the ventral horns of the gray matter contain cell
bodies of motor neurons of the somatic (voluntary) nervous system, which send their axons
out the ventral root of the cord – the dorsal and ventral roots fuse to form the spinal nerves
o Spinal nerve proper – very short and splits into dorsal and ventral rami
o Ventral rami –supply the muscles between the ribs and the skin and muscles of the
anterior and lateral trunk
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o Dorsal rami – serve the posterior body trunk
o both rami, like the spinal nerves, contain both motor and sensory fibers
Discuss the distribution of the dorsal and ventral rami of spinal nerves.
o Dorsal rami – serve the skin and muscles of the posterior body trunk
o Central rami – form the intercostal nerves, which supply the muscles between the ribs and
the skin and muscles of the anterior and lateral trunk
 The ventral rami of all other spinal nerves form complex networks of nerves called
plexuses, which serve the motor and sensory needs of the limbs
Name the four major nerve plexuses, give the major nerves of each, and describe their
distribution.
o See table 7.2 on pages 235-236 for more detail
o Cervical – C1 – C5
o Brachial – C5 – C8 and T1
o Lumbar – L1 – L4
o Sacral – L4 – L5 and S1 – S4
Identify the site of origin and explain the function of the sympathetic and parasympathetic divisions
of the autonomic nervous system.
o Parasympathetic division – the first neurons are located in brain nuclei of several cranial
nerves – III, VII, IX, X and in the S2 – S4 level of the spinal cord – is active when the body
is at rest and not threatened in any way – the resting and digesting system is chiefly
concerned with promoting normal digestion and elimination of feces and urine, and with
conserving body energy, and decreasing demands on the cardiovascular system
o Sympathetic division – the first neurons are in the gray matter of the spinal cord from T 1
– L2 – the fight-or-flight system – works at full speed when you are emotionally upset and
when you are physically stresses – effects of sympathetic nervous system activation
continue for several minutes until its hormones are destroyed by the liver explaining why
we need a calm down time even though the effects of the nerve impulses may act only
briefly
Contrast the effect of the parasympathetic and sympathetic divisions on the following organs:
heart, lungs, digestive system, blood vessels.
o The sympathetic nervous system increases heart rate, blood pressure, and blood glucose
levels, dilates the bronchioles of the lungs, dilated blood vessels in skeletal muscles and
withdraws blood from the digestive organs and shunts blood to the heart, brain, and
skeletal muscles
o The parasympathetic nervous system reverses the affects of the sympathetic nervous
system – decreases the demands on the cardiovascular system to low normal levels, the
digestive tract is actively digesting food and the skin is warm, the eye pupils are constricted
to protect the retinas from excessive light and lenses are set for close vision
Developmental Aspects of the Nervous System
 List several factors that may have harmful effects on brain development.
o Maternal measles (rubella), lack of oxygen, smoking, radiation, various drugs such as
alcohol, opiate, and cocaine
 Briefly describe the cause, signs, and consequences of the following congenital disorders: spina
bifida, anencephaly, cerebral palsy.
o Spina bifida – results when the vertebrae form incompletely (typically in the lumbosacral
region) – in the most severe cases, meninges, nerve roots, and even parts of the spinal
cord protrude from the spine, rendering the lower part of the spinal cord functionless and
the child is unable to control the bowels or bladder, and the lower limbs are paralyzed
Anencephaly – a failure of the cerebrum to develop, results in a child who cannot hear,
see, or process sensory inputs
o Cerebral palsy – neuromuscular disability in which the voluntary muscles are poorly
controlled and spastic due to brain damage - may result from the lack of oxygen during
delivery – half the victims have seizures, are mentally retarded, and/or have impaired
hearing or vision
 Explain the decline in brain size and weight that occurs with age.
o The brain reaches its maximum weight in the young adult but over the next ~60 years,
neurons are damages and die, and since they cannot replace themselves, our store of
neurons continually decreases – however, an unlimited number of neural pathways are
always available and ready to be developed
o Eventual shrinking of the brain is normal – however, certain activities can accelerate the
process such as boxing and chronic alcoholism
 Define senility and list some possible causes.
o Results from a gradual lack of oxygen due to the aging process characterized by
forgetfulness, irritability, difficulty in concentrating and thinking clearly and confusion
o Low blood pressure can be one cause – orthostatic hypotension is a type of low blood
pressure resulting from quick changes in body position such as standing up too fast after
sitting for a long period and the sympathetic nervous system does not have time to act
quickly enough to counteract the pull of gravity by activating the vasoconstrictor fibers
causing blood to pool in the feet resulting in the person becoming lightheaded or fainting
o