Introduction
- A. The nervous system is composed of neurons and neuroglia.
- 1. Neurons transmit nerve impulses along nerve fibers to other
neurons.
- 2. Nerves are made up of bundles of nerve fibers.
- 3. Neuroglia carry out a variety of functions to aid and protect
components of the nervous system.
- B. Organs of the nervous system can be divided into the central
nervous system (CNS), made up of the brain and spinal cord, and the
peripheral nervous system (PNS), made up of peripheral nerves that connect
the CNS to the rest of the body.
- C. The nervous system provides sensory, integrative, and motor
functions to the body.
- 1. Motor functions can be divided into the consciously controlled
somatic nervous system and the unconscious autonomic system.
General Functions of the Nervous System
- A. Sensory receptors at the ends of peripheral nerves gather
information and convert it into
nerve impulses.
- B. When sensory impulses are integrated in the brain as perceptions,
this is the integrative function of the nervous system.
- C. Conscious or subconscious decisions follow, leading to motor
functions via effectors.
Supporting cells Fig.
- A. Classification of Neuroglial Cells
- 1. Neuroglial cells fill spaces, support neurons, provide structural
frameworks, produce myelin, and carry on phagocytosis. Four are
in the CNS and the last in the PNS.
- 2. Microglial cells are small cells that phagocytize bacterial
cells and cellular debris.
- 3. Oligodendrocytes form myelin in the brain and spinal cord.
- 4. Astrocytes are near blood vessels and support structures,
aid in metabolism, and respond to brain injury by filling in spaces.
- 5. Ependyma cover the inside of ventricles and form choroid
plexuses within the ventricles.
- 6. Schwann cells are the myelin-producing neuroglia of the
peripheral nervous system.
Neuron Structure Figs. 9.3 - 9.4
- A. A neuron has a cell body with mitochondria, lysosomes, a Golgi
apparatus, chromatophilic substance (Nissl bodies) containing rough
endoplasmic reticulum, and neurofibrils.
- B. Nerve fibers include a solitary axon and numerous dendrites.
- 1. Branching dendrites carry impulses from other neurons (or
from receptors) toward the cell body.
- 2. The axon transmits the impulse away from the axonal hillock
of the cell body and may give off side branches.
- 3. Larger axons are enclosed by sheaths of myelin provided
by Schwann cells and are myelinated fibers.
- a. The outer layer of myelin is surrounded by a neurilemma
(neurilemmal sheath) made up of the cytoplasm and nuclei of
the Schwann cell.
- b. Narrow gaps in the myelin sheath between Schwann cells
are called nodes of Ranvier.
- 4. The smallest axons lack a myelin sheath and are unmyelinated
fibers.
- 5. White matter in the CNS is due to myelin sheaths in this
area.
- 6. Unmyelinated nerve tissue in the CNS appears gray.
- 7. Peripheral neurons are able to regenerate because of the
neurilemma but the CNS axons are myelinated by oligodendrocytes
thus lacking neurilemma and usually do not regenerate.
Classification of Neurons
- A. Neurons can be grouped in two ways: on the basis of structural
differences (bipolar, unipolar,
and multipolar neurons), and by functional differences (sensory neurons,
interneurons, and motor neurons).
- B. Classification of Neurons 9.5-9.6
- 1. Bipolar neurons are found in the eyes, nose, and ears, and
have a single axon and a single dendrite extending from opposite
sides of the cell body.
- 2. Unipolar neurons are found in ganglia outside the CNS and
have an axon and a dendrite arising from a single short fiber extending
from the cell body.
- 3. Multipolar neurons have many nerve fibers arising from their
cell bodies and are commonly found in the brain and spinal cord.
- 4. Sensory neurons (afferent neurons) conduct impulses from
peripheral receptors to the CNS and are usually unipolar, although
some are bipolar neurons.
- 5. Interneurons are multipolar neurons lying within the CNS
that form links between other neurons.
- 6. Motor neurons are multipolar neurons that conduct impulses
from the CNS to effectors.
Cell Membrane Potential
- A. A cell membrane is usually polarized, with an excess of negative
charges on the inside of the membrane; polarization is important to
the conduction of nerve impulses.
- B. Distribution of Ions 9.7
- 1. The distribution of ions is determined by the membrane channel
proteins that are selective for certain ions.
- 2. Potassium ions pass through the membrane more readily than
do sodium ions, making potassium ions a major contributor to membrane
polarization.
- C. Resting Potential 9.8-9.8b, 9.9a
- 1. Due to active transport, the cell maintains a greater concentration
of sodium ions outside and a greater concentration of potassium
ions inside the membrane.
- 2. The inside of the membrane has excess negative charges,
while the outside has more positive charges.
- 3. This separation of charge, or potential difference, is called
the resting potential.
- D. Potential Changes
- 1. Stimulation of a membrane can locally affect its resting
potential.
- 2. When the membrane potential becomes less negative, the membrane
is depolarized.
- 3. If sufficiently strong depolarization occurs, a threshold
potential is achieved as ion channels open.
- 4. At threshold, action potential is reached.
- 5. Action potential may be reached when a series of subthreshold
stimuli summate and reach threshold
- E. Action Potential 9.9b
- 1. At threshold potential, membrane permeability to sodium
suddenly changes in the region of stimulation.
- 2. As sodium channels open, sodium ions rush in, and the membrane
potential changes and becomes depolarized.
- 3. At the same time, potassium channels open to allow potassium
ions to leave the cell, the membrane becomes repolarized, and resting
potential is reestablished.
- 4. This rapid sequence of events is the action potential.
- 5. The active transport mechanism then works to maintain the
original concentrations of sodium and potassium ions.
Nerve Impulse Fig.
9.10
- A. A nerve impulse is conducted as action potential is reached
at the trigger zone and spreads by a local current flowing down the
fiber, and adjacent areas of the membrane reach action potential.
- B. Impulse Conduction
- 1. Unmyelinated fibers conduct impulses over their entire membrane
surface.
- 2. Myelinated fibers conduct impulses from node of Ranvier
to node of Ranvier, a phenomenon called saltatory conduction.
- 3. Saltatory conduction is many times faster than conduction
on unmyelinated neurons.
- C. All-or-None Response
- 1. If a nerve fiber responds at all to a stimulus, it responds
completely by conducting an impulse (all-or-none response).
- 2. Greater intensity of stimulation triggers more impulses
per second, not stronger impulses.
The Synapse Fig.
9.11
- A. Nerve impulses travel from neuron to neuron along complex nerve
pathways.
- B. The junction between two communicating neurons is called a synapse;
there exists a synaptic cleft between them across which the impulse
must be conveyed.
- C. Synaptic Transmission 9.12
- 1. The process by which the impulse in the presynaptic neuron
is transmitted across the synaptic cleft to the postsynaptic neuron
is called synaptic transmission.
- 2. When an impulse reaches the synaptic knobs of an axon, synaptic
vesicles release neurotransmitter into the synaptic cleft.
- 3. The neurotransmitter reacts with specific receptors on the
postsynaptic membrane.
- D. Excitatory and Inhibitory Actions
- 1. Neurotransmitters that increase postsynaptic membrane permeability
to sodium ions may trigger impulses and are thus excitatory.
- 2. Other neurotransmitters may decrease membrane permeability
to sodium ions, reducing the chance that it will reach threshold,
and are thus inhibitory.
- 3. The effect on the postsynaptic neuron depends on which presynaptic
knobs are activated.
- D. Neurotransmitters
- 1. At least 50 kinds of neurotransmitters are produced by the
nervous system, most of which are synthesized in the cytoplasm of
the synaptic knobs and stored in synaptic vesicles.
- 2. When an action potential reaches the synaptic knob, calcium
ions rush inward and, in response, some synaptic vesicles fuse with
the membrane and release their contents to the synaptic cleft.
- 3. Enzymes in synaptic clefts and on postsynaptic membranes
rapidly decompose the neurotransmitters after their release.
- 4. Destruction or removal of neurotransmitter prevents continuous
stimulation of the postsynaptic neuron.
Impulse Processing
- A. How impulses are processed is dependent upon how neurons are
organized in the brain and
spinal cord.
- B. Neuronal Pools
- 1. Neurons within the CNS are organized into neuronal pools
with varying numbers of cells.
- 2. Each pool receives input from afferent nerves and processes
the information according to the special characteristics of the
pool.
- C. Facilitation
- 1. A particular neuron of a pool may receive excitatory or
inhibitory stimulation; if the net effect is excitatory but subthreshold,
the neuron becomes more excitable to incoming stimulation (a condition
called facilitation).
- D. Convergence Fig. 9.14a
- 1. A single neuron within a pool may receive impulses from
two or more fibers (convergence), which makes it possible for the
neuron to summate impulses from different sources.
- E. Divergence 9.14b
- 1. Impulses leaving a neuron in a pool may be passed into several
output fibers (divergence), a pattern that serves to amplify an
impulse.
Types of Nerves Fig. 9.15)
- A. A nerve is a bundle of nerve fibers held together by layers
of connective tissue.
- B. Nerves can be sensory, motor, or mixed, carrying both sensory
and motor fibers.
Nerve Pathways
- A. The routes nerve impulses travel are called pathways, the simplest
of which is a reflex arc.
- B. Reflex Arcs
- 1. A reflex arc includes a sensory receptor, a sensory neuron,
an interneuron in the spinal cord, a motor neuron, and an effector.
- C. Reflex Behavior 9.16-9.17;
- 1. Reflexes are automatic, subconscious responses to stimuli
that help maintain homeostasis (heart rate, blood pressure, etc.)
and carry out automatic responses (vomiting, sneezing, swallowing,
etc.).
- 2. The knee-jerk reflex (patellar tendon reflex) is an example
of a monosynaptic reflex (no interneuron).
- 3. The withdrawal reflex involves sensory neurons, interneurons,
and motor neurons.
- a. At the same time, the antagonistic extensor muscles
are inhibited.
Meninges 9.18-9.19
- A. The brain and spinal cord are surrounded by membranes called
meninges that lie between the bone and the soft tissues.
- B. The outermost meninx is made up of tough, white dense connective
tissue, contains many blood vessels, and is called the dura mater.
- 1. It forms the inner periosteum of the skull bones.
- 2. In some areas, the dura mater forms partitions between lobes
of the brain, and in others, it forms dural sinuses.
- 3. The sheath around the spinal cord is separated from the
vertebrae by an epidural space.
- C. The middle meninx, the arachnoid mater, is thin and lacks blood
vessels.
- 1. It does not follow the convolutions of the brain.
- 2. Between the arachnoid and pia maters is a subarachnoid space
containing cerebrospinal fluid.
- D. The innermost pia mater is thin and contains many blood vessels
and nerves.
- 1. It is attached to the surface of the brain and spinal cord
and follows their contours.
Spinal Cord 9.20
- A. The spinal cord begins at the base of the brain and extends
as a slender cord to the level of the intervertebral disk between the
first and second lumbar vertebrae.
- B. Structure of the Spinal Cord 9.21
- 1. The spinal cord consists of 31segments, each of which gives
rise to a pair of spinal nerves.
- 2. A cervical enlargement gives rise to nerves leading to the
upper limbs, and a lumbar enlargement gives rise to those innervating
the lower limbs.
- 3. Two deep longitudinal grooves (anterior median fissure and
posterior median sulcus) divide the cord into right and left halves.
- 4. White matter, made up of bundles of myelinated nerve fibers
(nerve tracts), surrounds a butterfly-shaped core of gray matter
housing interneurons.
- 5. A central canal contains cerebrospinal fluid.
- C. Functions of the Spinal Cord 9.22-9.23
- 1. The spinal cord has two major functions: to transmit impulses
to and from the brain, and to house spinal reflexes.
- 2. Tracts carrying sensory information to the brain are called
ascending tracts; descending tracts carry motor information from
the brain.
- 3. The names that identify nerve tracts identify the origin
and termination of the fibers in the tract.
- 4. Many spinal reflexes also pass through the spinal cord.
Brain 9.24
- A. The brain is the largest, most complex portion of the nervous
system, containing 100 billion multipolar neurons.
- B. The brain can be divided into the cerebrum (largest portion
and associated with higher mental functions), the diencephalon (processes
sensory input), the cerebellum (coordinates muscular activity), and
the brain stem (coordinates and regulates visceral activities).
- C. Structure of the Cerebrum
- 1. The cerebrum is the largest portion of the mature brain,
consisting of two cerebral hemispheres.
- 2. A deep ridge of nerve fibers called the corpus callosum
connects the hemispheres.
- 3. The surface of the brain is marked by convolutions, sulci,
and fissures.
- 4. The lobes of the brain are named according to the bones
they underlie and include the frontal lobe, parietal lobe, temporal
lobe, occipital lobe, and insula.
- 5. A thin layer of gray matter, the cerebral cortex, lies on
the outside of the cerebrum and contains 75% of the cell bodies
in the nervous system.
- 6. Beneath the cortex lies a mass of white matter made up of
myelinated nerve fibers connecting the cell bodies of the cortex
with the rest of the nervous system.
- D. Functions of the Cerebrum
- 1. The cerebrum provides higher brain functions, such as interpretation
of sensory input, initiating voluntary muscular movements, memory,
and integrating information for reasoning.
- 2. Functional Regions of the Cerebral Cortex 9.25
- a. The functional areas of the brain overlap, but the cortex
can generally be divided into motor, sensory, and association
areas.
- b. The primary motor areas lie in the frontal lobes, anterior
to the central sulcus and in its anterior wall.
- c. Broca's area, anterior to the primary motor cortex, coordinates
muscular activity to make speech possible.
- d. Above Broca's area is the frontal eye field that controls
the voluntary movements of the eyes and eyelids.
- e. The sensory areas are located in several areas of the
cerebrum and interpret sensory input, producing feelings or sensations.
- f. Sensory areas for sight lie within the occipital lobe.
- g. Sensory and motor fibers alike cross over in the spinal
cord or brain stem so centers in the right hemisphere are interpreting
or controlling the left side of the body, and vice versa.
- h. The various association areas of the brain analyze and
interpret sensory impulses and function in reasoning, judgment,
emotions, verbalizing ideas, and storing memory.
- i. Association areas of the frontal lobe control a number
of higher intellectual processes.
- j. A general interpretive area is found at the junction of
the parietal, temporal, and occipital lobes, and plays the primary
role in complex thought processing.
- 3. Hemisphere Dominance 9.26
- a. Both cerebral hemispheres function in receiving and
analyzing sensory input and sending motor impulses to the opposite
side of the body.
- b. Most people exhibit hemisphere dominance for the language-related
activities of speech, writing, and reading.
- c. The left hemisphere is dominant in 90% of the population,
although some individuals have the right hemisphere as dominant,
and others show equal dominance in both hemispheres.
- d. The non-dominant hemisphere specializes in nonverbal
functions and controls emotions and intuitive thinking.
- e. The basal ganglia are masses of gray matter located
deep within the cerebral hemispheres that relay motor impulses
from the cerebrum and help to control motor activities by producing
inhibitory dopamine.
- f. Basal ganglia include the caudate nucleus, the putamen,
and the globus pallidus.
- E. Ventricles and Cerebrospinal Fluid 9.28
- 1. The ventricles are a series of connected cavities within
the cerebral hemispheres and brain stem.
- 2. The ventricles are continuous with the central canal of
the spinal cord, and are filled with cerebrospinal fluid.
- 3. Choroid plexuses, specialized capillaries from the pia mater,
secrete cerebrospinal fluid.
- a. Most cerebrospinal fluid arises in the lateral ventricles.
- 4. Cerebrospinal fluid has nutritive as well as protective
(cushioning) functions.
- F. Diencephalon 9.29
- 1. The diencephalon lies above the brain stem and contains
the thalamus and hypothalamus.
- 2. Other portions of the diencephalon are the optic tracts
and optic chiasma, the infundibulum (attachment for the pituitary),
the posterior pituitary, mammillary bodies, and the pineal gland.
- 3. The thalamus functions in sorting and directing sensory
information arriving from other parts of the nervous system, performing
the services of both messenger and editor.
- 4. The hypothalamus maintains homeostasis by regulating a wide
variety of visceral activities and by linking the endocrine system
with the nervous system.
- a. The hypothalamus regulates heart rate and arterial blood
pressure, body temperature, water and electrolyte balance, hunger
and body weight, movements and secretions of the digestive tract,
growth and reproduction, and sleep and wakefulness.
- 5. The limbic system, in the area of the diencephalon, controls
emotional experience and expression.
- a. By generating pleasant or unpleasant feelings about
experiences, the limbic system guides behavior that may enhance
the chance of survival.
- G. Brain Stem 9.24 & 9.29
- 1. The brain stem, consisting of the midbrain, pons, and medulla
oblongata, lies at the base of the cerebrum, and connects the brain
to the spinal cord.
- 2. Midbrain
- a. The midbrain, located between the diencephalon and pons,
contains bundles of myelinated nerve fibers that convey impulses
to and from higher parts of the brain, and masses of gray matter
that serve as reflex centers.
- b. The midbrain contains centers for auditory and visual
reflexes.
- 3. Pons
- a. The pons, lying between the midbrain and medulla oblongata,
transmits impulses between the brain and spinal cord, and contains
centers that regulate the rate and depth of breathing.
- 4. Medulla Oblongata
- a. The medulla oblongata transmits all ascending and descending
impulses between the brain and spinal cord.
- b. The medulla oblongata also houses nuclei that control
visceral functions, including the cardiac center that controls
heart rate, the vasomotor center for blood pressure control,
and the respiratory center that works, along with the pons,
to control the rate and depth of breathing.
- c. Other nuclei in the medulla oblongata are associated
with coughing, sneezing, swallowing, and vomiting.
- 5. Reticular Formation
- a. Throughout the brain stem, hypothalamus, cerebrum, cerebellum,
and basal ganglia, is a complex network of nerve fibers connecting
tiny islands of gray matter; this network is the reticular formation.
- b. Decreased activity in the reticular formation results
in sleep; increased activity results in wakefulness.
- c. The reticular formation filters incoming sensory impulses.
- H. Cerebellum 9.30
- 1. The cerebellum is made up of two hemispheres connected by
a vermis.
- 2. A thin layer of gray matter called the cerebellar cortex
lies outside a core of white matter.
- 3. The cerebellum communicates with other parts of the central
nervous system through cerebellar peduncles.
- 4. The cerebellum functions to integrate sensory information
about the position of body parts and coordinates skeletal muscle
activity and maintains posture.
Peripheral Nervous System
- A. The peripheral nervous system (PNS) consists of the cranial
and spinal nerves that arise from the central nervous system and travel
to the remainder of the body.
- B. The PNS is made up of the somatic nervous system that oversees
voluntary activities, and the autonomic nervous system that controls
involuntary activities.
- C. Cranial Nerves . 9.31;
- 1. Twelve pairs of cranial nerves arise from the underside
of the brain, most of which are mixed nerves.
- 2. The 12 pairs are designated by number and name and include
the olfactory, optic, oculomotor, trochlear, trigeminal, abducens,
facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and
hypoglossal nerves.
- 3. Refer to Figure 9.31 and Table 9.6 for cranial nerve number,
name, type, and function.
- D. Spinal Nerves 9.32
- 1. Thirty-one pairs of mixed nerves make up the spinal nerves.
- 2. Spinal nerves are grouped according to the level from which
they arise and are numbered in sequence, beginning with those in
the cervical region.
- 3. Each spinal nerve arises from two roots: a dorsal, or sensory,
root, and a ventral, or motor, root.
- 4. The main branches of some spinal nerves form plexuses.
- 5. Cervical Plexuses
- a. The cervical plexuses lie on either side of the neck
and supply muscles and skin of the neck.
- 6. Brachial Plexuses
- a. The brachial plexuses arise from lower cervical and
upper thoracic nerves and lead to the upper limbs.
- 7. Lumbrosacral Plexuses
- a. The lumbrosacral plexuses arise from the lower spinal
cord and lead to the lower abdomen, external genitalia, buttocks,
and legs.
Autonomic Nervous System
- A. The autonomic nervous system has the task of maintaining homeostasis
of visceral activities without conscious effort.
- B. General Characteristics
- 1. The autonomic nervous system includes two divisions: the
sympathetic and parasympathetic divisions, which exert opposing
effects on target organs.
- a. The parasympathetic division operates under normal conditions.
- b. The sympathetic division operates under conditions of stress
or emergency.
- C. Autonomic Nerve Fibers 9.33
- 1. In the autonomic motor system, motor pathways include two
fibers: a preganglionic fiber that leaves the CNS, and a postganglionic
fiber that innervates the effector.
- 2. Sympathetic Division 9.34
- a. Fibers in the sympathetic division arise from the thoracic
and lumbar regions of the spinal cord, and synapse in paravertebral
ganglia close to the vertebral column.
- b. Postganglionic axons lead to an effector organ.
- 3. Parasympathetic Division 9.35)
- a. Fibers in the parasympathetic division arise from the brainstem
and sacral region of the spinal cord, and synapse in ganglia close
to the effector organ.
- 4. Autonomic Neurotransmitters 9.36;
- a. Preganglionic fibers of both sympathetic and parasympathetic
divisions release acetylcholine.
- b. Parasympathetic postganglionic fibers are cholinergic fibers
and release acetylcholine.
- c. Sympathetic postganglionic fibers are adrenergic and release
norepinephrine.
- d. The effects of these two divisions, based on the effects
of releasing different neurotransmitters to the effector, are generally
antagonistic.
- 6. Control of Autonomic Activity
- a. The autonomic nervous system is largely controlled by reflex
centers in the brain and spinal cord.
- b. The limbic system and cerebral cortex alter the reactions
of the autonomic nervous system through emotional influence.
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