Download Nervous System Lect/96

Anatomy and Human Biology 2214
August 6, 2008
Michael Hall
The Nervous System
After you study this Nervous System lecture, you should be able to:
1. Distinguish between the ‘central’ (CNS) and ‘peripheral’ (PNS) divisions of the
nervous system
2. Understand the distinction between the ‘somatic’ and ‘autonomic’ divisions of the
nervous system
3. List the general functions of the sympathetic, parasympathetic and enteric
divisions of the autonomic nervous system
4. Explain the functions of sensory and motor nerves; distinguish between afferent
and efferent impulses
5. Describe the structure and function of a ‘reflex arc’
6. Understand the distinction between ‘preganglionic’ and ‘postganglionic’ neurons
and where their cell bodies are located
7. Distinguish between an autonomic and a sensory (spinal) ganglion
8. Describe the structure of a peripheral nerve, including its connective tissue
coverings
9. Describe the structure of a typical nerve fiber (cell)
10. Understand the process of myelination in the PNS
11. Understand the function of a Node of Ranvier
12. Understand the distribution of ‘gray’ and ‘white’ matter in the spinal cord.
13. Describe the general organization of the spinal cord in relation to afferent and
efferent nerves
14. Understand the general structure of a sensory ganglion and the function and origin
of the nerve fibers passing through it
15. Understand the division of the ANS into sympathetic, parasympathetic and enteric
divisions
16. Understand the organization and synaptic connections of nerves in the ANS
The human nervous system is by far the most complex system in the body, and is
formed by a network of over 100 million nerve cells (neurons) assisted by many more
glial cells, which support and protect the neurons. Anatomically the nervous system is
divided into the central nervous system (CNS) consisting of the brain, spinal cord and
retina, and the peripheral nervous system (PNS) composed of nerve fibers and small
aggregates of nerve cell bodies called ganglia. The PNS conveys impulses in both
directions between the CNS (brain and spinal cord) and the rest of the body.
Neurons respond to environmental changes (stimuli) by altering electric potentials
that exist between the inner and outer surfaces of their membranes, generating a nerve
impulse. This impulse is capable of traveling long distances, and transmitting information
to other neurons, muscles, glands, blood vessels etc.
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The Peripheral Nervous System
General Features of Neurons.
Neurons are responsible for the 1. reception, transmission and processing of
stimuli; 2. the release of neurotransmitters and 3. the triggering of certain cell activities.
Most neurons consist of three parts:
a). dendrites, which are specialized for receiving stimuli from the environment, sensory
epithelial cells or other neurons. Dendrites are usually short and divide like the branches
of a tree. Most neurons have numerous dendrites which considerably increase the
receptive area of a cell. This arborization of dendrites makes it possible for one neuron to
receive and integrate a great number of axon terminals from other nerve cells.
b). cell body (perikaryon, soma) which represents the trophic (integrative/metabolic)
center for the whole nerve cell. It is also receptive to stimuli. The cell body (soma,
perikaryon) is the part of the neuron that contains the nucleus and surrounding cytoplasm.
Most nerve cells have a spherical, large, euchromatic nucleus with a prominent nucleolus.
This indicates that they are actively transcribing large stretches of their DNA. The cell
body contains a highly developed rER and numerous free and bound ribosomes. Under
the light microscope, the rER and free ribosomes appear as basophilic granular areas
called Nissl bodies.
c). axon which is a single process specialized for generating or conducting nerve
impulses to other cells (nerve, muscle or glands). The distal portion of the axon is usually
branched and constitutes the terminal arborization. Each branch of this arborization
terminates in end bulbs (boutons) which interact with other neurons or non-nerve cells,
forming synapses. Synapses transmit information to the next cell in the circuit. Most
neurons have only one axon; a very few have no axon at all. Axons may be short, or very
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long; the axons of the motor cells of the spinal cord that innervate the foot muscles, may
have a length of up to a meter.
Typical Neuron
Structural classes of Neurons
Neurons and their processes are extremely variable in size and shape.
According to the size and shape of their processes, most neurons can be classified as
either:
a). multipolar neurons, which have more than two cell processes, one process
being the axon and the others dendrites;
b). bipolar neurons, with one dendrite entering and one axon leaving the cell
body
c). pseudounipolar (unipolar) neurons, which have a single process extending
from the cell body, which then divides into two branches. One branch extends to a
peripheral nerve ending and the other towards the central nervous system. Each branch
resembles an axon in its morphology, and is capable of transmitting information.
Most neurons of the body are multipolar. Bipolar neurons are found in the ear,
retina and olfactory mucosa, while pseudounipolar neurons are found in the dorsal root
ganglia and in some cranial nerve ganglia. They transmit information from sense organs
to the brain or spinal cord
Neurons can also be classified according to their functional roles.
Motor (efferent) neurons control effector organs such as muscle fibers and exocrine and
endocrine glands. They transmit information from the spinal cord to the periphery.
Sensory (afferent) neurons are involved in the reception of sensory stimuli from the
environment and from within the body. They transmit information from the periphery to
the spinal cord.
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Interneurons form a communicating and integrating network between the sensory and
motor neurons, and with other interneurons. Highly developed functions of the nervous
system cannot be ascribed to simple neuronal circuits; rather they depend on complex
interactions established by the integrated functions of many neurons. The majority of the
neurons in the body are interneurons.
Arrangement of Sensory and Motor nerves
The synapse is responsible for the unidirectional transmission of nerve impulses.
Synapses are the sites where contact occurs between neurons and effector cells (other
neurons, muscles or glands, blood vessels etc.). Most synapses transmit the impulse by
releasing neurotransmitters at the axon terminal. The synapse is formed by: 1) an axon
terminal (presynaptic terminal) that delivers the impulse; 2) a thin intercellular space
called the synaptic cleft; 3) a part of another cell (this can be the dendrite, the cell body,
or the axon) where a new impulse is generated (postsynaptic terminal) or a non-neuronal
cell, such as a muscle, gland or other cell;
Most synapses are chemical synapses and transmit nerve impulses through
neurotransmitters. A few synapses transmit impulses through gap junctions, and are
called electrical synapses.
Diagram of a typical synapse
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The Peripheral Nervous System (PNS)
The PNS consists of nerve cell bodies and nerve fibers, and supporting cells (Schwann
cells and satellite cells) that are distributed outside of the CNS. Aggregates of nerve cell
bodies in the PNS are called ganglia. Bundles (fascicles) of nerve fibers (axons) are
known as nerves. Large peripheral nerves may contain several fascicles. Ganglia serve
as relay stations to transmit nerve impulses; one nerve enters and another exits from each
ganglion. The direction of the nervous impulse determines whether the ganglion will be a
sensory or a motor ganglion. Peripheral nerves also contain blood vessels and connective
tissue.
A. Peripheral Nerves and Schwann cells
1) A peripheral nerve possesses afferent and efferent fibers traveling to and from the
CNS. Afferent fibers carry the information obtained from the interior of the body and
the environment to the CNS. Efferent fibers carry impulses from the CNS to the effector
organs. Nerves possessing only sensory fibers are called sensory nerves; those composed
only of fibers carrying impulses to the tissue or organ are called motor nerves. Most
nerves carry both sensory and motor fibers and are called mixed nerves; these nerves
have both myelinated and unmyelinated axons. The cell bodies and dendrites of peripheral
nerves may be located in the CNS or in peripheral ganglia. Their axons pass to the
periphery and innervate various organs and body tissues. These nerves establish
communication between the CNS and the sense organs and effectors (muscles, glands
etc.).
2) Peripheral nerves are almost always surrounded by CT, consisting of 3 layers which
surround the nerves, fascicles and individual fibers.
a) Epineurium is a dense irregular connective tissue layer which surrounds the
entire nerve and fills the space between the nerve fascicles (bundles of nerve fibers). The
epineurium is composed of collagen and elastic fibers with fibroblasts, macrophages and
mast cells. This layer also contains numerous blood vessels, small lymphatics and a few
small nerves, which innervate the blood vessels.
b) Perineurium consists of loose connective tissue and flattened epithelial cells
which surround the nerve fascicle. The epithelial cells are connected at their edges by
tight junctions (zonulae occludens) which form an effective barrier to most substances.
The thickness of the perineurium is dependent on the size of the nerve fascicle, with
thicker nerve fascicles having multiple epithelial cell layers.
c) Endoneurium consists of a thin layer of connective tissue surrounding
individual nerve fibers. It contains reticular fibers, and a basal lamina, both of which are
synthesized by the Schwann cells. Occasional fibroblasts and capillaries are found.
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3) Schwann cells surround all of the axon except the initial segment and the axon
terminal. Axons can be myelinated or non-myelinated—but in both cases they are
surrounded by Schwann cells.
a) A myelinated axon refers to the situation when an individual axon is
surrounded by multiple layers of a single Schwann cell. The general rule is--the larger the
axon, the thicker the myelin sheath.
Myelinated axon
Unmyelinated axons
Myelin consists of a lipid-protein complex that is characterized by myelin basic protein
and phospholipids. Myelin can be demonstrated by staining with osmium tetroxide,
which preserves the myelin and stains it black.
Many Schwann cells are distributed along a nerve and the points at which they
meet are called nodes of Ranvier. Adjacent Schwann cells interdigitate at the node.
When an axon potential passes along a myelinated nerve, it jumps from node to node,
significantly increasing conduction rates. This is called saltatory conduction.
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Nodes of Ranvier along a myelinated axon
b) Unmyelinated axons lie in grooves or furrows formed by Schwann cells--however no myelin sheath is formed. These axons are generally 1 m in diameter or
smaller. As is the case for myelinated axons, many Schwann cells aligned end-to-end
surround these axons along their entire length. Unmyelinated axons conduct slowly.
A single Schwann cell can envelop one or a number of unmyelinated axons.
B. Organization of the Spinal Cord
The spinal cord is obviously not a part of the PNS. However, afferent nerves which enter
the spinal cord from the periphery, and efferent nerves which leave the spinal cord on
their way to the periphery, make up the bulk of the nerves of PNS. Thus a short
description of the spinal cord may help to elucidate the structural relationships between
the CNS and the PNS.
In cross section the spinal cord exhibits a butterfly-shaped, gray inner substance,
the gray matter, and a whitish peripheral substance, the white matter. The gray matter
contains neuronal cell bodies and their dendrites, as well as axons and glial cells. The
white matter contains only myelinated and unmyelinated axons traveling to and from
other parts of the spinal cord and the brain, and axons traveling to and from the periphery,
and glial cells.
The spinal cord is continuous with the brain and is divided into a number of
segments. Each segment is connected to a pair of spinal nerves and each spinal nerve is
joined to its segment of the cord by a number of roots which are grouped either as
posterior (dorsal) or anterior (ventral) roots. The dorsal roots contain afferent (sensory)
nerves, while the ventral roots contain efferent (motor) nerves.
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Cross section of spinal cord
Somatic and Autonomic nerve tracts
C. Sensory Ganglia and Sensory (afferent) Fibers
Sensory ganglia receive afferent impulses that go to the CNS. Two types of sensory
ganglia exist. Some are associated with cranial nerves (cranial ganglia); others are
associated with the dorsal roots of the spinal nerves and are called spinal ganglia or
dorsal root ganglia. They contain the cell bodies and fibers of pseudounipolar neurons
transmitting information from the periphery to the CNS. Dorsal root ganglion cells
synapse with interneurons in the dorsal horn of the spinal cord and may be associated
with a variety of receptors in the periphery (muscles, glands, vessels, sensory cells etc).
The cell bodies of neurons in the dorsal root ganglia have a round appearance and
vary in size. They are surrounded by smaller, cuboidal shaped glial cells known as
satellite cells. These help to maintain a controlled environment around the neuronal cell
body, providing electrical insulation as well as a pathway for metabolic exchange.
Sensory ganglia are surrounded by a connective tissue layer that is continuous
with the epineurium of the nerve.
Remember that a single pseudounipolar neuron connects the sensory organ (eg.
skeletal muscle, gland, skin, blood vessel etc) through a sensory ganglion (e.g. dorsal root
ganglion), to the spinal cord or brain stem.
D. Autonomic Nervous System (ANS)
The autonomic (=visceral) nervous system regulates visceral functions, and is especially
important in the control of cardiac and smooth muscles, blood vessels and glands.
The ANS is subdivided into three anatomical divisions--sympathetic,
parasympathetic and enteric.
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In the ANS, a chain of two neurons connects the CNS to peripheral organs and
tissue. These two neurons synapse in a peripheral ganglion. The first neuron is
preganglionic (feeds into the ganglion) and the second is postganglionic (feeds out of
the ganglion). The preganglionic neurons of the ANS have their cell bodies in specific
locations in the CNS, either in the brainstem, or in the spinal cord while the
postganglionic neurons have their cell bodies in a peripheral ganglion The axons of the
postganglionic neurons then travel to the effector organs, innervating them directly.
1) Sympathetic division. Preganglionic cell bodies are located in the spinal cord.
Postganglionic cell bodies are located in ganglia lying close to the spinal cord
(paravertebral or sympathetic chain ganglia) or in the celiac and mesenteric ganglia.
These ganglia contain multipolar neurons and numerous satellite cells that surround the
cell bodies. Each preganglionic neuron makes contact with more than one postganglionic
neuron. Peripheral ganglia are surrounded by connective tissue which is continuous with
the epineurium.
Location of parasympathetic and sympathetic ganglia
2) Parasympathetic division. As with the sympathetic system, a chain of two
neurons connects the CNS to the target organ. Preganglionic cell bodies are located in the
brainstem and sacral division of the spinal cord. Postganglionic cell bodies are located in
ganglia situated near to or in their target tissue.
The sympathetic and parasympathetic divisions of the ANS often supply the same
organs. In these cases the actions of the two are usually antagonistic. An example of this
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is the innervation of cardiac muscle; sympathetic stimulation increases its activity, while
parasympathetic stimulation inhibits its activity. However they may also work together.
3) Enteric division. This division of the ANS includes both sympathetic and
parasympathetic innervation. It describes those nerves which stimulate the
gastrointestinal system. Enteric ganglia are located between the longitudinal and
circular smooth muscles and in the submucosa of the gastrointestinal tract. These ganglia
contain multipolar neurons and glial cells. Enteric neurons innervate smooth muscle as
well as the mucosa, submucosa and blood vessels of the digestive system.
Enteric ganglia in the gut
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