Notes: 11A. Nervous System
INTRODUCTION
NERVOUS TISSUES
There are two types of nervous tissues–the neurons (nerve cells) and glia (neuroglia). See paragraph 2-17. The neuron is the basic structural unit of the nervous system. The glia are cells of supporting tissue for the nervous system. There are several different types of glia, but their general function is support (physical, nutritive, etc.).
SPECIALIZATION
Nervous tissues are specialized to:
a.
Receive Stimuli. Cells receiving stimuli are said to be “irritable” (as are all living cells to a degree).
b.
Transmit Information.
c.
“Store” Information. The storing of information is called memory.
THE NEURON AND ITS “CONNECTIONS”
DEFINITION
A neuron is a nerve cell body and all of its processes (branches).
NEURON CELL BODY
The neuron cell body is similar to that of the “typical” animal cell described in lesson 1.
NEURON PROCESSES
There are two types of neuron processes–dendrites and axons.
a.
Dendrite. A dendrite is a neuron process which carries impulses toward the cell body. Each neuron may have one or more dendrites. Dendrites receive information and transmit (carry) it to the cell body.
b.
Axon. An axon is a neuron process which transmits information from the cell body to the next unit. Each neuron has only one axon.
c.
Information Transmission. Information is carried as electrical impulses along the length of the neuron.
d.
Coverings. Some neuron processes have a covering which is a series of Schwann cells, interrupted by nodes (thin spots). This gives the neuron process the appearance of links of sausage. The Schwann cells produce a lipid (fatty) material called myelin. This myelin acts as an electrical insulator during the transmission of impulses.
TYPES OF NEURONS
Neurons may be identified according to shape, diameter of their processes, or function.
a.
According to Shape. A pole is the point where a neuron process meets the cell body. To determine the type according to shape, count the number of poles.
(1) Multipolar neurons. Multipolar neurons have more than two poles (one axon and two or more dendrites).
(2) Bipolar neurons. Bipolar neurons have two poles (one axon and one dendrite).
(3) Unipolar neurons. Unipolar neurons have a single process which branches into a T-shape. One arm is an axon; the other is a dendrite.
b.
According to Diameter (Thickness) of Processes. Neurons may be rated according to the thickness of myelin surrounding the axon. In order of decreasing thickness, they are rated A (thickest), B, and C (thinnest). The thickness affects the rate at which impulses are transmitted. The thickest are fastest. The thinnest are slowest.
c.
According to Function.
(1) Sensory neurons. In sensory neurons, impulses are transmitted from receptor organs (for pain, vision, hearing, etc.) to the central nervous system (CNS).
(2) Motor neurons. In motor neurons, impulses are transmitted from the CNS to muscles and glands (effector organs).
(3) Interneurons. Interneurons transmit information from one neuron to another. An interneuron “connects” two other neurons.
(4) Others. There are other, more specialized types, for example, in the CNS.
NEURON “CONNECTIONS”
A neuron may “connect” either with another neuron or with a muscle fiber. A phrase used to describe such “connections” is “continuity without contact.” Neurons do not actually touch. There is just enough space to prevent the electrical transmission from crossing from the first neuron to the next. This space is called the synaptic cleft. Information is transferred across the synaptic cleft by chemicals called neurotransmitters. Neurotransmitters are manufactured and stored on only one side of the cleft. Because of this, information flows in only one direction across the cleft.
a.
The Synapse. A synapse is a “connection” between two neurons.
(1) First neuron. An axon terminates in tiny branches. At the end of each branch is found a terminal bulb. Synaptic vesicles (bundles of neurotransmitter) are located within each terminal bulb. That portion of the terminal bulb which faces the synaptic cleft is thickened and is called the presynaptic membrane. This is the membrane through which neurotransmitters pass to enter the synaptic cleft.
(2) Synaptic cleft. The synaptic cleft is the space between the terminal bulb of the first neuron and the dendrite or cell body of the second neuron.
(3) Second neuron. The terminal bulb of the first neuron lies near a site on a dendrite or the cell body of the second neuron. The membrane at this site on the second neuron is known as the postsynaptic membrane. Within the second neuron is a chemical that inactivates the used neurotransmitter.
b.
The Neuromuscular Junction. A neuromuscular junction is a “connection” between the terminal of a motor neuron and a muscle fiber. The neuromuscular junction has an organization identical to a synapse. However, the bulb is larger. The postsynaptic membrane is also larger and has foldings to increase its surface area.
(1) Motor neuron. The axon of a motor neuron ends as it reaches a striated muscle fiber (of a skeletal muscle). At this point, it has a terminal bulb. Within this bulb are synaptic vesicles (bundles of neurotransmitter). The presynaptic membrane lines the surface of the terminal bulb and lies close to the muscle fiber.
(2) Synaptic cleft. The synaptic cleft is a space between the terminal bulb of the motor neuron and the membrane of the muscle fiber.
(3) Muscle fiber. The terminal bulb of the motor neuron protrudes into the surface of the muscle fiber. The membrane lining the synaptic space has foldings and is called the postsynaptic membrane. Beneath the postsynaptic membrane is a chemical which inactivates the used neurotransmitter.
THE HUMAN CENTRAL NERVOUS SYSTEM
GENERAL
The major divisions of the human nervous system are the central nervous system (CNS), the peripheral nervous system (PNS), and the autonomic nervous system (ANS). The CNS is made up of the brain and spinal cord. Both the PNS and the ANS carry information to and from the central nervous system. The PNS is generally concerned with the innervation of skeletal muscles and other muscles made up of striated muscle tissue, as well as sensory information from the periphery of the body.
The ANS is that portion of the nervous system concerned with control of smooth muscle, cardiac muscle, and glands. The CNS (figure 11-4) is known as central because its anatomical location is along the central axis of the body and because the CNS is central in function. If we use a computer analogy to understand that it is central in function, the CNS would be the central processing unit and other parts of the nervous system would supply inputs and transmit outputs.
a.
Major Subdivisions of the CNS. The major subdivisions of the CNS are the brain and the spinal cord.
b.
Coverings of the CNS. The coverings of the CNS are skeletal and fibrous.
c.
Cerebrospinal Fluid (CSF). The CSF is a liquid thought to serve as a cushion and circulatory vehicle within the CNS.
THE HUMAN BRAIN
The human brain has three major subdivisions: brainstem, cerebellum, and cerebrum. The CNS is first formed as a simple tubelike structure in the embryo. The concentration of nervous tissues at one end of the human embryo to produce the brain and head is referred to as cephalization. When the embryo is about four weeks old, it is possible to identify the early forms of the brainstem, cerebellum, and cerebrum, as well as the spinal cord. As development continues, the brain is located within the cranium (Lesson 4) in the cranial cavity.
a. The Brainstem. The term brainstem refers to that part of the brain that would remain after removal of the cerebrum and cerebellum. The brainstem is the basal portion (portion of the base) of the brain. The brainstem can be divided as follows:
| FOREBRAINSTEM | thalamus hypothalamus |
| MIDBRAINSTEM | corpora quadrigemina cerebral peduncles |
| HINDBRAINSTEM | pons medulla |
(1) The brainstem is continuous with the spinal cord. Together, the brainstem and the spinal cord are sometimes known as the neuraxis.
(2) The brainstem provides major relays and controls for information passing up or down the neuraxis.
(3) The 12 pairs of cranial nerves connect at the sides of the brainstem.
b.
Cerebellum. The cerebellum is a spherical mass of nervous tissue attached to and covering the hindbrainstem. It has a narrow central part called the vermis and right and left cerebellar hemispheres.
(1) Peduncles. A peduncle is a stem-like connecting part. The cerebellum is connected to the brainstem with three pairs of peduncles.
(2) General shape and construction. A cross section of the cerebellum reveals that the outer cortex is composed of gray matter (cell bodies of neurons) with many folds and sulci (shallow grooves). More centrally located is the white matter (myelinated processes of neurons).
(3) Function. The cerebellum is the primary coordinator/integrator of motor actions of the body.
c.
Cerebrum. The cerebrum consists of two very much enlarged hemispheres connected to each other by a special structure called the corpus callosum. Each cerebral hemisphere is connected to the brainstem by a cerebral peduncle. The surface of each cerebral hemisphere is subdivided into areas known as lobes. Each lobe is named according to the cranial bone under which it lies: frontal, parietal, occipital, and temporal.
(1) The space separating the two cerebral hemispheres is called the longitudinal fissure. The shallow grooves in the surface of the cerebrum are called sulci (sulcus, singular). The ridges outlined by the sulci are known as gyri (gyrus, singular).
(2) The cerebral cortex is the gray outer layer of each hemisphere. The occurrence of sulci and gyri helps to increase the amount of this layer. Deeper within
the cerebral hemispheres, the tissue is white. The “gray matter” represents cell bodies of the neurons. The “white matter” represents the axons.
(3) The areas of the cortex are associated with groups of related functions.
(a) For example, centers of speech and hearing are located along the lateral sulcus, at the side of each hemisphere.
(b) Vision is centered at the rear in the area known as the occipital lobe.
(c) Sensory and motor functions are located along the central sulcus, which separates the frontal and parietal lobes of each hemisphere. The motor areas are located along the front side of the central sulcus, in the frontal lobe. The sensory areas are located along the rear side of the central sulcus, in the parietal lobe.
d.
Ventricles. Within the brain, there are interconnected hollow spaces filled with cerebrospinal fluid (CSF). These hollow spaces are known as ventricles. The right and left lateral ventricles are found in the cerebral hemispheres. The lateral ventricles are connected to the third ventricle via the interventricular foramen (of Monroe). The third ventricle is located in the forebrainstem. The fourth ventricle is in the hindbrainstem. The cerebral aqueduct (of Sylvius) is a short tube through the midbrainstem which connects the third and fourth ventricles. The fourth ventricle is continuous with the narrow central canal of the spinal cord.
THE HUMAN SPINAL CORD
a.
Location and Extent. The typical vertebra has a large opening called the vertebral (or spinal) foramen. Together, these foramina form the vertebral (spinal) canal for the entire vertebral column. The spinal cord, located within the spinal canal, is continuous with the brainstem. The spinal cord travels the length from the foramen magnum at the base of the skull to the junction of the first and second lumbar vertebrae.
(1) Enlargements. The spinal cord has two enlargements. One is the cervical enlargement, associated with nerves for the upper members. The other is the lumbosacral enlargement, associated with nerves for the lower members.
(2) Spinal nerves. A nerve is a bundle of neuron processes which carry impulses to and from the CNS. Those nerves arising from the spinal cord are spinal nerves. There are 31 pairs of spinal nerves.
b.
A Cross Section of the Spinal Cord. The spinal cord is a continuous structure which runs through the vertebral canal down to the lumbar region of the column. It is composed of a mass of central gray matter (cell bodies of neurons) surrounded by peripheral white matter (myelinated processes of neurons). The gray and white matter are thus considered columns of material. However, in a cross section, this effect of columns is lost.
(1) Central canal. A very narrow canal, called the central canal, is located in the center of the spinal cord. The central canal is continuous with the fourth ventricle of the brain.
(2) The gray matter. In the cross section of the spinal cord, one can see a central H-shaped region of gray matter. Each arm of the H is called a horn, resulting in two posterior horns and two anterior horns. The connecting link is called the gray commissure. Since the gray matter extends the full length of the spinal cord, these horns are actually sections of the gray columns.
(3) The white matter. The peripheral portion of the spinal cord cross section consists of white matter. Since a column of white matter is a large bundle of processes, it is called a funiculus.
COVERINGS OF THE CNS
The coverings of the CNS are skeletal and fibrous.
a.
Skeletal Coverings.
(1) Brain. The bones of the cranium form a spherical case around the brain. The cranial cavity is the space inclosed by the bones of the cranium.
(2) Spinal cord. The vertebrae, with the vertebral foramina, form a cylindrical case around the spinal cord. The overall skeletal structure is the vertebral column (spine). The vertebral (spinal) canal is the space inclosed by the foramina of the vertebrae.
b.
Meninges (Fibrous Membranes). The brain and spinal cord have three different membranes surrounding them called meninges. These coverings provide protection.
(1) Dura mater. The dura mater is a tough outer covering for the CNS. Beneath the dura mater is the subdural space, which contains a thin film of fluid.
(2) Arachnoid mater. To the inner side of the dura mater and subdural space is a fine membranous layer called the arachnoid mater. It has fine spiderweb-type threads which extend inward through the subarachnoid space to the pia mater. The subarachnoid space is filled with cerebrospinal fluid (CSF).
ARACHNOID = spider-like
(3) Pia mater. The pia mater is a delicate membrane applied directly to the surface of the brain and the spinal cord. It carries a network of blood vessels to supply the nervous tissues of the CNS.
BLOOD SUPPLY OF THE CNS
a.
Blood Supply of the Brain. The paired internal carotid arteries and the paired vertebral arteries supply blood rich in oxygen to the brain. Branches of these arteries join to form a circle under the base of the brain. This is called the cerebral circle (of Willis). From this circle, numerous branches supply specific areas of the brain.
(1) A single branch is often the only blood supply to that particular area. Such an artery is called an end artery. If it fails to supply blood to that specific area, that area will die (stroke).
(2) The veins and venous sinuses of the brain drain into the paired internal jugular veins, which carry the blood back toward the heart.
b.
Blood Supply of the Spinal Cord. The blood supply of the spinal cord is by way of a combination of three longitudinal arteries running along its length and reinforced by segmental arteries from the sides.
CEREBROSPINAL FLUID (CSF)
A clear fluid called cerebrospinal fluid (CSF) is found in the cavities of the CNS. CSF is found in the ventricles of the brain, the subarachnoid space, and the central canal of the spinal cord. CSF and its associated structures make up the circulatory system for the CNS.
a.
Choroid Plexuses. Choroid plexuses are special collections of arterial capillaries found in the roofs of the third and fourth ventricles of the brain. The choroid plexuses continuously produce CSF from the plasma of the blood.
b.
Path of the CSF Flow. Blood flows through the arterial capillaries of the choroid plexuses. As CSF is produced by the choroid plexuses, it flows into all four ventricles. CSF from the lateral ventricles flows into the third ventricle and then through the cerebral aqueduct into the fourth ventricle. By passing through three small holes in the roof of the fourth ventricle, CSF enters the subarachnoid space. From the subarachnoid space, the CSF is transported through the arachnoid villi (granulations) into the venous sinuses. Thus, the CSF is formed from arterial blood and returned to the venous blood.
THE PERIPHERAL NERVOUS SYSTEM (PNS)
GENERAL
a.
Definitions.
(1) The peripheral nervous system (PNS) is that portion of the nervous system generally concerned with commands for skeletal muscles and other muscles made up of striated muscle tissue, as well as sensory information from the periphery of the body. The sensory information is carried to the CNS where it is processed. The PNS carries commands from the CNS to musculature.
(2) A nerve is a collection of neuron processes, together and outside the CNS. (A fiber tract is a collection of neuron processes, together and inside the CNS.)
b.
General Characteristics of the Peripheral Nerves. The PNS is made up of a large number of individual nerves. These nerves are arranged in pairs. Each pair includes one nerve on the left side of the brainstem or spinal cord and one nerve on the right side. The nerve pairs are in a series, each pair resembling the preceding, from top to bottom.
c.
Categories of PNS Nerves. PNS nerves include cranial nerves and spinal nerves.
(1) Cranial nerves. The 12 pairs of nerves attached to the right and left sides of the brainstem are called cranial nerves. Each cranial nerve is identified by a Roman numeral in order from I to XII and an individual name. For example, the Vth (”fifth”) cranial nerve is known as the trigeminal nerve (N.).
TRI = three
GEMINI = alike
TRIGEMINAL = having three similar major branches
(2) Spinal nerves. Attached to the sides of the spinal cord are 31 pairs of spinal nerves. The spinal nerves are named by:
(a) The region of the spinal cord with which the nerve is associated.
(b) An Arabic numeral within the region. For example, T-5 is the fifth spinal nerve in the thoracic region.
A “TYPICAL” SPINAL NERVE
In the human body, every spinal nerve has essentially the same construction and components. By learning the anatomy of one spinal nerve, you can understand the anatomy of all spinal nerves.
a. Parts of a “Typical” Spinal Nerve. Like a tree, a typical spinal nerve has roots, a trunk, and branches (rami).
(1) Coming off of the posterior and anterior sides of the spinal cord are the posterior (dorsal) and anterior (ventral) roots of the spinal nerve. An enlargement on the posterior root is the posterior root ganglion. A ganglion is a collection of neuron cell bodies, together, outside the CNS.
(2) Laterally, the posterior and anterior roots of the spinal nerve join to form the spinal nerve trunk. The spinal nerve trunk of each spinal nerve is located in the appropriate intervertebral foramen of the vertebral column. (An intervertebral foramen is a passage formed on either side of the junction between two vertebrae.)
(3) Where the spinal nerve trunk emerges laterally from the intervertebral foramen, the trunk divides into two major branches. These branches are called the anterior (ventral) and posterior (dorsal) primary rami (ramus, singular). The posterior primary rami go to the back. The anterior primary rami go to the sides and front of the body and also to the upper and lower members.
b.
Neurons of a “Typical” Spinal Nerve. A nerve is defined above as a collection of neuron processes. Thus, neuron processes are the components that make up a nerve. These processes may belong to any of several different types of neurons: afferent (sensory), efferent (motor), and visceral motor neurons of the ANS.
(1) The afferent neuron and the efferent neuron are the two types we will consider here. An afferent neuron is one which carries information from the periphery to the CNS.
A = toward
FERENT = to carry
An efferent neuron is one which carries information from the CNS to a muscle or gland.
E = away from
FERENT = to carry
(2) The afferent neuron is often called the sensory neuron because it carries information about the senses to the CNS. The efferent neuron is often called the motor neuron because it carries commands from the CNS to cause a muscle to act.
(3) A stimulus acts upon a sensory receptor organ in the skin or in another part of the body. The information is carried by an afferent (sensory) neuron through merging branches of the spinal nerve to the posterior root ganglion. The afferent (sensory) neuron’s cell body is located in the posterior root ganglion. From this point, information continues in the posterior root to the spinal cord. The efferent (motor) neuron carries command information from the spinal cord to the individual muscle of the human body.
(4) Visceral motor neurons of the ANS (see section V), which innervate visceral organs of the body’s periphery, are distributed along with the peripheral nerves.
c.
The General Reflex Arc.
(1) Definitions.
(a) An automatic reaction to a stimulus (without first having conscious sensation) is referred to as a reflex. (As an example: The withdrawal of the hand from a hot object.)
(b) The pathway from the receptor organ to the reacting muscle is called the reflex arc.
(2) Components of the general reflex arc. The pathway of a general reflex arc involves a minimum of five structures.
(a) The stimulus is received by a receptor organ.
(b) That information is transmitted to the CNS by the afferent (sensory) neuron.
(c) Within the spinal cord, there is a special neuron connecting the afferent neuron to the efferent neuron. This special connecting neuron is called the internuncial neuron, or interneuron.
INTER = between
NUNCIA = messenger
INTERNUNCIAL = the carrier of information between
(d) The efferent (motor) neuron carries the appropriate command from the spinal cord to the reacting muscle.
(e) The reacting muscle is called the effector organ.
THE AUTONOMIC NERVOUS SYSTEM (ANS)
GENERAL
The autonomic nervous system (ANS) is that portion of the nervous system generally concerned with commands for smooth muscle tissue, cardiac muscle tissue, and glands.
a.
Visceral Organs.
(1) Definition. The term visceral organs may be used to include:
(a) The various hollow organs of the body whose walls have smooth muscle tissue in them. Examples are the blood vessels and the gut.
(b) The glands.
(2) Distribution. The visceral organs are located in the central cavity of the body (example: stomach) and throughout the periphery of the body (example: sweat glands of the skin).
(3) Control. It has always been thought that the control of visceral organs was “automatic” and not conscious. However, recent researches indicate that proper training enables a person to consciously control some of the visceral organs.
b.
Efferent Pathways. Earlier, we said that each neuron in the PNS extended the entire distance from the CNS to the receptor or effector organ. In the ANS, there are always two neurons (one after the other) connecting the CNS with the visceral organ. The cell bodies of the second neurons form a collection outside the CNS, called a ganglion.
(1) The first neuron extends from the CNS to the ganglion and is therefore alled the preganglionic neuron.
(2) Cell bodies of the second neuron make up the ganglion. The second neuron’s processes extend from the ganglion to the visceral organ. Thus, the second neuron is called the post-ganglionic neuron.
c.
Major Divisions of the Human ANS. The efferent pathways of the ANS fall into two major divisions:
(1) The thoraco-lumbar outflow (sympathetic nervous system).
(2) The cranio-sacral outflow (parasympathetic nervous system).
d.
Major Activities of the Human ANS.
(1) The ANS maintains visceral activities in a balanced or stable state. This is called homeostasis.
(2) When subjected to stress, such as a threat, the body responds with the “fight-or-flight reaction.” That is, those activities of the body necessary for action in an emergency are activated and those not necessary are deactivated. This is the primary function of the sympathetic portion of the ANS.
THE THORACO-LUMBAR OUTFLOW (SYMPATHETIC NERVOUS SYSTEM)
a. Remember the H-shaped region of gray matter in the cross section of the spinal cord. Imagine extending the cross link of the H slightly to the left and right of the vertical arms; the extended ends would correspond to the intermediolateral gray columns. Cell bodies of the first neurons of the sympathetic NS make up those columns between the T-1 and L-2 levels of the spinal cord, a total of 14 levels. Here, we are speaking of preganglionic sympathetic neurons.
b. Cell bodies of the second neurons make up various sympathetic ganglia of the body. These ganglia include the trunk or chain ganglia and the pre-aortic or “central” ganglia. Here, we are speaking of post- ganglionic sympathetic neurons.
c. The sympathetic NS innervates:
(1) Peripheral visceral organs (example: sweat glands).
(2) Central visceral organs (examples: lungs and stomach).
d. The neurons innervating the peripheral visceral organs are distributed to them by being included in the nerves of the PNS.
e. The sympathetic NS activates those visceral organs needed to mobilize energy for action (example: heart) and deactivates those not needed (example: gut).
THE CRANIO-SACRAL OUTFLOW (PARASYMPATHETIC NERVOUS SYSTEM)
a. Cell bodies of the first neurons of the parasympathetic NS make up the intermediolateral gray columns in the sacral spinal cord at the S-2, S-3, and S-4 levels. Cell bodies of the first neurons also make up four pairs of nuclei in the brainstem; these nuclei are associated with cranial nerves III, VII, IX, and X. Here, we are speaking of preganglionic parasympathetic neurons.
b. Cell bodies of the second neurons make up intramural ganglia within the walls of the visceral organs. These second neurons innervate the central visceral organs. They do NOT innervate peripheral visceral organs. Here, we are speaking of the post-ganglionic parasympathetic neurons.
c. The parasympathetic NS has the opposite effect on visceral organs from that of the sympathetic NS. (Example: The heart is accelerated by the sympathetic NS and decelerated by the parasympathetic NS.)
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