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THE PERIPHERAL AND AUTONOMIC (ANS) NERVOUS SYSTEM (PNS)
(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.
(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)
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.)
is your video Lesson for the PNS - Peripheral Nervous System
Video Lesson for the ANS - Autonomic Nervous System