Chapter 8. The Spinal Cord, Spinal Nerves, and Autonomic Nervous System
The spinal cord begins below the medulla and ends just above the small of the back at the conus medularis. The area within the vertebral column beyond the end of the spinal cord is called the cauda equina.
The spinal cord is protected by the vertebrae and the meninges. The dura mater, arachnoid mater and pia mater of the spinal cord are continuous with those of the brain. Cerebrospinal fluid is in the subarachnoid space that lies between the arachnoid and pia mater and in the central canal, a space in the middle of the gray matter of the cord. It provides a hydraulic cushion for the spinal cord.
Dorsal (sensory) and ventral (motor) horn cells
When the cord is viewed in a cross-section, its gray matter is "H" shaped or, as described by Bhatnagar, 2002, butterfly shaped. It has two ventral and two dorsal horns. The white matter surrounding the cell bodies of the cord is made up of ascending and descending fibers. Motor tracts are found on the ventral and lateral aspects of the cord while sensory tracts run along its dorsal area.
Motor neurons
These lower motor neurons are located on the ventral aspect of the cord. They are either alpha or gamma cells.
Alpha cells are the principle lower motor neurons of the spinal cord and form the main portion of the final common pathway. They conduct rapid motor impulses, with each alpha cell innervating approximately 200 muscle fibers.
Gamma neurons are also part of the final common pathway according to some sources but they are only half as numerous as alpha cells. Gamma cells conduct slow motor impulses. Their major function is to stretch muscle spindles.
Association neurons
Interneurons connect the anterior and posterior horns of the gray matter and are involved in the reflex arc. They work within the same segment of the spinal cord, with a segment being defined as the horizontal section of the cord that gives rise to one pair of spinal nerves.
Internuncial Neurons travel between segments, sending projections up to the brain stem and cerebellum. They project in an ascending, not descending manner.
These association neurons are found throughout the central nervous system. They are much more numerous than motor neurons; the ratio between the two types of cells is 30:1.
The main function of the association neurons in the spinal cord is that of inhibitory control. They also interconnect other cells with one another.
Some sources, including Bhatnager and Andy, (1995), do not distinguish between interneurons and internuncial neurons. Even if these two types of association neurons are grouped together, they should definitely be distinguished from the spinal nerves which are lower motor neurons, forming a final common pathway for information descending from the brain.
There are thirty-one pairs of spinal nerves. These nerves are mixed, having both a sensory and a motor aspect. Their motor fibers begin on the ventral part of the spinal cord at the anterior horns of the gray matter. The roots of their sensory fibers are located on the dorsal side of the spinal cord in the posterior root ganglia. When the motor and sensory fibers exit the spinal column through the intervertebral foramina and pass through the meninges, they join together to form the spinal nerves.
Spinal nerves receive only contralateral innervation from first order neurons.
Eight pairs of spinal nerves are located in the uppermost, cervical region of the cord:
Twelve pairs are found in the thoracic region.
Five pairs are in the lumbar area.
Five pairs are in the sacral area.
One pair is found in the most inferior, coccygeal region.
These second order lower motor neurons, the spinal nerves, form part of the final common pathway for information traveling from the central nervous system to the periphery. The spinal nerves provide innervation to body areas below the neck while cranial nerves (also second order neurons) carry impulses only to the head and neck, except for the vagus. (You will understand shortly that cranial nerves can be sensory, motor or both).
Also, the sensory and motor fibers of the spinal nerves form a reflex arc. This type of reflexive behavior occurs when a message from afferent fibers causes a motor reaction before going to the brain. For example, if you touch a hot burner on the stove, sensory information about the temperature of the burner travels along spinal nerves to your spinal cord and are carried directly to their motor nuclei by interneurons; the motor command goes out along the axons of the lower motor neuron causing you to move your hand away from the stove. As messages do not have to travel up to the brain to be processed, reactions mediated by this reflex arc can occur very rapidly. Of course you WILL feel pain shortly thereafter (milliseconds) as the information gets to the parietal lobe via the thalamus
The Autonomic (self regulating) Nervous System
The autonomic nervous system is involved in the control of the heart, glands and smooth muscles of the body and plays a major role in regulating unconscious, vegetative functions. It works together with the endocrine system to control the secretion of hormones and is itself controlled by the hypothalamus.
Because motor fibers make up the bulk of the autonomic system, some anatomists consider it to be purely motoric although it does include some afferent axons that carry information from the viscera.
Although the autonomic nervous system is considered to be one of the three main divisions of the human nervous system in its own right, parts of both the central nervous systems and the peripheral nervous systems play a role in its functions.
The autonomic nervous system has two components, the sympathetic system and the parasympathetic system. These two aspects have antagonistic functions.
The sympathetic system prepares the body for fight or flight reactions. Action of this system results in accelerated heart rate, increased blood pressure and blood flow away from the periphery and digestive system toward the brain, heart and skeletal muscles. It also causes adrenaline to be released, temporarily increasing physical strength.
The parasympathetic system brings the body back to a state of equilibrium. It slows heart rate and decreases the release of hormones into the blood stream. The activity of the parasympathetic system causes more localized reactions than does the sympathetic system as much of its output is to specific organs.
The autonomic nervous system consists of four chains of nuclei or ganglia, two of which are located on either side of the spinal cord. The outer chains of nuclei form the parasympathetic division of the system while those closest to the spinal cord make up its sympathetic element.
The rami of the autonomic nervous system are the axons of pre-ganglionic and ganglionic fibers. Most of the axons of pre-ganglionic fibers are myelinated. Their cell bodies are found in the gray matter of the brain stem and spinal cord. Their axons synapse with neurons within the two ganglionic chains.
Pre-ganglionic cells of the autonomic nervous system are neurons located in some of the cranial nerves of the brain stem and in some of the spinal nerves that project to the ganglionic chains of the autonomic nervous system. The autonomic nervous system is closely connected with the central and peripheral nervous systems.
Ganglionic cells originate within the ganglia. They project to post-ganglionic neurons.
Post-ganglionic cells are neurons that are located in the target organs and muscles of the autonomic nervous system.
It can be said that the motor pathways of the autonomic nervous system are made up of its pre-ganglionic and ganglionic cells.
The fibers of the ganglionic chain of the parasympathetic system are not as well-defined as those of the sympathetic chain. All pre-ganglionic neurons of the sympathetic system synapse with the sympathetic chain. This is not true of the parasympathetic pre-ganglionic cells, however. Some of them synapse with the chain, but others go directly to end organs or muscles.