Lecture 3: PERIPHERAL NERVOUS SYSTEM, Chapter 10 reading Flashcards
All Levels of the Spinal Cord Have a Similar Cross-Sectional Structure
In cross section the spinal cord consists of a roughly H-shaped area of gray matter that floats like a butterfly in a surround of white matter.
Spinal Cord Cross-Section
The gray matter can be divided into horns and the white matter into funiculi (from the Latin funiculus, meaning “string”).
The spinal cord is, to a great extent, a longitudinally organized structure, even though it is most conveniently studied in cross section. For example, the posterior gray horns are continuous cell columns rather than a series of discrete nuclei, and at any given level the posterior horn cells interact with cells from many other levels.
anterior median fissure
extends almost to the center of the cord; at the apex of this fissure, only a thin zone of white matter (the anterior white commissure) and a thin zone of gray matter separate the central canal from subarachnoid space.
posterior median sulcus
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posterior intermediate sulcus
Found at cervical and upper thoracic levels.
A glial septum projects from this sulcus, partially subdividing each posterior funiculus.
The Spinal Cord Is Involved in Sensory Processing, Motor Outflow, and Reflexes
Afferent fibers enter the cord via the dorsal roots and then end almost exclusively on the ipsilateral side of the CNS. They may reach their site of termination either by synapsing on neurons in the ipsilateral gray matter of the spinal cord or by ascending directly and uncrossed to relay nuclei in the medulla.
The relay cells in the spinal gray matter or the medulla then project their axons through defined sensory pathways to more rostral structures.
It may sometimes sound as if a particular primary afferent synapses on only one relay cell and sends its information into only one pathway. However, it is important to realize that each primary afferent fiber gives rise to many branches and feeds into more than one ascending sensory pathway and into local reflex circuits as well. It is estimated, for example, that a single Ia afferent from a muscle spindle may give rise to 500 or more branches within the spinal cord.
motor neurons that innervate skeletal muscles
The motor neurons that innervate skeletal muscles are located in the anterior horns, and many preganglionic autonomic neurons are located in the intermediate gray matter of some segments. The axons of these motor neurons leave the cord in the ventral roots. Activity in these neurons is modulated by local reflex circuits and by pathways that descend through the spinal white matter from the cerebral cortex and from various brainstem and diencephalic nuclei.
reflex
An involuntary, stereotyped response to a sensory input. All reflex circuits (other than axon reflexes) involve at least a primary afferent and a lower motor neuron (or a preganglionic and postganglionic autonomic neuron). With the exception of the stretch reflex, all reflex arcs also include one or more interneurons.
Many of these involve neural circuitry that is wholly contained within the spinal cord.
The Posterior Horn Contains _____ Interneurons and ____ Neurons
The Posterior Horn Contains Sensory Interneurons and Projection Neurons
The posterior horn consists mainly of interneurons whose processes remain within the spinal cord and of projection neurons whose axons collect into long, ascending sensory pathways. This area of gray matter contains two prominent parts, the substantia gelatinosa and the body of the posterior horn, both present at all spinal levels.
substantia gelatinosa
A distinctive region of gray matter, surmounted by Lissauer’s tract, that caps the posterior horn of the spinal cord at all levels.
The substantia gelatinosa looks pale in myelin-stained material because its inputs are poorly myelinated or unmyelinated. It deals mostly with pain and temperature sensation, and is an important site formodulating the entry of this information into ascending pathways.
Between the substantia gelatinosa and the surface of the cord is a relatively pale-staining area of white matter called Lissauer’s tract.
Lissauer’s tract.
A pale-staining area of white matter between the substantia gelatinosa (capping the posterior gray horn of the spinal cord) and the pial surface of the cord. Lissauer’s tract stains more lightly than the rest of the spinal white matter because it contains finely myelinated and unmyelinated pain and temperature fibers (derived from the lateral division of each dorsal root filament) which then distribute into the underlying gelatinosa over several segments.
The Anterior Horn Contains Motor Neurons
The anterior horn contains the cell bodies of the large motor neurons that supply skeletal muscle. These cells are alpha motor neurons, also referred to as lower motor neurons.
alpha motor neurons
Lower motor neurons that innervate the extrafusal fibers of skeletal muscle. They were named for their large axons, which are in the Aα range.
Alpha motor neurons occur in longitudinally oriented, cigar-shaped groups, each group innervating an individual muscle. Hence in cross sections they appear to be arranged in clusters, separated from one another by areas of interneurons; the clusters that innervate axial muscles are medial to those that innervate limb muscles. In the cervical and lumbar enlargements, which innervate the limbs, the anterior horns are enlarged laterally to accommodate the additional motor neurons. Smaller gamma motor neurons are interspersed with alpha motor neurons in all such groups.
lower motor neurons
alpha motor neurons, (AKA lower motor neurons), are the only means by which the nervous system can exercise control over body movements, whether voluntary or involuntary; a number of different parts and pathways of the nervous system can influence these lower motor neurons, but they alone can elicit muscle contraction.
Destruction of the lower motor neurons supplying a muscle or interruption of their axons causes complete paralysis of that muscle. Lower motor neuron lesions cause flaccid paralysis, indicating that the muscle is limp and uncontracted. Reflex contractions can no longer be elicited, and the muscle slowly atrophies (owing to a lack of trophic factors normally delivered to it by motor axons). This occurs, for example, in poliomyelitis (a viral disease that attacks the motor neurons of the anterior horn) and in injuries in which ventral roots are damaged.
flaccid paralysis
Lower motor neuron lesions cause paralysis of a type called flaccid paralysis, indicating that the muscle is limp and uncontracted. Reflex contractions can no longer be elicited, and the muscle slowly atrophies (owing to a lack of trophic factors normally delivered to it by motor axons). This occurs, for example, in poliomyelitis (a viral disease that attacks the motor neurons of the anterior horn) and in injuries in which ventral roots are damaged.
