Introduction to the Nervous System; Peripheral Nervous System
ANNOUNCEMENTS 1. As a rule of thumb for this course, concentrate your study on the material outlined on these web pages, in your course syllab us and in your textbook. The links to other internet sites and references to other study materials, for example, are provided in case you have a special interest in a particular area.
ASSIGNMENTS Moore's Clinically Oriented Anatomy: 38-45 Grant's Dissector: none
WHEN YOU FINISH THIS SECTION, YOU SHOULD BE ABLE TO PERFORM THE FOLLOWING TASKS AND ANSWER THESE QUESTIONS 1. 2. 3. 4. 5. Diagram and label a cross section of the spinal cord showing a typical spinal nerve. What is a dermatome? What is the clinical significance of a dermatome? What is the difference in function between a dorsal root and a dorsal ramus? What functional losses occur when a nerve or one of its major branches is injured? What is the prognosis (prognosis: the forecast for the outcome of a disease or injury) for recovery of function in a crushi ng nerve injury? In a cutting nerve injury? What is a rhizotomy?
6.
GENERAL ORGANIZATION OF THE NERVOUS SYSTEM Central
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Brain Spinal cord
Peripheral
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Somatic Cranial nn. Spinal nn. Ganglia Autonomic
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NEURON Structure
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Cell body (perikaryon) Processes Axon Dendrites
Types of neurons
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Multipolar Pseudounipolar
Where are neurons found in the body?
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Central nervous system (brain and spinal cord) Peripheral nervous system (ganglia)
Where are axons found in the body?
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Central nervous system, especially in white matter Peripheral nn.
STRUCTURE OF A PERIPHERAL NERVE Axons Connective tissues
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Endoneurium: Surrounds individual axons Perineurium: Surrounds a fascicle (bundle) of axons Epineurium: Surrounds an entire nerve (many fascicles bo und together)
Gross anatomically, you cannot dissect the connective tissues described above. However, it is these layers of connective tiss ue that give the peripheral nerve its strength and bulk. When you handle nerves in the gross lab, you are touching the epineurium of those nerves.
ANATOMY OF A TYPICAL SPINAL NERVE Organization of a cross section of the spinal cord
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Gray matter (neurons located here) White matter (axons located here)
A spinal nerve is formed by the joining of a
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Ventral root Dorsal root Dorsal root ganglion
Functional differences between dorsal roots and ventral roots
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Ventral roots are motor Dorsal roots are sensory
The two primary branches of a spinal nerve are called the
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Ventral primary ramus - both motor and sensory Dorsal primary ramus - both motor and sensory
Comparison of the function of roots vs. the function of rami
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Roots are EITHER motor OR sensory (ventral = motor, dorsal = sensory) Rami are BOTH motor AND sensory (contain a mixture of motor and sensory axons)
DISTRIBUTION OF SPINAL NERVES Plexuses Dermatomes NERVE DAMAGE Because nutrients travel away from the nerve cell body along its processes, injury results in the disconnection of the peripheral portion of the process from the cell and nerve function is lost distal to the site of injury. We will make use of this simple concept throughout the course when we discuss nerve injuries in clinical case settings.
Anatomy of the Typical Spinal Nerve Nerve spinal n. Source formed at the point where the dorsal and ventral roots meet; it ends where the dorsal and ventral primary rami diverge first branch off of the dorsal side of the spinal nerve Branches dorsal primary ramus; ventral primary ramus Motor to skeletal mm.; some levels carry preganglionic sympathetic axons (T1-L2) or preganglionic parasympathetic axons (S2-S4) Sensory general sense (touch, pressure, pain, heat, cold, etc.) from the entire surface of the neck, trunk and extremities; visceral pain (via the white ramus communicans and the sympathetic nervous system) Notes there are 31 pairs of spinal nerves 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal
Dorsal primary ramus
numerous
to the deep back mm.; sympathetic innervation to the skin
general sense (touch, pressure, pain, heat, cold, etc.) to the skin of the back; proprioceptive sense from muscles, tendons and joints general sense (touch, pressure, pain, heat, cold, etc.); proprioceptive sense from muscles, tendons and joints
a mixed nerve containing both motor and sensory fibers
Dorsal root
from the dorsal surface of the spinal cord to the formation of the spinal nerve see notes
none
none
dorsal root is entirely sensory in function; it is located dorsal to the denticulate ligament
Dorsal root ganglion
see notes
none
a dermatome on one side of the body; it takes two dorsal root ganglia (right and left) at one segmental level to supply both sides of the body general sense (touch, pressure, pain, heat, cold, etc.) to the skin of the trunk (except the back) and extremities; visceral pain via the white rami of the sympathetic nervous system; proprioceptive sense from muscles, tendons and joints (T1-L2) none
a sensory ganglion; located on the dorsal root - one per spinal nerve; location of the cell bodies of somatic afferent (sensory) neurons
ventral primary ramus
first branch off of the ventral side of the spinal nerve
numerous
to skeletal mm. of the neck, trunk and extremities; sympathetic innervation to the skin
a mixed nerve containing both motor and sensory fibers
ventral root
from the ventral horn of the spinal cord to the spinal nerve
none
to skeletal mm.; preganglionic sympathetic axons (T1-L2); preganglionic parasympathetic axons (S2-S4)
entirely motor in function; located ventral to the denticulate ligament; at all spinal core levels it contains GSE for skeletal mm.; at levels T1-L2 it contains GVE (preganglionic sympathetic) for blood vessels, sweat glands, thoracic viscera, abdominal viscera, pelvic viscera