Download The Spinal Cord and Spinal Nerve

The Spinal Cord and Spinal Nerve
Grouping of Neural Tissue
White Matter
- groups of neurons myelinated by:
- Schwann cells in the PNS
- Oligodendrocytes in the CNS
Gray Matter
- nerve cell bodies and dendrites or
- unmyelinated axons and neuroglia
Nerve - bundle of fibers (axons or dendrites) outside the CNS
Ganglia - collections of nerve cell bodies outside the CNS
Tract - a bundle of fibers in the CNS
Ascending Tract - sensory tracts going to the brain
Descending Tract - motor tracts coming from the brain
Nucleus - a mass of unmyelinated nerve cell bodies in the CNS
Horns - chief areas of gray matter in spinal cord
SPINAL CORD
Meninges
Epidural space - between the dura mater and the wall of the vertebral canal - filled
with adipose tissue which serves as padding
Dura Mater - runs from the foramen magnum to S2 where it fuses with the filum
terminale - continuous with the dura mater of the brain
Subdural Space - contains serous fluid
Arachnoid Mater - avascular delicate layer continuous with the arachnoid of the
brain - adheres to the dura mater
Subarachnoid Space - where Cerebral Spinal Fluid flows
Pia Mater - adheres to the surface of the brain and spinal cord
- Denticulate Ligaments - pieces of pia mater that suspend the spinal cord in the
middle of the dural space and protect it against injury
General Features of the Spinal Cord
Enlargements - regions where the spinal cord is thicker
- cervical
- lumbar
Conus Medullaris - pointed end of the cord near L1 - L2
Filum Terminale - pia mater strand that attaches spinal cord to the bottom of the
vertebral canal.
Cauda Equina - nerves that arise from the lower portion of the cord occupy the space
below L2
Structure of the Spinal Cord in Cross-section
Functions of the spinal cord
1. To convey sensory impulses from the PNS to the brain.
2. To conduct motor impulses from the brain to the PNS.
3. Integration of reflexes.
Reflex Center
- spinal cord is center for some reflex actions
- the basic components are:
1. Receptor - responds to a stimulus and initiates a nerve impulse in a sensory neuron
dendrite.
2. Sensory Neuron - has the cell body in the dorsal root ganglion (see diagram above)
Passes nerve impulse into the spinal cord through the dorsal root to the posterior
horn of the gray matter.
3. Center - region of the spinal cord where the incoming sensory information
generates an outgoing motor impulse - usually contains internuncial neurons
4. Motor Neuron - transmits impulses to muscle or gland through the ventral root to
the spinal nerve.
5. Effector - the organ (gland or muscle) that responds to the impulse from the motor
neuron
Types of Reflexes
- reflexes are fast responses to certain changes in the internal or external environment
that allow the body to maintain homeostasis
- there are several types of reflexes
- Spinal reflexes - carried out by the spinal cord alone
- Somatic reflexes - result in the contraction of skeletal muscle
- Cranial reflexes - involve brain centers and cranial nerves
- Visceral (autonomic) reflexes - cause contraction of smooth or cardiac muscle,
or secretion by glands
Reflexes of Clinical Significance
1. Patellar Reflex
- knee jerk
- relates to damage to the 2nd, 3rd, or 4th lumbar vertebra
- absent in people with chronic diabetes and neurosyphilis
- exaggerated in disease or injury to corticospinal tracts
2. Achilles Tendon
- ankle jerk
- relates damage to lumbosacral region of the spine
- absent in people with chronic diabetes, neurosyphillis, alcoholism, subarachnoid
hemorrhages
- exaggerated with cervical cord compression, or a lesion of the motor tracts of the
first or second sacral segments of the cord
3. Babinski Sign
- outer margin of the sole of the foot is stimulated
- in children under l 1/2 years of age the great toe is extended with or without fanning
of the toes - after 1 1/2 years may indicate corticospinal damage
- in adults the response is to curl the toes under
4. Abdominal Reflex
- stroke the side of the stomach and the umbilicus (belly-button) deviates laterally to
the opposite side
- absence indicates damage to corticospinal tracts, lesions of reflex centers in the
thoracic area of the cord, or multiple sclerosis
Spinal Nerves
- 31 pairs of spinal nerves
- named according to region and level of the spinal cord from which they emerge
- 8 cervical - 12 thoracic - 5 lumbar - 5 sacral - a coccyx
- spinal nerves attach to the spinal cord in two places
1. dorsal root - with sensory fibers
2. Ventral root - with motor fibers
- therefore each spinal nerve is a mixed nerve with both sensory and motor fibers