The Spinal Cord and Spinal Nerves Concepts

Question 1

Describe the protective structures and the gross anatomical features of the spinal cord.

Answer 1

Cerebrospinal fluid (CSF) surrounds the spinal cord, which is also shielded by three protective layers called the meninges (dura, arachnoid, and pia mater).

Question 2

Identify key structures seen in a transverse section of the spinal cord, and the functions of those structures

Answer 2

Gray matter: lateral movement of nervous signaling, between the CNS & PNS at the level of one set of spinal nerves

posterior= cell bodies and axons of interneurons & axons of incoming (afferent sensory neurons)

anterior= cell bodies of somatic (efferent) motor neurons

lateral= cell bodies of autonomic (efferent) motor neurons

White matter: contains bundles of myelinated axons (tracts in CNS) going up & down the column

Sensory tracts ascend in mostly posterior and lateral white columns

Motor tracts descend in the lateral and anterior white columns

Question 3

Trace the path of a generalized reflex arc, like the patellar stretch reflex arc. Explain how reciprocal innervation is involved.

Answer 3

1. A slight tap on the patellar ligament stretches the quads, which is sensed by muscle spindles, forming graded potentials (EPSPs)
2. Excited sensory neuron caries affective signals from the PNS to CNS. The cell body of this sensory neuron is in the posterior root ganglion
3. Within the gray matter of the spinal cord, sensory neuron synapses with a motor neuron to stimulate that motor neuron
4. The excited efferent motor neuron carries the AP back into the PNS to the NMJ of the motor neuron with the motor units in the quadriceps
5. The quadriceps is stimulated to contract to extend the lower leg at the knee to offset the sensed stretch

reciprocal innervation is involved as when the agonist contracts, the antagonist must relax for movement to take place. So, IPSP inhibits action potential from furthering to the motor neurons of the antagonist, so that the antagonist muscle won’t contract at the same time the agonist contracts.

Question 4

Explain why white matter to grey matter ratios change as you move up and down the spinal cord

Answer 4

White matter increases the more you move up on the spinal cord and decreases the more you move down. This is because the white matter is essentially the ascending and descending highways of the nervous system that all connect to the brain. As you move superiorly, the more ascending and descending nerve fibers there are because you are closer to the brain. The more inferior you are in the spinal column, the less information

Question 5

Give a VERY GENERAL overview of ascending sensor and descending motor tract through the white matter of the spinal cord, describing the functions of the major sensory and motor tracts of the spinal cord.

Answer 5

White matter: contains bundles of myelinated axons (tracts in CNS) going up & down the column
- Sensory tracts ascend in mostly posterior and later white columns
- Motor tracts descend in the lateral and anterior white columns

Question 6

Describe how spinal nerves are connected to the spinal cord, naming the structures moving medially to laterally as you move from the spinal cord out to the mixed spinal nerve.

Answer 6

Medially to lateral, rootlet, root, spinal nerve

Anterior and posterior rootlets of the spinal nerves fuse laterally to become the anterior and posterior roots, which also fuse laterally to become the spinal nerves that pass laterally through the intervertebral foramina

Question 7

connective tissue coverings of a spinal nerve

Answer 7

Endoneurium= innermost layer surrounding each axon in a fascicle. In addition to ubiquitous fibroblasts and collagen fibers of C.T., it also hosts macrophages

Perineurium= wraps the fascicle of nerve bundles - so the “middle” layer

Epineurium = Outer layer of the C.T. surrounding the entire nerve. It fuses with the spinal meninges as the spinal nerve passes out o the intervertebral foramen.

Question 8

branching (rami) of a spinal nerve.

Answer 8

Posterior (dorsal) ramus= most posterior branch of a spinal nerve serving the deep muscles of the back and the skin of the posterior trunk

Anterior (ventral) ramus= primary branch of a spinal nerve that serves the muscles and structures of the upper and lower limbs, as well as the skin of the lateral and anterior trunk

Rami Connunicantes= Most anterior projecting rami that are part of the sympathetic arm of the ANS

Meningeal branch= Tiny branch of a spinal nerve that turns medially, back into the vertebral column through the intervertebral foramen, to serve the vertebrae, meninges, and spinal cord itself

Question 9

Define plexus

Answer 9

bundle of intersecting nerves

Question 10

for each of the cervical, brachial, lumbar, and sacral plexuses, identify the spinal nerves feeding the plexus, the general location of the plexus, and at least one major nerve coming out of each plexus

Answer 10

Cervical: c1-c5, hypoglassal nerve

Brachial:c5-t1, ulnar nerve

Lumbar:l1-l4, obturator nerve

Sacral:l4-s4, gluteal nerve

Question 11

Describe the clinical significance of dermatomes.

Answer 11

Dermatomes are areas of skin on your body that rely on specific nerve connections on your spine. Certain segments of the skin are supplied by spinal nerves that carry somatic sensory nerve impulses to the brain. So when a certain section of the spinal cord is damaged, everything under it is also damaged.