B. SPINAL CORD & SPINAL NERVES
1. Part of peripheral nervous system taking messages to and from spinal cord. They are mixed, carrying both sensory and motor info. The caudal-most spinal nerves pass caudally together, resembling a horse’s tail, before exiting the appropriate intervertebral foramen.
2. Carry messages to and from brain, and spinal reflexes.
3. Vertebral foramen/canal. It runs from foramen magnum to about L2.
4. Arms & legs nerves entering & exiting spinal cord
5. General is meninges, specific is dura mater (outermost), arachnoid, then pia mater.
6. Subarachnoid space
7. Epidurals are given outside of the dura mater. L2 or below would be a safe place to administer the anesthetic without hitting the spinal cord.
8. The dorsal gray horns receive sensory information and the ventral gray horns transmit motor information. Since it is gray, it would contain neuron cell bodies, and unmyelinated neurons.
9. The lateral, dorsal (posterior) and ventral (anterior) columns.
10. Within the white columns are the ascending tracts carrying sensory information toward the brain and the descending tracts carrying motor information away from the brain.
11. Reflex is a fast repetitive response to a stimulus. It is involuntary i.e. not at the conscious level. If it involves the spinal cord it is a spinal reflex. A somatic reflex terminates in skeletal muscle, the autonomic reflex terminates in smooth muscle, cardiac muscle and glands.
12. Stimulus – Receptor – afferent sensory neuron – CNS (spinal cord) – efferent motor neuron – effector (such as a skeletal muscle)
13. One synapse in the spinal cord is monosynaptic reflex and would be very fast. The patellar reflex is monosynaptic and ipsilateral. A polysynaptic reflex has more than one synpase in the spinal cord – thus synapsing on interneurons or association neurons before synapsing on the motor neuron. These are slower reflexes and if they stay ipsilateral may describe the withdrawal reflex and if they go contralateral may describe the crossed extensor reflex.
If you hit the patellar tendon, you stretch the quad. muscle and stimulate the stretch receptor. It is an ipsilateral monosynaptic reflex ending with the quad. muscle contracting, thus extending the knee. If you step on the nail, the pathway is the withdrawal reflex resulting in an ipsilateral contraction of your flexors so that you pull away from the nail and a compensatory extension of the contralateral leg so that you can support your weight (crossed extensor reflex).
14. Spinal nerves described under one. It has a dorsal root carrying sensory info into the dorsal gray horn and a ventral root carrying motor info exiting the ventral gray horn. After the mixed spinal nerve leaves the vertebra, it branches into a dorsal ramus (branch) going toward your back and a ventral ramus going everywhere else in your body, including limbs and viscera. The rami are mixed.
15. Network of successive spinal nerves.
16. Cervical (about C1-4); Brachial (about C5-T1), Lumbar (about L1-4) and Sacral (about L4-S4)
17. Radial nerve – Brachial plexus innervates pectoral limb extensors
Ulnar nerve – Brachial plexus innervates distal medial flexors (e.g. flexor carpi ulnaris)
Musculocutaneous nerve – Brachial plexus innervates elbow flexors (e.g. biceps brachii)
Median nerve – Brachial plexus innervates distal lateral flexors (e.g. flexor carpi radialis)
Femoral nerve – Lumbar plexus innervates hip flexors and knee extensors (e.g. quadriceps)
Sciatic nerve – Sacral plexus innervates knee flexors (hamstrings) as well as all distal to your knee.
Phrenic nerve – Cervical plexus innervates your diaphragm and needed to breathe!