SPINAL CORD, SPINAL NERVES, & REFLEXES

Question 1

What are the 4 essential functions of the spinal cord?

Answer 1

1. Relays sensory info from periphery to brain, somatic & visceral.
2. Contains motor neurons both somatic & visceral.
3. Direct (local) connections between motor & sensory info: (basis of reflexes).
4. Relays motor info from brain to muscles, somatic & visceral.

Question 2

Describe the divisions/areas of the gray matter and white matter in the spinal cord.

Answer 2

Outer layer: white matter (fiber tracts = axons & their myelin).
Inner layer: gray matter (neuronal cell bodies, proximal dendrites & astrocytes).

Question 3

Do all 3 meningeal layers cover the spinal cord?

Answer 3

Yes. In the brain, the dura has two layers (periosteal & meningeal layer), but the vertebral canal only has the deep meningeal layer.

Question 4

What is the Conus Medullaris?

Answer 4

End of the spinal cord, L1-L2 level.

Question 5

What is the Dural Sac?

Answer 5

Tubular sheath containing spinal cord formed by dura mater.

Question 6

What is the Cauda Equina?

Answer 6

Horse’s tail, bundle of spinal nerves traveling to their appropriate exit points.

Question 7

What is the Filum Terminale, Internus?

Answer 7

Thread-like extension of the pia mater from the conus medullaris of the spinal cord, runs down and terminates at the lower end of the dural sac at S2, where it becomes the filum terminale externum.

Question 8

What is the Filum Terminale, Externus?

Answer 8

Thread-like extension of the dura mater that continues below the end of the dural sac (which terminates around the S2 level), attaches to the coccyx (forms coccygeal ligament).

Question 9

What are the different types of neurons that make up the spinal cord ventral horn?

Answer 9

• Alpha Motor Neurons
• Gamma Motor Neurons
• Preganglionic Sympathetic Neurons (visceral T1-L2)
• Preganglionic Parasympathetic Neurons (visceral S2-S4)
• Interneurons

Question 10

How many enlargements are in the spinal cord? Where are they located?

Answer 10

2, cervical and lumbar regions.

Question 11

Name the nuclear groups (columns) in the dorsal horn of the spinal cord, and name their functions.

Answer 11

Dorsal horn:
• Substantia Gelatinosa
• Nucleus Proprius
• Dorsal Nucleus

All are sensory.

Question 12

Do all of the nuclear groups in the spinal cord gray matter continue the entire length of the cord?

Answer 12

No.

Do NOT run the length of the cord:
• Phrenic Nucleus
• Accessory Nucleus
• Ventral Horn Enlargements
• Intermediolateral Cell Column
• Nucleus Dorsalis

DO run the length of the cord:
• Ventral Horn Medial
• Substantia Gelatinosa
• Nucleus Proprius.

Question 13

Describe the somatotopic map of the spinal cord ventral horn.

Answer 13

• Medial motor neurons innervate Proximal muscles.
• Lateral motor neurons innervate Distal muscles.
• Dorsal motor neurons (within the ventral horn) innervate flexor muscles.
• Ventral motor neurons (within the ventral horn) innervate extensor muscles.

Question 14

What types of fibers travel in the spinal cord white matter?

Answer 14

Myelinated axons, organized into tracts or funiculi (dorsal, ventral, and lateral) - can be ascending (sensory) and/or descending (motor).

Question 15

Describe the blood supply of the spinal cord.

Answer 15

Vertebral Arteries Branch Into:
• 1 Ventral/Anterior spinal artery
• 2 Dorsal/Posterior spinal arteries

Supplemented by branches from the aorta (aka segmental branches).

Question 16

What modalities travel in a spinal nerve?

Answer 16

Both sensory AND motor info. The spinal root and rootlets are PURELY sensory OR motor, but the spinal NERVE is BOTH.

Dorsal = sensory (afferent)
Ventral = motor (efferent).

Question 17

How many pairs of spinal nerves are there?

Answer 17

31 pairs:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal.

Question 18

What is a plexus?

Answer 18

The ventral rami of spinal nerves form complex networks of nerves called plexuses, which serve the motor and sensory needs of the body.

Question 19

What is a dermatome?

Answer 19

The area of the body’s surface innervated by one pair of spinal nerves, refers to the SENSORY part of the spinal nerves.

Made up of MANY receptive/cutaneous fields.

Question 20

What is a receptive/cutaneous field?

Answer 20

An area of skin innervated by ONE Dorsal Root Ganglia Neuron (aka a peripheral nerve, e.g., median nerve).

Many of these put together form a dermatome.

Question 21

What is a motor unit?

Answer 21

A total number of skeletal muscle fibers (can be a few or many) innervated by ONE Alpha Motor Neuron.

Question 22

What are the terminal branches of Cervical, Brachial, Lumbar, and Sacral Plexes?

Answer 22

Cervical: mixed
Brachial: mixed
Lumbar: mixed
Sacral: mixed.

Question 23

What types of cells and tissues do you find in a peripheral nerve?

Answer 23

Tissues:
- Epineurium (surrounds each nerve, SNs exit the intervertebral foramen followed by the dural sheath, transition into epineurium).
- Perineurium (surrounds fascicles/bundles of axons within the nerve).
- Endoneurium (wraps around axons, secreted by Schwann cells).
- Internode (surrounded by Schwann cells).
- Node of Ranvier (area between internodes, responsible for electrophysiology, AP jumps from node to node).

Cells:
- Fibroblasts
- Schwann cells
- Macrophages.

Question 24

Cervical Plexus

Answer 24

C1-C5

Motor branches:
• Phrenic Nerve: Innervates the diaphragm.

Sensory branches:
• Lesser Occipital n.: skin behind the ear.
• Great Auricular n.: skin in front of the ear.
• Transverse Cervical n.: skin of anterior neck.
• Supraclavicular n.: skin above clavicles.

Question 25

Brachial Plexus

Answer 25

C5-T1 Robert Taylor Drinks Cold Beer R= roots T= trunks D= divisions C= cords B= branches Branches: • Musculocutaneous= Elbow flexors. • Axillary= Deltoid. • Median= Wrist/finger flexors. • Radial= Elbow, wrist, finger extensors. • Ulnar= Wrist/finger flexors.

Question 26

Lumbar Plexus

Answer 26

L1-L5 • Illiohypogastric: skin and muscle of anterior body wall. • Illioinguinal: skin and muscle of anterior body wall. • Genitofemoral: skin of the scrotum and labia majora, cremaster muscle. • Lateral cutaneous: skin of lateral thigh. • Femoral: skin of anterior thigh, knee extensors. • Obturator: skin of medial thigh, adductors (medial compartment).

Question 27

Sacral Plexus

Answer 27

L4-S5 • Superior gluteal n.: Gluteus medius and minimus. • Inferior gluteal n.: Gluteus maximus. • Pudendal n.: • Sciatic n.: Muscles of posterior thigh. ## Footnote Sciatic splits into tibial and common fibular nerves... • Tibial n.: Posterior leg. • Common fibular n.: Anterior and lateral leg.

Question 28

What are the components of the Tendon/Stretch/Patellar Reflex?

Answer 28

Monosynaptic (because there is no interneuron between the central projection of the sensory neuron and the responding motor neuron). - Tap the patellar tendon (stimulus). - In response, muscle stretches, activating muscle spindles. - Muscle spindle sends signals to DRG (where the cell body of the sensory neuron resides). - DIRECTLY synapses on alpha motor neuron in ventral horn. - Causes muscle to contract.

Question 29

What are the components of the Flexor/Pain Reflex?

Answer 29

Polysynaptic, involves contraction of flexor muscles and inhibition of extensor muscles. Steps: - Pain/noxious/gross stimulus. - Picked up by peripheral process of DRG. - Impulse relayed from peripheral process, through the cell body, through the DORSAL (sensory) horn. - Interneuron. - Signals motor neuron. - Motor neuron sends axon out through spinal nerve. - Signal travels to muscle, tells it to flex/contract away from stimulus. ## Footnote The afferents branch into multiple axons/interneurons and spread activation to adjacent spinal segments, results in recruitment of all flexors of the limb and inhibition of the extensors.

Question 30

What are the components of the Crossed Extensor Reflex?

Answer 30

Supplements flexor/pain reflex when weight-bearing component is involved, involves the activation of the extensors and inhibition of the flexors (OPPOSITE of the flexor/pain reflex). Steps: Same as flexor/pain reflex, but information crosses the midline via interneurons that send axons to the contralateral (site of stimulus) to the ventral horn. Activate: • Excitatory interneurons to motor neurons that innervate extensors. • Inhibitory interneurons to motor neurons that innervate flexors.

Question 31

Sensory vs Motor, Dorsal vs Ventral

Answer 31

Dorsal = sensory (afferent). Ventral = motor (efferent).