Chapter 12: Spinal Cord and Spinal Nerves

Spinal cord labelled

The spinal cord consists of:

- The foramen magnum to the 2nd lumbar vertebra
- Segments (cervical, thoracic, lumbar, & sacral)
- 31 pairs of spinal nerves

Cervical enlargement

the non-uniform diameter of the spinal cord that supplies nerves to the upper limbs

Lumbosacral enlargement

the non-uniform diameter of the spinal cord that supplies nerves to the lower limbs

Conus medullaris

the tapered inferior end of the spinal cord

Cauda equina

origin of spinal nerves (includes lumbosacral enlargement and conus medullaris)

Filum terminale

anchors spinal cord to coccyx

Meninges

connective tissue membranes

Dura mater

- The most superficial and thickest meninge that is continuous with dura mater around the brain and spinal nerves
- Forms a thecal sac that surrounds the spinal cord from the foramen magnum to the end of the 2nd sacral vertebra

Arachnoid mater

A meninge that is thin and wispy, like a spiderweb

Pia mater

The deepest meninge, bound tightly to surface of the spinal cord.

Denticulate ligaments

attach spinal cord to dura mater

Epidural space

The space between the dura mater and periosteum that is filled with blood vessels, areolar C. T, adipose, spinal nerve roots

Which space in the spinal cord is anesthesia injected into?

epidural

Subarachnoid space

The spcae between the arachnoid mater and pia mater that contains CSF, blood vessels, and web-like strands of arachnoid

Subdural space

The space between the dura mater and arachnoid mater that contains serous fluid

Cross section of the spinal cord

Anterior median fissure

a deep cleft on the anterior side of the spinal cord partially separating the left and right halves

Posterior median sulcus

a deep cleft on the posterior side of the spinal cord partially separating the left and right halves

White matter of spinal cord

myelinated axons that are divided into columns (funiculi - ventral, dorsal, lateral) that are divided into tracts (fasciculi)

Ventral column of white matter

Lateral column of white matter

Dorsal column of white matter

A collection of axons inside the CNS

a tract

A collection of axons outside the CNS

a nerve

Gray matter of spinal cord

neuron cell bodies, dendrites, axons that are divided into horns (posterior, anterior, and lateral)

Posterior horn of gray matter

dorsal

Anterior horn of gray matter

ventral

Lateral horn of gray matter

associated with ANS

Commissures

connections between halves

Gray commissure

central canal in center

White commissure

Roots

the combined rootlets that arise from spinal nerves

Dorsal (posterior) root ganglion

collections of cell bodies of pseudo-unipolar sensory neurons

Ventral (anterior) root

Two roots merge laterally to form the

spinal nerve

Motor neuron cell bodies are in

anterior (motor) and lateral (autonomic) horns of gray matter

What do the motor neuron cell bodies innervate?

muscles/glands

Axons of motor neurons form

ventral roots and pass into spinal nerves

Cell bodies for spinal sensory neurons are located in the
A. anterior horn of spinal cord gray matter.
B. lateral horn of spinal cord gray matter.
C. dorsal root ganglia
D. posterior columns

C. dorsal root ganglia

The spinal cord extends from the
A. What's a spinal cord?
B. Level of the third cervical vertebra to the coccyx.
C. Level of the axis to the lowest lumbar vertebra.
D. Medulla oblongata to the level of the 2nd lumbar vertebra.

D. Medulla oblongata to the level of the 2nd lumbar vertebra.

Reflex

an automatic response to a stimulus produced by a reflex arc; it is the simplest portion capable of receiving a stimulus & producing response.

Automatic reflex

response to a stimulus without conscious thought (ex: BP, Blood CO2)

Somatic reflexes

remove the body from painful stimuli or keep the body from falling (etc.)

Reflex arc path

Action potentials produced in sensory receptors transmitted to -> sensory neuron -> interneuron -> motor neuron -> effector organ, which responds with a reflex

3 types of reflexes

1. Stretch Reflex
2. Golgi Tendon Reflex
3. Withdrawal Reflex

Stretch Reflex

when muscles contract due to a stretching force applied to them (example: knee jerk)

What is unique about the stretch reflex? And why?

there is no interneuron, because the sensory neurons synapse with alpha motor neurons

Alpha motor neurons

motor neurons in the spinal cord that innervate the muscle in which the muscle spindle is embedded

Muscle spindle

a sensory receptor made of specialized skeletal muscle fibers that respond to stretch

What is muscle spindle innervated by?

specific motor neurons (like gamma motor neurons) that control sensitivity of muscle spindle

Where are muscle spindle cells innervated?

the noncontractile centers

Golgi Tendon Reflex

prevents contracting muscles from applying excessive tension to tendons so they aren't damaged by excess tension

Golgi tendon organ

a sensory receptor that is an encapsulated nerve ending located near the muscle tendon junction

What produces sudden relaxation of the muscles, like a weightlifter suddenly dropping a heavy weight?

the Golgi tendon reflex

What do sudden movements like "clean and jerk" do?

put so much tension on tendons like the Achilles tendon, they could break

Withdrawal Reflex

Removes body limb from a pain sensation

Reciprocal innervation

relaxation of extensor muscle when flexor contracts in withdrawal reflex, but is also involved in stretch reflex

Crossed extensor reflex

when a withdrawal reflex is initiated in one lower limb, the crossed extensor reflex causes extension of opposite lower limb

Relationship of Brain and Spinal Cord Reflexes

Sensory information (pain) goes to brain, then --> Descending tracts from brain carry info to reflexes

How do neurotransmitters affect the sensitivity of the reflex?

by either stimulating or inhibiting the motor neuron

Review:
1. Define reflex.
2. Name the three types of reflexes and their sensory receptors.

1. An automatic response to a stimulus produced by a reflex arc.
2. Stretch reflex - muscle spindle, Golgi Tendon reflex - Golgi tendon organ, & Withdrawal reflex - pain receptor

What 3 things do peripheral nerves consist of?

1. Axon bundles
2. Schwann cells
3. Connective tissue surrounding the epineurium, perineurium, and endoneurium

Epineurium

entire nerve (continuous with the Dura Mater of the CNS)

Perineurium

axon groups form fascicles

Endoneurium

individual neurons

What is the name of the covering that covers axon groups to make up fascicles?

A. epineurium
B. endoneurium
C. perineurium

How many pairs of spinal nerves are there?

31

Where is the first pair of spinal nerves?

between skull and atlas

Where do nerves of the sacrum exit?

through the sacral foramina

Where do all other nerves (besides the ones of the sacrum) exit?

the intervertebral foramina

How many pairs of cervical nerves are there?

8

How many pairs of thoracic nerves are there?

12

How many pairs of lumbar nerves are there?

5

How many pairs of sacral nerves are there?

5

How many pairs of coccygeal nerves are there?

1

Dermatomal map

skin area supplied with sensory innervation by spinal nerves

How many cervical vertebra are in the spine?
A. 12
B. 7
C. 5
D. The cervical vertebra number equal the cervical vertebra nerves.

B. 7

A collection of spinal nerves that join together after leaving the spinal cord is called a
A. ganglion
B. nucleus
C. projection nerve
D. plexus

D. plexus

Branches of Spinal Nerves (3 things)

A. Dorsal Ramus
B. Ventral Ramus
C. Communicating Rami

Dorsal Ramus

innervates deep muscles of trunk and functions in movements of the vertebral column and sensation of the skin near the middle of the back.

Ventral Ramus - 2 ways of distribution

1. intercostal nerves
2. plexuses

Ventral ramus intercostal nerves

intercostal nerves in the thoracic region that innervate intercostal muscles and skin over thorax

Ventral ramus plexuses (5 things)

1. Cervical plexus (C1-C4)
2. Brachial plexus (C5-T1)
3. Lumbar plexus (L1-L4)
4. Sacral plexus (L4-S4)
5. Coccygeal plexus (S5-CO)

Cervical plexus

A. C1-C4
B. Innervates superficial neck structures, skin of the neck, the posterior portion of the head

Ansa cervicalis

the loop between C2 and C3

Phrenic nerve

A. From C3-C5 (in the cervical and brachial plexuses)
B. Innervates diaphragm

Brachial plexus

A. Originates from spinal n. C5-T1
B. Five ventral rami form three trunks, that separate into six divisions, then form cords that give rise to branches of smaller nerves (ARMUM)

STOP slide 60

Axillary nerve

1. Laterally rotates arm - teres minor
2. Abducts arm - deltoid
3. Skin: inferior lateral shoulder

Radial nerve

1. Movements at elbow and wrist
2.. Thumb movements
3. Skin: posterior surface of arm and forearm & lateral 2/3 of dorsum of hand

Musculocutaneous nerve

1. Movements of flexion at the shoulder, elbow and wrist
2. Supination of the forearm and hand
3. Skin: lateral surface of forearm

Ulnar nerve

1. Movements at wrist & fingers
2. Most of intrinsic hand m.
3. Skin: medial 1/3 of hand, little finger, and medial ½ of ring finger

Which nerve is the most easily damaged of all spinal nerves and why?

ulnar nerve - because ______.

Median nerve

1. Movement of hand, wrist, fingers, thumb
2. Skin: lateral 2/3 palm, thumb, index and middle fingers + lateral ½ of ring finger and dorsal tips of same fingers

Carpal Tunnel syndrome

Tingling, burning, numbness in hand, especially the thumb and middle fingers, cause by damage to median nerve

Which nerve is involved when you hit your Funny bone?
A. Ulnar
B. Radial
C. Median

A. Ulnar

Which nerve is compressed in Carpal Tunnel Syndrome?
A. Ulnar
B. Radial
C. Median

C. Median

Smaller nerves in the brachial plexus that innervate muscles acting on scapula and arm and supply cutaneous innervation of arm and forearm: (3 things)

1. Pectoral nerve
2. Subscapular nerve
3. Suprascapular nerve

Lumbosacral plexus

A. Lumbar plexus: ventral rami of L1-L4
B. Sacral plexus: ventral rami of L4-S4
C. Usually considered together because of their close relationship