BIOL 214- Chapter 13: Spinal Cord

4 functions of the spinal cord?

1. conduction

2. neural integration

3. locomotion

4. reflexes

conduction

nerve fibers conduct sensory and motor information up and down the spinal cord

neural integration

spinal neurons receive input from multiple sources, integrate it, and execute appropriate output

ex: bladder control

locomotion

spinal cord contains central pattern generators (groups of neurons that coordinate repetitive sequences of contractions for walking)

reflexes

involuntary responses to stimuli that are vital to posture, coordination, and protection

spinal cord

cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull

- occupies the upper 2/3 of the vertebral canal

- ends at L1 or cauda equina

- gives rise to 31 pairs of spinal nerves

segment

part of the spinal cord supplies by each pair of spinal nerves

ascending tracts

carry sensory information up

descending tracts

carry motor information down

decussation

crossing of the midline that occurs in many tracts so that brain senses and controls contralateral side of brain

contralateral

when the origin and destination of a tract are on opposite sides of the body

- perpendicular

ipsilateral

when the origin and destination of a tract are on the same side of the body, does not decussate

- parallel

sensory signals travel across:

3 neurons from origins (receptors) to destinations in the sensory areas of the brain

first-order neurons (sensory ascending)

detect stimulus and transmit signal to spinal cord or brainstem

second-order neurons (sensory ascending)

continues to the thalamus at the upper end of the brainstem

third-order neurons (sensory ascending)

carries the signal the rest of the way to the sensory region of the cerebral cortex

descending tracts involve:

2 motor neurons

upper motor neuron (descending)

originates in cerebral cortex or brainstem and terminates on a lower motor neuron

lower motor neuron (descending)

neurosoma is in brainstem or spinal cord

- axon leads to muscle or other target organ

corticospinal tracts

carry signals from cerebral cortex for precise, finely coordinated movements

pyramids

ridges on anterior surface of medulla oblongata formed from fibers of this system

most fibers decussate in:

lower medulla, forming the lateral corticospinal tract on contralateral side of spinal cord

- the rest will decussate lower down in the spinal column

nerve

a cord-like organ composed of numerous nerve fibers (axons) bound together by connective tissue

mixed fibers contain:

afferent (sensory) and efferent (motor) fibers

nerve fibers of peripheral nervous system are surrounded by:

Schwann cells forming neurilemma and myelin sheath

endoneurium

loose connective tissue external to neurilemma

perineurium

layers of overlapping squamous cells that wrap fascicles: bundles of nerve fibers

epineurium

dense irregular connective tissue that wraps entire nerve

sensory (afferent) nerves

carry signals from sensory receptors to the CNS

motor (efferent) nerves

carry signals from CNS to muscles and glands

mixed nerves

consists of both afferent and efferent fibers

posterior root conducts signals:

towards CNS

anterior root conducts signals:

away from CNS

ganglion

cluster of neurosomas outside the CNS

- enveloped in an endoneurium continuous with that of the nerve

- no blood brain barrier

31 pairs of mixed spinal nerves

8 cervical

12 thoracic

5 lumbar

5 sacral

1 coccygeal

posterior root

sensory input to spinal cord

posterior root ganglion

contains the neurosomas of sensory neurons carrying signals to the spinal cord

anterior root

motor output of spinal cord

cauda equina

formed from roots arising from L2 to Co1

anterior ramus

in thoracic region, it gives rises to intercostal nerve

in other regions, anterior rami form plexuses

posterior ramus

innervates the muscles and joints in that region of the spine and the skin of the back

meningeal branch

reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments

cervical plexus

in the neck, C1 to C5

- supplies neck and phrenic nerve to diaphragm

brachial plexus

C5-T1

- supplies upper limb and some of shoulder and neck

median nerve

in between radius and ulna

- carpal tunnel syndrome

lumbar plexus

L1-L4

- supplies abdominal wall, anterior thigh, and genitalia

sacral plexus

L4-S4

- supplies remainder of lower trunk and lower limb

- sciatic nerve

coccygeal plexus

S4, S5, and Co1

- supplies perineal area

somatosensory function

carry sensory signals from bones, joints, muscles, and skin

proprioception

brain receives information about body position and movements from nerve endings in muscles, tendons, and joints

- gymnasts and athletes have very good

motor function

primarily to stimulate motor contraction

dermatome

a specific area of skin that conveys sensory input to a spinal nerve

dermatome map

a diagram of the cutaneous regions innervated by each spinal nerve

specifications of dermatomes

- overlap their edges as much as 50%

- necessary to anesthetize 3 successive spinal nerves to produce a total loss of sensation in one dermatome