Chapter 13- Spinal Cord and Somatic Reflexes

Spinal Cord Functions

conduction, neural integration, locomotion, reflexes

Central Power Generators

Groups of neurons that coordinate repetitive muscle movements

Where does the spinal cord begin?

superiorly at the brain stem (foramen magnum)

Where does the spinal cord end?

L1-L2

The lower 1/3 of the spinal column is known as what?

cauda equina (L2-S5)

four regions of the spinal cord

cervical, thoracic, lumbar, sacral

How many spinal nerves arise from the spinal cord?

31 pairs

Cervical Enlargement of Spinal Cord

Responsible for supplying nerves to the upper limb

Lumbar Enlargement of Spinal Cord

Responsible for supplying nerves to the lower limb

Protective coverings of spinal cord (superficial to deep)

bone (vertebrae), epidural space, meninges

epidural space

fatty space between the vertebrae and dura mater

Meninges layers

dura mater, arachnoid mater, pia mater

dura mater (dural sheath)

most superficial of the three meninges

arachnoid mater

weblike middle layer of the three meninges

subarachnoid space

a space in the meninges beneath the arachnoid membrane and above the pia mater that contains the cerebrospinal fluid

pia mater

thin, delicate inner membrane of the meninges

In the spinal cord, gray matter is surrounded by

white matter

gray matter of spinal cord

-composed of somas, dendrites, and trigger zones

-site of synapses (integraton) for local potentials

white matter of spinal cord

-surrounds gray matter

-myelinated axons bundled together into tracts

-myelination gives white matter its pearly white color

Decussation

The anatomical crossing over of neurons from left to right

Ascending Spinal Tracts

carry sensory information toward the brain

Ascending Tracts usually transverse 3 neurons:

1. First-order neuron

2. Second-order neuron

3. Third-order neuron

First-Order Neuron

sensory neuron that detects stimulus and sends to spinal cord or brain stem

Second-Order Neuron

Interneuron that carries signal to the thalamus

Third-Order Neuron

Interneuron that carries signal from thalamus to the cerebral cortex

What is the thalamus also called?

gateway to cerebral cortex or the "mailroom"

descending tracts

carry motor information down the brainstem and spinal cord to effectors

Descending Tracts transverse 2 neurons:

1. Upper Motor Neuron

2. Lower Motor Neuron

Upper Motor Neuron

Soma is in the cerebral cortex (gray matter); its axon synapses with a lower motor neuron

Lower Motor Neuron

carries signal from spinal cord to the effector

Nerves are made up of

bundles of axons

Spinal Nerve Anatomy

endoneurium, perineurium, epineurium

Endoneurium

surrounds nerve fibers

Perineurium

surrounds nerve fiber fascicles

Epineurium

surrounds the entire nerve

The epineurium is important for

preventing the nerve from stretching

Nerves can only be:

sensory, motor, or mixed

nerve FIBERS can be:

afferent, efferent, somatic, visceral, general, or special

Where do spinal nerves arise?

roots proximal to the spinal cord

Spinal Nerve Anatomy:

-sensory input travels through the dorsal root

-motor output travels through the ventral root

-dorsal root ganglia house the soma of sensory neurons

what type of fibers do spinal nerves contain?

afferent and efferent fibers

4 traits of reflexes

1. require stimulation

2. must be quick

3. involuntary

4. stereotyped

somatic reflexes

activate skeletal muscle

visceral reflexes

involuntary muscles and glands

Anatomy of Somatic Reflex

-utilizes a reflex arc

-somatic receptors in skin, muscles, or tendons

-afferent nerve fiber

-integration center

-efferent nerve fiber

-effectors

example of a stretch reflex

patellar (knee-jerk) reflex

stretch reflex

-brain sets length for a muscle and stretch reflex maintains that length

The stretch reflex is important for

-maintaining muscle tone and adjusting it without consciousness.

-large muscles involved in posture

The Flexor (Withdraw) Reflex

-involves 2 reflex arcs simultaneously

-in response to a painful stimulus

-often accompanied by a crossed-extensor reflex

A flexor reflex

withdrawals limb

Crossed-extensor Reflex

stabilizes body during withdrawal

Testing reflexes demonstrates what?

health/integrity of the spinal cord

Plantar Reflex (Babinski)

tests the integrity of the spinal cord from L4 to S2

An infant up until age 1 will exhibit the Babinski sign due to

incomplete myelination of their nervous system at birth

The Babinski sign in older individuals indicates?

spinal cord or brain disease

Polio

-Destroys motor neurons in brain stem and spinal cord

-Causes muscle pain, weakness, loss of reflexes, atrophy, and ultimately paralysis

-spread through contaminated water

Lou Gehrig's Disease (ALS)

-degeneration of motor neurons and muscle atrophy

-astrocytes fail to reabsorb glutamate (neurotransmitter) and it accumulates to toxic levels

-cause is unknown

Rabies

-a viral infection from animal bites

-virus replicates in muscle tissue

-spreads via somatic nerve fibers to the CNS and then the autonomic nerve fibers

-causes seizures, coma, and death

-essential a 100% fatality rate if not treated before reaching the CNS