Ch 13- The Spinal Cord, Spinal Nerves, and Somatic Reflexes

Conduction, neural integration, locomotion.

What are the three principal functions of the spinal cord?

Nerve fibers conduct sensory and motor info up and down the cord.

What is conduction in the spinal cord?

Spinal neurons receive input from multiple sources, integrate it, and execute output (e.g., bladder control).

What is neural integration in the spinal cord?

Central pattern generators coordinate repetitive sequences of contractions for walking.

What is locomotion in the spinal cord?

Involuntary responses to stimuli important for posture, coordination, and protection.

What are reflexes in the spinal cord?

Starts at foramen magnum, ends around L1.

Where does the spinal cord start and end?

31 pairs.

How many pairs of spinal nerves arise from the cord?

• Cervical enlargement (upper limb nerves)

• lumbosacral enlargement (pelvic/lower limb nerves)

Which areas of the spinal cord are thicker?

• Medullary cone (conus medullaris) – inferior point of cord

• Cauda equina – bundle of nerve roots L2–S5, horse-tail shape

What is the medullary cone and cauda equina?

• Dura mater

• arachnoid mater

• pia mater

What are the three meninges of the spinal cord?

epidural space

Space between dura mater and vertebrae; contains fat, blood vessels, connective tissue; used for epidural anesthesia.

lumbar cistern

CSF-filled space inferior to the medullary cone; site for spinal taps.

denticulate ligaments

Pia mater extensions anchoring cord to dura mater.

gray matter

Neuron cell bodies, dendrites, proximal axons; dull color, butterfly-shaped in cord.

white matter

Myelinated axons forming tracts; surrounds gray matter.

• Dorsal – sensory

• Ventral – motor. Lateral horns in T2–L1 for sympathetic output.

What are the dorsal and ventral horns?

central canal

Fluid-filled channel in center of gray commissure.

• Ascending – carry sensory info to brain

• Descending – carry motor info from brain.

What is the difference between ascending and descending tracts?

decussation

Crossing of midline

• contralateral = opposite side

• ipsilateral = same side

• 1st-order: receptor → spinal cord/brainstem

• 2nd-order: spinal cord → thalamus

• 3rd-order: thalamus → cerebral cortex

What are the three orders of neurons in ascending tracts?

Posterior funiculus: gracile fasciculus (lower body), cuneate fasciculus (upper body)

Which tract carries vibration, touch, proprioception?

Spinothalamic tract (anterolateral system).

Which tract carries pain, temperature, tickle, itch?

Posterior and anterior spinocerebellar tracts.

Which tracts carry proprioception to cerebellum?

Lateral and anterior corticospinal tracts.

Which tracts carry motor signals for precise movements?

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

How many spinal nerves are there?

• Posterior (dorsal) root – sensory

• Anterior (ventral) root – motor

What are the two roots of a spinal nerve?

• Anterior ramus (plexuses/intercostal)

• Posterior ramus (back muscles/skin)

• Meningeal branch (meninges/vertebrae)

What are the distal branches of a spinal nerve?

• Cervical (C1–C5)

• Brachial (C5–T1)

• Lumbar (L1–L4)

• Sacral (L4–S4)

• Coccygeal (S4–Co1)

What are the five nerve plexuses?

dermatome

Area of skin innervated by a single spinal nerve; edges overlap ~50%.

reflex

Quick, involuntary, stereotyped reaction to stimulus.

Somatic – skeletal muscle; Visceral – organs.

What is a somatic reflex vs visceral reflex?

Receptor → afferent fiber → integrating center → efferent fiber → effector.

What are the components of a reflex arc?

muscle spindle

Stretch receptor in muscle; monitors length/movement (proprioception).

Muscle contracts in response to stretch; e.g., knee-jerk reflex.

What is the stretch (myotatic) reflex?

reciprocal inhibition

Prevents antagonistic muscles from contracting during reflex.

flexor (withdrawal) reflex

Ipsilateral, polysynaptic; withdraws limb from painful stimulus.

crossed extension reflex

Contralateral reflex; extensor muscles of opposite limb contract to maintain balance.

tendon reflex

Response to excessive tendon tension; prevents tendon/muscle injury.

complete transection

Full severance of cord; immediate loss of motor control below injury.

spinal shock

Temporary loss of reflexes after injury.

• Paraplegia – lower limbs

• Quadriplegia – all four limbs

• Hemiplegia – one side

• Paresis – partial weakness

Types of paralysis

poliomyelitis

Virus destroys motor neurons in anterior horn/brainstem; causes muscle weakness, paralysis, atrophy.

endoneurium

Loose connective tissue outside the neurilemma that surrounds each nerve fiber.

fascicle

A bundle of nerve fibers.

the entire nerve; made of dense irregular connective tissue.

What does the epineurium wrap?

Mixed nerves

consist of both afferent and efferent fibers

• most common 

varicella-zoster virus.

What virus causes chickenpox?

An itchy rash that usually clears without complications.

What happens during chickenpox?

shingles (herpes zoster)

• A reactivation of the varicella-zoster virus when immunity is weakened.

• symptoms- painful trail of skin discoloration and fluid-filled vesicles along a dermatome (usually one side of chest or waist).

Intersegmental reflex

one in which the input and output occur at different levels (segments) of the spinal cord