The Spinal Cord and Spinal Nerves

General Organization of the Nervous System

Divisions of the Nervous System

• CNS

• Brain and spinal cord

• In the white matter, axons arranged in tracts and columns

• PNS

• Remainder of nervous tissue

Gross Anatomy of the Spinal Cord

Adult spinal cord

• Localized enlargements provide innervation to limbs

• 31 segments

• each segment has a pair of dorsal roots and a pair of ventral roots

• Filum terminale

• Conus medularis

• Spinal nerves extend off cord

• Mixed nerves

Spinal meninges

• Provide physical stability and shock absorption

• Three layers

• Dura mater

• Arachnoid

• Pia mater

Dura mater

• Covers spinal cord

• Tapers to coccygeal ligament

• Epidural space separates dura mater from walls of vertebral canal

Arachnoid

• Interior to dura mater are the subdural space, the arachnoid and the subarachnoid space

• Subarachnoid space contains CSF

Pia mater

• Meshwork of elastin and collagen fibers

• Innermost meningeal layer

• Denticulate ligaments extend from pia mater to dura mater

Sectional anatomy of the spinal cord

• White matter is myelinated and unmyelinated axons

• Gray matter is cell bodies, unmyelinated axons and neuroglia

• Projections of gray matter toward outer surface of cord are horns

Horns of spinal cord

• Posterior gray horn contains somatic and visceral sensory nuclei

• Anterior gray horns deal with somatic motor control

• Lateral gray horns contain visceral motor neurons

• Gray commissures contain axons that cross from one side to the other

White matter

• Divided into six columns (funiculi) containing tracts

• Ascending tracts relay information from the spinal cord to the brain

• Descending tracts carry information from the brain to the spinal cord

Spinal Nerves

31 pairs of spinal nerves

• Nerves consist of:

• Epineurium

• Perineurium

• Endoneurium

Spinal nerves

• White ramus (myelinated axons)

• Gray ramus (unmyelinated axons that innervate glands and smooth muscle)

• Dorsal ramus (sensory and motor innervation to the skin and muscles of the back)

• Ventral ramus (supplying ventrolateral body surface, body wall and limbs)

• Each pair of nerves monitors one dermatome

Nerve plexus

• Complex interwoven network of nerves

• Four large plexuses

• Cervical plexus

• Brachial plexus

• Lumbar plexus

• Sacral plexus

Principles of Functional Organization

General organization

• Sensory neurons

• Deliver information to CNS

• Motor neurons

• Distribute commands to peripheral effectors

• Interneurons

• Interpret information and coordinate responses

Neuronal pools

• Functional group of interconnected neurons

• Neural circuit patterns

• Divergence

• Convergence

• Serial processing

• Parallel processing

• Reverberation

An introduction to reflexes

• Reflexes are rapid automatic responses to stimuli

• Neural reflex involves sensory fibers to CNS and motor fibers to effectors

Reflex arc

• Wiring of a neural reflex

• Five steps

• Arrival of stimulus and activation of receptor

• Activation of sensory neuron

• Information processing

• Activation of motor neuron

• Response by effector

Reflex classification

• According to

• development

• Site of information processing

• Nature of resulting motor response

• Complexity of neural circuit

• Innate reflexes

• Result from connections that form between neurons during development

• Acquired reflexes

• Learned, and typically more complex

More reflex classifications

• Cranial reflexes

• Reflexes processed in the brain

• Spinal reflexes

• Interconnections and processing events occur in the spinal cord

• Somatic reflexes

• Control skeletal muscle

• Visceral reflexes (autonomic reflexes)

• Control activities of other systems

• Monosynaptic reflex

• Sensory neuron synapses directly on a motor neuron

• Polysynaptic reflex

• At least one interneuron between sensory afferent and motor efferent

• Longer delay between stimulus and response

Spinal Reflexes

Spinal Reflexes

• Range from simple monosynaptic to complex polysynaptic and intersegmental

• Many segments interact to form complex response

Monosynaptic Reflexes

• Stretch reflex automatically monitors skeletal muscle length and tone

• Patellar (knee jerk) reflex

• Sensory receptors are muscle spindles

• Postural reflex maintains upright position

Polysynaptic reflexes

• Produce more complicated responses

• Tendon reflex

• Withdrawal reflexes

• Flexor reflex

• Crossed extensor reflex

Polysynaptic reflexes

• Involve pools of interneurons

• Are intersegmental in distribution

• Involve reciprocal inhibition

• Have reverberating circuits to prolong the motor response

• Several reflexes may cooperate to produce a coordinated response

Integration and Control of Spinal Reflexes

Control of spinal reflexes

• Brain can facilitate or inhibit motor patterns based in spinal cord

• Motor control involves a series of interacting levels

• Monosynaptic reflexes are the lowest level

• Brain centers that modulate or build on motor patterns are the highest

Reinforcement and inhibition

• Reinforcement = facilitation that enhances spinal reflexes

• Spinal reflexes can also be inhibited

• Babinski reflex replaced by planter reflex