CH 14 HA

CHAPTER 14: THE NERVOUS SYSTEM II: SPINAL CORD AND SPINAL NERVES

SPINAL CORD FUNCTION

  • Conduction

    • Sensory information ascends to the brain

    • Motor commands descend from the brain

  • Neural integration

    • Processing of information from diverse sources

  • Locomotion

    • Central pattern generators coordinate simple repetitive movements

  • Reflexes

    • Involuntary stereotyped responses to stimuli

SURFACE ANATOMY OF THE SPINAL CORD

  • Regions:

    • Cervical

    • Thoracic

    • Lumbar

    • Sacral

  • Enlargements:

    • Cervical enlargement

    • Lumbosacral enlargement

  • Medullary cone:

    • The tapered end of the spinal cord

  • Cauda equina:

    • Bundle of spinal nerves and lumbosacral plexus below the termination of the spinal cord

SPINAL MENINGES

  • Dura mater

    • Tough, outer layer forming the dural sheath

  • Epidural space

    • Space between the dura mater and vertebrae

  • Arachnoid mater

    • Middle layer of the meninges

  • Subarachnoid space

    • Contains cerebrospinal fluid (CSF)

  • Pia mater

    • Delicate, inner layer firmly attached to the spinal cord

  • Coccygeal and denticulate ligaments

    • Help anchor the spinal cord within the vertebral canal

CROSS SECTIONAL ANATOMY

  • Gray matter

    • Contains little myelin

    • Composed of:

    • Two posterior (dorsal) horns

    • Two anterior (ventral) horns

    • Two lateral horns (present in thoracic and lumbar regions)

  • White matter

    • Contains myelinated axons

    • Organized into three pairs of columns containing tracts (fasciculi):

    • Posterior (dorsal) funiculi

    • Lateral funiculi

    • Anterior (ventral) funiculi

SPINAL TRACTS

  • Ascending tracts

    • Carry sensory information to the brain

  • Descending tracts

    • Carry motor information from the brain

  • Many tracts cross the midline at a decussation:

    • The tract origin is contralateral to its destination

    • Example: A right brain stroke impairs function on the left side of the body

  • Some tracts do not cross the midline:

    • The tract origin is ipsilateral to its destination

MAJOR SPINAL TRACTS

  • Tract | Funiculus | Decussation | Functions

    • Ascending (Sensory) Tracts

    • Gracile fasciculus | Posterior | In medulla | Sensations of limb and trunk position and movement, deep discriminative touch, vibration, and visceral pain (below level T6)

    • Cuneate fasciculus | Posterior | In medulla | Same as gracile fasciculus, but from level T6 up

    • Spinothalamic | Lateral and anterior | In spinal cord | Sensations of light touch, tickle, itch, temperature, pain, and pressure

    • Spinoreticular | Lateral and anterior | In spinal cord (some fibers) | Sensation of pain from tissue injury

    • Posterior spinocerebellar | Lateral | None | Feedback from muscles (proprioception)

    • Anterior spinocerebellar | Lateral | In spinal cord | Same as posterior spinocerebellar

    • Descending (Motor) Tracts

    • Lateral corticospinal | Lateral | In medulla | Fine control of limbs

    • Anterior corticospinal | Anterior | In spinal cord | Fine control of limbs

    • Tectospinal | Anterior | In midbrain | Reflexive head-turning in response to visual and auditory stimuli

    • Lateral reticulospinal | Lateral | None | Balance and posture; regulation of awareness of pain

    • Medial reticulospinal | Anterior | None | Same as lateral reticulospinal

    • Lateral vestibulospinal | Anterior | None | Balance and posture

    • Medial vestibulospinal | Anterior | In medulla (some fibers) | Control of head position

ASCENDING TRACTS

  • Structure of sensory pathways:

    • Contains first-, second-, and third-order nerve fibers

  • Posterior funiculus tracts:

    • Two tracts ascend within each posterior funiculus to end in the medulla:

    • Gracile fasciculus: from the lower body

    • Cuneate fasciculus: from the upper body

    • Both tracts contain first-order neurons sensing body position, discriminative touch, and pressure on the same side of the body

    • Decussation occurs in the medulla

SOME ASCENDING TRACTS

  • The spinothalamic tract ascends in the anterior and lateral funiculi to end in the thalamus:

    • Contains second-order neurons sensing pain and temperature from contralateral sensory neurons

    • Decussation occurs in the spinal cord

  • Additional ascending tracts include:

    • Spinoreticular tract

    • Posterior spinocerebellar tract

    • Anterior spinocerebellar tract

DESCENDING TRACTS

  • Structure of motor pathways:

    • Contains upper and lower motor neurons

  • Corticospinal tracts:

    • Carry signals for precise limb movements from motor cortex (upper motor neurons)

    • Lateral corticospinal tract: large pathway in lateral funiculus (decussates in the medulla)

    • Anterior corticospinal tract: small pathway in anterior funiculus (decussates in the spinal cord)

  • Tectospinal tract: involved in reflex turning of the head in response to sight and sound

  • Additional descending tracts:

    • Medial and lateral reticulospinal tracts

    • Medial and lateral vestibulospinal tracts

GENERAL ANATOMY OF NERVES AND GANGLIA

  • Nerve: A bundle consisting of several axons wrapped in connective tissue

    • Endoneurium: wraps an individual axon

    • Perineurium: wraps a fascicle of axons

    • Epineurium: wraps the entire nerve

CLASSIFICATION OF NERVE FIBERS

  • Class | Description

    • Afferent fibers | Carry sensory signals from receptors to the CNS

    • Efferent fibers | Carry motor signals from the CNS to effectors

    • Somatic fibers | Innervate skin, skeletal muscles, bones, and joints

    • Visceral fibers | Innervate blood vessels, glands, and viscera

    • General fibers | Innervate widespread organs such as muscles, skin, glands, viscera, and blood vessels

    • Special fibers | Innervate localized organs in the head, including the eyes, ears, olfactory and taste receptors, and muscles of chewing, swallowing, and facial expression

ANATOMY OF A GANGLION

  • Posterior root ganglion:

    • Contains cell bodies of somatosensory neurons

  • Anterior root:

    • Contains motor nerve fibers

  • Signal conduction:

    • Sensory pathway:

    • Afferent nerve fibers → Posterior root ganglion → Spinal cord

    • Motor pathway:

    • Efferent nerve fibers → To peripheral receptors and effectors

  • Ganglion:

    • Cluster of cell bodies outside the CNS

SPINAL NERVES

  • Total pairs: 31

    • C1–C8, T1–T12, L1–L5, S1–S5, Co1

  • Proximal branches:

    • Ventral root

    • Dorsal root

    • Dorsal root ganglion

  • Distal Branches:

    • Posterior ramus

    • Anterior ramus

    • In the thorax: becomes intercostal nerve

    • In other regions: contributes to a plexus

    • Meningeal branch

RAMI OF THE SPINAL NERVES

  • Components:

    • Posterior and anterior rootlets of spinal nerve

    • Posterior root

    • Anterior root

    • Spinal nerve

    • Anterior ramus of spinal nerve

    • Posterior ramus of spinal nerve

    • Sympathetic chain ganglion

    • Communicating rami

NERVE PLEXUSES

  • Function:

    • Plexuses receive fibers from anterior rami and give rise to peripheral nerves

  • Cervical plexus:

    • Located in the neck

  • Brachial plexus:

    • Located in the shoulder

  • Lumbar plexus:

    • Located in the lower back

  • Sacral plexus:

    • Located below lumbar

  • Coccygeal plexus:

    • Located at the lower sacrum and coccyx

CERVICAL AND BRACHIAL PLEXUS

  • Somatosensory Nerves:

    • Lesser occipital n.

    • Greater occipital n.

    • Transverse cervical n.

    • Supraclavicular n.

  • Motor Nerves:

    • Ansa cervicalis

  • Mixed Nerves:

    • Phrenic n.

    • Musculocutaneous n.

    • Axillary n.

    • Radial n.

    • Medial n.

    • Ulnar n.

LUMBAR AND SACRAL/COCCYGEAL PLEXUSES

  • Somatosensory Nerves:

    • Lateral femoral cutaneous n.

  • Mixed Nerves:

    • Iliohypogastric n.

    • Ilioinguinal n.

    • Genitofemoral n.

    • Femoral n.

    • Obturator n.

  • Somatosensory Nerves:

    • Posterior cutaneous n.

  • Mixed Nerves:

    • Superior gluteal n.

    • Inferior gluteal n.

    • Tibial n.

    • Fibular n.

    • Pudendal n.

CUTANEOUS INNERVATION AND DERMATOMES

  • Dermatome:

    • Area of skin innervated by a particular spinal nerve

  • Dermatome map:

    • Diagram of the areas of skin innervated by each spinal nerve

SOMATIC REFLEXES VS. VISCERAL REFLEXES

  • Properties of a reflex:

    • Requires stimulation

    • Quick

    • Involuntary

    • Stereotyped responses

  • Visceral reflex:

    • Involves glands, cardiac muscle, and smooth muscle

    • Controlled by the autonomic nervous system

  • Somatic reflex:

    • Involves skeletal muscles

    • Controlled by the somatic nervous system

REFLEX ARC

  • Components:

    1. Somatic receptors

    2. Afferent nerve fibers

    3. Integrating center (spinal cord)

    4. Efferent nerve fibers

    5. Skeletal muscles (effectors)

GENERAL REFLEX CATEGORIES

  • Ipsilateral reflex:

    • Sensory input and motor output on the same side

  • Contralateral reflex:

    • Sensory input from the opposite side as the motor output

  • Intersegmental reflex:

    • Sensory signal in one level; motor output at a higher or lower level

TYPES OF SOMATIC REFLEXES

  • Stretch reflex

    • Increased muscle tension in response to stretch

    • Serves to maintain equilibrium and posture, stabilize joints, and coordinate joint actions

    • Example: Knee-jerk reflex (patellar reflex)

  • Flexor reflex

    • Contraction of flexor muscles resulting in withdrawal from an injurious stimulus

    • Example: Withdrawal from a burn or pinprick

  • Crossed extension reflex

    • Contraction of extensor muscles in one limb when flexor muscles of the opposite limb contract

    • Function: Stiffens one leg when the opposite leg is lifted to prevent falling

  • Tendon reflex

    • Inhibition of muscle contraction when a tendon is excessively stretched

    • Purpose: Prevent tendon injuries

CLINICAL PERSPECTIVE

  • Spinal shock

    • Syndrome of paralysis and absence of reflexes after spinal cord transection

  • Neurogenic shock

    • Dangerous drop in blood pressure due to lack of sympathetic nervous system activity

  • Paraplegia

    • Paralysis of both lower limbs due to lesions between T1 and L1

  • Quadriplegia

    • Paralysis of all four limbs due to lesions above C5

  • Hemiplegia

    • Paralysis on one side of the body, usually caused by stroke or brain lesions