Spinal Cord Anatomy and Functions

Introduction

• Thousands of Americans are paralyzed each year due to spinal cord injury
• The spinal cord serves as the information highway, connecting the brain to the lower body
• This chapter focuses on the spinal cord and its spinal nerves

13.1 The Spinal Cord

Functions of the Spinal Cord

• Conduction involves nerve fibers transmitting sensory and motor information up and down the spinal cord
• Neural integration occurs as spinal neurons receive input from various sources, integrate said input, and generate appropriate output
• Locomotion involves spinal cord containing central pattern generators that coordinate repetitive muscle contraction sequences for walking
• Reflexes are involuntary responses to stimuli crucial for posture, coordination, and protection

Surface Anatomy

• The spinal cord is a cylinder of nervous tissue originating from the brainstem at the foramen magnum of the skull
• It occupies approximately the upper two-thirds of the vertebral canal
• The inferior margin concludes at L1 or slightly beyond
• The spinal cord averages 1.8 cm thick and 45 cm long
• Spinal cord gives rise to 31 pairs of spinal nerves
• A segment relates to the portion of the spinal cord supplied by each pair of spinal nerves
• Longitudinal grooves present on the anterior and posterior sides
• These grooves include the anterior median fissure and the posterior median sulcus
• The spinal cord divides into cervical, thoracic, lumbar, and sacral regions
• Two areas of the spinal cord appear thicker than elsewhere
• Cervical enlargement involves nerves to the upper limb
• Lumbosacral enlargement involves nerves to the pelvic region and lower limbs
• The medullary cone (conus medullaris) is the tapered, pointed inferior end of the spinal cord that occurs inferior to lumbosacral enlargement
• The cauda equina is a collection of nerve roots that occupy the vertebral canal from L2 to S5

Meninges of the Spinal Cord

• Meninges consist of three fibrous membranes enclosing the brain and spinal cord
• These membranes separate soft tissue of the central nervous system from the bones of the cranium and vertebral canal
• From superficial to deep, the meninges consist of the dura mater, arachnoid mater, and pia mater
• The dural sheath surrounds the spinal cord, separated from vertebrae by the epidural space
• The arachnoid membrane attaches to the dura and separates from the pia by fibers within the subarachnoid space filled with cerebrospinal fluid (CSF)
• In a lumbar puncture (spinal tap), a sample of CSF is taken
• Pia mater is a delicate membrane following the spinal cord's contours, continuing inferiorly as the fibrous terminal filum fusing with the dura to form the coccygeal ligament

Cross Sectional Anatomy

• The central area consists of gray matter shaped like a butterfly
• This shaped matter is surrounded by white matter in three funiculi
• Gray matter is made of neuron cell bodies containing very little myelin
• It is the site of information processing and synaptic integration
• White matter primarily consists of abundantly myelinated axons
• Its primary function is to carry signals from one part of the CNS to another

Gray Matter

• The spinal cord core consists of gray matter shaped like a butterfly or the letter H in cross section
• Gray matter has anterior and posterior (dorsal) horns
• The posterior (dorsal) root of a spinal nerve carries only sensory fibers
• Gray matter has thicker anterior (ventral) horns
• The anterior (ventral) root of a spinal nerve exclusively carries motor fibers
• The gray commissure connects the right and left sides
• Contains a central canal lined with ependymal cells and filled with CSF
• The lateral horn is visible from T2 through L1
• It contains sympathetic nervous system neurons

White Matter

• White matter in the spinal cord surrounds the gray matter
• Axon bundles course up and down the cord, facilitating communication between different levels of the CNS
• Funiculi (columns) consist of three white matter pairs bundles
• Posterior (dorsal), lateral, and anterior (ventral) funiculi are on each side
• Tracts (fasciculi) are subdivisions of each funiculus

Spinal Tracts

• Fibers in a particular tract share a similar origin, destination, and function
• Ascending tracts carry sensory information up
• Descending tracts carry motor information down
• Decussation is the crossing of the midline that occurs in numerous tracts, enabling the brain to sense and control the contralateral side of the body
• Contralateral indicates that the origin and destination of a tract are on opposite sides of the body
• Ipsilateral indicates that the origin and destination of a tract are on the same side of the body and do not decussate

Ascending Tracts

• Gracile fasciculus, cuneate fasciculus, spinothalamic tract, spinoreticular tract, posterior (dorsal), and anterior (ventral) spinocerebellar tracts carry sensory signals up the spinal cord
• Sensory signals travel across three neurons between origin (receptors) to destinations within the brain's sensory areas
• First-order neurons: detect stimulus and transmit signal to spinal cord or brainstem
• Second-order neurons: continues to the thalamus at the upper end of the brainstem
• Third-order neurons: carries the signal for the route to the sensory region of the cerebral cortex

Gracile Fasciculus

• Gracile fasciculus carries signals from midthoracic and lower parts of the body
• It comprises the entire posterior funiculus below T6
• The gracile fasciculus is accompanied by the cuneate fasciculus at T6 and above
• Consists of first-order nerve fibers traveling up the ipsilateral side of the spinal cord
• Gracile fasciculus terminates at the gracile nucleus of the medulla oblongata
• Gracile fasciculus carries signals for vibration, visceral pain, deep and discriminative touch, and proprioception from lower limbs and lower trunk
• Proprioception involves a nonvisual sense of the position and movements of the body

Cuneate Fasciculus

• At T6 and above, the cuneate fasciculus occupies the lateral portion of the posterior funiculus
• Cuneate fasciculus pushes the gracile fasciculus medially
• It contains first order neurons carrying the same type of sensory signals as the gracile fasciculus
• Its signals originate from upper limb and chest
• Ends in the cuneate nucleus of the ipsilateral medulla oblongata
• Second order neurons from the gracile and cuneate nuclei decussate
• Second order neurons form the medial lemniscus, a tract leading to the thalamus
• Third-order neurons go from the thalamus to the cerebral cortex, transporting signals to the cerebral hemisphere
• The left hemisphere processes stimuli from the right side of the body and vice versa
• This occurs because the second-order neurons cross over

Spinothalamic Tract

• The spinothalamic tract is part of the anterolateral system that ascends within the anterior and lateral funiculi of the spinal cord
• It carries signals for pain, pressure, temperature, light touch, tickle, and itch
• The spinothalamic tract comprises axons of second-order neurons
• First-order neurons end in the posterior horn of the spinal cord
• Second-order neurons initiate in the posterior horn then decussate, forming the spinothalamic tract
• Third-order neurons continue from there to the cerebral cortex
• Due to the crossing of second-order neurons, signals are sent to the cerebral hemisphere contralateral to the stimulus site

Spinoreticular Tract

• The spinoreticular tract ascends within the anterolateral system
• Carries pain signals that originate from tissue injury
• It comprises axons of second-order neurons
• First-order neurons enter the posterior horn and immediately form a synapse with second-order neurons
• Second-order neurons decussate to the opposite anterolateral system
• Second-order neurons ascend the cord and terminate in reticular formation: a loosely organized core of gray matter in the medulla and pons
• Third-order neurons continue from the pons to the thalamus
• Fourth-order neurons complete the path to the cerebral cortex

Spinocerebellar Tracts

• Anterior and posterior spinocerebellar tracts travel through the lateral funiculus
• Transports proprioceptive signals from the limbs and trunk up to the cerebellum
• Spinocerebellar tracts form axons of second-order neurons
• First-order neurons originate within the muscles and tendons before terminating in the posterior horn of the spinal cord
• Second-order nerves ascend within the spinocerebellar tracts and terminate within the cerebellum, offering necessary feedback for coordinating movements
• The posterior spinocerebellar tract remains ipsilateral
• Anterior spinocerebellar tracts cross over and ascend the contralateral side, but cross back to terminate in the ipsilateral cerebellum

Descending Tracts

• Descending tracts carry motor signals down the brainstem and spinal cord
• Descending tracts involve two motor neurons
• Upper motor neuron originates in the cerebral cortex or brainstem, terminating on a lower motor neuron
• Lower motor neuron neurosoma is in the brainstem or spinal cord
• The axon of the lower motor neuron directs the target organ

Corticospinal Tracts

• Corticospinal tracts transport signals from the cerebral cortex
• Corticospinal tracts facilitate precise, finely coordinated movements
• Pyramids are ridges located on the anterior surface of the medulla oblongata, formed by fibers of this system
• The fibers decussate in the lower medulla, forming the lateral corticospinal tract on the contralateral side of the spinal cord
• Fibers form the anterior (ventral) corticospinal tract that travels down the ipsilateral side of the spinal cord
• It decussates inferiorly (like the lateral tract), ultimately controlling contralateral muscles

Descending Tracts

• The tectospinal tract originates in the tectum region of the midbrain
• It intersects the contralateral side of the midbrain to elicit reflex turning of the head in response to sights and sounds
• Lateral and medial reticulospinal tracts start within the reticular formation of the brainstem
• The tracts control muscles in upper and lower limbs, especially those for posture and balance
• The tracts contain descending analgesic pathways that reduce pain signals transmitted to brain
• Lateral and medial vestibulospinal tracts commence in brainstem vestibular nuclei
• They receive impulses for balance from the inner ear
• The tracts control extensor limb muscles to maintain balance control

Neurological Disorders

• Poliomyelitis is caused by the poliovirus
• Poliovirus destroys motor neurons in the brainstem and anterior horn of the spinal cord
• Polio symptoms include muscle pain, weakness, and reflex loss
• Patients experience paralysis, muscular atrophy, and respiratory arrest
• Polio spreads through fecal contamination of water
• Amyotrophic Lateral Sclerosis (ALS) is also known as Lou Gehrig disease
• ALS is from the destruction of motor neurons and muscular atrophy
• Sclerosis (scarring) of lateral spinal cord regions also occurs
• Astrocytes are unable to reabsorb the neurotransmitter glutamate from tissue fluid, accumulating to toxic levels
• ALS early symptoms include muscular weakness, difficulty speaking, swallowing, and using hands
• ALS does not affect sensory and intellectual functions

13.2 The Spinal Nerves

General Anatomy of Nerves and Ganglia

• The spinal cord communicates with the remainder of the body via spinal nerves
• Nerves are cord-like organs composed of numerous nerve fibers (axons) bound by connective tissue
• Mixed nerves comprise both afferent (sensory) and efferent (motor) fibers
• Nerve fibers are surrounded by Schwann cells, establishing the neurilemma and myelin sheath around the axon
• The endoneurium involves loose connective tissue exterior to the neurilemma
• The perineurium encompasses layers of overlapping squamous cells that wrap fascicles, which are bundles of nerve fibers
• The epineurium is dense and irregular connective tissue that wraps entire nerves
• Blood vessels pass through connective tissue coverings
• They supply a plentiful blood supply

Nerve Types

• Sensory (afferent) nerves: carry signals from sensory receptors to the CNS
• Motor (efferent) nerves: carry signals from the CNS to muscles and glands
• Mixed nerves: consist of both afferent and efferent fibers
• Both sensory and motor fibers can also have fibers for somatic/visceral functions, as well as general/special functions

Ganglion

• A ganglion is a neurosomas cluster occurring outside the CNS
• It occurs enveloped in an endoneurium continuous with that of the nerve
• Neurosurgeries include neurosomas containing bundles of nerve fibers leading into and out of the ganglion
• Posterior root ganglion is associated with spinal nerves

Spinal Nerve Distribution

• There are 31 pairs of spinal nerves
• 8 cervical (C1-C8)
  • First cervical nerve exits between skull and atlas
  • Others exit at intervertebral foramina
• 12 thoracic (T1-T12)
• 5 lumbar (L1-L5)
• 5 sacral (S1-S5)
• 1 coccygeal (Co1)

Proximal Branches

• Each spinal nerve originates in two proximal roots
• The posterior (dorsal) root involves sensory input to the spinal cord
  • Posterior (dorsal) root ganglion contains the neurosomas of sensory neurons which transport signals to the spinal cord
  • Six to eight rootlets terminate at the posterior horn of the spinal cord
• The anterior (ventral) root is motor output originating in the spinal cord
  • Six to eight rootlets leave the spinal cord and converge to create the anterior root
• The cauda equina is from L2 to Co1

Distal Branches

• Beyond the vertebra, the nerve splits into distal branches
• Anterior rams
• In the thoracic region, gives rise to the intercostal nerve
• In other regions, anterior rami form plexuses
• The posterior rams innervates the muscles/joints around the region of the spine and the skin of the back
• The mining branch reenters the vertebral canal to innervate the mininges vertebrae and spinal ligaments

Nerve Plexuses

• Anterior rami branch and anastomose repeatedly to create five nerve plexuses
• Cervical plexus (C1-C5) is found in the neck, and supplies the phrenic nerve to the diagphram
• Brachial plexus (C5-T1) is found near the shoulder, and supplies the upper limb and some of the shoulder/neck
• Median nerve-carpal tunnel syndrome
• Lumbar plexus (L1-L4) is located in the lower back, supplies the abdonimal wall,anterior thigh, and the gentalia
• Sacral plexus (L4, L5 and S1-S4) is found in the pelvis
• Supplies the remainder of the lower trunk and limb
• Coccygeal plexus (S4, S5, and Co1)

Nerve Plexuses Function

• Somatosensory- carry signals about bones, joints, muscles and the skin
• Proprioception- Brain receives input from nerve endings in muscles, tendons and joints (body position and movement)
• Motor function - stimulate muscle contraction

Cutaneous Innervation and Dermatomes

• A dermatome is a specific area of skin that conveys sensory input to a spinal nerve
• A dermatome map is a diagram of cutaneous regions innervated by each spinal nerve
• Dermatomes overlap their edges as much as 50%
• It's necessary to anesthetize three successive nerves to produce a total loss of sensation in one dermatome

13.3 Somatic Reflexes

The Nature of Reflexes

• Reflexes: quick, involuntary, stereotyped gland or muscle reactions to stimulation
• Require stimulation or responding actions/sensory input -Are quick /synaptic delay
• Occur without intent and are difficult to surpress
• Occur essentially the same way every time
• Reflexes consist of glandular secretion and contraction to all three types of muscle
• Somatic reflexes consist of the sematic nervous system innervating the skeletal muscle

Pathways of a Sematic Reflex Arc

• Sematic receptor- skin, muscles and tendons
• Afferent nerve fibers- carry information from receptors to posterior horn of spinal cord/brainstem
• Integrating center- gray part of brain between neurons
  • Determines whether efferent neurons issue a signal to muscles
• Efferent nerve fibers- carries motor impulse to muscles
• Effectors- carry out the response ( the muscles)

Muscle Spindles

• Muscle spindles are stretch receptors found in skeletal muslces
• Monitor bodily position and movement
• Enablebrain to send moter commands back to the muscles that controlled coordinated moments (corrective reflexes), posture, and muscle tone.

Spindle Details

• Contains seven to eight intrafusal muscle fibers within it
  • Extrafusal fibers generate force for movement
• A gamma moter neuron keeps an intrafusal fiber taut
• Middle of fiber
  • Primary efferent fibers monitor fiber length and speed/ length changes
  • Secondary efferent fibers monitor fiber length only

The Stretch Reflex

• The stretch reflex fights back when contracted
• Important role is to maintain equilibrum and posture
• Muscles contract and maintain equilibrium
• A form of balancing with the flexor to smooth movement
• Mediated primarily by the brain, brain more pronounced with tendons /knee jerks
• Monosynaptic reflex and afferent / efferent neurons

The Flexor (Withdrawal) Reflex

• The quick contraction of flexor that result in removing yourself from a injury
• Pathways in which signals travel over many synapses on the way to the muscle

The Crossed Extension Reflex

• Contraction of extensor
• Extend other leg for balance
• Ipsilateral reflex arc vs Contralateral reflex arc

Tendon Reflexes

• Proprioceptors in a tendon near it's junction
• 1mm of nerve fibers entwined in collagen of said tendon
• Inhibits muscles from excessive tension
• Inhibits contraction
• Contraction occurs before it tears a tendon or moves the muscle/bone