Spinal Cord Functions & Anatomy

Spinal Cord Functions

• The spinal cord is responsible for conduction, neural integration, locomotion, and reflexes
• Nerve fibers conduct information up and down the spinal cord in bundles
• Spinal neurons integrate inputs from multiple sources and execute appropriate outputs
• Central pattern generators coordinate continued walking, initiated by brain motor neurons
• Spinal reflexes are crucial for posture, motor coordination, and protective responses

Spinal Cord Anatomy

• The spinal cord extends from the brainstem at the foramen magnum through the vertebral canal
• It terminates slightly beyond the inferior margin of the first lumbar vertebra
• The spinal cord initially spans the entire vertebral column during fetal development
• Due to differential growth rates, it only reaches the L3 vertebra by birth
• There are 31 pairs of spinal nerves originating from the cord
• Each pair supplies a spinal segment, even though the cord lacks visible segmentation
• The longitudinal grooves on the spinal cord include the anterior median fissure and the posterior median sulcus
• Spinal cord regions are named after the vertebral column level where spinal nerves emerge
• The cervical enlargement in the inferior cervical region gives rise to nerves of the upper limbs
• The lumbar enlargement in the lumbosacral region leads to nerves of the pelvic region and lower limbs
• Inferior to the lumbar enlargement, the spinal cord tapers into the medullary cone (conus medullaris)
• The lumbar enlargement and medullary cone create the cauda equina, nerve roots occupying the vertebral canal from L2 to S5

Spinal Cord Meninges

• Meninges, the dura mater, arachnoid mater, and pia mater, enclose the spinal cord and brain
• The dura mater creates a dural sheath around the spinal cord, forming a loose-fitting sleeve
• The epidural space lies between the dural sheath and vertebral bones, containing blood vessels, adipose tissue, and loose connective tissue
• Anesthetics are introduced into the epidural space to block pain signals
• The arachnoid mater is simple squamous epithelium adhering to the dura's inner surface with fibers connecting it to the pia mater
• The subarachnoid space between the arachnoid and pia mater contains cerebrospinal fluid (CSF)
• Inferior to the medullary cone is the lumbar cistern, a subarachnoid space filled with the cauda equina and CSF
• The pia mater, closely follows the spinal cord's contours
• Inferior to the medullary cone, the pia mater continues as the terminal filum in the lumbar cistern
• The pia mater anchors to vertebra Co1 at S2, forming the coccygeal ligament
• Denticulate ligaments, extensions of the pia, attach to the dura via the arachnoid, anchoring the cord

Spinal Cord Structure

• Like the brain, the spinal cord is composed of gray and white matter
• The gray matter, contains somas, dendrites, and unmyelinated axons of neurons
• The white matter, contains myelinated axons organized into tracts
• The H-shaped central core of gray matter in the spinal cord consists of posterior (dorsal) and anterior (ventral) horns
• The right and left sides are connected by a gray commissure
• The central canal within the gray commissure is lined with ependymal cells and filled with CSF
• Spinal nerves attach to the cord by a posterior (dorsal) root and an anterior (ventral) root
• Sensory nerve fibers pass through the posterior root to the posterior horn
• The anterior horn houses somatic motor neuron somas, whose axons lead to skeletal muscles
• In thoracic and lumbar segments, a lateral horn contains sympathetic nervous system neurons
• Axons running up and down the spinal cord form three pairs of white matter columns or funiculi
• The columns, posterior (dorsal), lateral, and anterior (ventral), consists of tracts or fasciculi

Spinal Tracts

• Spinal tracts include ascending and descending tracts
• Ascending tracts carry sensory information up the spinal cord
• Descending tracts conduct motor impulses down the spinal cord
• Nerve fibers in a tract share origin, destination, and function
• Some tracts originate in the brainstem, which supports the cerebellum and cerebral hemispheres
• Decussation occurs when tracts cross from one side of the body to the other
• Contralateral tracts have origins and destinations on opposite sides
• Ipsilateral tracts' fibers do not decussate
• Sensory signals travel across three neurons in ascending tracts
• First-order neurons detect stimuli and signal the spinal cord or brainstem
• Second-order neurons continue to the thalamus
• Third-order neurons signal the cerebral cortex
• Axons of these neurons are the first- through third-order nerve fibers
• The gracile fasciculus handles signals from the midthoracic and lower body
• Below T6, it is joined by the cuneate fasciculus
• Signals for vibration, visceral pain, discriminative touch & proprioception are carried by first-order nerve fibers that terminate in the gracile nucleus of the medulla oblongata
• The cuneate fasciculus joins the gracile fasciculus at T6, carrying sensory signals from the upper limbs and chest
• Its fibers end in the cuneate nucleus of the medulla oblongata
• Second-order fibers from the gracile and cuneate systems decussate in the medulla and form the medial lemniscus
• Third-order fibers run from the thalamus to the contralateral cerebral hemisphere
• The spinothalamic tract and smaller tracts form the anterolateral system
• Carries pain, temperature, pressure, tickle, itch & touch signals
• First-order neurons end in the spinal cord's posterior horn, synapsing with second-order neurons and ascends contralaterally to the cerebral cortex
• Spinoreticular tracts carry pain signals from tissue injury
• First-order sensory neurons synapse with second-order neurons in the posterior horn
• They decussate and ascend to the reticular formation in the medulla
• Third-order neurons continue to the thalamus, then fourth-order neurons complete the path to the cerebral cortex
• Spinocerebellar tracts carry proprioceptive signals from the limbs and trunk to the cerebellum
• First-order neurons originate in muscles/tendons and end in the posterior horn
• Second-order neurons send fibers up to cerebellum
• Posterior tract fibers travel ipsilaterally
• Anterior tract fibers cross, travel contralaterally then cross back to enter the ipsilateral side of the cerebellum

Descending Tracts

• Motor information is carried from the brainstem to the spinal cord
• Upper motor neurons begin in the cerebral cortex or brainstem
• The upper motor neuron synapses with the lower motor neuron in the brain stem or spinal cord
• The axon of the lower motor neuron innervates the muscle or target organ
• Descending tracts include: corticospinal, tectospinal, reticulospinal and vestibulospinal tracts
• The corticospinal tracts carry motor signals from the cerebral cortex for coordinated limb movements
• Their fibers form ridges called pyramids
• Most fibers decussate in the medulla and form the lateral corticospinal tract
• Some fibers remain uncrossed, forming the anterior corticospinal tract
• The tectospinal tract starts in the midbrain's tectum and crosses to the contralateral side
• It descends to the upper spinal cord and facilitates reflex head turning
• The reticulospinal tracts originate in the reticular formation in the brainstem
• They control posture and balance
• These tracts reduce pain signal transmission through analgesic pathways
• The vestibulospinal tracts start in the brainstem vestibular nuclei
• They control balance and facilitate control of limb extensor muscles to stiffen/straighten limbs
• The medial vestibulospinal tract splits into ipsilateral and contralateral fibers for head control
• The rubrospinal tracts are prominent in other mammals, almost nonexistent in humans

Spinal Nerves Structure

• Nerves, cord like organs, contain of nerve fibers bundled by connective tissue
• A nerve contains a few to millions of nerve fibers
• Peripheral nerve fibers are ensheathed in Schwann cells, forming a neurilemma
• External to the neurilemma, each fiber is surrounded by a basal lamina and endoneurium
• Nerve fibers are bundled into fascicles wrapped in a perineurium composed of multilayered epithelium
• Fascicles are bundled in an epineurium (dense irregular connective tissue)
• Nerves have a high metabolic rate and plentiful blood supply
• Peripheral, PNS fibers are sensory (afferent) and/or motor (efferent), somatic or visceral, general or special
• Nerves can be sensory, motor & mixed
• Sensory nerves (afferent) are rare, including for smell and vision
• Motor nerves (efferent)
• Mixed nerves (afferent and efferent) conduct signals in two directions but each fiber only conducts signals in one direction

Ganglia

• A ganglion is a cluster of neurosomas outside the CNS
• Ganglia are enclosed in an epineurium continuous with the nerve
• Bundles of nerve fibers lead into and out of the ganglion, passing neurosomas
• The human body has 31 pairs of spinal nerves which include 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerves
• The first cervical spinal nerve emerges between the skull and atlas (C1)
• Cervical spinal nerve numbering corresponds to the vertebra inferior to it
• For all other spinal nerves, the nerves emerge inferiorly to the corresponding numbered vertebrae
• Spinal nerves merge from proximal branches prior to the intervertebral foramen
• Six to eight anterior rootlets emerge from the spinal cord converging to form the anterior root
• Six to eight posterior rootlets emerge from the spinal cord converging to form the posterior root
• Distal to the posterior root ganglion, the anterior and posterior roots merge where they leave the dural sheath
• Spinal nerves proper leave the vertebral canal through the intervertebral foramen, forming a mixed nerve
• Anterior and posterior roots are shortest in the cervical region
• Roots of segments L2 to Co1 of the cord form the cauda equina
• Viruses may invade the central nervous system (CNS) via the spinal nerve routes
• After emerging from the intervertebral foramen, spinal nerves divide into the intervertebral foramen, the anterior ramus, posterior ramus, and meningeal branch immediately

Spinal Nerves Branches

• Immediately after emerging from the intervertebral foramen, spinal nerves divide into an anterior ramus, posterior ramus, and meningeal branch
• Spinal nerves branch on both ends
• The meningeal branch reenters the vertebral canal and innervates meninges, vertebrae, and spinal ligaments
• The posterior ramus innervates muscles and joints in spine and skin of back
• The anterior ramus innervates anterior and lateral skin/muscles of trunk and gives rise to the limb nerves
• Anterior rami form intercostal nerves in the thoracic region or nerve plexuses elsewhere
• The anterior ramus gives off communicating rami, which connect to sympathetic chain ganglia in spinal nerves T1 through L2
• Anterior rami form nerve plexuses, which include cervical, brachial, lumbar, sacral & coccygeal, everywhere except the thoracic region
• Spinal nerve roots form plexuses and may have smaller branches like trunks, anterior divisions, posterior divisions, & cords
• Spinal nerves have somatosensory and motor functions
• Proprioception involves the reception and processing of body position and movements
• Skeletal muscle contractions are stimulated by motor function

Dermatomes

• Dermatomes, are skin areas where each spinal nerve (except C1) receives sensory input
• Dermatomes can overlap by 50%
• Loss of sensation from one dermatome requires severing three spinal nerves

Characteristics of Somatic Reflexes

• Somatic reflexes are quick, involuntary, stereotyped reactions to stimuli
• Reflexes require stimulation from sensory input and are responses, they are not spontaneous
• Reflexes are quick, involving minimal synaptic delay
• Reflexes are involuntary, they occur without awareness
• Reflexes are stereotyped, they occur the same way every time
• Reflexes include glandular secretion and all 3 types of muscle contraction
• Somatic reflexes are skeletal muscle reflexes mediated by the brainstem and spinal cord with involvement of the somatic nervous system
• The somatic reflex arc contains a pathway of somatic receptors, afferent nerve fibers, an integrating center (spinal cord or brainstem gray matter), efferent nerve fibers, & effectors

Muscle Spindles

• Muscle spindles involve the central nervous system and stretch receptors
• Muscle spindles inform the brain of muscle length and body movements
• Muscle spindles are abundant in muscles that require fine control muscles
• Muscle spindles are bundles of modified muscle fibers in an elongated fibrous capsule
• Located near the tendons at the muscle ends
• Intrafusal fibers are modified muscle fibers within the spindle
• Extrafusal fibers are the rest of the muscle
• Spinal cord gamma motor neurons innervate the contractile poles of intrafusal fibers
• Alpha motor neurons innervate extrafusal fibers
• The midportion of intrafusal fibers lacks sarcomeres, is non-contractile, & supplied by sensory nerve fibers
• Primary afferent fibers monitor change in muscle length
• Secondary afferent fibers monitor muscle length
• Brain constantly monitors length and tension in skeletal muscles

Components of Somatic Reflexes

• The length and tension of nearly every skeletal muscle utilizes constant brain and subconscious monitoring
• The stretch (myotatic) reflex causes muscle contractions to compensate
• Stretch reflexes feed back to synergists and antagonists
• The flexion of a joint creates a stretch reflex in the extensors
• Stretch reflexes facilitate smooth & coordinated movements
• Sudden stretches often trigger a stretch reflex, mediated primarily by the brain
• Patellar (knee-jerk) reflex is an example of a tendon reflex
• Monosynaptic reflex arcs return to the muscle directly via spinal cord primary afferent fibers, with minimal synaptic delay
• Somatic reflex testing is helpful in diagnosing disorders that cause exaggeration, inhibition, or absence of somatic reflexes
• Reciprocal inhibition, a reflex phenomenon that prevents muscle contraction, occurs in stretch reflexes
• Spinal cord interneurons inhibit an antagonist's alpha motor neurons in the sensory fibers from one muscle spindle

Types of Reflexes

• The quick contraction of flexor muscles enables withdrawal of a limb (flexor or withdrawal reflex)
• Involved in this reflex is contraction of flexors and relaxation of extensors in the stimulated limb
• Complex neural pathways characterize the flexor reflex
• Sustained contraction is produced by the polysynaptic reflex arc
• Some signals reach the muscles promptly, others at a delay resulting in prolonged output from the spinal cord
• Extensor muscle contraction in the opposite limb enables balance in the crossed extension reflex
• Branches of the afferent nerve fibers cross to the contralateral side and synapse to excite/inhibit motor neurons
• Ipsilateral reflex arcs enable the flexor reflex and contralateral employs crossed extension reflex arcs
• Muscles at different locations are affected and require input/output for spinal cord segments in an intersegmental reflex arc
• The response to excessive tension on a tendon is the tendon reflex
• Proprioceptors, also known as tendon organs, are located at a tendon's junction
• A tendon organ consists of an encapsulated bundle of collagen fibers & nerve fibers that penetrate the capsule
• Signals pass from the squeezed nerve endings to the spinal cord, feeding back to the central nervous system
• The tendon reflex inhibits alpha motor neurons to prevents a muscle to contract
• By doing so, it prevents tendons from injury
• Before reflexes occur, tendon injuries may occur that may be caused by strong muscles and hasty movements
• Some muscle parts work more than others when a tendon reflex occurs resulting in workload distributed evenly across the muscle