Spinal Cord: Function and Anatomy

• Thousands of Americans experience spinal cord injuries leading to paralysis each year.
• The spinal cord serves as the "information highway", connecting the brain to the lower body.

Functions of the Spinal Cord

• Conduction is where nerve fibers carry sensory and motor information to and from the brain.
• Neural integration is where spinal neurons process input from various sources to execute appropriate outputs.
• Central pattern generators within the spinal cord coordinate repetitive muscle contractions, enabling locomotion.
• Reflexes are involuntary responses vital for posture, coordination, and protection.

Spinal Cord Anatomy

• The spinal cord is a cylinder of nervous tissue extending from the brainstem at the foramen magnum
• The spinal cord's inferior margin ends at the L1 vertebra or slightly beyond.
• Thirty-one pairs of spinal nerves arise from the spinal cord.
• A segment of the spinal cord is the area supplied by each pair of spinal nerves.
• The spinal cord occupies the upper two-thirds of the vertebral canal
• The spinal cord averages 1.8 cm in thickness and 45 cm in length
• Anterior median fissure and posterior median sulcus exist as longitudinal grooves on the spinal cord.
• The spinal cord has cervical, thoracic, lumbar, and sacral regions.
• Cervical enlargement represents thicker areas with nerves to the upper limb.
• Lumbosacral enlargement involves nerves to the pelvic region and lower limbs
• The medullary cone (conus medullaris) is where the cord tapers inferior to the lumbosacral enlargement.
• The Cauda equina is a bundle of nerve roots occupying the vertebral canal from L2 to S5.

Spinal Meninges

• Meninges are three fibrous membranes enclosing the brain and spinal cord.
• Meninges separate the central nervous system's soft tissue from the cranium and vertebral canal bones.
• The meninges from superficial to deep are: dura mater, arachnoid mater, and pia mater.
• The dural sheath surrounds the spinal cord and is separated from vertebrae by the epidural space.
• Arachnoid membrane adheres to the dura and is filled with cerebrospinal fluid (CSF)
• In a lumbar puncture (spinal tap) CSF is extracted.
• Pia mater is a delicate membrane following the spinal cord's contours.

Spinal Cord: Cross-Sectional Anatomy

• The spinal cord has a central area of gray matter shaped like a butterfly, surrounded by white matter.
• Gray matter consists of neuron cell bodies with little myelin
• White matter consists of abundantly myelinated axons.
• Gray matter is the site of information processing and synaptic integration.
• White matter carry signals within the CNS.
• The gray matter has dorsal and posterior horns
• The dorsal root of the spinal nerve carries only sensory fibers.
• Thicker anterior (ventral) horns are a feature of gray matter
• Gray matter's anterior (ventral) root of the spinal nerve carries only motor fibers.
• The gray commissure connects the right and left sides, with a central canal lined with ependymal cells and filled with CSF
• The lateral horn contains neurons of the sympathetic nervous system and is visible from T2 through L1.

Spinal Cord: White Matter

• White matter surrounds the gray matter.
• Axons course up and down the cord, providing communication between levels of the CNS.
• Funiculi (columns) are three pairs of white matter bundles: posterior (dorsal), lateral, and anterior (ventral).
• Tracts (fasciculi) are the subdivisions within each funiculus.

Spinal Tracts

• Fibers in a tract have similar origins, destinations, and functions.
• Ascending tracts carry sensory information up.
• Descending tracts carry motor information down.
• Decussation is when a tract crosses the midline so the brain senses and controls the contralateral side of the body.
• Contralateral is when the origin and destination of a tract are on opposite sides of the body.
• Ipsilateral is when the origin and destination of a tract are on the same side of the body; meaning it does not decussate.

Ascending Tracts

• Ascending tracts carry sensory signals up the spinal cord
• Ascending tracts include the gracile fasciculus, cuneate fasciculus, spinothalamic tract, spinoreticular tract, and spinocerebellar tracts.
• First-order neurons detect stimuli and transmit signals to the spinal cord or brainstem.
• Second-order neurons continue to the thalamus.
• Third-order neurons carry the signal to the sensory region of the cerebral cortex.

Gracile Fasciculus

• The gracile fasciculus carries signals from the midthoracic and lower parts of the body.
• Below T6, the gracile fasciculus composes the entire posterior funiculus.
• Above T6, the gracile fasciculus is accompanied by the cuneate fasciculus.
• First-order nerve fibers travel up the ipsilateral side of the spinal cord as part of the gracile fasciculus.
• The gracile fasciculus terminates at the gracile nucleus of the medulla oblongata and carries signals for vibration, visceral pain, deep touch, and proprioception from the lower limbs and trunk.
• Proprioception involves the nonvisual sense of position and movements of the body.

Cuneate Fasciculus

• The cuneate fasciculus originates at T6 and above, occupying the lateral portion of the posterior funiculus.
• The cuneate fasciculus contains first-order neurons conveying the same sensory signals as the gracile fasciculus, but from the upper limb and chest.
• Fibers in the cuneate fasciculus end in the cuneate nucleus of the ipsilateral medulla oblongata.
• Second-order neurons from the gracile and cuneate nuclei decussate and form the medial lemniscus, a tract leading to the thalamus.
• Third-order neurons from the thalamus then go to the cerebral cortex, carrying signals to the cerebral hemisphere.
• The left hemisphere processes stimuli from the right side of the body (and vice versa) because of the decussation of second-order neurons.

Spinothalamic Tract

• The Spinothalamic tract is part of the anterolateral system
• It passes up the anterior and lateral funiculi of the spinal cord
• The spinothalamic tract carries signals for pain, pressure, temperature, light touch, tickle, and itch.
• This tract is made up of axons of second-order neurons
• First-order neurons end in the posterior horn of the spinal cord.
• Second-order neurons start in the posterior horn, then decussate to form the spinothalamic tract.
• Third-order neurons go to the cerebral cortex
• Signals cross, they proceed to the cerebral hemisphere contralateral to the stimulus site.

Spinoreticular Tract

• The spinoreticular tract travels up the anterolateral system.
• It carries pain signals from tissue injury and is made up of axons of second-order neurons
• The first-order neurons enter the posterior horn and synapse with second-order neurons.
• The second-order neurons decussate to the opposite anterolateral system, ascend the cord, and then end in the reticular formation located 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.
• These tracts carry proprioceptive signals from the limbs and trunk up to the cerebellum.
• They are made up of axons of seconder order neurons
• First-order neurons originate in muscles and tendons, ending in the posterior horn of the spinal cord.
• Second-order nerves ascend and end in cerebellum, needed to coordinate movements.
• The posterior spinocerebellar tract stays ipsilateral.
• Anterior spinocerebellar tracts cross over, travel up contralateral side, and cross back to end in ipsilateral cerebellum.

Descending Tracts

• Descending tracts carry motor signals down the brainstem and spinal cord.
• Descending tracts involve two motor neurons
• The upper motor neuron originates in the cerebral cortex or brainstem and terminates on a lower motor neuron.
• The lower motor neuron's neurosoma resides in the brainstem or spinal cord and its axon leads to a muscle or target organ.

Corticospinal Tracts

• The corticospinal tracts conduct signals from the cerebral cortex utilized for precise, coordinated movements.
• Pyramids are ridges located on the anterior surface of the medulla oblongata that consist of fibers from this system.
• Fibers decussate in the lower medulla, forming the lateral corticospinal tract on the contralateral side of the spinal cord.
• Some fibers form the anterior (ventral) corticospinal tract, which descends ipsilaterally and then decussates inferiorly to control contralateral muscles.

Other Descending Tracts

• The tectospinal tract begins in the tectum of the midbrain.
• The tectospinal tract crosses to the contralateral side of the midbrain, triggering reflex turning of the head in response to sights and sounds.
• Lateral and medial reticulospinal tracts originate in the brainstem's reticular formation.
• The reticulospinal tracts control muscles of the upper and lower limbs, especially those for posture and balance; reduce pain transmission.
• Lateral and medial vestibulospinal tracts begin in the brainstem's vestibular nuclei and receive balance impulses from the inner ear while controlling limb extensor muscles for balance.

Poliomyelitis

• Poliomyelitis is caused by the poliovirus.
• Poliomyelitis destroys motor neurons in the brainstem and anterior horn of the spinal cord.
• Symptoms of poliomyelitis include muscle pain, weakness, and loss of reflexes.
• Followed by paralysis, muscular atrophy, and respiratory arrest.
• The poliovirus spreads through fecal contamination of water.

Amyotrophic Lateral Sclerosis

• Amyotrophic lateral sclerosis (ALS), also referred to as Lou Gehrig disease, involves the destruction of motor neurons, which leads to muscular atrophy.
• Sclerosis (scarring) also occurs in the lateral regions of the spinal cord.
• Sensory and intellectual functions remain unaffected.
• Early signs of ALS are muscular weakness; difficulty speaking, swallowing, and using hands.

Spinal Nerves

• Spinal nerves connect the spinal cord to the rest of the body
• A nerve is a cord-like organ containing nerve fibers (axons) bound together by connective tissue.
• Mixed nerves contain afferent (sensory) and efferent (motor) fibers.
• Nerve fibers of the peripheral nervous system are surrounded by Schwann cells forming a neurilemma and myelin sheath.
• Endoneurium is loose connective tissue external to the neurilemma.
• Perineurium are layers of overlapping squamous cells wrap fascicles: bundles of nerve fibers
• Epineurium is dense irregular connective tissue that wraps the entire nerve.
• Blood vessels penetrate the connective tissue coverings, for plentiful blood supply.
• Ganglion is a cluster of neurosomas outside the CNS
• A ganglion is enveloped in an endoneurium, continuous with that of the nerve, and contains bundles of nerve fibers.
• Posterior root ganglion is associated with spinal nerves.
• There are thirty-one pairs of spinal nerves (mixed nerves).
• Eight cervical nerves (C1–C8) in which the first cervical nerve exits between the skull and atlas, while others exit at intervertebral foramina
• There are twelve thoracic nerves (T1–T12), five lumbar nerves (L1–L5), five sacral nerves (S1–S5), and one coccygeal nerve (Co1).

Spinal Nerves: Branches

• Spinal nerves connect with the spinal cord via proximal roots.
• The posterior (dorsal) root carries sensory input to the spinal cord.
• Six to eight nerve rootlets enter the posterior horn of the spinal cord
• The posterior (dorsal) root ganglion contains neurosomas of sensory neurons.
• The anterior (ventral) root expresses motor output from the spinal cord
• The roots that arise from the spinal cord between L2 and C01 form the cauda equina.
• The anterior ramus innervates the intercostal nerve in the thoracic region or forms plexuses in other regions.
• The posterior ramus innervates the muscles and joints of the spine region as well as the corresponding skin.
• The meningeal branch of the spinal nerve, which then innervates the meninges, vertebrae, and ligaments of the spinal column.

Spinal Nerves: Distal Branches

• Beyond the vertebra, the nerve divides into distal branches
• The anterior ramus in the thoracic region gives rise to the intercostal nerve
• the anterior ramus forms plexuses in other regions
• The posterior ramus innervates the muscles and joints in that region of the spine and the skin of the back.
• The meningeal branch reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments.

Shingles

• Virus remains for life in the posterior root ganglia, but is kept in check by the immune system.
• Shingles (herpes zoster) is a localized disease caused by the virus traveling down sensory nerves, due to a decline in the immune system.
• Shingles appears as a painful trail of skin discoloration and fluid-filled vesicles along the nerve path.
• Childhood chickenpox vaccinations can reduce the risk of shingles later in life.

Nerve Plexus

• Anterior rami branch and anastomose to form the cervical, brachial, lumbar, sacral, and coccygeal nerve plexuses.
• The cervical plexus lies in the neck (C1-C5), supplying the neck and phrenic nerve to the diaphragm.
• The brachial plexus is near the shoulder (C5-T1), supplying the upper limb.
• The median nerve is involved with carpal tunnel syndrome
• The lumbar plexus is in the lower back (L1-L4), supplying the abdominal wall, anterior thigh, and genitalia.
• The sacral plexus is in the pelvis (L4, L5, S1-S4), supplying lower trunk and lower limb.
• The coccygeal plexus is at S4, S5, and Co1.
• Carry sensory signals from bones, joints, muscles, and skin, is a somatosensory function.
• It’s propriopception that allows the brain to receive data of body position and limb movements.
• Primary signal for motor function is allowing muscle contraction

Other Nerve Injuries

• Radial nerve injury is where the nerve passes through the axilla, and involves crutch paralysis and wrist drop.
• Sciatic nerve injury triggers sharp pain (sciatica) that travels from the gluteal region down the posterior side of the thigh and leg to ankle.
• 90% of sciatica cases result from herniated intervertebral discs or osteoporosis of the lower spine.

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 the cutaneous regions innervated by each spinal nerve.
• Dermatomes overlap their edges by as much as 50%.
• Thus, to achieve total loss of sensation in one dermatome, three successive spinal nerves must be anesthetized.

Somatic Reflexes

• Reflexes are quick, involuntary, stereotyped reactions of glands or muscle to stimulation.
• Unlike reactions, reflexes require stimulation and are involuntary.
• Reflexes involve one or more interneurons, so there's a minimum synaptic delay
• Reflexes are stereotyped, happening nearly the same way each time

Somatic Reflexes: Types

• Somatic reflexes affect somatic nervous system-innervated skeletal muscles.
• Somatic receptors are in skin, muscles, or tendons.
• Afferent nerve fibers carry data from the receptors to the posterior horn of the spinal cord or to the brainstem.
• The integrating center is a point of synaptic contact between neurons in the gray matter of cord or a brainstein.
• This integrating center determines whether efferent neurons issue a signal.
• Efferent nerve fibers then carry motor impulses to muscles
• Effectors are the muscles that carry out the response.
• Muscle spindles are stretch receptors embedded in skeletal muscles and are proprioceptors.

Muscle Spindles

• Muscle spindles are specialized sense organs that monitor body parts.
• Muscle spindles inform the brain of muscle length and body movement.
• Enables the brain to motor commands back to the muscles to control coordinated movement, corrective reflexes,muscle tone, and posture.
• A spindle contains seven or eight intrafusal muscle fibers within it
• The rest of the muscle’s fibers (generating force) are extrafusal fibers.
• Sensory nerve fibers are held and activated in the midportion of the intrafusal fiber

Muscle Spindles: Nerve Fibers

• A gamma motor neuron innervates the ends of an intrafusal fiber this maintaining to keeps it taut
• Primary afferent fibers are responsible for monitoring fiber length
• Secondary afferent fibers solely monitor length.
• Spindle function example assist with staying upright when standing on a boat

Stretch Reflex

• When a muscle is stretched, the stretch (myotatic) reflex is where it “fights back” and contracts
• A factor that helps sustain equilibrium is posture
• The head may start to tip forward at sleep
• Muscles contract to raise the head
• Stabilize joints by balancing tension in extensors and flexors and by smoothing muscle actions.
• The higher stretch reflex is mediated by the brain, but the knee jerk is the spinal component that can be more pronounced.

Stretch Reflex: Knee Jerk

• The knee-jerk (patellar) reflex is a monosynaptic reflex (one synapse between the afferent and efferent neurons).
• Testing somatic reflexes helps diagnose many diseases.
• Reciprocal inhibition is a reflex phenomenon that prevents muscles that are working against each other by inhibiting an antagonist when an agonist is excited.

Flexor Reflex

• The quick contraction of flexor muscles that results in the withdrawal of a limb from an injurious stimulus is the flexor reflex
• The flexor or withdrawal reflex involves trigger of flexor contraction and extensor relaxation in that limb
• Polysynaptic reflex arc is the method of operation, where signals travels over many synapses

Crossed Extension Reflex

• Muscles that are contracted are extensors in the limb opposite, is the crossed extension reflex
• The other leg maintains balance when extending
• A flexor reflex uses an ipsilateral pathway arc: stimulus and response on the same side whereas, the other uses a contralateral one arc: The opposite arc, where the input and output are not on the same side.
• Intersegmental reflex are where the signal passes different levels or segments of a point of spine
• One example is pain in the foot causing contraction of the abdominal muscles

Tendon Reflex

• Tendon organs are proprioceptors adjacent to a junction by a muscle to tendon Golgi tendon organ is 1 mm in length with entwined nerve fibers with collagen
• Tendon reflex is in response to excessive tension The reflex inhibits the muscles for contracting and moderates as a reflex to moderate muscle contraction that tears a tendon from bone from a pull