Spinal Cord and Peripheral Nervous System PDF

The Spinal Cord and Peripheral Nervous System Functions of the spinal cord ● Conduction - nerve fibers conduct sensory and motor information up and down the spinal cord ● Neural integration - spinal neurons receive input from multiple sources, integrate it, and execute appropriate output (for exampl...

The Spinal Cord and Peripheral Nervous System Functions of the spinal cord ● Conduction - nerve fibers conduct sensory and motor information up and down the spinal cord ● Neural integration - spinal neurons receive input from multiple sources, integrate it, and execute appropriate output (for example, bladder control) ● Locomotion - spinal cord contains central pattern generators: groups of neurons that coordinate repetitive sequences of contractions for walking ● Reflexes - involuntary responses to stimuli that are vital to posture, coordination, and protection Spinal cord anatomy ● Divided into the cervical, thoracic, lumbar, and sacral regions ● Two areas of the cord are thicker than elsewhere: ○ Cervical enlargement ○ Lumbosacral enlargement ● Medullary cone (conus medullaris) ● Cauda equina Cross-section of the spinal cord ● Gray matter ○ Two posterior horns - sensory ○ Two anterior horns - motor ○ Lateral horns T2-L1 - ANS sympathetic ○ Commissural fibers connect two sides ● White matter ○ Bundles of axons - posterior, lateral, and anterior funiculus on each side ■ Subdivided into tracts Spinal tracts Ganglion ● Cluster of nerve cell bodies outside the CNS Spinal Nerves ● ● 31 pairs ○ 8 cervical (C1 to C8) ○ 12 thoracic (T1 to T12) ○ 5 lumbar (L1 to L5) ○ 5 sacral (S1 to S5) ○ 1 coccygeal (Co1) 5 plexuses ○ Cervical (C1 to C5) ○ Brachial (C5 to T1) ○ Lumbar (L1 to L4) ○ Sacral (L4 to S4) ○ Coccygeal (S4 to Co1) Dermatome Autonomic Nervous System (ANS) General properties of ANS ● A motor nervous system that controls glands, cardiac muscles, and smooth muscle ○ The primary target organs of the ANS ■ Viscera of thoracic and abdominal cavities ■ Some structures of the body wall - cutaneous blood vessels, sweat glands, and arrector muscles ○ Carries out actions involuntary Divisions of ANS ● Sympathetic division ○ Prepares the body for physical activity: exercise, trauma, arousal, competition, anger, or fear; fight or flight reaction ○ Increases heart rate, BP, airflow, blood glucose levels, etc; reduces blood flow to the skin and digestive tract ● Parasympathetic division ○ Calms many body functions reducing energy expenditure and assists in bodily maintenance; resting and digesting state ○ Digestion and waste elimination ● Autonomic tone ○ Normal background rate of activity that represents the balance of the two systems according to the body's needs Efferent pathways Sympathetic nervous system ● Thoracolumbar division -------------------------------------------------> ● Short preganglionic/long postganglionic fibers ○ Preganglionic fibers leave T1 - L2 ○ Lead to chain ganglia parallel to spinal cord ■ 3 cervical ■ 11 thoracic ■ 4 lumbar ■ 4 sacral ■ 1 coccygeal ● Neuronal divergence ● Craniosacral division --------------------------------------------------> ● Long preganglionic/short postganglionic fibers ○ Preganglionic fibers leave ■ CN3 ■ CN7 ■ CN9 ■ CN10 ■ S2-S4 ○ End in terminal ganglia near target organ ● Very little neural divergence Enteric plexus ● The nervous network of the digestive tract ○ Composed of 500 million networks neurons found in the walls of the digestive tract (5x more neurons than spinal cord) ○ Unlike the rest of the ANS, it does not arise from the brainstem or spinal cord ○ Like the rest of ANS, it innervates smooth muscle, glands ● Has its own ganglia and reflex arcs ● Regulates motility of the esophagus, stomach, and intestines and secretion of digestive enzymes and acid ● Normal digestive function also requires regulation by sympathetic and parasympathetic systems Neurotransmitter and their receptors ● Opposing effects in ANS ○ Sympathetic and parasympathetic fibers secrete different neurotransmitters (norepinephrine and acetylcholine) ○ The receptors on target cells vary ● Neurotransmitters ○ ACh - all preganglionic fibers and parasympathetic postganglionic ○ Norepinephrine - sympathetic postganglionic ● Receptors ○ Cholinergic ■ Muscarinic and nicotinic ○ Adrenergic ■ Alpha and beta Dual innervation Cortical Pathways Communication tracts Somatic sensory pathways ● Somatotopy = functional map of primary somatosensory cortex ○ Sensory homunculus (“little human”) = somatotope showing the relative size of cortex devoted to any specific body area ○ ● ● Motor homunculus = functional map of primary motor cortex ■ Proportions reflect the number of motor units innervated and degree of fine motor control in corresponding body region Three major somatic sensory pathways ○ Spinothalamic pathway ○ Posterior column pathway ○ Spinocerebellar pathway Spinothalamic pathway ○ Anterior spinothalamic tracts ■ Crude touch and pressure sensations from body ○ Lateral spinothalamic tracts ■ Pain and temperature sensations from body ○ First-order neuron - from receptor to spinal cord; synapses with second-order neuron in posterior gray horns ○ Second-order neuron - from posterior gray horn; crosses spinal cord; ascends to thalamus ○ Third-order neuron - from thalamus to primary sensory cortex ● Posterior column pathway ○ Highly localized (“fine”) touch, pressure, vibration, proprioception ○ From peripheral receptor to primary somatosensory cortex ○ Sensory axons ascend in medial gracile fasciculus and lateral cuneate fasciculus ○ Medial lemniscus - tract leading from the gracile nucleus and cuneate nucleus to the thalamus ● Spinocerebellar pathway ○ Proprioceptive information about position of skeletal muscles, joints, and tendons; goes to the cerebellum ○ Axons of posterior spinocerebellar tracts do not cross to the other side - pass through the inferior cerebellar peduncle on the same side ○ Anterior spinocerebellar tract axons do cross to opposite side of the spinal cord Somatic motor pathways ● Always involve at least two motor neurons ○ Upper motor neuron ■ Cell body in a CNS processing center ○ Lower motor neuron ■ Cell body in a nucleus of brainstem or spinal cord ■ Upper motor neuron synpases on lower motor neuron which then innervates a single motor unit of a skeletal muscle Coricospinal pathway ● Voluntary control of skeletal muscles ● Sometimes called the pyramidal system - upper motor neurons start at pyramidal cells in primary motor cortex ● Upper motor neuron axons descend into brainstem and spinal cord ● Synapse with lower motor neurons that control skeletal muscles Tracts of the corticospinal pathway ● Corticobulbar (bulbar, brainstem) tracts - conscious control of skeletal muscles for eye, jaw, face, some muscles of neck/pharynx ○ Synapse on lower motor neurons in cranial nerve motor nuclei ● Corticospinal tracts - conscious control of skeletal muscles ○ Visible along ventral surface of medulla oblongata as pair of thick bands (pyramids) ■ About 15 percent descend uncrossed as anterior corticospinal tracts; cross over in anterior white commissure before synapsing on lower motor neurons in anterior gray horns Two main pathways for subconscious motor commands ● Commands issues from cerebrum, diencephalon, brainstem ○ Lateral pathway - muscle tone/precise movements of distal limb muscles ■ Upper motor neurons start in red nucleus ■ Axons cross to opposite side of the brain; descend through rubrospinal (ruber, red) tracts ○ Medial pathway - gross movements of trunk, proximal limb ■ Upper motor neurons located in vestibular nuclei, superior and inferior colliculi, and reticular formation ■ Three major sets of tracts in medial pathway ● Reticulospinal tracts ● Vestibulospinal tracts ● Tectospinal tracts ■ Reticulospinal tracts ● Axons of upper motor neurons in reticular formation ● Reticular formation receives input from almost every ascending/descending pathway and has extensive connections with cerebrum, cerebellum, and brainstem ■ Vestibulospinal tracts ● Start at vestibular nuclei of CN VIII (vestibulocochlear nerve) ● Get sensory information from internal ear about head position/movement; send motor commands to alter muscle tone and position of neck, head, eyes, and limbs ■ Tectospinal tracts ● Upper motor neurons in superior/inferior colliculi (midbrain) ● Reflex changed in position of head, neck, upper limb in response to bright light, sudden movement, and loud noises Multiple levels of somatic motor control ● Many nuclei in spinal cord and brain are involved in controlling skeletal muscle contractions ● Generally, the closer the motor center is to the cerebral cortex, the more complex the motor activity ● Cerebellum coordinates motor activities at multiple levels ● ● Preparing for movement ○ Decision to move begins relay of information from: ■ Frontal lobes → premotor cortex → basal nuclei/cerebellum As movement begins: ○ Information is sent from premotor cortex to primary motor cortex ○ Commands are modified by feedback from cerebellum/ basal nuclei ● Effects of primary motor cortex damage ○ Person loses fine motor control over skeletal muscles ○ Some voluntary movements will still be controlled by basal nuclei with input from prefrontal cortex ○ Cerebellum cannot fine-tune movements because corticospinal pathway is inoperative ■ An individual is able to stand, balance, and walk, but movements are hesitant, awkward, and poorly controlled Reflexes Reflexes ● Quick, involuntary, stereotyped reactions of glands or muscle to stimulation ○ Reflexes requiem stimulation ■ Not spontaneous actions, but responses to sensory input ○ Reflexes are quick ■ Involve few, if any, interneurons and minimum synaptic delay ○ Reflexes are involuntary ■ Occur without intent and are difficult to suppress ○ Reflexes are stereotyped ■ Occur essentially the same way every time ○ Reflexes include glandular secretion and contraction of all three types of muscles ■ Somatic reflexes: reflexes involving the somatic nervous system, innervating skeletal muscles ■ Visceral reflexes: reflexes involving organs such as heart and intestines A somatic reflex involves a reflex arc with the following pathway 1. Somatic receptors in skin, muscles, or tendons 2. Afferent nerve fibers carry information from receptors to posterior horn of spinal cord or to the brainstem 3. Integrating center - a point of synaptic contact between neurons in gray matter of cord or brainstem a. Determines whether efferent neurons issue signal to muscles 4. Efferent nerve fibers carry motor impulses to muscles 5. Effectors - the muscles that carry out the response A muscle spindle and its innervation ● Many somatic reflected involve muscle spindles - stretch receptors embedded in skeletal muscles ● Serve as proprioceptors - specialized sense organs to monitor position and movement of body parts ● Inform the brain of muscle length and body movement ● Enable the brain to send motor commands back to the muscles that control coordinated movement, corrective reflexes, muscle tone, and posture Stretch reflex and reciprocal inhibition of the antagonistic muscle ● Stretch (myotactic) reflex - when a muscle is stretched, it “fights back” and contracts ○ Helps maintain equilibrium and posture ○ Head starts to tip forward as you fall asleep, muscles contract to raise the head ○ Stabilize joints by balancing tension in extensors and flexors, smoothing muscle actions ○ Stretch reflex is mediated primarily by the brain, but its spinal component can be more pronounced if muscle is suddenly stretched by a tendon tap (knee jerk) The flexor and crossed extension reflexes ● Flexor reflex - the quick contraction of flexor muscles resulting in the withdrawal of a lumb from an injurious stimulus ○ Triggers contraction of the flexors and relaxation of the extensors in that limb ○ Polysynaptic reflex arc - pathway in which signals travel over many synapses on their way to the muscle ● Crosses extension reflex - contraction of extensor muscles in limb opposite of the one that is withdrawn ○ Maintains balance by extending other leg ○ Flexor reflex uses an ipsilateral reflex arc (stimulus and response on same side) whereas crossed extension reflex uses a contralateral reflex arc (input and output are on opposite sides) ○ Intersegmental reflex - one in which the input and output occur at different levels (segments) of the spinal cord The tendon reflex ● Tendon organs - proprioceptors in a tendon near its junction with a muscle; involved in the tendon reflex ○ Is 0.5mm long, consists of encapsulated bundle of collagen fibers and one or more nerve fibers ● Tendon reflex - response to excessive tension on the tendon ○ Inhibits the muscle from contracting strongly ○ Moderates muscle contraction before it tears the tendon or pulls it loose from the muscle or bone