Spinal Cord Organization January 12, 2011
Spinal Cord 31 segments terminates at L1-L2 special components - conus medullaris - cauda equina no input from the face
Spinal Cord, Roots & Nerves Dorsal root Ventral root Spinal nerve
Dorsal Root Ganglion Cell bodies of 1 st order sensory neurons Afferent
Spinal Cord Cross Section Dorsal root sensory afferent Ventral root motor efferent
Cross Section of Spinal Cord White matter peripheral Gray matter central Central canal
Volume of Gray & White Matter Cervical 5 Thoracic 7 Lumbar 4 Sacral 3 Coccygeal 1 Gray matter enlarges at cervical and lumbar regions. White matter increases at higher levels of the cord.
Central Gray Matter Motor neurons 2 nd order sensory neurons Interneurons Motor neuron = lower motor neuron = anterior horn cell = second order motor neuron Glial cells
Organization of Gray Matter Pain Temperature
Organization of Gray Matter Position sense Vibration Pressure Touch
Organization of Gray Matter Neck & base of Posterior Horn Position sense from legs Between T1 and L2 enlarged Dorsal nucleus of Clarke 2 nd order sensory neuron going to cerebellum
Organization of Gray Matter Intermediolateral Horn Interomediolateral horn Between T1 and L2 enlarged Sympathetic neurons (preganglionic)
Sympathetic Innervation
Organization of Gray Matter Intermediolateral Horn. At S2, S3, and S4 Parasympathetic neurons (preganglionic)
Parasympathetic Innervation
Organization of Gray Matter Modulates motor activity via gamma motor neurons Adjusts briskness of deep tendon reflexes
Organization of Gray Matter Nuclei of alpha and gamma motor neurons Innervates muscle spindles and muscles
Lower Motor Neurons Alpha motor neurons originate from Rexed lamina IX ventral dorsal organization ventral extensor muscles dorsal flexor muscles medial lateral organization medial proximal muscles lateral distal muscles
Nature of Somatic Reflexes Quick, involuntary, stereotyped reactions of glands or muscle to sensory stimulation automatic responses to sensory input that occur without our intent or often even our awareness Functions by means of a somatic reflex arc stimulation of somatic receptors afferent fibers carry signal to dorsal horn of spinal cord one or more interneurons integrate the information efferent fibers carry impulses to skeletal muscles skeletal muscles respond 12-19
The Muscle Spindle Sense organ (proprioceptor) that monitors length of muscle and how fast muscles change in length Composed of intrafusal muscle fibers, afferent fibers and 12-20 gamma motorneurons
Stretch Reflex 12-21
Somatotopic Organization gamma reflex loop
Coordinating Role of Interneurons flexor withdrawal reflex crossed extensor reflex
Flexor Withdrawal Reflexes Occurs during withdrawal of foot from pain Polysynaptic reflex arc Neural circuitry in spinal cord controls sequence and duration of muscle contractions 12-24
Crossed Extensor Reflexes Maintains balance by extending other leg Intersegmental reflex extends up and down the spinal cord Contralateral reflex arcs explained by pain at one foot causes muscle contraction in other leg 12-25
Breathing Nuclei of alpha and gamma motor neurons LMNs from spinal levels C3, C4 & C5 innervate the phrenic nerve Controls diaphragm
Control Functions Nuclei of alpha and gamma motor neurons At levels S1 - S4 LMNs form Onuf s nucleus Innervates anal and urethral spincters; needed for sexual function
Sacral Cord
Configuration of Gray Matter Abundant white matter high in the spinal cord Enlargement of gray matter at the cervical level Innervates arm muscles
Configuration of Gray Matter Enlargement in thoracic region for sympathetic neurons Enlargement in lumbar region for motor neurons to the legs Minimal white matter at the lower cord
Descending Motor Pathway Upper motor neuron 1 st order neuron synapses on LMN Crossed pathway Lateral corticospinal Uncrossed pathway Anterior corticospinal
Somatotopic Organization
White Matter in the Spinal Cord Divided into three funiculi (columns) posterior, lateral, and anterior Each column (funiculus) contains several Fiber tracts are either Ascending (sensory) Descending (motor) Fiber tract names often reveal their origin and destination 12-33
White Matter: Pathway Generalizations Tracts may decussate (cross-over) Most consist of two or three neurons Pathways are paired (one on each side of the spinal cord or brain) Contralateral means origin and destination are on opposite sides while ipsilateral means on same side 12-34
Spinal Cord Tracts Unconscious position sense movement feedback
Ascending & Descending Tracts Sensory Gracile tract Leg position & vibration Cuneate tract Arm position & vibration Dorsal spinocerebellar tract Strength & muscle speed Ventral spinocerebellar tract Modulation; interneurons Lateral spinothalamic tract Pain & temperature Anterior spinothalamic tract Light touch Spinocervical thalamic tract Kinesthetic movement & discriminative touch Motor Corticospinal tract Speed & agility Reticulospinal tract Differential facilitation of motor neurons Rubrospinal tract Fix movement errors Lateral vestibulospinal tract Extensor & posture Medial vestibulospinal tract Flexor & head position Tectospinal tract Head turning
Ascending Tracts Sensory Position Vibration Sensory Pain Temperature
Ascending Tracts Carry sensory signals up to the spinal cord Typically uses 3 neurons 1st order neuron - detects stimulus and carries it to spinal cord 2nd order neuron - within s.c.; continues to the thalamus (the sensory relay station) 3rd order neuron - carries signal from thalamus to sensory region of cerebral cortex Most have names with prefix spino- 12-38
Dorsal Column Ascending Pathway Carries sensations related to discriminative touch, visceral pain, vibration, and proprioception 1st order neuron - detects stimulus Fasciculus gracilis Carries sensation from below T6 Fasciculus cuneatus Carries sensation from T6 or higher 2nd order neuron synapses with 1st in medulla and decussates 3rd order neuron synapses with 2nd in thalamus and carries signal to cerebral cortex (postcentral gyrus) System is contralateral 12-39
Spinothalamic Pathway Carries sensations of pain, pressure, temperature, light touch, tickle and itch Located in the anterior and lateral columns Decussation of the second order neuron occurs in spinal cord Third order neurons arise in thalamus and continue to cerebral cortex of the postcentral gyrus 12-40
Spinoreticular Tract Pain signals from tissue injury Decussate in spinal cord and ascend with spinothalamic fibers End in reticular formation (medulla and pons) 3 rd and 4 th order neurons continue to thalamus and cerebral cortex 12-41
Spinocerebellar Pathway 1st order neurons originate in muscles and tendons 2nd order neurons ascend in ipsilateral lateral column Terminate in cerebellum (a large motor control are of the brain) Transmit proprioceptive signals from limbs and trunk 12-42
Descending (Motor) Pathways Descending tracts deliver efferent impulses from the brain to the spinal cord, and are divided into two groups Direct pathways equivalent to the pyramidal tracts Indirect pathways, essentially all others Motor pathways involve two neurons Upper motor neuron (UMN) Begins with soma in cerebral cortex or brainstem Its axon terminates ON the LMN in anterior horn Lower motor neuron (LMN) Soma in anterior horn; axon leads to muscle aka anterior horn motor neuron (also, final common pathway) 12-43
The Direct (Pyramidal) System Direct pathways originate with the pyramidal neurons in the precentral gyri (aka, primary motor area). Pyramidal neuron is the UMN; it forms the corticospinal tract (cortico =cortex; spinal - s.c.) UMN synapses in the anterior horn with LMN LMN (anterior horn motor neurons) activates skeletal muscles The direct pathway regulates fast and fine (skilled) movements Lateral corticospinal tracts: UMN decussates in pyramids of medulla Anterior corticospinal tracts: UMN decussates at the spinal cord level 12-44
The Direct (Pyramidal) System 12-45
Indirect (Extrapyramidal) System Upper motor neuron (UMN) originates in nuclei deep in cerebrum (not in cerebral cortex);.e., in brain stem, UMN does not pass through the pyramids LMN is an anterior horn motor neuron This system includes the rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts These motor pathways are complex and multisynaptic 12-46
Descending Motor Tracts Extrapyramidal Tracts Tectospinal tract (tectum of midbrain) reflex turning of head in response to sights and sounds Reticulospinal tract (reticular formation) controls limb movements important to maintain posture and balance Vestibulospinal tract (brainstem nuclei) postural muscle activity in response to inner ear signals Rubrospinal tracts originate in red nucleus of midbrain; control flexor muscles (see next slide) 12-47
Indirect (Extrapyramidal) System 12-48 b
Spinal Cord Injury Position Pain Brown Sequard Syndrome
Spinal Cord Trauma and Disorders Severe damage to ventral root results in flaccid paralysis. Skeletal muscles cannot move either voluntarily or involuntarily Without stimulation, muscles atrophy. When only UMN of primary motor cortex is damaged, spastic paralysis occurs. Spinal motor neurons remain intact, muscles continue to be stimulated irregularly by spinal reflex activity. Muscles remain healthy longer but their movements are no longer subject to voluntary control. Muscles commonly become permanently shortened. Transection (cross sectioning) at any level results in total motor and sensory loss in body regions inferior to site of damage. If injury in cervical region, all four limbs affected (quadriplegia) If injury between T1 and L1, only lower limbs affected (paraplegia) 12-50
Transverse vs Hemi Cord Syndrome
Anterior vs Posterior Cord Syndromes
Traumatic Spinal Cord Injury DUI: $100 addt l fine South Carolina Spinal Cord Injury Research Board
Spinal Cord Trauma and Disorders Spinal shock - transient period of functional loss that follows the injury Results in immediate depression of all reflex activity caudal to lesion. Bowel and bladder reflexes stop, blood pressure falls, and all muscles (somatic and visceral) below the injury are paralyzed and insensitive. Neural function usually returns within a few hours following injury If function does not resume within 48 hrs, paralysis is permanent. Amyotrophic Lateral Sclerosis (aka, Lou Gehrig s disease) Progressive destruction of anterior horn motor neurons and fibers of the pyramidal tracts. Lose ability to speak, swallow, breathe. Death within 5 yrs Cause unknown (90%); others have high glutamate levels Poliomyelitis Virus destroys AHMN Victims die from paralysis of respiratory muscles Virus enters body in feces-contaminated water (public swimming pools) 12-54
Blood Supply to Spinal Cord