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Spinal Cord Spinal Cord • • • Runs through the vertebral canal Extends from foramen magnum to second lumbar vertebra Regions – – – – – • Cervical Thoracic Lumbar Sacral Coccygeal Gives rise to 31 pairs of spinal nerves – All are mixed nerves • • Not uniform in diameter – Cervical enlargement: supplies upper limbs – Lumbar enlargement: supplies lower limbs Conus medullaris- tapered inferior end – Ends between L1 and L2 • Cauda equina - origin of spinal nerves extending inferiorly from conus medullaris. • Connective tissue membranes Meninges – Dura mater: outermost layer; continuous with epineurium of the spinal nerves – Arachnoid mater: thin and wispy – Pia mater: bound tightly to surface • Forms the filum terminale – anchors spinal cord to coccyx • Forms the denticulate ligaments that attach the spinal cord to the dura • Spaces – Epidural: external to the dura • Anesthestics injected here • Fat-fill – Subdural space: serous fluid – Subarachnoid: between pia and arachnoid • Filled with CSF • Cross Section of Spinal Cord Anterior median fissure and posterior median sulcus – deep clefts partially separating left and right halves • • Gray matter: neuron cell bodies, dendrites, axons – Divided into horns • Posterior (dorsal) horn • Anterior (ventral) horn • Lateral horn White matter – Myelinated axons – Divided into three columns (funiculi) • Ventral • Dorsal • lateral – Each of these divided into sensory or motor tracts Cross section of Spinal Cord • • Commissures: connections between left and right halves – Gray with central canal in the center – White Roots – Spinal nerves arise as rootlets then combine to form dorsal and ventral roots – Dorsal and ventral roots merge laterally and form the spinal nerve Organization of Spinal Cord Gray Matter • Recall, it is divided into horns – Dorsal, lateral (only in thoracic region), and ventral • Dorsal half – sensory roots and ganglia • Ventral half – motor roots • Based on the type of neurons/cell bodies located in each horn, it is specialized further into 4 regions – – – – Somatic sensory (SS) - axons of somatic sensory neurons Visceral sensory (VS) - neurons of visceral sensory neur. Visceral motor (VM) - cell bodies of visceral motor neurons Somatic motor (SM) - cell bodies of somatic motor neurons Gray Matter: Organization White Matter in the Spinal Cord • Divided into three funiculi (columns) – posterior, lateral, and anterior – Columns contain 3 different types of fibers (Ascend., Descend., Trans.) • Fibers run in three directions – Ascending fibers - compose the sensory tracts – Descending fibers - compose the motor tracts – Commissural (transverse) fibers - connect opposite sides of cord White Matter Fiber Tract Generalizations • Pathways decussate (most) • Most consist of a chain of two or three neurons • Most exhibit somatotopy (precise spatial relationships) • All pathways are paired – one on each side of the spinal cord White Matter: Pathway Generalizations Descending (Motor) Pathways • Descending tracts deliver motor instructions from the brain to the spinal cord • Divided into two groups – Pyramidal, or corticospinal, tracts – Indirect pathways, essentially all others • Motor pathways involve two neurons – Upper motor neuron (UMN) – Lower motor neuron (LMN) • aka ‘anterior horn motor neuron” (also, final common pathway) Pyramidal (Corticospinal) Tracts • Originate in the precentral gyrus of brain (aka, primary motor area) – I.e., cell body of the UMN located in precentral gyrus • Pyramidal neuron is the UMN – Its axon forms the corticospinal tract • UMN synapses in the anterior horn with LMN – Some UMN decussate in pyramids = Lateral corticospinal tracts – Others decussate at other levels of s.c. = Anterior corticospinal tracts • LMN (anterior horn motor neurons) – Exits spinal cord via anterior root – Activates skeletal muscles • Regulates fast and fine (skilled) movements Corticospinal tracts 1. 2. 3. 4. Location of UMN cell body in cerebral cortex Decussation of UMN axon in pyramids or at level of exit of LMN Synapse of UMN and LMN occurs in anterior horn of s.c. LMN axon exits via anterior root Extrapyramidal Motor Tracts • Includes all motor pathways not part of the pyramidal system • Upper motor neuron (UMN) originates in nuclei deep in cerebrum (not in cerebral cortex) • UMN does not pass through the pyramids! • LMN is an anterior horn motor neuron • This system includes – – – – Rubrospinal Vestibulospinal Reticulospinal Tectospinal tracts • Regulate: – Axial muscles that maintain balance and posture – Muscles controlling coarse movements of the proximal portions of limbs – Head, neck, and eye movement Extrapyramidal Tract Note: 1. UMN cell body location 2. UMN axon decussates in pons 3. Synapse between UMN and LMN occurs in anterior horn of sc 3. LMN exits via ventral root 4. LMN axon stimulates skeletal muscle Extrapyramidal (Multineuronal) Pathways • Reticulospinal tracts – originates at reticular formation of brain; maintain balance • Rubrospinal tracts – originate in ‘red nucleus’ of midbrain; control flexor muscles • Tectospinal tracts - originate in superior colliculi and mediate head and eye movements towards visual targets (flash of light) Main Ascending Pathways • The central processes of first-order neurons branch diffusely as they enter the spinal cord and medulla • Some branches take part in spinal cord reflexes • Others synapse with second-order neurons in the cord and medullary nuclei Three Ascending Pathways • The nonspecific and specific ascending pathways send impulses to the sensory cortex – These pathways are responsible for discriminative touch (2 pt. discrimination) and conscious proprioception (body position sense). • The spinocerebellar tracts send impulses to the cerebellum and do not contribute to sensory perception Nonspecific Ascending Pathway • • • • • • Include the lateral and anterior spinothalamic tracts Lateral: transmits impulses concerned with pain and temp. to opposite side of brain Anterior: transmits impulses concerned with crude touch and pressure to opposite side of brain 1st order neuron: sensory neuron 2nd order neuron: interneurons of dorsal horn; synapse with 3rd order neuron in thalamus 3rd order neuron: carry impulse from thalamus to postcentral gyrus Specific and Posterior Spinocerebellar Tracts • Dorsal Column Tract 1. AKA Medial lemniscal pathway 2. Fibers run only in dorsal column 3. Transmit impulses from receptors in skin and joints 4. Detect discriminative touch and body position sense =proprioception • 1st order neuron - a sensory neuron • synapses with 2nd order neuron in nucleus gracilis and nucleus cuneatus of medulla • 2nd order neuron.- an interneuron • decussate and ascend to thalamus where it synapses with 3rd order neuron • 3rd-order (thalamic neurons) •transmits impulse to somatosensory cortex (postcentral gyrus) Spinocerebellar Tract • Transmit info. about trunk and lower limb muscles and tendons to cerebellum • No conscious sensation Spinal Cord Trauma and Disorders • Severe damage to ventral root results in flaccid paralysis (limp and unresponsive) • Skeletal muscles cannot move either voluntarily or involuntarily • Without stimulation, muscles atrophy. • When only UMN of primary motor cortex is damaged • spastic paralysis occurs - muscles affected by persistent spasms and exaggerated tendon reflexes • 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) 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 anterior horn motor neurons • Victims die from paralysis of respiratory muscles • Virus enters body in feces-contaminated water (public swimming pools) 2 Primary Systems Dorsal column-Medial Lemniscal System Corticospinal Tract Dorsal Column (SC) -Medial Lemniscal (brain stem) System • 1° sensory function – brings info from sensory receptors in the periphery all the way to the 1° somatic sensory cortex. • Refer to the postcentral gyrus of parietal lobe! • Via the SC, brainstem, and thalamus. • A 3-neuron-circuit (sites of synaptic contact): Dorsal Column (SC) -Medial Lemniscal (brain stem) System i. DRG cells – bipolar (pseudounipolar) neurons receive info from peripheral sensory receptors and bring it to SC and bs, where the info is 1st processed (through dorsal column). *synapse at relay nucleus in medulla: dorsal column nucleus. ii. Axons of these neurons from the dorsal column nucleus cross over (decussate) here at the medulla and continue as the medial lemniscus thalamus. iii. These next thalamic neurons send their axons into the internal capsule (white mattter underlying the cortex) synapse at 1° somatic sensory cortex. Corticospinal Tract • This pathway serves a 1° motor function – bringing information down from the primary motor cortex (where?) all the way down to skeletal muscle in the periphery (activating muscle contraction). • This pathway contains only 1 synapse (unlike the DCML system, which contains __ (how many)? Corticospinal Tract i. 1° motor cortical neuron (pre-central gyrus of the frontal lobe) internal capsule ventral surface of midbrain pyramids on ventral surface of medulla. These axons decussate at caudal medulla and travel in the lateral column of the SC synapse at motor neurons in the ventral horn these axons travel to the spinal nerve (through ventral nerve roots) to synapse at the muscles. Overview of SC Anatomy –as it relates to these systems • Gray matter: - dorsal (sensory) and ventral (motor) horns. - intermediate zone – integration of sensory and motor functions (inter-neurons, reflexes), which serve direct spinal reflexes. • White matter: - nerve tracts (myelin) - dorsal column (contains dorsal column) - lateral column (contains motor descending axons) - ventral column (will discuss later). Many of the CNS slices we will view, such as this next slide, are myelin-stained – so white mater appears dark. Central canal – lowest component of ventricular system Myelin-Stained Spinal Cord Sect.