Spinal Cord.docx

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Anatomy and Cell Biology
Anatomy and Cell Biology 3319
Kem Rogers

Spinal Cord • Functions in three ways: o Through spinal nerves that attach – involved in sensory & motor innervation of body (not head) o Through ascending & descending tracts traveling within its white matter – provides a two way conduction pathway for signals between the body & the brain o Through sensory & motor integration in its gray matter – major centre for reflexes • Runs through the vertebral canal of the vertebral column • Vertebral canal - formed from the successive vertebral foramina of the articulated vertebrae • Extends form foramen magnum at base of the skull’s occipital bone superiorly to level of first or second lumbar vertebrae (L1 or L2) inferiorly • At its inferior end, spinal cord tapers into conus medullaris – cone of spinal cord • Long connective tissue – filum terminale extends from conus medullaris & attaches to coccyx inferiorly – anchoring spinal cord • 31 pairs of spinal nerves (PNS) attach to spinal cord through dorsal & ventral nerve roots • Nerves lie in the intervertebral foramina – from which they send lateral branches throughout the body • Spinal nerves are named based on the vertebral locations in which they lie o Cervical (8 nerves)  Even though there are only 7 vertebrae – first nerve exits above vertebrae  Each subsequent cervical spinal nerve exits inferior to a cervical vertebra o Thoracic (12 nerves) o Lumbar (5 nerves) o Sacral (5 nerves) o Coccygeal (1 group) • Thoracic, lumbar & sacral spinal nerves exit below the vertebra for which they are named • In cervical & lumbar regions of spinal cord – nerves to upper & lower limbs arise – cord shows obvious cervical & lumbar enlargements • Cord does not reach inferior end of vertebral column – lumbar & sacral nerve roots must descend within vertebral canal for some distance before reaching their corresponding intervertebral foramina • Collection of nerve roots at inferior end of vertebral canal cauda equine • Spinal cord does not extend the full length of the vertebral canal because of prenatal events • Until the 3 month of development – spinal cord does extend to coccyx but grows more slowly than caudal vertebral column • As vertebral column elongates – cord assumes progressively more rostral position • By the time of birth – cord ends at the level of lumbar vertebra3L • During childhood, attains its adult position – terminating at level of intervertebral disc betwe1n L 2 L o Varies from person to person from inferior margin of T12o superior margin of L 3 • Cord forms from neuroectoderm & its segmented appearance reflects pattern of adjacent somites • Spinal cord segment o Indicate region of spinal cord from which axonal processes that form given spinal nerve emerge o Each spinal cord segment is designated by spinal nerve that arises from it st st o Ex. 1 thoracic segment (spinal cord segment T ) 1 where 1 thoracic nerve (spinal nerve T ) 1 • Segments are located superior to where corresponding spinal nerves emerge through intervertebral foramina because spinal cord does not extend to the end of the column • Ex. o Spinal cord segment T i5 located at the level of vertebra T4 o Lumbar cord segment L is1located at vertebra T 11 o Sacral cord segment S is at vertebra L 1 1 • Wider laterally than anteroposterioly • Decreasing white matter caudally • Two grooves: - Divide it into right & left halves o Dorsal (posterior) median sulcus o Ventral (anterior) median fissure - wider White Matter of Spinal Cord • Outer region of white matter • Composed of myelinated & unmyelinated axons – allow communication between different parts of the spinal cord & between cord & brain • Fibres are classified by being one of three types according to which direction they carry nerve impulses: o Ascending – carry sensory information from sensory neurons of the body up to the brain o Descending – carry motor instruction form brain to spinal cord – stimulate contraction of body’s muscles & secretion from its glands o Commissural – carry information from one side of the spinal cord to the other • One each side of spinal cord is divided into three funiculi (long ropes) o Dorsal (posterior funiculus) o Ventral (anterior) funiculus o Lateral funiculus • Ventral & lateral funiculi are continuous with each other & divided by imaginary line that extends from the ventral horn of gray matter • Funiculi contain many fibre tracts • Each fibre tract is composed of axons that all have similar destinations & functions • Sensory & motor pathway integrate the PNS with CNS Gray Matter of Spinal Cord & Spinal Roots • Mixture of neuron cell bodies, short, nonmyelinated neurons & neuroglia • Shaped like letter H Crossbar of H – gray commissure o Composed of unmyelinated axons that cross from one side of CNS to the other • Within gray commissure is narrow central cavity of spinal cord – central canal • Two posterior arms of H are dorsal (posterior) horns & ventral (anterior) horns • In 3D – arms from columns of gray matter that run the entire length of spinal cord • Only in thoracic & superior lumbar segments of spinal cord – small lateral gray matter columns – lateral horns are visible • Neurons in spinal gray matter are organized functionally • Dorsal horns consist entirely of interneurons • Interneurons receive information from sensory neurons whose cell bodies lie outside the spinal cord in dorsal root ganglia & whose axons reach the spinal cord via dorsal roots - alar plate • Ventral (& the lateral) horns contain cell bodies of motor neurons that send their axons out of the cord via the ventral roots to supply muscles & glands – basal plate o Lateral horn – visceral motor neurons  blood vessels & sweat glands & organs o Ventral horn - somatic motor neurons  muscles • Interneurons are also present in the ventral horns (but not emphasized in) • Size of ventral motor horns varies along the length of spinal cord – reflecting the amount of skeletal musculature innervated at each level o Ventral horns are largest in cervical & lumbar regions segments – innervate upper & lower limbs o Segments form the previously described cervical & lumbar enlargements of spinal cord Sympathetic ganglion: many neurons from visceral motor control – for sympathetic nervous system (go out to skin & muscle) – rapid communicant – have information going into sympathetic ganglion - synapse of sympathetic information – post ganglion goes to join spinal cord • Gray matter can be further classified according to innervation of somatic & visceral regions of the body • Scheme recognizes four zones: o Somatic sensory (SS) o Visceral sensory (VS) o Visceral motor (VM) o Somatic motor (SM) • Equivalent to functional divisions of PNS Protection of Spinal Cord • Protected by: o Bone (vertebral column) o Meninges o Cerebrospinal fluid • Touch dura mater – spinal dural sheath – differs from dura around brain o Does not attach to surrounding bone o Corresponds only to the meningeal layer of the brain’s dura matter • Just external to spinal dura is epidural space (large) o Cushioning fat & network of veins • Anesthetics are often injected into epidural space to block nerve impulses in the spinal cord & relieve pain in the body regions inferior to injection • Subdural space, arachnoid mater, subarachnoid space & pia mater around spinal cord are continuous with the corresponding elements around the brain • Inferiorly, dura & arachnoid extend to the level o2 S , well beyond the end of spinal cord • Pia mater extends into coccyx – covering filum terminale • Lateral extension of pia mater called denticulate ligaments anchor spinal cord laterally to dura mater throughout the length of the cord Clinical Application • Meningitis o Inflammation of meninges caused by bacterial or viral infection o Infection can spread to the underlying nervous tissue & cause brain inflammation or encephalitis o Usually diagnosed by taking a sample of cerebrospinal fluid from subarachnoid space & examining it for microbes • Because adult spinal cord ends at level of vertebra 1 or 2 – needle can safely enter the subarachnoid space inferior to that location – lumbar puncture or spinal tap o Either between L & L or L & L
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