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Anatomy and Physiology Biology 2401 Chapter-13 The Spinal Cord and Spinal Nerves Spinal Cord – Location, Protection and Functions Location: • Placed in the vertebral canal • Extends from just above the foramen magnum to between L1 and L2 vertebrae Functions: • Center of reflexes – quick response to stimuli • Integration (summation of inhibitory and excitatory) nerve impulses • Has tracks for upward and downward travel (to the brain) of sensory and motor information Protection: • Bony protection – Vertebrae that form a column around the spinal cord • Connective tissue membranes – meninges – Outer Dura mater – Middle Arachnoid mater – Inner Pia mater Spinal Cord – Meninges and Spaces Going from outside to inside: • • • • • • • • Vertebrae – cervical, thoracic and first lumbar Epidural space – filled with fat Dura mater – membrane that forms a jacket Subdural space – minimal space, filled with interstitial fluid Arachnoid mater – membrane that almost sticks to dura mater Subarachnoid space – filled with Cerebrospinal fluid (CSF) Pia mater – membrane that adheres to the spinal cord – extends inferiorly (filum terminale) to anchor spinal cord to coccyx – extends laterally (denticulate ligaments) hold spinal cord inside vertebral column Spinal cord Spinal Cord – External Aantomy Cervical and lumbar enlargements – where the nerves to the upper and lower extremities come out Conus medullaris – cone-shaped end of spinal cord Filum terminale – thread-like extension of pia mater – stabilizes spinal cord in canal Cauda equina (horse’s tail) – dorsal & ventral roots of lowest spinal nerves Spinal segment – area of cord from which each pair of spinal nerves arises Spinal Cord – Internal Structure Anterior median fissure: • deeper groove on the anterior surface of the spinal cord Posterior median sulcus: • shallow groove on the posterior side of the spinal cord Roots: bundles of axons that enter or exit the spinal cord – Posterior root - sensory axons that enter the spinal cord • Posterior root ganglion – contains the cell body of the sensory neuron – Anterior root - motor axons that exit the spinal cord Spinal Cord – Internal Structure Gray matter: Shaped like the letter H or a butterfly • Posterior gray horn – where sensory fibers enter the spinal cord • Anterior gray horn – where the motor neurons are located and motor fibers exit the spinal cord • Lateral gray horn – where the association or interneurons are located • Gray commissure – connects the right and the left sides of the spinal cord • Central canal – located in the gray commissure and contains CSF Spinal Cord – Internal Structure White matter: – surround the gray matter – divided into columns – anterior, posterior and lateral columns – each column contains tracts • Tracts are bundles of nerve fibers that have a common origin or destination and carry similar information – Ascending tracts…take impulses from spinal cord to the brain – Descending tracts…bring impulses from brain to the spinal cord Spinal Nerves 31 pairs of spinal nerves: C1 – C8 cervical nerves T1 – T12 thoracic nerves L1 – L5 lumbar nerves S1 – S5 sacral nerves Co1 coccygeal nerve Spinal Nerves Spinal nerves: Bundles of sensory & motor fibers plus connective tissue Formed by the combination of posterior and anterior roots Arrangement of fibers is similar to muscle fibers in a muscle • Endoneurium – connective tissue that wraps around each nerve fiber • Perineurium – connective tissue that surrounds group of nerve fibers forming a fascicle • Epineurium – connective tissue that covers the entire nerve Spinal Nerves - Branches Each spinal nerve forms 4 branches: • Posterior (dorsal) ramus - supplies to the skin & muscles of the back • Anterior (ventral) ramus – supplies to the extremities and anterior wall of the trunk • Meningeal ramus – supplies to the meninges and vertebrae • Rami communicantes (autonomic nerves) – supplies to the internal organs Spinal Nerves - Plexus Anterior (ventral) rami branches of spinal nerves form a network by joining with other anterior rami branches….plexus Found in neck, arm, lower back & sacral regions Spinal Nerves – Cervical Plexus • Formed by the anterior/ventral rami of spinal nerves (C1 to C5) • Supplies nerves to skin and muscles of head, neck & shoulders • Also forms phrenic nerve that goes to the diaphragm – damage to spinal cord above C3 causes respiratory arrest Spinal Nerves – Brachial Plexus • Formed by the anterior/ventral rami of spinal nerves (C5 to T1) • Supplies nerves to skin and muscles of neck, shoulders and upper extremities Spinal Nerves – No Thoracic Plexus No plexus is formed by the anterior/ventral rami branches of thoracic spinal nerves Become intercostal nerves to innervate intercostal muscles Spinal Nerves – Lumbar Plexus • Formed by the anterior/ventral rami of spinal nerves (L1 to L4) • Supplies to the skin and muscle of abdominal wall, external genitals & anterior/medial thigh – Injury to femoral and obturator nerves causes inability to extend leg & loss of sensation in thigh Spinal Nerves – Sacral Plexus • Formed by the anterior/ventral rami of spinal nerves (L4, L5, S1-S4) • Supplies to the skin and muscle of buttocks and lower extremities – Injury to sciatic nerves causes pain from buttocks to below the knee Reflex and Reflex Arcs Reflex: • • a fast, predictable, automatic response to changes in the environment helps to maintain homeostasis Reflex arc: • • Specific pathway followed by a nerve impulse in a reflex 5 components of reflex arc: – receptor that detects stimulus – sensory neuron that sends impulse to CNS – integrating center, the CNS – motor neuron that takes impulse away from CNS – effector that responds Reflex and Reflex Arcs - Types Reflex and Reflex Arcs - Types Spinal Reflex: • When integration takes place in the spinal cord Cranial Reflex: • When integration takes place in the brain Somatic Reflex: • Which result in contraction of skeletal muscles Autonomic/Visceral Reflex: • Where the involuntary effectors respond…smooth or cardiac muscles or glands Monosynaptic Reflex Arc: • A pathway that has only one synapse in CNS Polysynaptic Reflex Arc: • A route that has 2 or more synapse in CNS Ipsilateral Reflex Arc: • Where the sensory impulse enters the spinal cord on the same side the motor impulse exits Contralateral Reflex Arc: • Where the sensory impulse enters the spinal cord on the opposite side the motor impulse exits Clinical Applications - Disorders Neuritis – inflammation of nerves – caused by injury, chemicals or infection Shingles – infection of peripheral nerve by chicken pox virus – causes pain and line of skin rash Poliomyelitis – viral infection causing motor neuron – Paralysis of muscles possible death from cardiac failure or respiratory arrest Spinal cord injury – Due to accidents, falls, sports, or violence • Monoplegia – paralysis of one limb • Paraplegia – paralysis of both lower limbs • Quadriplegia – paralysis of all four limbs Meningitis – Inflammation of meninges