Spinal Cord
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- general nervous system function
- sensory information to information processing center to motor information transmitted to muscles then to muscles
- embryonic spinal cord
- develops from caudal neural tube; plates expand both dorsally and ventrally
- alar plate neurons
- become interneurons and sprout axons that become white matter
- basal plate neurons
- become motor neurons and sprout axons to effector organs
- h shaped
- center; gray matter; central canal; mostly cell bodies
- gray matter
- cell bodies
- white matter
- myelinated tracts
- dorsal root ganglia
- develop from neural crest cells; contain cell bodies from sensory neurons; receive sensory input; axons go to dorsal spinal cord; afferent fibers from sensory neurons form dorsal root and carry information here
- spinal cord
- extends from brain stem through foreman magnum to first or second lumbar vertebra; ends just inferior to the ribs; 42 inches long, 3/4 inch thick; elastic and flexible but delicate
- protect spinal cord
- bone, spinal fluid, meninges
- meninges
- pia mater, arachnoid, dura mater
- dura mater
- single, fibrous layer, unattached to bone
- arachnoid
- delicate, weblike structure
- pia mater
- contiguous with outer layer of spinal cord
- between bone and dura
- epidural space contains fat padding and veins
- between arachnoid and pia
- subarachnoid space contains cerebral spinal fluid
- inferior cord
- conus medullaris and filum terminale
- conus medullaris
- tapering cone shaped structure
- filum terminale
- fibrous extension of conus, covered with ; anchors spinal cord to coccyx-prevents jostling of cord
- denticulate ligaments
- dura and tooth like pia that secure the cord in place
- epidural space
- cord ends between L1 and L2; subarachnoid space within allows penetration by needles without damage to to cord
- cauda equina
- collection of nerve roots at the inferior end of spinal cord angle sharply downward and inferiorly
- fetal development
- vertebral column grows faster than spinal cord; lower nerve roots catch up to exit points
- spinal nerves
- 31 pairs which all exit from the intravertebral foramen; travel to corresponding body region; define a spinal cord segment
- cervical/lumbar enlargements
- increased innervation to arms (cervical), increased innervation to legs (lumbar), function determines form
- internal structure
- slightly flattened from front to back; anterior- ventral median fissure; posterior- dorsal median sulcus(more shallow); partially divide spinal cord in half; connected by central gray commissure
- dorsal horn
- cell bodies of interneurons and projection neurons, receives efferent sensory information,sends sensory info to brain
- dorsal root ganglia axons
- may project to the dorsal horn interneurons or projection neurons; axons may go directly to tracts in white matter and then to the brain
- ventral roots
- motor neurons in this horn send their axons out through these roots that fuse to become the ventral roots of spinal nerves
- gray matter organization
- divided into 10 layers (laminae); each layer receives different information; I-VI and X are sensory; VII-IX generally are motor
- 4 zones of relative involvement of lamina
- input from somatic sensory neurons; input from visceral sensory; visceral motor neurons; somatic motor neurons
- white matter organization
- bundles of myelinated and unmyelinated axons; ascending, descending and transversely; bundles divided on each side into white columns or funiculi (dorsal, lateral and ventral)
- ascending
- up cord or to brain (sensory)
- descending
- from brain or higher cord (motor)
- transversely
- from one side to the other (commissural fibers)
- white matter tracts
- multineuron pathways (peripheral neurons, spinal cord neurons, brain neurons); decussation, relay, somatotopy and symmetry in common
- decussation
- most paths cross from one side to the contralateral side; usually some axons also travel on the ipsilateral side; sensory tracts takes place at sensory input or at the medulla; motor tracts take place at the medulla
- pyramidal tract
- motor tract of the medulla
- relays
- most pathways have a chain of two or three neurons that relay information along the pathway or tract
- somatotopy
- a precise spatial relationship among tract fibers that demonstrates an orderly mapping of the body
- receptor neurons
- for extensors are ventral to receptor neurons for the flexors
- motor information
- to the appendages is lateral to motor information to the trunk
- sensory information
- from the arm enters lateral to that of the leg; carried through different fibers in the tract
- cerebellum somatotopy
- more complex
- brain somatotopy
- extremely specific
- symmetry
- all pathways and tracts are paired; one of the pair is on each side of the spinal cord or brain
- first order neurons
- ascending pathway; from periphery
- second order neurons
- ascending pathway; with cell bodies in dorsal horn or medulla that project to brain
- third order neurons
- ascending pathway; with cells in thalamus or elsewhere that project to other parts of the brain
- dorsal column-medial lemniscus tract
- ascending tract; discriminative touch, vibration and conscious proprioception; pathway formed by paired tracts of fascicles cuneatus and fascicles gracilis and the medial lemniscus; pathway decussates in medulla
- spinothalamic tract
- ascending tract; pain, temperature and coarse touch; primary afferent neurons synapse with second order projection neurons in dorsal horn; terminates in thalamus; decussates at the level of the first order neuron
- spinocerebellar tract
- ascending tract; muscle or tendon stretch from golgi tendon organs, muscle fibers and joint receptors to cerebellum, subconscious proprioception
- medial lemniscus
- begins in the medulla and ends in the thalamus; info from thalamus transmitted to the primary somatosensory cortex
- ventral
- spinocerebellar tracts; crossed fibers that cross back again, transmit information from trunk and legs to same side of cerebellum
- dorsal
- spinocerebellar tracts; fibers do not cross; transmit information from trunk and less to same side of body to cerebellum
- direct system (pyramidal)
- descending pathway; pyramidal and corticospinal tracts only; no synapses until the ventral horn; contacts and influences the extrapyramidal system; decussates in medulla; only synapse in neutral horn on lower motor neurons
- indirect system (extrapyramidal)
- descending pathway; all brain and brain stem motor nuclei pathways except pyramidal tract; multineuronal
- motor pathways
- consist of two neurons; upper motor and lower motor
- upper motor neurons
- pyramidal cells of the motor cortex; subcortical neurons that give rise to the descending motor tracts to the ventral horn; do not project to muscles; damage=spastic paralysis (multiple sclerosis)
- lower motor neurons
- ventral horn neurons in the spinal cord ventral horn that innervate skeletal muscle; damage=flaccid paralysis (polio)
- direct system organization
- originates from cells in the primary motor cortex; fibers become in the pyramidal tract in the brain stem; regulates fast, precise and skilled movements
- indirect system organization
- influence ventral horn neurons; receive input from the premotor areas and basal ganglia; not part of the corticospinal tract
- rubrospinal tract
- indirect pathway; begins in red nucleus of midbrain and ends in ventral horn; does not synapse with direct pathway; regulates muscle tone and control flexors
- tectospinal tract
- indirect pathway; begins in the superior colliculus of the midbrain and ends in the ventral horn of the cervical spine; helps head move during visual tracking; does not synapse with corticospinal tract
- reflexes
- inborn vs learned
- inborn
- intrinsic reflex; rapid predictable response to a stimulus, unconscious (maintained by brain stem and spinal cord); postural changes, avoiding pain, controlling visceral activities
- learned
- acquired reflex; rapid predictable response to a stimulus from practice or experience; driving a car and getting dressed in the morning
- spinal reflexes processing
- serial processing and parallel processing
- serial processing
- reflex arcs; the whole system works together to bring about a predictable response
- parallel processing
- allows conscious responses; input from reflex ascends and allows decision making
- reflex arcs
- highly specific neural pathways; have five components; somatic or autonomic reflexes
- somatic reflexes
- activate muscle
- autonomic reflexes
- activate visceral effectors (smooth muscle or glands)
- receptor
- site of stimulus
- sensory neuron
- transmits impulse
- integration center
- may be monosynaptic or polysynaptic; decision center
- motor neuron
- conducts efferent impulses
- effector
- muscle fiber or gland
- myotatic reflex (stretch)
- maintains muscles length set by brain; maintains muscle tone; all monosynaptic but have polysynaptic aspects; all ipsilateral
- patellar reflex
- keeps your knees from buckling; if knees buckle, quad lengthens with conscious thought
- stretch reflex homeostasis
- positive reflex response= sensory and motor components are intact; vigor of response indicates the level of excitability of spinal cord
- hypoactive response
- peripheral nerve damage or ventral horn injury
- hyperactive response
- lesions of upper motor neurons can reduce inhibition
- spinal cord function homeostasis
- receives sensory input and transmits to brain; processes some sensory information; receives motor info from brain and transmits to muscles; reflex center; reflex learning
- spinal cord function
- damage to spinal cord produces functional loss
- cervical
- 7; head, neck, diaphragm, wrist extensors, triceps, hand
- thoracic
- 12; chest, abdominal muscles
- lumbar
- 5; legs
- sacral
- 5; bowel, bladder, sexual function
- spinal cord transection
- complete loss of sensory and motor function below level of damage
- spinal injury
- spinal shock; transient loss of all function below injury (motor and visceral, bowel and bladder, reflexes)
- cervical vertebra damage
- quadriplegia
- damage between T1 and L1
- paraplegia
- stroke
- hemiplegia
- polio
- viral attack of ventral horn cells; irreversible flaccid paralysis and atrophy; deadly; preventable with oral vaccination
- anencephaly
- without brain; cerebrum and brain stem did not develop in utero; child is vegetative; muscles flaccid and no voluntary movement
- spina bifida
- forked spine; incomplete formation of vertebral arches (lumbosacral); maternal folic acid inadequacy diet