spinal cord: meninges the spinal meninges (dura mater, arachnoid mater, and pia mater) are layers of...
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Spinal Cord: Meninges• The spinal meninges (dura
mater, arachnoid mater, and pia mater) are layers of connective tissue that protect the spinal cord and supply it with nutrients
• They are continuous with the cranial meninges, which perform the same functions for the brain
• Between the arachnoid mater and the pia mater is the subarachnoid space, which contains cerebrospinal fluid (CSF)
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Spinal Cord: Spinal Tap• A spinal tap (lumbar puncture) is a
procedure in which a needle is inserted into the arachnoid space in the lumbar region to withdraw CSF or administer medication
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Spinal Cord: External Anatomy• There are 31 pairs of spinal nerves (8
cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal)
• Bundles of axons called roots connect each nerve to the spinal cord
• The dorsal root is sensory, the ventral root is motor
• Each dorsal root has a “swelling” composed of cell bodies of sensory neurons, called the dorsal root ganglion
• Nerves to and from the upper limbs form the cervical enlargement, nerves to and from the lower limbs form the lumbar enlargement
• The spinal cord tapers at its end to form the conus medullaris
• Lumbar, sacral, and coccygeal nerves hang below the conus medullaris to form the cauda equina (“horse’s tail”)
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Spinal Cord: Internal Anatomy• The white matter is
permeated by the anterior median fissure and the posterior median sulcus
• The gray matter forms an “H” or butterfly, with the central canal in the middle of the gray commissure
• The ventral horns contain somatic motor nuclei, whereas the dorsal horns contain somatic and autonomic sensory nuclei
• The anterior white commissure connects the white matter on right and left sides
• The ventral and dorsal gray horns divide the white matter into the ventral white columns, dorsal white columns, and lateral white columns
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Spinal Nerve Anatomy
• Nerves are similar in organization to muscles
• Groups of axons are bundled in fascicles
• The entire nerve is wrapped in the epineurium
• The perineurium surrounds each fascicle
• The endoneurium surrounds each axon
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Spinal Nerve Anatomy
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Spinal Cord Tracts
• Spinal tracts are the “highways” for information traveling between the brain and the body
• Sensory tracts take information from sensory organs to the brain
• Motor tracts carry impulses from the brain to muscles and target organs
• Spinal cord gray matter is a site where excitatory and inhibitory impulses are summed
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Dermatomes• Dermatomes provide sensory input to the
CNS via the dorsal roots of spinal nerves, or via the trigeminal nerve (cranial nerve V)
• Can be used diagnostically to assess spinal nerve damage
• Can explain referred pain
• Can be used to determine how to administer anesthesia to portions of the body (e.g. phantom pain)
• May be a remnant of our earlier segmentation
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Reflex Arcs
• Integration takes place in the spinal cord, conveying the impulse to a motor neuron through a monosynaptic or polysynaptic reflex arc
• The motor neuron conveys the impulse to the part of the body that will respond
• The effector organ (an organ, muscle, or gland) responds to the motor impulse
• Somatic reflexes involve skeletal muscles, autonomic reflexes involves smooth muscle and glands
• The pathway followed by a nerve impulse that produces a reflex is a reflex arc
• A sensory receptor responds to a stimulus
• A sensory neuron conveys the impulse to the spinal cord
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Reflex Arcs
• Reflex Arcs provide an illustration of homeostasis
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Stretch Reflex
• An excitatory motor impulse is conveyed along the motor neuron to the muscle, which contracts
• The sensory neuron also synapses with another motor neuron
• An inhibitory motor impulse is sent to the antagonistic muscle
• The stretch reflex occurs in response to stretching of muscle
• The stretch reflex is monosynaptic and ipsilateral
• Stretching simulates the muscle spindle organ
• Impulse is sent along a somatic sensory neuron to the dorsal horn
• In the spinal cord the sensory neuron synapses with a motor neuron in the ventral horn
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Stretch Reflex• The muscle spindle organ
“notifies” the spinal cord if it is being stretched
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Crossed Extensor Reflex
• Motor neurons on the opposite side cause extension of the opposite limb
• Inhibitory motor neurons cause inhibition of necessary antagonistic muscles on both sides (not shown)
• The crossed extensor reflex is polysynaptic and contralateral
• The crossed extensor reflex is an intersegmental reflex
• Painful stimulus to right foot sends impulse along sensory neuron
• In the spinal cord the sensory neuron synapses with several neurons on the same side and on the opposite side
• Motor neurons on the same side cause flexion of the limb
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Movements of the Lower Leg
Involuntary movement
Voluntary movement