Download Spinal nerves

Spinal Cord
• CNS tissue is enclosed within vertebral column;
begins at foramen magnum and ends at L1 or L2
• Functions
– Provides two-way communication to and from
brain
– Contains spinal reflex centers
– Protected by bone, meninges, and CSF
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• Ends in conus medullaris
• Filum terminale (fibrous extension of conus medullaris)
covered with pia mater which extends to coccyx and anchors
the cord so it is not damaged by movements
• Cauda equina
– Collection of nerve roots at inferior end of
vertebral canal
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Spinal nerves (Part of PNS)
– 31 pairs
• Spinal nerves split into dorsal and ventral branches
or “rami” almost as soon as they leave vertebral
column.
– Spinal nerves are often named for bone or structure they
pass near
• Plexuses are groups of spinal nerves.
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• Cervical plexus (C1 to C4) serves muscles and skin of
neck, except for phrenic nerve which stimulates
diaphragm.
• Brachial plexus (C5-8, T1): serves skin and muscles of
arm & shoulder area
• Lumbar plexus (L1-4): serves skin and muscles of (upper)
leg & pelvic area.
• Sacral plexus (L4, L5, S1-4): serves skin and muscles of
(lower) leg & pelvic area.
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Cross-sectional Anatomy
• Two lengthwise grooves partially divide cord
into right and left halves
– Ventral or Anterior median fissure
– Dorsal or Posterior median sulcus
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Gray Matter-consists of cell bodies, non-myelinated
processes and neuroglia
• In cross section, looks like “H” or a butterfly
• Gray commissure—connects masses of gray matter;
encloses central canal
• Dorsal horns - interneurons that receive somatic and
visceral sensory input
• Ventral horns - some interneurons; somatic motor
neurons; axons exit cord via ventral roots
• Lateral horns (only in thoracic and superior lumbar
regions) - sympathetic neurons
• Dorsal roots – sensory input to cord
• Dorsal root (spinal) ganglia—cell bodies of sensory
neurons
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White Matter
• Divided into three white columns (funiculi) on each side
– Dorsal, lateral, and ventral
– Each spinal tract composed of axons with similar
destinations and functions
• The tracts in funiculi are typically named for start and end
points of their axonsAscending=spinal cord  brain;
Descending=brain  spinal cord.
– Lateral funiculi have a mix of ascending and descending
tracts.
• Distribution: primarily sensory tracts in posterior;
descending motor tracts in the anterior; (recall that sensory
information comes in back side (dorsal root), and motor responses
go out front door (ventral root).
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Spinothalamic Pathways (Ascending)
• Lateral and ventral spinothalamic tracts
• Provide discriminatory touch (pain, temperature, coarse
touch, pressure) and conscious proprioception
Spinocerebellar Tracts (Ascending)
• Ventral and dorsal tracts terminate in cerebellum
• Convey information about muscle or tendon stretch
to cerebellum
– Used to coordinate muscle activity
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Dorsal Column–Medial Lemniscal Pathways
(Ascending)
• Composed of paired fasciculus cuneatus and
fasciculus gracilis in spinal cord and medial lemniscus
in brain (medulla to thalamus)
• Transmit input to somatosensory cortex for
discriminative touch and vibrations
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Descending Pathways
• Reticulospinal and vestibulospinal tracts—maintain
balance
• Rubrospinal tracts—control flexor muscles
• Tectospinal tracts mediate head movements in
response to visual stimuli
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Reflexes
• rapid, involuntary, predictable motor response to stimulus
– Can involve just peripheral nerves and spinal cord or may also involve
higher brain centers
• Direct transfer of stimulus from sensory neuron to motor
neuron (sometimes with an interneuron in between) allows
for rapid response to stimuli.
• May be:
– Inborn (intrinsic)
• Example – maintain posture, control visceral activities
• Can be modified by learning and conscious effort
--Learned (acquired) reflexes result from practice or
repetition,
• Example – driving skills
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• Input travels along one pathway, called a reflex arc,
to a specific destination
• System works in all-or-none manner to produce
specific response
• Particular stimulus always causes same response
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Reflex Arc
• Components
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to
CNS
3. Integration center—either monosynaptic or
polysynaptic region within CNS
4. Motor neuron—conducts efferent impulses from
integration center to effector organ
5. Effector—muscle fiber or gland cell that
responds to efferent impulses by contracting or
secreting
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Stretch and Tendon Reflexes
• To smoothly coordinate skeletal muscle, nervous system
must receive constant proprioceptor input about
– Length of muscle (from muscle spindles)
– Muscle tone: Amount of tension in muscle and
tendons
– allows muscle to maintain correct muscle tone and
adjust it reflexively
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LABWORK
1. Describe the important structures of spinal
cord.
2. Describe cross-sectional features of spinal
cord (models and cross-section slide).
3. Describe spinal nerves and plexuses
associated with the spinal cord (models).
4. Describe reflexes and basics of reflex
physiology.
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