reflexes the typical pattern of a neural “circuit” is an arc. sensory afferents integration in...
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Reflexes
• The typical pattern of a neural “circuit” is an arc.
sensory afferents integration in CNS motor efferents effector tissue
• Reflexes are the simplest form of integration by CNS.– spinal cord and brain stem only– monosynaptic– polysynaptic
• Somatic reflexes– effector tissue = skeletal muscle
Fig. 13.21
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Skeletal Muscle Proprioceptors
• Muscle Spindles– detect muscle length
• inhibit muscle stretch
• Golgi Tendon Organs– detect muscle tension
• inhibit over-contraction
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Golgi Tendon Organ
Fig. 13.23
Type Ib fibers
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Muscle Spindlesfus- = spindle intrafusal fibers – specialized muscle fibers of the spindleextrafusal fibers – regular skeletal muscle fibers
A
AType II fiberType II fiber
Type Ia fiber
Fig. 13.20
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• The (Ia) sensory nerve ending is a stretch receptor, wrapped around the center of an intrafusal fiber.
• The intrafusal fibers have sarcomeres only on their ends.
• A stimulation of intrafusal fibers will cause contraction of the ends and stretch of the middle.
• There is always some tonic discharge by the A motor neurons.
• Therefore, the stretch receptors are always slightly stimulated – warmed up and ready to respond to any stretch of the muscle.
Fig. 6-3 Ganong
Muscle Spindles
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Stretch Reflex
• a monosynaptic reflexa monosynaptic reflex
• e.g., the knee-jerk reflex
• negative feedback• ipsilateral stimulation
of extension (simultaneous inhibition
of flexion – polysynaptic)
A
Fig. 13.21
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Polysynaptic Reflexes
• via interneurons– sensory neuron from skin
[interneuron(s)]
A motor neuron
• examples– withdrawal (flexor) reflex– crossed extensor reflex
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Withdrawal Reflex + = stimulatory = stimulatory
- = inhibitory= inhibitory
Fig. 13.22
• a flexor reflex– ipsilateral
• stimulation of flexion
• inhibition of extension
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Crossed Extensor
Reflex
Fig. 13.22
• an extensor reflex– contralateral
• stimulation of extension
• inhibition of flexion
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Somatic Motor Pathways(from brain)
• organizing principles– phylogenetic
• lateral and medial motor systems– medial tracts are more primitive (see ventral/anterior columns)
» control proximal muscles: posture and gross movements– lateral tracts are more advanced
» control distal muscles: fine, skilled movements
– clinical• pyramidal and extrapyramidal systems
– pyramidal system (direct pathway)– extrapyramidal systems (indirect pathways)
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Somatic Motor System
• The frontal lobe is a major control center for somatic motor functions.– precentral gyrus
• 30-60% of cortical motor fibers originate here.
Fig. 14.23
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Pyramidal System
• direct pathway– The axon of one
neuron travels from the cerebral cortex to lower motor neurons (such as the A motor neurons).
– two paths• lateral corticospinal
tracts• ventral corticospinal
tracts
Fig. 13.6
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• lateral corticospinal tracts– lateral motor system– major importance only in primates– precise control of voluntary
movements in distal muscles– In humans, 80-90% of cortical
motor fibers travel in these tracts.– cross over in the pyramids of the
medulla• giving the “pyramidal” system its
name
• ventral corticospinal tracts– medial motor system– cross over (via interneurons) in
the spinal cord, at the level of the lower motor neuron
Fig. 12-2Ganong
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Medullary Pyramids
Fig. 14.27
Fig. 14.9