Spinal Mechanisms And Reflexes Flashcards
(37 cards)
Subsystems of neural circuits responsible for control of movement
. Local circuitry of spinal cord (lower motor neurons, local circuit interneurons that receive proprioceptive sensory inputs and descending projections from higher centers)
. Motor cortex and brainstem (upper motor neuron pathways)
. Cerebellum and basal ganglia (regulate activity of upper motor neurons)
Spinal interneurons
. Form networks (central pattern generators) that can produce a pattern of rhythmic outputs
. Play role in carrying out rhythmic movements that are performed relatively automatically
. Able to coordinate timing and sequence of activation and inhibition btw motor neurons innervating different mm.
Local motor control
. Provide relay btw cortical motor program and lower motor neurons
. Use proprioceptive info from sensory receptors in mm., tendons, and joints
Alpha motor neurons
. Innervate extrafusal mm. Fibers that generate force
. Cell bodies located in the ventral horn of the spinal cord, and they receive input from descending motor pathways and spinal reflex circuits
Gamma motor neurons
. Innervate intrafusal mm. Fibers that are components of mm. Spindles
. Too small to generate any significant force
. Co-activated w/ corresponding alpha motor neuron
. Cell bodies in ventral horn of spinal cord
. Receive input from descending motor pathways and spinal reflex circuits
Muscle spindles
. Muscle stretch receptors that monitor both absolute mm. Length, and rate of change of mm. Length
. Spindles contribute to kinesthesia, which is the sense of position and movement of limbs, locomotion, providing feedback to the CNS on muscle length to help the CNS correct movement errors, muscle tone, degree to which a muscle resists passive lengthening
Muscle spindle structure
. Located deep in belly of a mm.
. Innervate the capsule that surrounds intrafusal mm. Fibers
. Arranged in parallel to extrafusal fibers
. Types: nuclear chain and nuclear bag fibers
. W/in each capsule there are twice as many nuclear chain fibers (5 per capsule) than nuclear bag (2-3 capsule)
Muscle spindle innervation
. Afferent: primary endings and secondary endings
. Efferent: motor innervation by gamma motor neurons
Primary afferent ending
Innervate both chain and bad fibers
. Selectively sensitive to onset of mm. Stretch
. Discharge at slower rate if stretch is maintained
. Code for both steady state mm. Length and velocity of length change
Afferent secondary endings
. Primarily innervates nuclear chain fibers
. Not very sensitive to the onset of stretch
. Very sensitive to absolute length
Length monitoring
. External force that passively stretches a mm. Pulls on intrafusal fibers
. Activates mm. Spindle and inc. rate of stretch receptors firing
. Active mm. Contraction of extrafusal fibers removes tension from. Spindles and slows the rate of stretch receptor firing
. When mm. Shortens, there is alpha-gamma coactivation so the loss of intrafusal fiber information by contracting both intrafusal and extrafusal fibers
. Contraction of intrafusal fibers maintains tension on mm. Stretch receptors and afferent info about mm. Length continues to reach CNS
Golgi tendon organs
. Monitor tension in muscle
. Depends on muscle length, load, and degree of muscle fatigue
Golgi tendon structure
. Located ant junction of tendon w/ its assoc. muscle
Golgi tendon organ innervation
. Single group Ib afferent axon
. Axon loses its myelination upon entering the golgi tendon organ
. Branches of the axon are intertwined among the collagen fibers that connect the tendon to the mm.
Tension monitoring system
. Golgi tendon organs are connected in series w/ extrafusal fibers
. When mm. Contracts tension is placed on tendon
. Golgi tendon organ discharges in response to tension generated by contracting mm.
. Activation causes widespread inhibition of contracting mm. And stimulation of antagonistic mm.
. Protects mm. When exceptionally large and potentially damaging tension is generated
Why does passive stretching not activate Golgi tendon organs?
. Passive stretching does not greatly activate then bc during a passive stretch most energy is taken up by the more compliant muscle fibers rather than a stiff tendon so stretch doesn’t deform musculocutaneous-tendon junction
Differences btw muscle spindles and golgi tendon organs
. Spindle: info on length and dynamic changes in length
. Golgi: muscle tension
Large afferent fiber neuropathy
. Causes gait impairment and gross errors in positioning and force
Spinal reflex characteristics
. Fast
. Automatic
. Stereotyped: specific stimulus elicits predictable result
Stretch reflexes
. Automatic contractions of mm. In response to passive lengthening of same mm.
. Include contraction of homonymous mm., synergistic mm., and inhibition of antagonistic mm.
. Important for the constant automatic corrections performed during movements and static postures
Myotactic reflex
. Extrafusal and intrafusal mm. Fibers and assoc. muscle spindles are passively and simultaneously stretched
. Muscle spindle stretch causes reflex mm. Activation
. Components: autogenic excitation, synergistic activation, and reciprocal innervation
Autogenic excitation of stretch reflex
. Involves only monosynaptic connection btw group 1a mm. Spindle afferent and alpha-motor neuron that innervates same mm.
. Connections are excitatory causing contraction of same mm.
Synergistic activation of stretch reflex
. Group 1a afferents from primary mm. Make excitatory connections w/ alpha-motor neurons innervating synergistic mm.
. Number and strength of connections vary from mm. To mm. But are not as strong as autogenic excitation
. Net effect may be reflex contraction of synergistic mm.
Reciprocal innervation of stretch reflex
. Group 1a afferents make excitatory connection on inhibitory spinal cord interneurons
. Inhibitory interneurons synapse on alpha-motor neurons innervating mm. That are antagonistic to original stretched mm.
. Cause inhibition of alpha-motor neurons to antagonistic mm.
