Spinal Mechanisms And Reflexes

Question 1

Subsystems of neural circuits responsible for control of movement

Answer 1

. 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)

Question 2

Spinal interneurons

Answer 2

. 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.

Question 3

Local motor control

Answer 3

. Provide relay btw cortical motor program and lower motor neurons
. Use proprioceptive info from sensory receptors in mm., tendons, and joints

Question 4

Alpha motor neurons

Answer 4

. 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

Question 5

Gamma motor neurons

Answer 5

. 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

Question 6

Muscle spindles

Answer 6

. 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

Question 7

Muscle spindle structure

Answer 7

. 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)

Question 8

Muscle spindle innervation

Answer 8

. Afferent: primary endings and secondary endings

. Efferent: motor innervation by gamma motor neurons

Question 9

Primary afferent ending

Answer 9

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

Question 10

Afferent secondary endings

Answer 10

. Primarily innervates nuclear chain fibers
. Not very sensitive to the onset of stretch
. Very sensitive to absolute length

Question 11

Length monitoring

Answer 11

. 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

Question 12

Golgi tendon organs

Answer 12

. Monitor tension in muscle

. Depends on muscle length, load, and degree of muscle fatigue

Question 13

Golgi tendon structure

Answer 13

. Located ant junction of tendon w/ its assoc. muscle

Question 14

Golgi tendon organ innervation

Answer 14

. 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.

Question 15

Tension monitoring system

Answer 15

. 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

Question 16

Why does passive stretching not activate Golgi tendon organs?

Answer 16

. 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

Question 17

Differences btw muscle spindles and golgi tendon organs

Answer 17

. Spindle: info on length and dynamic changes in length

. Golgi: muscle tension

Question 18

Large afferent fiber neuropathy

Answer 18

. Causes gait impairment and gross errors in positioning and force

Question 19

Spinal reflex characteristics

Answer 19

. Fast
. Automatic
. Stereotyped: specific stimulus elicits predictable result

Question 20

Stretch reflexes

Answer 20

. 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

Question 21

Myotactic reflex

Answer 21

. 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

Question 22

Autogenic excitation of stretch reflex

Answer 22

. 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.

Question 23

Synergistic activation of stretch reflex

Answer 23

. 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.

Question 24

Reciprocal innervation of stretch reflex

Answer 24

. 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.

Question 25

Clinical stretch reflexes

Answer 25

. Patellar tendon stretched and activates group Ia primary afferents in mm. Spindles of quads
. Burst of APs enters spinal cord and monosynaptically activates alpha-motor neuron of quads and assoc. synergistic extensor mm.
. Also may be inhibition of assoc. antagonistic mm.
. Result is active contraction of the upper leg mm.

Question 26

Negative feedback control loops in spinal cord

Answer 26

. Common
. Maintain stability of a system by returning variable to setpoint or dampening oscillations in perturbed system
. Muscle spindles and Golgi tendon organs participate in neg. feedback control of mm. Length and tension

Question 27

Recurrent inhibition

Answer 27

. Motor neuron is excited by interneurons or sensory afferent and Renshaw cell is activated
. Cell synapses back onto same body of alpha-motor neuron and dec. that neuron’s activity
. When firing rate of motor neuron inc. the recurrent inhibition via renshaw cell inhibits the inc. in firing rate
. Helps stabilize firing of neuron

Question 28

Renshaw cells

Answer 28

Inhibitory interneurons that use Gly as NT
. Located in spinal cord and produce recurrent inhibition of motor neurons
. Dampens the changes in activity of motor neurons controlling synergistic and antagonistic mm. In reflex arc

Question 29

Tetanus toxin effect on reflexes

Answer 29

. Selectively inhibit the release of Gly by Renshaw cells

. Inc. in muscle tension and rigidity

Question 30

Strychnine

Answer 30

. Poison that acts on Renshaw cell’s ability to control alpha-motor neuron firing by binding to Gly receptors on motor neuron
. Leads to tetanic contractions

Question 31

Reciprocal inhibition

Answer 31

. Relaxation of antagonistic mm. During movements enhances speed and efficiency
. Simplifies the control of voluntary mm. Since higher centers to not have to send separate commands to opposing mm.

Question 32

Co-contraction

Answer 32

. Prime mover and antagonistic mm. Contract at same time
. Has stiffening effect on joint
. Most useful when precision and joint stabilization are critical

Question 33

Tendon reflex

Answer 33

. Inverse myotactic reflex/golgi tendon reflex
. Contraction of extrafusal m. Fibers inc. tension in attached golgi
. Group Ib afferents enter spinal cord and synapse on inhibitory interneurons in spinal cord
. Autogenic inhibition occurs
. Group Ib also excite alpha motor neurons that innervate antagonistic mm.
. Reflex reduces m. Tension or contributes to fine adjustments in force of m. Contraction during ordinary motor activities
. Complementary to stretch reflex in coordinated control of limb movement

Question 34

Autogenic inhibition

Answer 34

. Activation of inhibitory interneurons inhibits the firing of alpha-motor neuron that innervates contracting homonymous and possibly synergistic m.

Question 35

Flexion reflex pathways

Answer 35

. Spinal pathway (crossed-extension reflex)

. Flexor-withdrawal

Question 36

Spinal pathway for flexor reflexes

Answer 36

. Input causes excitatory spinal interneurons to activate alpha motor neurons that innervate flexors of ipsilateral limb
. Same afferents cause inhibitory spinal interneurons to inhibit alpha motor neurons in extensors
. Commissural interneurons evoke opposite pattern of motor activity in contralateral side of spinal cord
. Causes extension of contralateral limb that helps maintain balance

Question 37

Flexor withdrawal reflex

Answer 37

. Strong nociceptors stimulus resulting in reflex withdrawal of affected limb
. Protective reflex
. Reflex is directed toward area being stimulated and is tailored to the intensity of stimulus
. Modification of the reflex so it reflects stimulus is called local sign