Ch 3 - Motor Systems Flashcards
Posture & Movement depend on a combination of
- Involuntary reflexes coordinated by the spinal cord
- Voluntary actions controlled by higher brain center
- It is achieved by contraction & relaxation of various muscle groups
Motor Unit
Single motor neuron and the muscle fibers that it innervates
Degree of innervation & level activity of Fine Control
- Few fibers innervated
- Low threshold & hence fire 1st
- ex: ocular muscles
Degree of innervation & level activity of Larger movements
- Many fibers innervated
- High threshold, hence fire last
- ex: leg muscles
Motor neuron pool
Set of motor neurons innervating fibers within the same muscle
Size Principle
More motor units = greater tension/contraction
More motor units = greater tension/contraction
Size principal
Extrafusal fibers
- Bulk of the muscle
- Provides the greater force of contraction
- Innervated by alpha-motor neurons
Intrafusal fibers
- Smaller than & parallel to extrafusal fibers
- Generate insignificant force compared to extrafusal fibers
- Form muscle spindles (stretch receptors)
- Innervated by gamma motor neurons, Ia & II fibers
Muscle Spindles
- Stretch receptors
- Groups Ia & II afferents which are arranged in parallel to extrafusal fibers
- Detect changes in muscle length
Group Ia Afferents
Detect rate of change in muscle
Detect rate of change in muscle
Ia afferents
Group II Afferents
Detect actual change in muscle length
Detect actual change in muscle length
Group II Afferents
Golgi Tendon Organs
- Group Ib afferents which are arranged in series w/ extrafusal fibers
- Detects tension
Group Ib afferents that detect tension
Golgi Tendon Organs
How Does the Spindle Work?
Muscle spindle reflexes oppose (correct for) increases in muscle length (stretch). When a muscle is stretched, both extrafusal & intrafusal fibers are lengthened due to parallel arrangement. Increase in length of the intrafusal fibers is detected by the sensory afferent fibers innervating them. Group Ia afferents detects rate of change via nuclear bag & chain. Group II afferents detects change in length of fiber via nuclear chain. Group Ia & II fibers sends sensory info to the spinal cord, synapsing w/ the alpha motor neurons, which then innervate the extrafusal fibers to contract/shorten & return to normal length. Gamma motor neurons are coactivated w/ the alpha motor neurons. Ensures the muscle spindle will remain sensitive to changes in muscle length even during the contraction.
Gamma motor neurons
Ensures that the muscle spindle will remain sensitive to changes in muscle length even during the contraction
Stretch Reflex
- Monosynaptic
- aka myotatic reflex
- Muscle is stretched
- Innervated by group Ia & II fibers afferents
- Simultaneously, group Ia & II fibers synapses w/ alpha motor neuron causes contraction of synergistic muscle & synapses w/ interneuron causing relaxation of antagonistic muscle. The synergistic muscle returns to normal length. Firing rate of the group Ia afferents returns to baseline
- Ex: Knee-jerk reflex; tapping the patellar tendon, causing the quadriceps muscle to stretch
Stretch Reflex has how many synapses?
Monosynaptic
Golgi Tendon Reflex
- Dysynaptic
- Inverse of myotatic reflex
- Innervated by group Ib afferent fibers
- Group Ib afferents stimulate the inhibitory interneurons in spinal cord. Inhibition of alpha motor neurons results in relaxation of synergistic muscle
- Simultaneously, the antagonistic muscle contracts
- Ex: clasp-knife reflex = This reflex is abnormal and occurs when there is an increase in muscle tone
Flexor-Withdrawal Reflex
- Always for withdrawal from dangerous situations
- Polysynaptic
- Innervated by group II, III, IV afferents
- Ipsilateral flexion = relaxation of extensors & contraction of flexors
- Contralateral extension = contraction of extensors & relaxation of flexors
- After discharge. A persistent neural discharge. Contracted muscles remain contracted for a period of time after the reflex is activated
Extrafusal fibers are innervated by
Alpha-motor neurons
Intrafusal fibers are innervated by
Gamma, Ia & II motor neurons
Golgi tendon organs are innervated by
Group Ib neurons
Function of gamma motor neurons
Ensures the muscle spindle will remain sensitive to changes in muscle length even during the contraction
Golgi tendon reflex is innervated by
Group Ib
Is golgi tendon reflex mono-, dy, or poly snaptic?
Dysynaptic
The Flexor-Withdrawal Reflex is mono-, dy, or poly synaptic?
Polysnaptic
Flexor-Withdrawal Reflex is innervated by
Group II, III, & IV
