WEEK 9 (Control of muscle movement) Flashcards
What is a motor unit?
A single motoneuron and the muscle fibers that it innervates
What does Proper control of muscle function require?
- Excitation of the muscle by SPINAL CORD ANTERIOR MOTOR NEURONS
- CONTINUOUS FEEDBACK of sensory information from each muscle to the spinal cord indicating the functional status of each muscle at each instant
What information does each muscle feed back to the spinal cord?
- Length of muscle
- Tension
- How rapidly the length or tension is changing
What are the two types of Sensory receptors?
Muscle spindles & Golgi tendon organs
What is the difference between Muscle spindles and Golgi tendon organs?
MUSCLE SPINDLES
- distributed throughout the belly of the muscle
- send info to the nervous system about MUSCLE LENGTH/RATE OF CHANGE OF LENGTH
GOLGI TENDON ORGANS
- located in the muscle tendons
- transmit info about TENDON TENSION/RATE OF CHANGE OF TENSION
Signals from Muscle spindles and Golgi tendon organs are for the purpose of ____________ muscle control
Intrinsic
They operate almost completely at a subconscious level
What are the two types of muscle fibers?
Extrafusal & Intrafusal fibers
What are the properties of Extrafusal fibers?
- Make up the majority of skeletal muscle
- Innervated by alpha motoneurons
- Used to generate force
What are the properties of Intrafusal fibers?
- Innervated by gamma motoneurons
- Too small to generate significant force
- Encapsulated in sheaths
- Form muscle spindles that run parallel to the extrafusal fibers
What are Muscle spindles?
Sensory receptors (proprioreceptors) innervated by both sensory and motor nerve fibers
What are the two types of intrafusal fibers present in muscle spindles?
Nuclear bag fibers & Nuclear chain fibers
What is the difference between Nuclear bag fibers and Nuclear chain fibers?
NUCLEAR BAG FIBERS
- larger
- nuclei are accumulated in a central “bag” region
NUCLEAR CHAIN FIBERS
- smaller
- nuclei arranged in rows “chains”
- more plentiful
What does Sensory innervation of the muscle spindle consist of?
- A single GROUP 1A AFFERENT NERVE which innervates the central region of the nuclear bag fibers and the nuclear chain fibers
- GROUP II AFFERENT NERVES which innervate nuclear chain fibers
What does Motor innervation of the muscle spindle consist of?
Two types of gamma motoneurons: DYNAMIC and STATIC
What is the difference between Dynamic γ motoneurons and Static γ motoneurons?
Dynamic γ motoneurons synapse on nuclear bag fibers in “plate endings”
Static γ motoneurons synapse on nuclear chain fibers in “trail endings” which spread out over longer distances
How are γ Motoneurons different to α motoneurons?
γ Motoneurons are smaller and slower than the α motoneurons that innervate the extrafusal fibers
What is the function of γ Motoneurons?
To regulate the sensitivity of the intrafusal muscle fibers they innervate
What is the function of Muscle spindles?
Stretch receptors that correct changes in muscle length when extrafusal muscle fibers are either shortened (by contraction) or lengthened (by stretch). Muscle spindle reflexes therefore work to return muscle to its resting length after it has been shortened or lengthened.
What are Spinal cord reflexes?
Stereotypical motor responses to specific kinds of stimuli
What is the reflex arc?
The neuronal circuit that directs this motor response
What does the reflex arc include?
- Sensory receptors
- Sensory afferent nerves which carry info to the spinal cord
- Interneurons in the spinal cord
- Motoneurons which direct the muscle to contract or relax
What is the simplest of all the spinal cord reflexes?
THE STRETCH REFLEX
It has only one synapse between the sensory afferent nerves and motor afferent nerves
What is an example of a stretch reflex?
KNEE-JERK REFLEX
It is initiated by tapping the patellar tendon, causing the quadriceps muscle to stretch
Describe the knee-jerk reflex
1) When the muscle is stretched, group Ia afferent fibers (sensory afferent nerves) in the muscle spindle are activated and their firing rate increases. They enter the spinal cord and synapse directly on and activate alpha motoneurons. This pool of alpha motoneurons innervates the homonymous muscle.
2) When alpha motoneurons are activated, they cause CONTRACTION of the muscle that was originally stretched (HOMONYMOUS MUSCLE). When the muscle contracts, it shortens, decreasing stretch on the muscle spindle. The muscle spindle returns to its original length and the firing rate of group Ia afferents returns to baseline
3) Simultaneously, info is sent from the spinal cord to cause contraction of SYNERGISTIC MUSCLES and relaxation of ANTAGONISTIC MUSCLES
Describe the Golgi tendon reflex
1) When the muscle contracts, the extrafusal muscle fibers shorten which activates the Golgi tendon organs attached to them. The group Ib afferent fibers that synapse on INHIBITORY NEURONS in the spinal cord are activated and these inhibitory interneurons synapse on the alpha motoneurons
2) When the inhibitory interneurons are activated, they inhibit firing of the alpha motoneurons producing relaxation of the homonymous muscle
3) As HOMONYMOUS muscle relaxes, the reflex causes SYNERGISTIC MUSCLES to relax and ANTAGONISTIC MUSCLES to contract
What happens when Golgi tendon organs of a muscle tendon are stimulated by increased tension?
Signals are transmitted to the spinal cord to cause INHIBITORY REFLEX EFFECTS in the respective muscle
(provides a -ve feedback mechanism that prevents the development of too much tension on the muscle)
What happens when tension on the muscle/tendon becomes extreme?
Significant inhibitory effect leads to a sudden reaction in the spinal cord that causes INSTANTANEOUS relaxation of the entire muscle (LENGTHENING REACTION)
it is a protective mechanism to prevent tearing of the muscle or avulsion of the tendon from its attachments to the bone
What is the flexor-withdrawal reflex?
A polysynaptic reflex that occurs in response to a tactile, painful or noxious stimulus
Describe the Flexor-withdrawal reflex
1) When a limb touches a painful stimulus, flexor reflex afferent fibers are activated which synapse on multiple interneurons in the spinal cord
2) On the ipsilateral side of the pain stimulus, reflexes are activated that cause flexor muscles to contract and extensor muscles to relax. This produces flexion on the IPSILATERAL SIDE.
3) On the contralateral side of the pain stimulus, reflexes are activated that cause extensor muscles to contract and flexor muscles to relax. This produces extension on the contralateral side (CROSSDEXTENSION REFLEX).
4) A persistent neural discharge (AFTERDISCHARGE) occurs in the polysynaptic reflex circuits which causes contracted muscles to remain contracted for a period of time after the reflex is activated.
What are the differences between the different types of Reflexes?
STRETCH REFLEX
NO. OF SYNAPSES: one
STIMULUS FOR REFLEX: stretch (lengthening) of the muscle
SENSORY AFFERENT FIBERS: Ia
RESPONSES: contraction of the muscle
GOLGI TENDON REFLEX
NO. OF SYNAPSES: two
STIMULUS FOR REFLEX: contraction (shortening) of the muscle
SENSORY AFFERENT FIBERS: Ib
RESPONSES: relaxation of the muscle
FLEXOR-WITHDRAWAL REFLEX
NO. OF SYNAPSES: many
STIMULUS FOR REFLEX: pain, temp etc
SENSORY AFFERENT FIBERS: II, III and IV
RESPONSES: flexion on ipsilateral side & extension on contralateral side
