Spinal reflexes and upper motorneurons Flashcards
What are the Two sensory receptors associated with muscles monitor length and tension?
Muscle spindles
stretch (length) receptors
In parallel with muscle fibers
Golgi tendon organs
tension (force) receptors
In series with muscle fibers
What is Muscle spindles? and what type of receptor it has ?
– stretch receptors
i’s a fibrous, fluid filled capsule surrounds “intrafusal” muscle fibers
the spindle receives sensory and motor innervation
sensory axons have stretch-sensitive ion channels
(group la sensory neuron_
Intrafusal vs Extrafusal muscle fibres
Two fiber types are in parallel
Extrafusal fibres are responsible for force generation
Intrafusal fibres are responsible for changing spindle length (and therefore sensitivity)
Need to know where our limbs are can be achieved by
And what does it help with?
monitoring muscle length and muscle tension.
This also helps maintain constant muscle length and tension, preventing muscle overload and compensating for fatigue.
Particularly common in muscles used for fine movements (like in fingers)
Depend on stretch sensitive ion channel.
How muscle spindle encodes the length ?
Action potential rate encodes muscle length and/or velocity
Longer muscle / lengthening of muscle = higher firing rate
Shorter muscle / shortening of muscle = lower firing rate
Intrafusal fibres are responsible for
And how do are they involved to signal for change of length?
changing spindle length (and therefore sensitivity)
Fibrous capsule of the muscle spindle contains specialized “intrafusal” fibers.
They are contractile, so they have inputs from a motor neuron, but they’re also associated with sensory neurons.
These sensory neurons have stretch sensitive ion channels, to signal muscle length.
Fusiform => having a spindle shape
Gemma motor neurons and it’s difference with alpha motor neuron
Gamma motor neurons target intrafusal fibers, shortening the muscle spindle.
Alpha motor neurons target extrafusal muscle fibers, shortening bulk of muscle.
- alpha activation decreases sensory Ia activity;
- gamma activation increases sensory Ia activity.
How is Gamma co-activation important in Maintaining sensitivity of the spindles
Activation of the alpha motor neuron causes a contraction (shortening) of the extrafusal fibers. This reduces firing in the 1a axon.
Problem: after contraction of extrafusal fibers, muscle spindle is “slack” => loses its ability to signal muscle length
Co-activation of the gamma motor neuron shortens the intrafusal fibers.
This increases firing in the 1a axon.
Feedback loop model Gemma
Gamma feedback loop: tries to maintain a “set-point” of constant muscle length.
Simple feedforward system: co-activation of alpha and gamma to change set-point (like a form of adaptation).
Does CO-activation occur with unexpected change in muscle length?
NO, Co-activation only occurs when there are planned changes to muscle length. So if muscle has an unexpected change in length, the change in AP rate of the 1a sensory fibre signals that change.
Why not just have a spindle system that is sensitive throughout the complete range of muscle lengths?
Simply not sensitive enough. We have firing rate range of 0-100 spikes/s – can reasonably only code 100 different lengths. By changing sensitivity, we can effectively code 1000s of different lengths.
The myotatic reflex
Group Ia sensory neurons synapse on alpha motor neurons and interneurons.
A monosynaptic feedback loop mediates the myotatic reflex.
spinal-cord mediate reflexes are modulated by descending inhibition
The myotatic reflex is diagnostic test of :
- 1 - Normal spindle and sensory fiber function
- 2 – spinal cord synapses (and response gain)
- 3 – motorneuron function
- 4 – muscle contraction.
when tendon jerk is the largest?
and when is this relevent?
- Tendon jerk is largest when muscle length is optimal length for contraction. If muscle is short, tendon is slack. If muscle is long, spindles have high background activity and don’t signal additional stretch.
- Size of jerk is also susceptible to descending control from the brain (i.e., we can consciously inhibit it to some degree). This is the basis of the Jendrassik manouever.
- When is this relevant – maintaining posture. If body leans forwards, it stretches the ankle extensors, leading to contraction and the body leans back.
- Similarly, if the body leans too far back, ankle flexors are stretched, leading to contraction.
Reciprocal inhibition
Sensory inputs from Ia axons are used to inhibit contraction of antagonist
=> contraction of one muscle automatically relaxes the second .This also prevents the myotatic reflex from resisting intentional movement
occurs during voluntary muscle contraction
prevents the myotatic reflex to engage in voluntary movement
Reflex Action of Ia Fibre activation
- Monosynaptic excitation of homonymous muscle
- Excitation of synergist muscles
- Inhibition of antagonist muscles
- Restricted to muscles in the same “myotatic unit” (no effect at other joints).
why Myotatic reflex must be suppressed during voluntary movement?
Myotatic reflex must be suppressed during voluntary movement – otherwise the intentional contraction of one muscle, which leads to stretch of the antagonist muscle, will be counteracted.
What is Homonymous muscle, Synergist, Antagonist
Homonymous muscle = muscle from which original signal originated
Synergist = muscle performing the same action as the homonymous muscle
Antagonist = opposing muscle
primary role of muscle spindle
This reflex is not the primary role of the muscle spindles (which is to provide information about muscle length), but it illustrates a simple way that their signals can be used to modulate movement.
Golgi tendon organs signal muscle tension
where does is synapse to?
types of ion channel
what happens when tension increases?
Innervated by Ib sensory axons, which synapse onto interneurons in ventral horn.
Increased tension => increased firing rate
Golgi tendon organs are in series with extrafusal muscle fibers
have stretch-sensitive ion channels
During which type of contraction Golgi tendon organ only signal change and spindle doesn’t
- During an isometric contraction - No change in muscle length – this is what happens when we grip an object or hang from a bar.
- Thus muscle spindle doesn’t change its firing rate, but Golgi tendon organ signals change in tension.
