L19: Spinal Reflexes Flashcards
There are excitatory and inhibitory sensory receptors acting on spinal motoneurons. In the case of the inhibitory, they must act via an __________.
interneuron
Direct synaptic contacts = ___________
Indirect synaptic contacts = ___________
Direct synaptic contacts = monosynaptic
Indirect synaptic contacts = polysynaptic
Muscle spindle is a stretch receptor. What type of changes does it sense? How is it organized in relation to the extrafusal muscle fibers? What’s so special/unique about the muscle spindle?
Muscle spindle senses changes in length. It lies parallel to extrafusal muscle fibers It is the only sensory afferent in CNS that has its own nerve supply as in it’s the only sensory afferent that receives neuronal input by gamma-motoneurons.
What’s the relationship btw gamma-motoneuron and intrafusal fiber of muscle spindle?
gamma-motoneuron innervates the muscle spindle. gamma-motoneuron effectively has no force, but it affects the stiffness of the muscle spindle, making the muscle spindle more sensitive to length changes when it’s contracted (limp).
Because the muscle spindle is located in parallel with the extrafusal fibers, it will stretch along with the muscle. The muscle spindle signals muscle length and velocity to the CNS through two types of specialized sensory fibers that innervate the intrafusal fibers. These sensory fibers have stretch receptors that open and close as a function of the length of the intrafusal fiber. What are the 2 fibers?
Group Ia afferents (also called primary afferents) wrap around the central portion of intrafusal fibers. Group Ia afferents provide information about both LENGTH and VELOCITY; they sense dynamic changes in length with intense firing w/ stretch onset then slows down. (non-linear firing pattern)
Group II afferents (also called secondary afferents) innervate the ends of the nuclear chain fibers (distal regions). Because they do not innervate the central bag region, Group II afferents signal information about muscle LENGTH only. Group II afferents detect absolute changes in length (static changes), and have steady firing. (linear firing pattern)
Starting at equilibrium with forearm and arm 90 degrees to one another holding a cup, explain the activity of intrafusal fibers when liquid is added to the cup causing elbow extension.
Muscles work as agonists:antagonists pairs. When liquid is added to cup, there’s increased load, and the spindle fibers in biceps are stretched, which fires A.Ps to spinal cord activating motoneurons to increase excitability (recruitment). Biceps will contract. Therefore, biceps contracting and triceps relaxing will restore equilibrium.
**Negative feedback to restore original condition.
What happens to spindle fibers when muscles (extrafusal muscles) contract?
-muscle contraction will unload spindle, eliminating its ability to signal additional changes in muscle length. In other words, the muscle spindle is rendered silent/ineffective. Since an unstretched/unloaded spindle is of no use to CNS, we have a system to make these spindles stiff. Gamma-motoneurons will be activated to act upon the spindle to maintain sensitivity to further changes in length.
What is golgi tendon organ (aka GTO)? How is it organized in relation to extrafusal fibers?
The golgi tendon organ senses changes in tension and is organized in series with extrafusal fibers in btw the muscle and the tendon.
A Golgi tendon organ is made up of a capsule containing numerous collagen fibers The organ is innervated by primary afferents called Group Ib fibers, which have specialized endings that weave in between the collagen fibers. When force is applied to a muscle, the Golgi tendon organ is stretched, causing the collagen fibers to squeeze and distort the membranes of the primary afferent sensory endings. As a result, the afferent is depolarized, and it fires action potentials to signal the amount of force.
Compare and contrast Group Ia and Group Ib afferents.
Group Ia is the sensory afferent in muscle spindle detecting changes in length and velocity whereas Group Ib is the sensory afferent in golgi tendon organ detecting active tension. Group Ia and Group Ib are both fast conducting, but Group Ia is faster.
Will passive force cause golgi tendon organ to fire?
No to very little; in other words, passive force has LITTLE EFFECT on GTO activity
Explain how the golgi tendon organ reflex works.
In contrast to muscle spindle…
GTO will reduce excitability of biceps & increase excitability of triceps by firing Ib inhibitory interneuron to inhibit homonymous muscle and firing dysynaptic pathway to excite antagonist muscles.
What is Flexion Crossed-Extension Reflex?
It is a reflex that serves a protective role in which limb can be withdrawn faster than voluntary rxn time. For ex., R foot stepped on nail will send sensory info to CNS, which will cause stimulated leg to flex to withdraw from nail (+ hamstrings, -quads). Info will be crossed to other side, causing left leg to extend to support (+ quads, - hamstrings).
What is the Central pattern regulator (CPG)?
It is a distributed network of neurons in the spinal cord that can produce coordinated movements in the absence of higher inputs (output without sensory feedback).
In experimental studies, when you transect spinal cord or transect brainstem at the colliculi (midbrain), hind limbs can be electrically stimulated to work. This is due to?
Central pattern regulator in spinal cord that can produce coordinated movements in the absence of higher inputs.
Can CPG ouput be modified by training?
Yes, it suggests that spinal cord has plasticity, activity dependent and could potentially be used for rehabilitation.
