Muscle Receptors and Spinal Reflexes

Summary, alpha motor neurons

-- Alpha Motor Neurons receive an enormous number of descending, local and sensory inputs long their dendrites

Withdrawal Reflex

-- incorporates additional sensory fibers into reflexes. -- The leg is off the ground even before the individual is consciously aware of it. This is the same circuitry we looked at before. If you are walking while you stepped on the nail, then the muscle the was inhibited in that leg has to be activated on the other leg.

Clonus

-- involuntary rythmic contractions in response to stretch. -- produced by alternating cycles of muscle spindles and golgi tendon organs being activated. -- A sudden stretch activates muscle spindles, triggering the stretch reflex, resulting in muscle contraction. The muscle tension activates Golgi tendon organs, which result in autogenic inhibition, relaxing the muscle. As the stretch continues, a new cycle of stretch reflex begins, followed by the tendon organ inhibition -- alternating cycles can occur 3-10 times per second, and can interfere with voluntary motor functions.

Joint Receptors

-- mechanoreceptors used for voluntary and involuntary movement and for perception of body position -- Information from joint receptors is combined with info from muscle spindle, Golgi tendon organs, skin receptors -- Most receptors are rapidly adapting, bring information about a moving joint

Alpha-Gamma Co-activation

1. Alpha and gamma motor neurons are co-activated for voluntary contraction. Thus, spindle and extrafusal muscle are co-contracted to retain length/acceleration sensing 2. In case of a heavy load, let's say the foot needs to be lifted 10 inches off the ground but the extrafusal fibers don't have enough input to do it, then the gamma fibers which originally lifted the 10 inches need will to supplement the input and stimulate the alpha motor neurons until they send enough stimulus to lift the foot 10 inches off the ground.

Group Ia and II sensory fibers respond differently to passive stretching of the muscle fiber:

1. Group IA respond to changes in length (dynamic status) These type of muscle fibers give you velocity. 2. Group II responds to the length (static parameter) These fibers give you actual length.

Neurogenic Diseases

1. Lower Motor Neuron diseases - e.g., peripheral neuropathies - axonal injuries, acute de-nervations 2.Upper Motor Neuron diseases - e.g. Multiple Sclerosis (MS), - Stroke - Amyotrophic Lateral Sclerosis (ALS)

Muscle tone

1. Physiologically, skeletal muscles are not in a fully relaxed state. Muscle tone is the tension present in relaxed muscle. Tone is also assessed as resistance to passive movement. 2. Spasticity is increased muscle tone that produces strong resistance to movement, and usually increases with velocity of passive movement 3. If descending inhibitory pathways are damaged, there can be dis-inhibition of γ-motor neurons, which causes the spindle to contract. This leads to continuous activity in spindle afferents and consequent contraction of the whole extrafusal muscle.

Renshaw (inhibitory) neurons and recurrent inhibition

1. This negative feedback loop may stabilize the firing rate of motor neurons. 2. Renshaw neurons are the targets of the bacterium that produces tetanus. These neurons produce glycine which keep the excitation of alpha motor neurons.

Myotatic reflex, summary:

1. activation of Ia and II afferents which then produce: 2. monosynaptic activation of α-motor neurons to the (homonymous) muscle that is stretched, 3. polysynaptic facilitation of α-motor neurons to agonist muscles, 4. polysynaptic inhibition of antagonist muscles.

How does the spindle remain functional when muscle contracts?

1. when extrafusal muscle contracts, spindle becomes lax and stops firing 2. therefore, spindle cannot report on muscle length and shortening. 3. the gamma motor neurons are much smaller than the alpha motor neurons. By stimulating both, the intrafusal muscle fibers contract the same amount that the extrafusal fibers do.

Stretch reflex is used for adjusting/correcting motor activity for a changing load:

Branching into synergistic muscles. Notice the three branches and the muscles they innervate. She is building upon the information of previous slides, these are not separate situations. The same reflex is in place as the weight of the cup increases, the synergistic and main muscle are excited while the antagonist muscle is inhibited. This system is NOT capable of making giant adjustments, for those you have to actually consciously act.

The muscle spindle

Except for the muscles of the face all the other muscles of the body have these spindles. With the exception perhaps of the mastication muscles.

Afferent axons of muscle receptors: conduction velocities

From fastest: 1. muscle spindle 2. Merkel, Meissner, Pacinian and Ruffini cells 3. Free nerve endings

Gamma Loop

Gamma loop refers to the functional circuit that includes spindle afferents, α- and γ-motor neurons, and regulated contraction of extrafusal and intrafusal muscle fibers. Gamma loop serves as a feedback loop to regulate the accuracy of a voluntary contraction.

Golgi tendon organs

Inside the tendon, the afferent Ib detect the tension inside the tendon. Senses tension not length.

Reciprocal innervation of antagonist muscle

More complex, the afferent fibers branch and synapse in an inhibitory interneuron which synapses on a motor fiber of the antagonist muscle, thus preventing the antagonist muscle from contracting. This is called reciprocal innervation.

Autogenic Inhibition by Golgi tendon organs:

Multiple inputs to spinal interneurons: - Primary sensory axons - Descending axons from brain - Collaterals of lower motor neuron axons A strongly contracting muscle produces inhibition of itself through this system. Protective mechanism, enough to avoid damage.

Is the center of the intrafusal fibers contractal or non-contractal?

Non-contractal

Comparison, active contraction:

Spindle is silent, golgi tendon organ is activated

Sequence of events in the myotatic reflex:

Stretch of muscle -- AP in afferent fibers -- posterior root ganglion -- posterior horn of spinal cord -- single synapse on motor neuron, AP activates NMJ muscle contraction 1. there is a small time delay between the firing of the Ia fibers and discharge of AMNs.

Dynamic and Static axons associated with intrafusal myofibers

The end of these muscle fibers are sarcomeres. Unlike the extrafusal fibers the intrafusal have sensory fibers. Above are the possibilities for the innervations.

Spinal pattern generator circuits

may be used for breathing, swallowing, walking motor patterns. Strong evidence exists in several mammalian species. -- the key here is inhibitory interneurons inhibiting themselves.

Comparison, passive stretch:

spindle is activated, golgi tendon organ is not. During the passive stretch the tendon does not experience tension, only the muscle does.