Motor Output-Reflexes

Describe what happens with spinal cord damage or severed.

-1st: spinal shock for 3-4wks, characterized by: flaccid paralysis, loss of autonomic function, loss of all sensation below injury site,, no resistance in the joint to passive movement -gradually, **reflexes return and become hyperactive** -voluntary mvmt/sensation may never return

What are the components of a reflex arc?

-sense organ/receptor -afferent neuron -central connection/synapses in the CNS -efferent neuron -effector tissue

What are the fiber type innervates a somatic muscle unit? Where are somas located? What is an NMJ? What response does this type of neuron produce in the effector? What is a somatic motor unit?

A alpha motor neurons Somas in the ventral horn -NMJ or neuromuscular junctions of skeletal muscle extrafusal fibers with skeletal muscle -the A alpha motor neurons produce contraction of the muscle Somatic motor unit: alpha motor neuron and its skeletal muscle fibers

What type(s) of fibers innervate A gamma efferents (which innervate the contractile/nonsensory portions of muscle spindles)?

Both Type 1a (muscle spindle afferent) and Type 1b (Golgi tendon organ afferent)

What are involuntary motor responses called? Where are they initiated?

Called Reflexes. Initiated at spinal cord, brain stem, cerebellum, and cortex.

What happens with increased output from A gamma fibers?

Causes increased contraction of the spindle, causing increased sensitivity to stretch.

What is the difference between intrafusal and extrafusal muscle fibers?

Extrafusal: contractile fibers Intrafusal: thinner fibers in the belly of muscles that make up the muscle spindle (monitor stretch of muscle)

Where do the majority of Type 1a/type 1b fibers run?

In the DCML pathway, providing proprioception.

Where are interneurons located in the spinal cord?

In the lateral or dorsal horns.

What are the basic requirements of the monosynaptic reflex arc?

It requires: -the sensory receptor, with sensory neuron soma in the dorsal root ganglion -synapses w/ Aalpha motor neuron in the ventral horn -effector organ (skeletal muscle for somatic) -NO interneurons

Distinguish between monosynaptic and polysynaptic reflex arcs.

Monosynaptic reflex arcs **do not require interneurons** -only one synapse Polysynaptic reflex arcs **require interneurons -More than one synapse

Which of the following reflexes are monosynaptic (in their most basic form)? Polysynaptic?

Stretch/myotatic: mono Inverse myotatic: poly Flexor W/drawal: poly

What is the key reflex in the maintenance of posture? What factor mediates the reflex? Describe the pathway involved.

The Myotatic reflex/stretch reflex is mediated by muscle spindles when the muscle is stretched (ex. knee jerk). The pathway is as follows: Type Ia fibers of the muscle spindle synapse directly with A alpha efferents in the ventral horn in a monosynaptic pathway, producing contraction of the stretched muscle. Collateral synapses can also occur with interneurons in the spinal cord, producing IPSPs onto those A alpha efferents that innervate the antagonist muscle, causing relaxation of that muscle in **reciprocal innervation.** As well, A gamma efferents are stimulated by the Type 1a fibers, producing contraction of the muscle spindle in the agonist muscle/relaxation in the antagonist.

Describe the pathway of muscle spindles sensing info.

The receptor registers excess stretch--> travels via afferent fiber --> synapse in spinal cord --> efferent fiber --> muscle contracts

Describe the Inverse myotatic reflex/negative stretch reflex. What type of receptors mediate the pathway? Give a general example of where this pathway could be found.

The inverse myotatic reflex, overall, results in extension of the agonist, and is mediated by Golgi tendon organs. These receptors respond to muscle tension, according to the following: contracted muscle -->golgi tendon organs at the ends of the muscle to lengthen-->golgi tendon organs say: relax, I am lengthened (muscle contracted) too much/far! Pathway: Type Ib Aalpha afferents (which mostly travel in DCML), have collateral branches that will synapse on interneurons in the spinal cord, producing IPSPs unto the agonist A alhpa motor neuron (the mucsle initiating the response), resulting in relaxation of the agonist. As well, EPSPs are produced unto the antagonist A alpha motor neuron, resulting in contraction of the antagonist. An example of this reflex: you pick up smtg that is too heavy, and you drop it.

What do A gamma efferent fibers innervate?

They innervate intrafusal fibers of the muscle spindle, determining the sensitivity of the spindle to stretch.

Describe the Flexor Withdrawal reflex/pain reflex/withdrawal reflex/nociceptive reflex.

This reflex is called **prepotent**, meaning it takes precedence over any other reflex activity at the moment. Pathway: Adelta nociceptor fiber from LST pathway synapses with its 2nd order neuron in the dorsal horn gray matter of the spinal cord. (The 2nd order neuron caries the pain info to the thalamus). **Collaterals from the PRIMARY LST Adelta pain nociceptor synapse with interneurons that: -cause flexion of the ipsilateral limb in **flexion reflex** (reciprocal innervations inhibit the extensors/antagonists of ipsilateral limb). -cause extension of the contralateral extensors limb (**crossed-extensor reflex**), and reciprocal innervations inhb the activity of contralateral flexors. -overall, ipsilateral limb contracts, and contralateral limb extends to help equalize muscle loads and stabilize the body

What types of intrafusal fibers afferently innervate muscle spindles?

Type Ia (Aalpha)- nuclear bag fibers-has rings on dendritic ends that transmits info about **muscle length and velocity of contraction** Type II(A beta)-nuclear chain fibers-dendritic ends transmit info about muscle **length**

Describe, in general, the path of somatic/voluntary motor info.

Voluntary motor responses initiated in motor cortex, then goes thru brain stem thru spinal cord to the skeletal muscle.