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Motor behavior.docx

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McGill University
PHGY 209
Erik Cook

Motor behavior • Purposeful or goal directed • Two types: o Voluntary movements o Reflexive: more important component. Thousands of little small muscle contractions in body, trunk that keep us sitting up. We would flop over if this whole motor component would be turned off. We’re not even aware of this component. Muscle control: Extension versus flexion • Extension: increase of angle around a joint. • Flexion: decrease of angle around a joint. • Both of these involve coordination between two muscle groups (in knees, elbows, etc.). • During extension, we contract the extensor muscle, but we relax the flexor muscles. • The agonist muscle is the one we contract and antagonist is the one we relax. • How do we relax a muscle? By inhibiting the motor neurons that innervate that muscle. • During flexion, the opposite happens. The flexor muscle becomes the agonist and contracts while the extensor muscle becomes the antagonist and relaxes. • To keep things at a constant position, we have to balance the forces on either side of the joint. Reciprocal innervation of muscles 1) Coordinated flexor and extensor muscle activation and relaxation. • Both of these things go off every time we move. 2) Limb position is maintained by a balance of flexor and extensor muscle tension. Motor neurons (-> neuromuscular junction) • Only excitatory (ACh) is released at the neuromuscular junction. o Motor neurons only cause muscles to contract. No relaxation caused directly, since that would involve inhibiting the motor neuron that is innervating it. • There are two types of motor neurons. o Alpha: innervate skeletal (extrafusal) muscle and cause them to contract. o Gamma: smaller, innervate muscle spindle (intrafusal), part of the proprioceptive system that monitors the length of the muscle. o Muscle fibers are thus part of the proprioceptive system and are innervated by gamma motor neurons. Why the case? Next lecture. • Cell bodies in ventral horn of spinal cord (spinal nerves) or brain stem (cranial nerves – axons leave the brain stem via the cranial or spinal nerves) o Receive inputs mostly from interneurons in the spinal cord. Spinal afferent (sensory) and efferent (motor) pathways • Spinal segment with nerves coming off from one side. • Touch and proprioception entering the cranial nerves for instance, they branch off to the dorsal root (ganglion= has their cell bodies), they go in and turn immediately up in the dorsal columns. There is also a branch off going into the gray matter of the spinal cord. This branch carries both the somatosensory information and proprioceptive information. • There are interneurons which innervate motor neurons. Motor neuron cell bodies are located in the ventral horn of the gray matter.Axons go out at the ventral root and join up with cranial nerves to innervate skeletal muscles. • Dorsal column = ipsilateral touch and proprioception. • Descending motor commands: come down from our brain down the white matter and come out to also innervate the interneurons that then innervate the motor neurons. These commands are part of our voluntary movement commands or also part of the pathways involved in reflexive control of posture. • Repetitive motions like walking involves the circuitry in the spinal cord. • We won’t cover motor lesions in the spinal cord. Spinal interneurons • Integrate a lot of information and innervate motor neurons. • They receive descending pathways (voluntary movements). They also receive information about the force/tension muscle is putting on (tension monitoring in tendon receptors). They receive information about pain. • They receive proprioceptive feedback about joint position. They get coordinated complex movements by information from other spinal levels up and down the spinal cord. (We will talk less about this). o Chicken without head continues walking for a while: all this circuitry in the spinal cord that produces these complex motor movements like walking. • Length monitoring: part of our proprioceptive feedback that tells us how long our muscles are that helps us determine body position. • All of this is integrated by the spinal interneurons: some of this consists of excitatory inputs, others inhibitory. Spinal reflexes 1) Clinical relevance: Important for the evaluation of the health of the entire nervous system (e.g. doctor tapping our knee) 2) Reflexes are mostly out of our control. 3) Withdrawal reflex: protects limbs from injury. This reflex has occurred throughout our life. a. Touching something hot and pulling back our arm right away. Due to spinal cord circuitry. But, if what we’re picking up hot is our dinner, you tend to hold it for a while despite the burning sensation and then drop it down. So, we can override some of these reflexes to some degree. 4) Stretch reflex: controls muscle length. a. Monosynaptic (primary) - b. Polysynaptic (secondary) - 5) Inverse stretch reflex: controls muscle tension/force. Reflexes can be modified • Most spinal reflexes can be overridden (to some degree). Flexion withdrawal reflex • E.g. picking up a hot plate and dropping it, withdrawing our hand. • The point of these reflexes is that they’re fast and don’t require us to think much. • For instance, you activate nociceptors in the big toe. CausesAPs to propagate up. • Their axons go all the way up our leg into the spinal nerve into the dorsal root (cell bodies in the dorsal ganglion). We all know that these afferents come in and synapse on second order neurons- this is where the synapse is important since the presynapse has opiate receptors. The axon from the 2 order neuron crosses the midline in the spinal segment and they go up the anterolateral column. • They also have collaterals that branch into the gray matter.And they excite excitatory interneurons that then excite motor neurons of the flexion muscle (the ipsilateral flexor). The flexor muscle contracts => the leg will move immediately away from the painful stimulus damaging our tissue. • Recall:Anytime we activate one motor group (e.g. ipsilateral flexor - agonist), we inhibit the ipsilateral extensor - antagonist. • How does our spinal cord inhibit the antagonist? These afferents also drive inhibitory interneurons (in red) that inhibit motor neurons that innervate the ipsilateral extensor. We relax the ipsilateral extensor by inhibiting that motor neuron that innervates it. • This reflex is polysynaptic: there is more than one synaptic connection between the afferent input and the motor output (here 2 synapses). • When we lift our foot, we would tip over unless there was something else compensating. • So, there is also a contralateral component, the cross extensor reflex (automatic) – excitatory interneurons that cross the midline and activate motor neurons that innervate and excite the contralateral leg so it can take more of our weight without tipping over (shift in center of gravity) -> they’re exciting the contralateral extensor. Through another set of inhibitory interneurons, they inhibit the contralateral flexor. • This reflex has an ipsilateral and contralateral components with opposite effects. • This happens in arm withdrawal. No change in weight, but center of gravity also shifts. Interesting properties of the withdrawing reflex • The magnitude of the withdrawal reflex depends on the magnitude of the pain stimulus. • Response development depends on the magnitude of the pain stimulus.As the pain increases, the response gets bigger inAthan B than C = irradiation. • Distance of limb withdrawal is related to pain stimulus due to the recruitment of interneurons in the spinal cord which drive more motor neurons that are innervating our flexors. • The more painful the stimulus is, the longer we keep the limb withdrawn (afterdischarge => due to feedback loops in the spinal cord that keep the withdrawal going or keeps the foot up for instance). Properties of withdrawal reflex • Polysynaptic: interneurons between the afferent input carrying the nociceptor, pain information and the motor output. • Afterdischarge: can last seconds. Spinal reflexes 1) Withdrawal reflex: protects limbs from injury. 2) Stretch reflex: controls muscle length. a. Monosynaptic (primary) b. Polysynaptic (secondary) 3) Inverse stretch reflex: controls muscle tension/force. Monosynaptic stretch (knee jerk) • Tapping the knee with a little hammer stretches the extensor muscle. When we stretch a muscle, it tends to pull back. And we’ll talk soon about how it occurs (transduction and change in muscle length when they get stretched). • Result of activating this proprioceptive input caused by stretching our muscles: o Many times, if we hit the patella tendon, it tends to stretch our extensor muscle in the leg at tiny bit. This activates the stretch receptor called the muscle spindle. This is part of our proprioceptive input coming to the right dorsal root (cell bodies in the dorsal root ganglion) and information goes to the brain via the dorsal columns carrying ipsilateral touch and proprioception. They again branch off from this stretch receptors and do something unique in the spinal cord; they have a motor synaptic connection to motor neurons. • Remember: most of the time, motor neurons are activated by interneurons that integrate all the info together (descending, voluntary commands to move our arms, reflexes from other areas of the spinal cord, and so on). In the withdrawal reflex, we saw an example of this integration. That reflex has interneurons mediating the integration of these signals, but the ***monosynaptic stretch reflex has a monosynaptic component (a single synapse between the afferent input and the activation of the motor neurons)***. • The motor neuron then innervates the same muscle that we stretched. • It activates and causes contraction of the ipsilateral extensor if we have stretched the ipsilateral extensor originally. • There is also a polysynaptic component through this inhibitor interneuron, because in order to cause one muscle to contract (the agonist muscle), you cause the antagonist muscle to relax by inhibiting the motor neuron that innervates the antagonist muscle, in this case it’s the inhibition of the motor neurons innervating the ipsilateral flexor, because we’re stretching the ipsilateral extensor. • You stretch muscle by tapping briefly on the tendon, it briefly pulls our extensor muscles (quadriceps, etc.) and the leg kicks. • We do that in any muscle of wrist, ankle, etc. • Why is your doctor doing this to us? This gives us a snapshot of the health of the nervous system. o If there are problems with spinal cord descending pathways that control voluntary movements, reflexes can get bigger - they become spastic. o Or, if there’s problems in the neuromuscular junction (the final output of motor neurons), these reflexes can go away. The great thing is we don’t need a conscious patient to tell us about that. Muscle spindle and Golgi tendon organ • Let’s talk about afference and transduction process for measuring muscle length. • This is called the muscle spindle.And this differentiates from another afferent Golgi tendon organ, important in the next reflex. • The muscle shown here is composed of extrafusal muscle fibers, activated by alpha motor neurons. • Muscles are attached to bones via tendons. In the tendon is the Golgi tendon organ, which measures muscle tension/force. (How much force the muscle is contracting?) • The length of the muscle is the muscle spindle. It’s in parallel with the extrafusal muscle fibers. The Golgi tendon organ is in series – it feels all the force the muscle puts on. The stretch receptors which are in parallel change their length as the muscle is changing length. When our muscles contract, they get much shorter and longer when the muscles relax. In short, the stretch receptor is measuring the length of the muscle. • The stretch receptor is in the middle where the transduction occurs. • On either side of the stretch receptors are small tiny intrafusal muscle fibers and these are innervated by the gamma motor neurons. These gamma motor neurons cause these muscles to contract to maintain sensitivity of the stretch receptor. • The output/afferent of the stretch receptor is muscle length. Muscle spindles • It has a covering: the capsule. • Gamma motor neurons are innervating the intrafusal muscle fibers from either side. • The afferents are coming out from the muscle spindle. There are too types: Ia and II. • Muscle spindle is getting longer and shorter as the extrafusal muscle fibers are getting longer and shorter. It’s activating Ia and II afferents. Response of Ia (primary) and II (secondary) afferents • The difference between those afferents is that Ia are RAPIDLY ADAPTING afferents. The type II are SLOWLY or NOT ADAPTING at all. • Remember:Afferent adaptation is the emphasis on when things change. o Ia afference tend to change their firing when the muscle is changing length. It may encode some static change in muscle length but not much. Rapidly adapting, signaling dynamic changes. o Type II not only signals static changes in muscle length, but slowly adapting or non adapting. • Tap = brief change in muscle length. o It’s only the Ia fibers that get activated, because these are the rapidly adapting and quickly responding to changes in muscle length. o Type II do nothing. Muscle spindles can lose sensitivity • Gamma motor neurons in the intrafusal muscle fibers. • Here’s the problem with the muscle spindles. They are a lot like the elastic (when too stretched and becomes floppy, stops working). If it’s too floppy, they don’t work any longer. • It’s the job of the intrafusal muscle fibers is to keep the muscle spindle (the transduction portion) at the right tension so it works. • E.g. We’re holding arm out. The weight we’re holding causes it to go down (-> extending my flexor). When these muscles lengthen, it activates the muscle spindle, and we see an increase inAPs. This signal goes back through the spinal cord and causes our flexion muscles to contract a bit to compensate. (min. 21) • But if we voluntarily flex, we decrease the length of our extrafusal muscle fibers, by a lot, the muscle spindles become floppy (no longer really parallel but more stretched out). They don’t work as well -> stop in activity leads to a reduction in sensitivity of the muscle spindle (it collapses). • This doesn’t really happen so to speak, because as the muscles are contracting and the flexor muscles are getting shorter because of alpha motor neuron activity, there is a coactivation of the gamma motor neurons that are innervating the intrafusal fibers on both sides of the muscle spindle. Gamma motor neurons maintain muscle spindle sensitivity • What that does is as the alpha motor neurons cause the muscles to shorten, the gamma motor neurons activity is tightening the spindle portion pulling on both sides of the spindle, causing both tiny muscles on the sides to contract, which stretches up the spindle in the middle. It maintains spindle sensitivity. • The intrafusal fibers are not very strong. They don’t affect the overall force of muscles. All they do is regulate the stretchy spindle portion in the middle and maintain its operating range – just at the right amount of stretchiness to keep working. • Alpha-gamma coactivation!Anytime alpha motor neurons are activated in the spinal cord, gamma motor neurons are activated as well. Alpha-gamma coactivation • Q/A: Where is the signal coming from to activate the gamma motor neurons? These gamma neurons are sitting for example in the spinal cord right next to alpha motor neurons that are all together. So, any signal that activates alpha motor neurons also activates gamma motor neurons. When you activate alpha motor neurons, you cause the muscles to contract. At the same time, you’re activating gamma motor neurons to maintain muscle spindle sensitivity which then can excite
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