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8. CNS Cogntiive Motor

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PHGY 209
Erik Cook

Naveen Sooknanan McGill Fall 2011 CNS: Cognitive/Motor The next area will study is motor behaviour, with an emphasis on the spinal cord because we know more about it than the brain Motor behaviour is goal or purpose oriented due to higher processing in the brain There are two types of motor behaviour: voluntary and reflexive o Voluntary motor behaviour is a very small portion of the overall system. These are the movements are think about doing o Reflexive movements are the majority of the system and involve the spinal cord and regions of the brain You dont think about these movements. For example, simply sitting upright requires 100s of microadjustments per second so you dont fall over The nervous system is required for movement. A very peculiar sea animal actually loses its nervous system and loses its motor function o At a young age, this organism has a nervous system and swims freely. However, at some point, it anchors down onto a surface and loses all motor function At this point the organism begins to digest its own nervous system for energy because it no longer needs it One important factor in muscle contraction is that, although we may not realize it, the flexing of one muscle always causes the relaxation of another. When you inhibit, or relax, a muscle, you are actually inhibiting the motor neurons that innervate the muscle The agonist is the muscle that contracts and the agonist is the muscle that relaxes (the same muscle can act as either of these in different situations) Extension and flexion of a muscle involve the rotation of muscles around a joint o We will only discuss joints that either flex or extend muscles, not complicated joints like the wrist of shoulder Extension around a joint increases the angle around the joint. This is caused by the contraction of an extensor muscle (in this case the tricep) and the relaxation of a flexor muscle (in this case the bicep) o Thus the extensor is the agonist here and the flexor is the antagonist Flexion around a joint decreases the angle around the joint. This is caused by the opposite as above: relaxation of the extensor and flexing of the flexor o In this case, the extensor acts as the antagonist and the flexor acts as the agonist This pairing between agonist and antagonist provides coordinated extensor and flexor muscle activation This is done by reciprocal innervation of the agonist and antagonist muscles (one is excited while one is inhibited) Limb position is given by the balance of these muscle pairs o When they are perfectly balance, the limb stays put 1 Naveen Sooknanan McGill Fall 2011 o When one muscle takes precedence over another, the limb begins to move in the direction of the dominant muscle Motor neurons are exclusively excitatory acetylcholine (ACh) neurons which cause muscles to contract, as we have already learned Alpha motor neurons innervate the extrafusal skeletal muscle (we will describe this next) which cause movement Gamma motor neurons innervate intrafusal muscle spindles which are tiny muscles that regulate muscle length These motor neurons have their cell bodies within the ventral horn of the spinal cord or the cranial nerves of the brain stem o These neurons synapse with interneurons before they go to their final target o Motor neurons receive most of their inputs from interneurons, which we will look at next Spinal afferents take the path of touch and proprioception, through the dorsal column (we wont have to consider lesions here though) The efferent (motor) pathway can be activated by descending motor commands from the brain, but they can also be controlled by interneurons coming directly from the input in the spinal cord o The motor efferent then travels to its destination Therefore, there is a motor component which is independent of the brain, which is not surprising because you dont need to think about every single muscle contraction you make o Both pathways, from the brain and from the input, synapse with an interneuron within the grey matter of the spinal cord o From there, the interneuron synapses with a motor neuron in the ventral horn which can then carry its information to the destination Spinal interneurons can be excited or inhibited by different receptors or pathways coming from different parts of the body Pain afferent from skin receptors can activate a reflex through activating interneurons o For example, putting your hand on a hot stove will cause you to pull you hand away very quickly Proprioceptive input from joint receptors can excite interneurons Descending pathways from the brain can activate interneurons based on voluntary movements o These movements are the ones you think about Other spinal levels can coordinate complex movements, such as running, independently from the brain by activating interneurons Muscle receptors from antagonistic muscles which monitor muscle length can inhibit the activity of the agonist muscle by inhibiting this interneuron 2 Naveen Sooknanan McGill Fall 2011 o This inhibition happens to prevent the agonist from over-exertion A tendon receptor, which monitors muscle tension, can inhibit interneurons when the muscle tension becomes too high It can be seen through a simple, yet gruesome, example that some motor movements can be mediated independent of the brain Before butchering, a chicken must have its head removed There is a period of time after head removal where the chicken can run around randomly This means that the motor movements and patterns required for running are independent of the brain because they can take place when the brain is no longer attached to the body Spinal reflexes are motor commands that happen without conscious planning (i.e. they are not activated through the brain). These reflexes have important clinical significance because reflexes can change with damage to the motor neuron. There are three reflexes which we will study The withdrawal reflex is meant to protect the limb from injury o When you experience something painful, your initial reaction is to pull the limb away as soon as possible o This reflex is independent is independent of the brain because it would take too long for the reflex to happen; the tissue would already be damaged The stretch reflex controls muscle length through proprioceptive control o This reflex can either be monosynaptic (no interneuron required) or polysynaptic (it synapses with an interneuron) o This reflex is important in clinical situations because damages to the nervous system here causes a change in muscle tone (how the muscle pulls) In inverse stretch reflex controls muscle tension in order to prevent the muscle from hyperextending These reflexes can be overridden to some degree by the brain For example, if you are holding a really hot dinner plate, your withdrawal reflex will tell you to drop the plate, but because you dont want to drop the plate, you will hold on to it just long enough to bring it to the table The flexion withdrawal reflex is a polysynaptic reflex, meaning the efferents in the spinal cord synapse with an interneuron before being sent to their final destination. For an example, let us take the reflex involved in stepping on a thumb tack The pain afferent will come in through the dorsal root ganglion, as we have learned. However, before going to the anterolateral column, the afferent branches onto 2 ipsilateral interneurons in the grey matter o One interneuron synapses with an inhibitory efferent that travels to the ipsilateral extensor (the quadricep in this case), and causes it to relax (becomes the antagonist) o The other interneuron connects to an excitatory efferent which contracts the corresponding flexor muscle (the hamstring in this case), thus becoming the agonist 3
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