2.3 Motor Recruitment.pdf

[00:00:00] >> Motor Recruitment and Sensory Signals. These are the objectives. A motor unit is a single motor neuron and all the muscle fibers it innervates. The number of muscle fibers innervated by a single neuron ranges from maybe three to one for an eye muscle to several hundreds to one for a li...

[00:00:00] >> Motor Recruitment and Sensory Signals. These are the objectives. A motor unit is a single motor neuron and all the muscle fibers it innervates. The number of muscle fibers innervated by a single neuron ranges from maybe three to one for an eye muscle to several hundreds to one for a limb muscle. [00:00:24] All muscle fibers in a motor unit act as one unit, contracting or relaxing at almost the same time. Muscle fibers of one motor unit are not all next to each other, they are distributed throughout the muscle. If the motor units nerve activates as muscle fibers to contract, all those fibers will contact maximally. [00:00:42] This is called all-o-none principle. The size principle of recruitment describes the fact that the smallest motor neurons are the first ones to be recruited and the largest motor neurons are recruited last. Since the smaller motor units have fewer muscle fibers per nerve, more must be recruited to produce a force of a specific level. [00:01:05] Smaller motor neurons participate in most sustained activities because they tend to innovate slow twitch type 1 muscle fibers that fatigue slowly. When muscle functions require more strength, the largest fast twitch and more quickly fatiguing motor units become active. In other words, motor units are normally recruited in an orderly fashion with those that produce the lowest force first, followed by the higher force producing units as force requirements increase. [00:01:32] The recruitment principle states that increasing the number of motor units activated simultaneously increases the overall muscle tension. The excitatory input or the rate coding principle states that increasing the frequency of stimulation of the individual motor units increases the percentage of time that each active muscle fiber develops maximum tension. [00:01:55] Firing of a single motor unit results in a twitch contraction of the stimulated muscle fibers. With an increase of that firing rate, the twitches summate or all add together to increase and sustain a force output. And individuals increase in muscle force by increasing both the number of active motor units and the firing rates of those motor units. [00:02:20] Many sensory signals are used to help with muscle activation. Proprioceptors detect changes in the tension and the position of the structures that they are in and transmit those nerve impulses to other areas. Additionally, sensory organs like the eyes and the ears' vestibular system, both of which provide input on position, balance, and motion, combine together with the proprioceptors. [00:02:46] And all those integrated sensory signals are then used by the motor control centers in the brain to automatically adjust the location, type, number, and frequency of motor unit activation, so the most appropriate muscle tension is developed to perform the desired movements. Proprioception describes the sensory input from receptors in muscle spindles, tendons, and joints that help tell joint position and joint movement, including the direction, the amplitude, and the speed, as well as the relative tension of the tendons. [00:03:22] Kinesthesia is a more specific term that describes the awareness of a dynamic joint motion, and position sense describes the awareness of a static position. Muscle spindles are in skeletal muscle, and act as a stretch receptor. Muscle spindles send sensory signals that kind of tell other neurons in the spinal cord and the brain of their length, and, therefore, the length of the muscle and the rate at which the muscle stretch is occurring. [00:03:53] There's varying numbers of muscle spindles in different muscles in the body. Muscle spindles are especially abundant in the small muscles of the eye, hand, and foot, because these muscles need to constantly be alerted to even small changes in movement. So in essence, muscle spindles kind of act as thermostats, comparing the length of the muscle spindle with the length of the muscle fiber that's around it. [00:04:18] And if the length of the muscle fiber is less than that of the muscle spindle, the frequency of the muscle spindle discharge nerve impulses is decreased since they're not being facilitated. However, when the muscle spindle is being stretched, its sensory receptors send more nerve impulses that excite an alpha motor neuron and activate the muscle fiber. [00:04:41] The constant state of input into the muscle spindles sets up a constant state of readiness. So although that the muscle is not activated, it's kind of on a steady state of alert and it's ready to act when needed. This state of readiness is also called muscle tone, which is characterized by an amount of muscle stiffness and resting tension. [00:05:04] Muscle tone is determined by the level of excitability of the entire pool of motor neurons controlling the muscle, the intrinsic stiffness of the muscle itself ,and the level of the sensitivity of the many different reflexes. The contributions of the muscle spindle is only one of the pieces that contribute to muscle tone. [00:05:22] But the mechanism is particularly important in the regulation and maintenance of postural muscle tone. Neuroconnections in the spinal cord contribute to a lot of the automatic control of movement. Specifically, the spinal region is a site for reflex motions, muscle synergy activations, and central pattern generators. Reflex motions include stretch reflex, reciprocal inhibition, and autogenic inhibition. [00:05:50] Spinal reflexes provide movement that is generated as a response to information arising from cutaneous, muscle, or joint receptors. Movements are stereotypical and predictable, but can be modified by the central nervous system. For example, the alertness of the person or the nerve arousal of the person will change a person's response to a stretch reflex. [00:06:15] Interneurons within the spinal cord link the motor neurons to the functional groups or muscle synergies. So you can see here, the receptors, they go into the spinal cord, you have an interneuron, and it goes back out to the muscle. The stretch reflex is a simple reflex arc that is mediated at the spinal cord. [00:06:38] An abrupt stretch of a muscle initiates a burst of impulses from the stretch receptors in the muscle spindle, which travels to the spinal cord and then excites activity in the motor units of the same muscle. We've all experienced this when we've gone to the physician and had the knee reflex done. [00:06:57] So looking at this more closely, you can see here that the reflex hammer is hitting the patellar tendon and the receptors generate a nerve impulse that then goes up to the spinal cord. And then an efferent neuron comes back down and conducts an impulse from the spinal cord to the muscle fibers and the muscle fiber responds and does a quick contraction. [00:07:30] This picture shows the stretch reflex regulation of a muscle length. In this case, the biceps is under the influence of a stretch reflex when the muscle is engaged in a steady-state flexion, and then it gets a sudden unexpected increase in load. So it causes those sensory endings on the biceps to go to the spinal cord, and it causes excitatory contraction to go back to the muscle to contract the biceps and hold that steady-state. [00:08:08] It can also send an inhibitory reaction back to the triceps to relax that muscle. These connections provide the basis for the rationale behind active stretching where maybe a person will be asked to actively contract a muscle to shut off the opposing muscle. For example, if you're stretching the hamstrings and you have someone actively contract the quadriceps muscle, that might help inhibit the hamstrings and allow a more effective stretch of it. [00:08:49] This can also be called reciprocal inhibition for this specific stretch reflex, where one muscle is activated and the opposing muscle is inhibited. There are, of course, more complicated nerve pathways than this one, and any muscle is supplied with many different motor nerve fibers and spindles. So the synaptic connections to even one single motor neuron are multiple. [00:09:17] Pattern generators are more complex muscle activation patterns that produce movement through neural connections at the spinal cord level. They are more complex than the simple stretch reflexes we looked at in the previous slides. The flexible networks of interneurons produce stepping and walking patterns that can be modified by cortical commands. [00:09:38] Adaptable networks of interneurons in the spinal cord activate the nerves to elicit the activation of flexor and extensor muscles at the hips, knees, and ankles. And the mechanism allows for efficiency of movement. These patterns are very sensitive to the changes in the task and the environment, and the body will adapt to responses in those changes. [00:10:01] These are the references, thank you.