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Chapter 10 Motor System

10.1 Spinal Motor Control and Proprioception

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The motor system refers to the nerve cells that are used to control our body. The key roles of the motor system are to plan, control, and execute voluntary (deliberate) movements, and to control involuntary (subconscious or automatic) functions, such as digesting food. The motor system is sometimes described as a top-down process: in a voluntary movement, neural activity in the frontal lobe sends commands down to motor neurons located in the brainstem or spinal cord, which in turn activate muscle groups. In reality, motor control is more of a loop, rapidly communicating between the sensory cortex and motor cortex. Sensory information about limb position, posture, and objects in contact with the skin inform the descending motor plan. Simultaneously, the motor plan provides predictions about upcoming movement.

There are multiple levels of control. Within the spinal cord, simple reflexes can function without higher input from the brain. Slightly more complex spinal control occurs when central pattern generators function during repetitive movements like walking. The motor and premotor cortices in the brain are responsible for the planning and execution of voluntary movements. And finally, the basal ganglia and cerebellum modulate the responses of the neurons in the motor cortex to help with coordination, motor learning, and balance.

This lesson explores the lowest level of control at the level of the spinal cord.

 

Spinal cord and brains showing regions of motor control.
Figure 32.1. Motor output is controlled at multiple levels: A. Spinal cord and spinal neurons, B. Motor (dark blue) and premotor (light blue) cortices, C. Basal ganglia (a subcortical structure shown in light blue) and cerebellum (yellow). ‘Motor Control Levels’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Alpha Motor Neurons

Muscle fibers are innervated by alpha motor neurons. Alpha motor neurons are also called lower motor neurons because they are not located in higher brain areas. These cells are the only cells that directly command muscle contraction. The cell bodies of the alpha motor neurons are located in the central nervous system in the ventral horn of the spinal cord. Their axons leave the spinal cord via the ventral roots and travel to the muscle via efferent peripheral spinal nerves.

Spinal cord showing location of alpha motor neuron in ventral horn.
Figure 10.2. Alpha motor neurons are located in the ventral horn of spinal cord. Their axons, which are efferent fibers, travel to the muscles via spinal nerves. ‘Alpha Motor Neurons’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Like the sensory systems, the motor system is also organized in a topographic fashion, referred to as ‘somatotopic organization’. Within the spinal cord, alpha motor neurons that innervate muscles in the arms and legs are located in the lateral portion of the ventral horn, whereas alpha motor neurons that innervate muscles in the trunk are located in the medial portion.

Illustration of ventral horn showing relative locations of motor neurons that innervate different muscles. Details in caption.
Figure 10.3. The ventral horn is organized in a topographic manner, with proximal muscles (like those in the trunk) located more medially than distal muscles (like the arms or legs). Additionally, motor neurons are organized by function with extensor motor neurons located together and flexor neurons located together. ‘Spinal Cord Map’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Location of Alpha Motor Neurons

The structure of the spinal cord is reviewed in Chapter 26 of Introduction to Neurobiology. Examining cross sections of the spinal cord at different levels reveals that there is a non-uniform distribution of lower motor neurons. This is determined by the size of the ventral horn at the different levels of the spinal cord. At the cervical enlargement the larger ventral horns are due to the presence of more lower motor neurons that function in movement of the arms. At the lumbar enlargement the larger ventral horns are due to the presence of more lower motor neurons that function in movement of the legs.

Figure 10.4. Ventral horn anatomy at different levels of the spinal cord. There are 4 divisions of the spinal cord: cervical, thoracic, lumbar, and sacral. The size of the ventral horn (colored in red), where α motor neuron cell bodies are located, changes at the different levels of the spinal cord. The ventral horn is enlarged in the cervical and sacral regions of the spinal cord due to the increased number of motor neurons that are needed for motor control of the arms and legs. The ventral horn is smaller in the thoracic region due to decreased muscle innervation in this area.

Neuromuscular Junction

The lower motor neurons communicate with muscle fibers (muscle cells) at the neuromuscular junction (NMJ). The NMJ is similar to other chemical synapses, however the postsynaptic cell is a muscle cell separated by about 30 nm. The presynaptic cell is the motor neuron and the postsynaptic site is the sarcolemma, the cell membrane of the long cylindrical muscle fibers (muscle cells). The neuromuscular junction is one of the largest synapses in the body and one of the most well-studied because of its peripheral location.

The neuromuscular junction is located midway down the length of the muscle fiber. The muscle fiber will contract in response to depolarization traveling down the sarcolemma of the muscle fiber. The synapse is located midway down the length of the muscle fiber so that the postsynaptic signal can travel in both directions down the long muscle fiber and quickly activate a contraction along the entire cell.

Acetylcholine is the neurotransmitter released at the neuromuscular junction (NMJ), and it acts upon ligand-gated, non-selective cation channels called nicotinic acetylcholine receptors that are present in postjunctional folds of the muscle fiber. Nicotinic acetylcholine receptors allow the influx of sodium ions into the muscle cell. The depolarization will cause nearby voltage-gated channels to open and fire an action potential in the muscle fiber. In a healthy system, an action potential in the motor neurons always causes an action potential in the muscle cell. The action potential leads to contraction of the muscle fiber.

Postjunctional folds on muscle fiber and ion flow after acetylcholine action.
Figure 10.5. The ionotropic nicotinic acetylcholine receptors in the postjunctional folds of the muscle fiber are non-selective cation channels that allow the influx of sodium and the efflux of potassium The depolarization of the cell by the sodium influx will activate nearby voltage-gated ion channels. ‘NMJ Ion Flow’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

To review, an action potential traveling down the motor neuron (presynaptic cell) will cause the release of acetylcholine into the synapse. Acetylcholine binds to postsynaptic nicotinic acetylcholine receptors that are located in the folded sarcolemma (increasing surface area), causing depolarization of muscle fibers and ultimately muscle contraction.

Acetylcholinesterase, an enzyme that breaks down acetylcholine and terminates its action, is present in the synaptic cleft of the neuromuscular junction. Muscle contraction must be tightly controlled. Thus, the actions of acetylcholinesterase are very important to cease muscle contraction quickly.

Neuromuscular junction.
Figure 10.6. The neuromuscular junction (NMJ) is the synapse between a motor neuron and a muscle fiber. Acetylcholine is released at the NMJ and acts on nicotinic acetylcholine receptors located in the postjunctional folds of the muscle fiber. Neurotransmitter action is terminated by breakdown by acetylcholinesterase. ‘Neuromuscular Junction’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Clinical Application: Myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder characterized by muscle weakness, resulting in difficulty with speech, trouble with movement and swallowing, drooping eyelids, and double vision. Each year, an estimated 20 out of a million people get diagnosed with MG.

The muscle weakness seen in MG results from immune system-mediated destruction of the nicotinic acetylcholine receptors expressed at the NMJ. Thus, when the lower motor neuron releases acetylcholine, the muscle cells are unable to detect this release, so they fail to contract appropriately.

One therapeutic strategy involves inhibition of acetylcholinesterase, the enzyme that degrades acetylcholine. This causes the synaptic acetylcholine to remain in the synapse longer, increasing the chance that receptors get activated. Alternatively, autoimmune diseases like MG can be improved with immunosuppressant therapy. With successful treatment, MG usually does not result in changes in lifespan.

An individual with Myasthenia Gravis.
10.7. Myasthenia Gravis. Individuals with Myasthenia Gravis can have symptoms such as drooping eye lids and weak facial muscles.

Motor Units

Importantly, there are many more muscles cells than there are motor neurons. One alpha motor neuron can innervate multiple fibers within one muscle due to the branching of motor neuron axons. Each axon terminal synapses (innervates) a single muscle fiber. A motor neuron and all the fibers innervated by it are called a motor unit. The muscle fibers within one motor unit are often spread throughout the muscle to spread the contraction throughout the full muscle. Further, motor units in a muscle usually contract asynchronously to help protect the muscle from fatigue. A graded contraction of the muscle is produced by activating varying numbers of motor units.

Motor units differ in size. Small motor units are motor units that innervate fewer muscle fibers and thus control fine movements. Small motor units are located in the eyes and fingers, both of which function in fine and precise movements. Large motor units innervate many muscle fibers and are typically found in weight-bearing muscles like the thighs.

The group of motor neurons that innervate all the fibers of one muscle is called a motor pool.

Motor neurons and muscle fibers.
Figure 10.8 Motor neurons can innervate more than one muscle fiber within a muscle. The motor neuron and the fibers it innervates are a motor unit. Three motor units are shown in the image: one blue, one green, one orange. Those three motor units innervate all the muscles fibers in the muscle and are the motor pool for that muscle. ‘Motor Unit and Pool‘by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Types of Motor Units

In addition to the size of the motor unit, the types of muscle fibers that are innervated by motor units can also differ. There are three different types of motor units:

  1. Slow motor units. Slow motor units are slow to contract and generate less force but can work for a long time. They are used in endurance exercise like jogging.
  2. Fast fatigue-resistant motor units. Fast fatigue-resistant motor units are quick to contract (though not as fast the fast fatigable motor units). These motor units generate more force that then slow motor units, but are much more resistant to fatigue than the fast fatigable motor units.
  3. Fast fatigable motor units. Fast fatigable motor units are quickest to contract and generate the most force. These motor units are more prone to fatigue due to decreased number of mitochondria within the muscles. Fast fatigable motor units generate a lot of force quickly, but also tire quickly. They are used mostly in high intensity exercise like lifting weights and sprinting.
Graph of force generated by different types of motor units.
Figure 10.9. Force generated by different types of motor units. Slow motor units are slow to contract and exert very little force (shown in pink). Fast-fatigue resistant motor units are quicker to contract and generate more force than slow motor units. Fast-fatigable motor units are fastest to contract and generate the most force. ‘Motor unit force’ by Valerie Hedges is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Muscle Activation

Action Potential Triggered in a Muscle

When an action potential is triggered in a muscle it causes a muscle twitch (contraction) that is followed by a period of relaxation. A muscle twitch shows an increase in tension after a short delay (latent period). During the contraction period, the muscle tension increased, and then has a long relaxation period, causing the muscle tension to be increased long after the initial stimulus.

Graph of a muscle twitch.
Figure 10.10. Muscle twitch. A stimulus will cause a muscle to experience a twitch. A twitch is characterized by a short latent period following the stimulation, followed by contraction, which increases muscle tension. Following contraction, the muscle has a longer relaxation period until tension returns to baseline.

When a stimulus occurs during the long relaxation period following a muscle twitch, the newly generated muscle twitch will summate to increase the strength of the overall muscle contraction. Shortening the time between stimuli will result in unfused tetanus and if stimuli are very close together will result in fused tetanus. Therefore, a higher rate of action potentials in the alpha motor neuron will generate more muscle contraction.

Graph comparing a single muscle twitch, unfused tetanus, and fused tetanus.
Figure 10.11. Unfused and fused tetanus in muscles. Due to the long relaxation period in muscles, if another stimulus is delivered during the relaxation period, the muscle twitches summate to produce an increase in muscle tension, referred to as ‘unfused tetanus’. When stimuli are delivered very close together in time, such that the muscle does not have time to relax, then maximal muscle tension results, also called ‘fused tetanus’.

Muscle Recruitment

In addition to the rate of action potentials changing the force of the muscle contraction, muscle recruitment can also increase the strength of contraction within a muscle. When generating motor activity, the smallest motor units will be activated to contract first due to their size and increased excitability to move the load. Increasingly larger motor units are recruited to lift heavier loads. Recruitment of motor units allows for us to generate appropriate muscle tension to move a given load. For example, if you need to pick up a pencil, then we do not need to use the same force as if we were trying to pick up a 20-pound weight. Only smaller motor units would be activated to pick up the pencil, and increasingly larger motor units would be recruited to lift the heavier load of the 20-pound weight.

Graph showing motor unit recruitment for three different motor units.
Figure 10.12. Muscle Recruitment. Motor units are recruited in size order to move increasingly heavier loads. For a light load that requires minimal muscle tension, only Motor Unit 1 (shown in red) is recruited. For a medium-heavy load, Motor Unit 2 (shown in blue) is recruited, in addition to Motor Unit 1. The heaviest loads require the additional recruitment of Motor Unit 3 (shown in orange).

Alpha Motor Neuron Inputs

The alpha motor neurons that directly cause muscle contraction receive inputs from three different sources.

  1. Sensory cells from the dorsal root ganglion that provide sensory information from the muscles through proprioception.
  2. Upper motor neurons from the motor cortex in the brain and brain stem that are responsible for initiating voluntary movement.
  3. Interneurons in the spinal cord. These represent the largest input to the alpha motor neurons and can either provide excitation or inhibition to the alpha motor neuron.
Different inputs for the alpha motor neurons in the spinal cord.
Figure 10.13. Alpha motor neuron inputs. Alpha motor neurons located in the ventral horn of the spinal cord receive inputs from the sensory neurons collecting information from the periphery, upper motor neurons from the brain and brainstem, and from a population of interneurons within the spinal cord. ‘Alpha motor neuron inputs’ by Valerie Hedges is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Proprioception

Raise your arms above your head. Even without seeing your arms, your nervous system has mechanisms that inform you about the location and position of your body parts, including how much your joints are bent. This sense is called proprioception and is critically important for coordinated movement and motor reflexes that contribute to those tiny, rapid adjustments that are made while maintaining balance. Proprioceptive information ascends through the spinal cord and into the brain via the dorsal column-medial lemniscus tract. Proprioception is also processed in the primary somatosensory cortex.

Proprioception refers to the “body sense” that informs us about how our bodies are positioned and moving in space. Proprioceptors are receptors that provide proprioception information.

There are two main types of proprioceptors:

  1. Muscle spindles measure muscle stretch (muscle length) and transmit this sensory information via 1a sensory afferent fibers. Muscle spindles are nested within and arranged parallel to the extrafusal muscle fibers.
  2. Golgi Tendon Organs measure muscle tension and transmit this sensory information via 1b sensory afferent fibers. Golgi tendon organs are located between the extrafusal muscle fibers and their points of attachment at the bone.

Muscle Spindles

Extrafusal and Intrafusal Muscle Fibers

Muscle spindles are fibrous capsules that are located within muscles. Intrafusal muscle fibers are special muscle fibers that are located within the fibrous capsule of the muscle spindle. The intrafusal muscle fibers are innervated by gamma motor neurons that will cause them to contract.

Extrafusal muscle fibers, however, make up the bulk of the muscle and are located outside of the muscle spindle. The extrafusal muscle fibers are stimulated to contract by the alpha motor neurons.

Illustration of a muscle spindle.
Figure 10.14. Type I primary afferent sensory axons wrap around the intrafusal fibers within the muscle spindle, located deep in the muscle. When the muscle stretches, these sensory neurons are activated. Extrafusal muscle fibers are located outside of the muscle spindle and make up the bulk of the muscle. ‘Muscle Spindle’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Muscle Spindles Function

Group 1a sensory afferent axons, which have a large diameter and are heavily myelinated, wrap around the intrafusal fibers contained within the muscle spindle. These sensory afferent fibers will signal when the intrafusal fibers of the muscle spindle are experiencing stretch and communicate information about muscle length.

Within the spinal cord, a single sensory 1a afferent axon synapses on every alpha motor neuron within the motor pool that innervates the muscle that contains the muscle spindle. This allows for a fast and powerful contraction of the muscle in response to a change in muscle stretch.

Gamma Motor Neuron Function

When the muscle experiences a stretch and the extrafusal fibers are stretched, the muscle spindle and the intrafusal fibers are also stretched (due to being within the muscle and surrounded by the extrafusal fibers). When the muscle spindle stretches, the 1a sensory axon will start to fire action potentials. The sensory axon synapses with an alpha motor neuron that will then cause the extrafusal muscle fibers to contract. As the extrafusal fibers contract and the muscle shortens, the muscle spindle goes slack, and the 1a axon will no longer fire action potentials as it is no longer being stretched. The gamma motor neuron is then activated that innervates the intrafusal muscle fibers, causing the intrafusal fibers to contract, allowing the muscle spindle to sense stretch again.  Therefore, the gamma motor neuron is critical for allowing the muscle spindle to continue providing information about muscle stretch even when the muscle has experienced contraction.

Series of images showing how gamma motor neurons allow for muscle spindle activity.
Figure 10.15. Gamma motor neuron function. A. A muscle spindle is shown within a muscle. A 1a sensory afferent is wrapped around the muscle spindle. An electrode is measuring an increase in 1a sensory afferent firing rate when the muscle is stretched. B. In response to stretch, the alpha motor neuron that innervates the extrafusal muscle fibers has an increase in firing rate. At this time, the muscle spindle goes slack and can no longer communicate muscle stretch information via the 1a sensory afferent. C. Gamma motor neuron activity increases to contract intrafusal muscle fibers within the muscle spindle. This allows for the muscle spindle to continue to communicate muscle length that is then sensed by the 1a sensory afferent. ‘Gamma motor neuron function’ by Valerie Hedges is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Golgi Tendon Organ

The Golgi tendon organ is a proprioceptor that measures muscle tension, or the force of contraction. They also contribute to our detection of weight, as we lift something heavy for example. Golgi tendon organs are located in the tendon that connects the muscle to bone. The tendon is made up of collagen fibrils and the Group 1b sensory axons are intertwined within the collagen fibrils. When the muscle experiences an increase in tension, the collagen fibrils surrounding the 1b sensory axon physically squeeze the 1b axon, opening mechanically-gated ion channels within the 1b sensory axon.

Structure of a Golgi tendon organ.
Figure 10.16. Golgi tendon organ. Golgi tendon organs are located within muscle tendons. A Group 1b sensory axon intertwines within the collagen fibrils of the muscle tendon. When the muscle experiences an increase in tension, the collagen fibrils squeeze the 1b sensory axon, causing a signal to travel down the 1b axon toward the spinal cord. ‘Golgi tendon organ’ by Valerie Hedges is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

The purpose of the Golgi tendon organ is to allow for an optimal range of tension for the muscle and to protect the muscle from injury due to being overloaded. This is accomplished through a negative feedback loop controlled by the Golgi tendon organ. When the muscle experiences an increase in muscle tension, the 1b sensory axon starts to fire action potentials. The sensory neuron synapses onto an inhibitory interneuron within the spinal cord, which when active will release GABA onto the alpha motor neuron that innervates the same muscle that experienced the increase in muscle tension to begin with. When GABA binds to the alpha motor neuron, it will decrease firing of the alpha motor neuron, leading to a decrease in contraction of the muscle. This is an example of negative feedback as the increased muscle tension ultimately leads to physiological changes that decrease muscle tension.

Golgi tendon organ signaling.
Figure 10.17. Golgi tendon organ signaling. An incrase in tension within the muscle, will cause activity of a 1b sensory axon that travels through the dorsal side of the spinal cord, where it synapses onto an inhibitory interneuron within the spinal cord. Activation of the inhibitory interneuron will cause it to release GABA onto the α motor neuron within the ventral horn of the spinal cord, inhibiting the α motor neuron from firing. This will lead to a decrease in contraction of the muscle that is innervated by the α motor neuron. ‘Golgi tendon organ signaling’ by Valerie Hedges is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Key Takeaways

  • Motor neuron cell bodies are located in the ventral horn of the spinal cord.
  • Motor neuron axons are located in the peripheral nervous system and travel to muscles via spinal nerves.
  • Acetylcholine is released at the neuromuscular junction and acts upon ionotropic nicotinic acetylcholine receptors.
  • The spinal cord is topographically organized.
  • Muscle twitches can summate to increase muscle tension.
  • A motor unit is an alpha motor neuron and all of the motor fibers that it innervates. Motor units differ in size, recruitment, and power.
  • Muscle spindles and Golgi tendon organs are proprioceptors that communicate information about the location and position of the body.

Test Yourself!

Additional Review

  1. What is the difference between a motor unit and a motor pool?

Attributions

This chapter is adapted from 32. Spinal Motor Control and Proprioception in Introduction to Neurobiology by Avinash Singh which is licensed under a  Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Which is adapted from “Spinal Motor Control and Proprioception” in Introduction to Neuroscience by Valerie Hedges which is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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