Neural circuits involved in movement production are very complex. Motor commands are sent to the corresponding muscles through the motor system or efferent connection (efferent - exit the brain, output). In turns, the sensation from the peripheral system, e.g. the visual system, bodily touch, and proprioception, make up what we called the sensory system or sensory afferent (afferent - affect the brain, input).
Sensory and motor systems are organized hierarchically. We have the spinal cord as the base, followed by the brainstem, and the cerebral cortex as the highest order. In addition, there are separate yet parallel processing pathways connecting these three so that information is processed in a timely fashion. A major function of the somatosensory system is to provide information for the purposeful movement of our limbs and body in relation to objects in the external world with which we interact. There are two critical sites where both systems make contact, each representing a different level of hierarchy:
How does the circuit look like? The cell bodies of motor neurons that directly control and move muscles are located inside the ventral root of the spinal cord. Motor neurons of the ventral horn are modulated by interneurons, which in turn are controlled by two sources. The first source of input to the spinal motor networks is the somatosensory afferents whose cell bodies lie inside the dorsal root ganglion. Cell bodies of spinal interneurons are situated in the intermediate zone in between ventral and dorsal horns. Most reflexes, however, are polynsynaptic, meaning that there are more than one spinal interneuron involved. The second input source is the descending fibers within the spinal cord that carry motor commands issued by the cerebral cortex. Voluntary movements initiated by the motor cortex are therefore executed by the same network of spinal neurons that are involved in reflex behavior. For an anatomical perspective, see below.
References
[1] Chapter 33 - The Organization of Movement. In Kandel E.R. et. al. (2000). Principles of Neural Science 4e, McGraw-Hill.
[2] Chapter 8 - Control of Movement. In Carlson, Neil R. (2013). Physiology of Behavior 11e, Pearson Education.
(1) The spinal cord represents the lowest motor hierarchy that is stereotyped and repetitive.
(2) The cerebral cortex at the level of the central sulcus provides voluntary control.
The spinal cord contains lower motor neurons
The spinal cord is composed of a central core region in the butterfly-shaped gray matter surrounded by white matter. The gray matter is subdivided into dorsal (posterior) and ventral (anterior) horns. The ventral horn contains cell bodies of alpha motor neurons, or lower motoneurons, that innervate muscles and produce movements. The intermediate zone is located in between the ventral and dorsal horn, and contains cell bodies of interneurons. In fact, it is the cell bodies that give the gray color to the structure. Low-level motor outputs are relatively stereotyped and the temporal accuracy is high. For instance, the spinal cord is where the simplest monosynaptic reflex circuit lies. Reflex movements are automatic and very fast, that is, with short latency (below 100 msec usually). Although technically reflexes do not involve the intervention of the cerebral cortex, central motor commands and cognitive processes have the capacity to alter synaptic transmission in spinal reflex pathways.
The spinal cord is composed of a central core region in the butterfly-shaped gray matter surrounded by white matter. The gray matter is subdivided into dorsal (posterior) and ventral (anterior) horns. The ventral horn contains cell bodies of alpha motor neurons, or lower motoneurons, that innervate muscles and produce movements. The intermediate zone is located in between the ventral and dorsal horn, and contains cell bodies of interneurons. In fact, it is the cell bodies that give the gray color to the structure. Low-level motor outputs are relatively stereotyped and the temporal accuracy is high. For instance, the spinal cord is where the simplest monosynaptic reflex circuit lies. Reflex movements are automatic and very fast, that is, with short latency (below 100 msec usually). Although technically reflexes do not involve the intervention of the cerebral cortex, central motor commands and cognitive processes have the capacity to alter synaptic transmission in spinal reflex pathways.
How does the circuit look like? The cell bodies of motor neurons that directly control and move muscles are located inside the ventral root of the spinal cord. Motor neurons of the ventral horn are modulated by interneurons, which in turn are controlled by two sources. The first source of input to the spinal motor networks is the somatosensory afferents whose cell bodies lie inside the dorsal root ganglion. Cell bodies of spinal interneurons are situated in the intermediate zone in between ventral and dorsal horns. Most reflexes, however, are polynsynaptic, meaning that there are more than one spinal interneuron involved. The second input source is the descending fibers within the spinal cord that carry motor commands issued by the cerebral cortex. Voluntary movements initiated by the motor cortex are therefore executed by the same network of spinal neurons that are involved in reflex behavior. For an anatomical perspective, see below.
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| Fig-1: Reflex mechanisms are produced by the sensorimotor system at the level of the spinal cord. |
The brainstem consists of three divisions
The 3 divisions of the brainstem include the midbrain, medulla, and pons. Complex reflexes and rhythmic movements such as breathing, heart rate, and alertness occur in the brainstem. To some extent, balance and postural control recruit brainstem nuclei. Most autonomic systems involve the brainstem. Voluntary motor behavior also depends on various motor nuclei in the brainstem bilaterally, e.g. the red nuclei, vestibular nuclei, and the reticular formation. Each nucleus is uniquely connected to the spinal pathways both in ascending and descending manner. The details won't be discussed here.
The highest level of motor behavior is the cerebral cortex of the forebrain. This "controller" collects and integrates sensory information and is influenced by memory, subcortical modulation (the basal ganglia), and error correction (the cerebellum).
Why is the hierarchical structure important?
There is a considerable amount of time delay due to multiple connectivities in between. This problem, fortunately, is already solved by the hierarchical organization itself. For instance, when you want to flex your arm, there is this set of commands sent down from the motor cortex. This command will, at the same time, inhibit the opposing extensor muscles through the inhibitory interneuron that is activated during the stretch reflex. The variety of low-level circuits in the spinal cord and brainstem simplify the commands that the cortex has to send. Just like the director of the company instructs the managers and workers, in the same way, the motor cortex makes use of the simpler circuits by exciting some connections and inhibiting the rest. In this way, temporal details and precision will be left with the low-level motor neurons.
The Descending Pathways
The motor cortex sends commands to the spinal cord through axons projecting downwards, also known as the upper motor neurons. These neurons project directly and indirectly through various descending pathways situated alongside afferent sensory ascending pathways. The uncovering of such anatomical connections is not an easy feat, but thanks to studies by e.g. Lawrence and Kuypers (1968), we now know there are two main motor pathways mentioned below. The corticospinal tract is the only direct descending pathway from the motor cortex. Axons in this tract synapse directly with the alpha motor neurons of the skeletal muscle. Other pathways go through the brain stem and interact with the nuclei there, implying there is a certain degree of neuronal modulation involved. Refer to Fig-2.
The corticospinal tract consists of axons of motor neurons with cell bodies situated in the regions of the primary motor cortex that control the limbs and trunk. These neurons are called the Betz cells, which are unique to the motor cortex, and are the largest and longest neurons in the nervous system. They travel down from the cortex forming pyramidal tracts that terminate in the grey matter part of the spinal cord. The axons then cross over or decussate at the caudal medulla. Those which travel to the contralateral columns of the spinal cord form the lateral corticospinal tract and are associated with distal limbs, fingers, lower feet, and toes. Those which stay ipsilaterally are called the anterior corticospinal tract and are associated with the trunk and upper leg.
Note that if you read electrophysiology and monkeys literature, you may encounter different terms carrying the same meaning, that is, cortical motor neuron whose cell body resides in the cortex Layer V: pyramidal tract neuron (PTN) or upper motor neuron (as opposed to the alpha or lower motor neuron). A part of these neurons that carries direct connections with the lower motor neurons is called corticomotorneurone (CM cells).
The corticobulbar tract projects to the cranial nerves and controls facial muscles, neck, and tongue (refer back to the homunculus drawing!) The reticulospinal tract is associated with motor neurons whose cell bodies are located in the reticular formation, a network of nuclei occupying the core of the brainstem from the upper midbrain to the lower medulla. This structure is related to various tasks such as movements, arousal & sleep, and some complex reflexes. Neurons in this area control more automatic movement such as postural adjustment, breathing, sneezing, coughing.
The final common pathway:
Coined by Sherrington, this term refers to the lower motor neurons that innervate the skeletal muscle to induce movements. It is named 'common' in the sense that no matter what descending tract the axons originated, eventually, they will synapse in the ventral horn with the alpha motor neurons. One neuron can make contact (innervate) to 5 - 1000 muscle fibers. As a whole, the lower motor neuron and its target muscle are called the motor unit.
The highest level of motor behavior is the cerebral cortex of the forebrain. This "controller" collects and integrates sensory information and is influenced by memory, subcortical modulation (the basal ganglia), and error correction (the cerebellum).
Why is the hierarchical structure important?
There is a considerable amount of time delay due to multiple connectivities in between. This problem, fortunately, is already solved by the hierarchical organization itself. For instance, when you want to flex your arm, there is this set of commands sent down from the motor cortex. This command will, at the same time, inhibit the opposing extensor muscles through the inhibitory interneuron that is activated during the stretch reflex. The variety of low-level circuits in the spinal cord and brainstem simplify the commands that the cortex has to send. Just like the director of the company instructs the managers and workers, in the same way, the motor cortex makes use of the simpler circuits by exciting some connections and inhibiting the rest. In this way, temporal details and precision will be left with the low-level motor neurons.
The Descending Pathways
The motor cortex sends commands to the spinal cord through axons projecting downwards, also known as the upper motor neurons. These neurons project directly and indirectly through various descending pathways situated alongside afferent sensory ascending pathways. The uncovering of such anatomical connections is not an easy feat, but thanks to studies by e.g. Lawrence and Kuypers (1968), we now know there are two main motor pathways mentioned below. The corticospinal tract is the only direct descending pathway from the motor cortex. Axons in this tract synapse directly with the alpha motor neurons of the skeletal muscle. Other pathways go through the brain stem and interact with the nuclei there, implying there is a certain degree of neuronal modulation involved. Refer to Fig-2.
- The lateral group: the lateral corticospinal tract, corticobulbar tract, and rubrospinal tract. This group is primarily involved in making an independent goal-directed movement of distal limbs. This is important for reaching and object manipulation.
- The ventromedial group: the anterior corticospinal tract, vestibulospinal tract, reticulospinal, and tectospinal tract. This group primarily controls the more proximal muscles and trunk related to postural control and locomotion.
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Fig-2: The lateral (left) and medial (right) descending pathways of the voluntary movement; picture from [2]. |
Note that if you read electrophysiology and monkeys literature, you may encounter different terms carrying the same meaning, that is, cortical motor neuron whose cell body resides in the cortex Layer V: pyramidal tract neuron (PTN) or upper motor neuron (as opposed to the alpha or lower motor neuron). A part of these neurons that carries direct connections with the lower motor neurons is called corticomotorneurone (CM cells).
The corticobulbar tract projects to the cranial nerves and controls facial muscles, neck, and tongue (refer back to the homunculus drawing!) The reticulospinal tract is associated with motor neurons whose cell bodies are located in the reticular formation, a network of nuclei occupying the core of the brainstem from the upper midbrain to the lower medulla. This structure is related to various tasks such as movements, arousal & sleep, and some complex reflexes. Neurons in this area control more automatic movement such as postural adjustment, breathing, sneezing, coughing.
The final common pathway:
Coined by Sherrington, this term refers to the lower motor neurons that innervate the skeletal muscle to induce movements. It is named 'common' in the sense that no matter what descending tract the axons originated, eventually, they will synapse in the ventral horn with the alpha motor neurons. One neuron can make contact (innervate) to 5 - 1000 muscle fibers. As a whole, the lower motor neuron and its target muscle are called the motor unit.
There are different sizes of cell bodies. Smaller neurons have smaller axons and thus higher total resistance. This, in turn, causes the post-synaptic potential to reach the threshold more easily. Innervation of the alpha motor neurons to muscles vary in quantity. The size principle of the motor unit recruitment states the following:
- Muscle fibers targeted by the small alpha motor neurons are recruited (activated) first. With increasing force demand, muscle fibers targeted by large motor neurons will then start to be active.
- Muscle fibers capable of producing large force get fatigued easily and they are fast-twitch. These fibers are able to generate different forces quickly. Conversely, muscles that produce smaller force are slow-twitch fibers, but more fatigue-resistant muscle fibers.
- Small motor neurons innervate fewer muscle fibers. Larger neurons innervate more fibers.
More on reflexes:
A large body of research dedicated to reflex patterns exists. Here, I just wanna summarize some famous reflexes, e.g. the withdrawal reflex, knee-jerk reflex. These are examples of the stretch reflex, also known as a myotactic reflex (roughly 30 – 80 msec latency). It happens when the muscle contracts in direct response to its stretch.
Another form of reflexes called the long-loop reflexes are in response to somatosensory stimuli that are believed to operate via the cerebral cortex, hence the term transcortical reflexes. By definition, long loop reflexes occur at 60 – 120 msec latencies too long to be mediated by the circuits within the spinal cord yet too short to be mediated volitionally. Voluntary movements occur probably with > 150 msec latency.
The Hoffman reflex, or H reflex, is a type of reflex elicited through electrical stimulation. Hence this reflex is generally considered the electrical equivalent of the knee-jerk, differing only in that the H reflex bypasses muscle spindle mechanisms. The H reflex can be used to assess the response of the nervous system to various neurologic conditions, neuromuscular injury, training, etc.
[1] Chapter 33 - The Organization of Movement. In Kandel E.R. et. al. (2000). Principles of Neural Science 4e, McGraw-Hill.
[2] Chapter 8 - Control of Movement. In Carlson, Neil R. (2013). Physiology of Behavior 11e, Pearson Education.
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