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13.5: Somatic Motor Responses

  • Page ID
    22345
  • By the end of this section, you will be able to:

    • List the cortical components of motor processing
    • Describe the pathway of descending motor commands from the cortex to the skeletal muscles
    • Compare different descending pathways, both by structure and function
    • List the structures and steps involved in a reflex arc
    • Describe several reflex arcs and their functional roles

    The defining characteristic of the somatic nervous system is that it controls skeletal muscles. Somatic senses inform the nervous system about the external environment, but the response to that is through voluntary muscle movement. The term “voluntary” suggests that there is a conscious decision to make a movement. However, some aspects of the somatic system use voluntary muscles without conscious control. One example is the ability of our breathing to switch to unconscious control while we are focused on another task. However, the muscles that are responsible for the basic process of breathing are also utilized for speech, which is entirely voluntary.

    Cortical Responses

    The information on sensory stimuli registered through receptor cells is relayed to the CNS along ascending pathways. In the cerebral cortex, the initial processing of sensory perception can lead to the incorporation of sensory perceptions into memory, but more importantly, it leads to a response. The completion of cortical processing through the primary, associative, and integrative sensory areas initiates a similar progression of motor processing, usually in different cortical areas.

    Prefrontal Cortex

    Whereas the sensory cortical areas are located in the occipital, temporal, and parietal lobes, motor functions are largely controlled by the frontal lobe. The most anterior regions of the frontal lobe—the prefrontal areas—are important for executive functions, which are cognitive functions that lead to goal-directed behaviors. These higher cognitive processes include working memory that can help organize and represent information that is not in the immediate environment. The prefrontal lobe is responsible for aspects of attention, such as inhibiting distracting thoughts and actions so that a person can focus on a goal and direct behavior toward achieving that goal. The functions of the prefrontal cortex are integral to the personality of an individual, because it is largely responsible for what a person intends to do and how they accomplish those plans.

    Secondary Motor Cortices

    In generating motor responses, the executive functions of the prefrontal cortex will need to initiate actual movements. One way to define the prefrontal area is any region of the frontal lobe that does not elicit movement when electrically stimulated. These are primarily in the anterior part of the frontal lobe. The regions of the frontal lobe that remain are the regions of the cortex that produce movement. The prefrontal areas project into the secondary motor cortices, which include the premotor cortex and the supplemental motor area. The premotor area aids in controlling movements of the core muscles to maintain posture during movement, whereas the supplemental motor area is hypothesized to be responsible for planning and coordinating movement. The supplemental motor area also manages sequential movements that are based on prior experience (that is, learned movements). Neurons in these areas are most active leading up to the initiation of movement. For example, these areas might prepare the body for the movements necessary to drive a car in anticipation of a traffic light changing.

    Adjacent to these two regions are two specialized motor planning centers. The frontal eye fields are responsible for moving the eyes in response to visual stimuli. There are direct connections between the frontal eye fields and the superior colliculus. Also, anterior to the premotor cortex and primary motor cortex is Broca’s area. This area is responsible for controlling movements of the structures of speech production.

    Primary Motor Cortex

    The primary motor cortex is located in the precentral gyrus of the frontal lobe. The primary motor cortex receives input from several areas that aid in planning movement, and its principle output stimulates spinal cord neurons to stimulate skeletal muscle contraction. The primary motor cortex is arranged in a similar fashion to the primary somatosensory cortex, in that it has a topographical map of the body, creating a motor homunculus. The neurons responsible for musculature in the feet and lower legs are in the medial wall of the precentral gyrus, with the thighs, trunk, and shoulder at the crest of the longitudinal fissure. The hand and face are in the lateral face of the gyrus. Also, the relative space allotted for the different regions is exaggerated in muscles that have greater enervation. The greatest amount of cortical space is given to muscles that perform fine, agile movements, such as the muscles of the fingers and the lower face. The “power muscles” that perform coarser movements, such as the buttock and back muscles, occupy much less space on the motor cortex.

    Descending Pathways

    The motor output from the cortex descends into the brainstem and to the spinal cord to control the musculature through motor neurons. Neurons located in the primary motor cortex, named Betz cells, are large cortical neurons that synapse with lower motor neurons in the spinal cord or the brainstem. The two descending pathways traveled by the axons of Betz cells are the corticospinal tract and the corticobulbar tract. Both tracts are named for their origin in the cortex and their targets—either the spinal cord or the brainstem (the term “bulbar” refers to the brainstem as the bulb, or enlargement, at the top of the spinal cord).

    These two descending pathways are responsible for the conscious or voluntary movements of skeletal muscles. Any motor command from the primary motor cortex is sent down the axons of the Betz cells to activate upper motor neurons in either the cranial motor nuclei or in the ventral horn of the spinal cord. The axons of the corticobulbar tract are ipsilateral, meaning they project from the cortex to the cranial motor nucleus on the same side of the nervous system. The corticobulbar tract controls the movement of muscles in the face, head and neck. Conversely, the axons of the corticospinal tract are largely contralateral, meaning that they cross the midline of the brainstem or spinal cord and synapse on the opposite side of the body. The corticospinal tract controls movement of muscles of limbs and trunk. Therefore, the right motor cortex of the cerebrum controls muscles on the left arm, for example, and vice versa.

    The corticospinal tract descends from the cortex through the deep white matter of the cerebrum. It then passes between the caudate nucleus and putamen of the basal nuclei as a bundle called the internal capsule. The tract then passes through the midbrain as the cerebral peduncles, after which it burrows through the pons. Upon entering the medulla, the tracts make up the large white matter tract referred to as the pyramids (Figure \(\PageIndex{1}\)). The defining landmark of the medullary-spinal border is the pyramidal decussation, which is where most of the fibers in the corticospinal tract cross over to the opposite side of the brain. At this point, the tract separates into two parts, which have control over different domains of the musculature.

    Sections of brain, midbrain, medulla and spinal cord with descending bundles of axons in each region
    Figure \(\PageIndex{1}\): Corticospinal Tract. The major descending tract that controls skeletal muscle movements is the corticospinal tract. It is composed of two neurons, the upper motor neuron and the lower motor neuron. The upper motor neuron has its cell body in the primary motor cortex located in the precentral gyrus of the frontal lobe. Its axon descend through the cerebral peduncle of the midbrain and crosses the midline at the medulla through the decussation of pyramids. Traveling down through the anterior and lateral corticospinal tracts, it then synapses on the lower motor neuron, which is in the ventral horn of the spinal cord and projects to the skeletal muscle in the periphery. (Image credit: "Corticospinal Pathway" by OpenStax is licensed under CC BY 3.0)

    Appendicular Control

    The lateral corticospinal tract is composed of the fibers that cross the midline at the pyramidal decussation (see Figure \(\PageIndex{1}\)). The axons cross over from the anterior position of the pyramids in the medulla to the lateral column of the spinal cord. These axons are responsible for controlling appendicular muscles.

    This influence over the appendicular muscles means that the lateral corticospinal tract is responsible for moving the muscles of the arms and legs. The lower cervical spinal cord and the lumbar spinal cord both have wider ventral horns, representing the greater number of muscles controlled by these motor neurons. The cervical enlargement is particularly large because there is greater control over the fine musculature of the upper limbs, particularly of the fingers. The lumbar enlargement is not as significant in appearance because there is less fine motor control of the lower limbs.

    Axial Control

    The anterior corticospinal tract is responsible for controlling the muscles of the body trunk (see Figure \(\PageIndex{1}\)). These axons do not decussate in the medulla. Instead, they remain in an anterior position as they descend the brainstem and enter the spinal cord. These axons then travel to the spinal cord level at which they synapse with a lower motor neuron. Upon reaching the appropriate level, the axons decussate, entering the ventral horn on the opposite side of the spinal cord from which they entered. In the ventral horn, these axons synapse with their corresponding lower motor neurons. The lower motor neurons are located in the medial regions of the ventral horn, because they control the axial muscles of the trunk.

    Because movements of the body trunk involve both sides of the body, the anterior corticospinal tract is not entirely contralateral. Some collateral branches of the tract will project into the ipsilateral ventral horn to control synergistic muscles on that side of the body, or to inhibit antagonistic muscles through interneurons within the ventral horn. Through the influence of both sides of the body, the anterior corticospinal tract can coordinate postural muscles in broad movements of the body. These coordinating axons in the anterior corticospinal tract are often considered bilateral, as they are both ipsilateral and contralateral.

    Extrapyramidal System

    Other descending connections between the brain and the spinal cord are called the extrapyramidal system. The name comes from the fact that this system is outside the corticospinal pathway, which includes the pyramids in the medulla. A few pathways originating from the brainstem contribute to this system.

    The tectospinal tract projects from the midbrain to the spinal cord and is important for postural movements that are driven by the superior colliculus (Figure \(\PageIndex{2}\)). The name of the tract comes from an alternate name for the superior colliculus, which is the tectum. The reticulospinal tract connects the reticular system, a diffuse region of gray matter in the brainstem, with the spinal cord. This tract influences trunk and proximal limb muscles related to posture and locomotion. The reticulospinal tract also contributes to muscle tone and influences autonomic functions. The vestibulospinal tract connects the brainstem nuclei of the vestibular system with the spinal cord. This allows posture, movement, and balance to be modulated on the basis of equilibrium information provided by the vestibular system. The pathways of the extrapyramidal system are influenced by subcortical structures. For example, connections between the secondary motor cortices and the extrapyramidal system modulate spine and cranium movements. The basal nuclei, which are important for regulating movement initiated by the CNS, influence the extrapyramidal system as well as its thalamic feedback to the motor cortex.

    Figure \(\PageIndex{2}\) summarizes both ascending and descending pathways.

    Section of spinal cord with red and blue sections in the white matter representing efferent and afferent pathways
    Figure \(\PageIndex{2}\): Ascending and Descending Tracts. A cross section of the spinal cord shows the sensory ascending tracts (in blue) and motor descending tracts (in red). Ascending sensory pathways include the dorsal column medial lemniscus system, the spinocerebellar tracts and anterolateral system. Descending motor pathways include pyramidal tracts and extrapyramidal tracts. (Image credit: "Spinal Cord Tracts - English" by Polarlys and Mikael Häggström is licensed under CC BY-SA 3.0)

    Ventral Horn Output

    The somatic nervous system provides output strictly to skeletal muscles. The lower motor neurons, which are responsible for the contraction of these muscles, are found in the ventral horn of the spinal cord. These large, multipolar neurons have a corona of dendrites surrounding the cell body and an axon that extends out of the ventral horn. This axon travels through the ventral nerve root to join the emerging spinal nerve. The axon is relatively long because it needs to reach muscles in the periphery of the body. The diameters of cell bodies may be on the order of hundreds of micrometers to support the long axon; some axons are a meter in length, such as the lumbar motor neurons that innervate muscles in the first digits of the feet.

    The axons will also branch to innervate multiple muscle fibers. Together, the motor neuron and all the muscle fibers that it controls make up a motor unit. Motor units vary in size. Some may contain up to 1000 muscle fibers, such as in the quadriceps, or they may only have 10 fibers, such as in an extraocular muscle. The number of muscle fibers that are part of a motor unit corresponds to the precision of control of that muscle. Also, muscles that have finer motor control have more motor units connecting to them, and this requires a larger topographical field in the primary motor cortex.

    Somatic Reflexes

    This chapter began by introducing reflexes as an example of the basic elements of the somatic nervous system. Simple somatic reflexes do not include the higher centers discussed for conscious or voluntary aspects of movement. Reflexes can be spinal or cranial, depending on the nerves and central components that are involved.

    The example described at the beginning of the chapter involved heat and pain sensations from a hot stove causing withdrawal of the arm through a connection in the spinal cord that leads to contraction of the biceps brachii (Figure \(\PageIndex{3}\)). The description of this withdrawal reflex was simplified, for the sake of the introduction, to emphasize the parts of the somatic nervous system. But to consider reflexes fully, more attention needs to be given to this example.

    As you withdraw your hand from the stove, you do not want to slow that reflex down. Consequently, as the biceps brachii contracts, the antagonistic triceps brachii needs to relax. In the hot-stove withdrawal reflex, this occurs through an interneuron in the spinal cord. The interneuron’s cell body is located in the dorsal horn of the spinal cord. The interneuron receives a synapse from the axon of the sensory neuron that detects that the hand is being burned. In response to this stimulation from the sensory neuron, the interneuron then inhibits the motor neuron that controls the triceps brachii. Without the antagonistic contraction, withdrawal from the hot stove is faster and keeps further tissue damage from occurring.

    Person touching a source of heat with one hand and withdrawing it. Skeletal muscle of the arm and spinal cord section shown too.
    Figure \(\PageIndex{3}\): Withdrawal Reflex Arc. Nerve impulse of a stimulus is sensed by two or more neurons in the skin of the fingers. The spinal cord receives sensory impulses of the body and, through an interneuron, sends impulses to the arm muscle through spinal nerves. After receiving the order, the muscle runs the command, in this case to remove the finger from the stimulus. (Image credit: "Imgnotraçat arc reflex eng" by MartaAguayo is licensed under CC BY-SA 3.0)

    Another type of reflex is a stretch reflex shown in Figure \(\PageIndex{4}\). In this reflex, when a skeletal muscle is stretched, a muscle spindle receptor is activated. The axon from this receptor structure will cause direct contraction of the muscle. A collateral of the muscle spindle fiber will also inhibit the motor neuron of the antagonist muscles. A common example of this reflex is the knee jerk that is elicited by a rubber hammer struck against the patellar ligament in a physical exam. The muscle is quickly stretched, resulting in activation of the muscle spindle that sends a signal into the spinal cord through the dorsal root. The fiber synapses directly on the ventral horn motor neuron that activates the muscle, causing contraction. The reflexes are physiologically useful for stability. If a muscle is stretched, it reflexively contracts to return the muscle to compensate for the change in length. In the context of the neurological exam, reflexes indicate that the lower motor neuron is functioning properly.

    Seated person flexing leg under patellar hammer. Skeletal muscle of the leg and section of spinal cord shown too.
    Figure \(\PageIndex{4}\): Stretch Reflex. When an extensor muscle is stretched (1), muscle spindles are stimulated (2) and send information to the spinal cord through a primary afferent neuron (3). The primary afferent neurons synapses on alpha motor neuron of the same extensor muscle (4) causing it to contract (5). At the same time, the primary afferent neuron stimulates an inhibitory interneuron (6) which inhibits the alpha motor neuron to the flexor muscle, preventing its contraction (7). Consequently, the flexor muscle (antagonist) relaxes. (Image credit: "Stretch Reflex" by Cenveo is licensed under CC BY 3.0)

    A specialized reflex to protect the surface of the eye is the corneal reflex, or the eye blink reflex. When the cornea is stimulated by a tactile stimulus, or even by bright light in a related reflex, blinking is initiated. The sensory component travels through the trigeminal nerve, which carries somatosensory information from the face, or through the optic nerve, if the stimulus is bright light. The motor response travels through the facial nerve and innervates the orbicularis oculi on the same side. This reflex is commonly tested during a physical exam using an air puff or a gentle touch of a cotton-tipped applicator.

    Concept Review

    The motor components of the somatic nervous system begin with the frontal lobe of the brain, where the prefrontal cortex is responsible for higher functions such as working memory. The integrative and associate functions of the prefrontal lobe feed into the secondary motor areas, which help plan movements. The premotor cortex and supplemental motor area then feed into the primary motor cortex that initiates movements. Large Betz cells project through the corticobulbar and corticospinal tracts to synapse on lower motor neurons in the brainstem and ventral horn of the spinal cord, respectively. These connections are responsible for generating movements of skeletal muscles.

    The extrapyramidal system includes projections from the brainstem and higher centers that influence movement, mostly to maintain balance and posture, as well as to maintain muscle tone. The superior colliculus and red nucleus in the midbrain, the vestibular nuclei in the medulla, and the reticular formation throughout the brainstem each have tracts projecting to the spinal cord in this system. Descending input from the secondary motor cortices, basal nuclei, and cerebellum connect to the origins of these tracts in the brainstem.

    All of these motor pathways project to the spinal cord to synapse with motor neurons in the ventral horn of the spinal cord. These lower motor neurons are the cells that connect to skeletal muscle and cause contractions. These neurons project through the spinal nerves to connect to the muscles at neuromuscular junctions. One motor neuron connects to multiple muscle fibers within a target muscle. The number of fibers that are innervated by a single motor neuron varies on the basis of the precision necessary for that muscle and the amount of force necessary for that motor unit. The quadriceps, for example, have many fibers controlled by single motor neurons for powerful contractions that do not need to be precise. The extraocular muscles have only a small number of fibers controlled by each motor neuron because moving the eyes does not require much force, but needs to be very precise.

    Reflexes are the simplest circuits within the somatic nervous system. A withdrawal reflex from a painful stimulus only requires the sensory fiber that enters the spinal cord and the motor neuron that projects to a muscle. Antagonist and postural muscles can be coordinated with the withdrawal, making the connections more complex. The simple, single neuronal connection is the basis of somatic reflexes. The corneal reflex is contraction of the orbicularis oculi muscle to blink the eyelid when something touches the surface of the eye. Stretch reflexes maintain a constant length of muscles by causing a contraction of a muscle to compensate for a stretch that can be sensed by a specialized receptor called a muscle spindle.

    Review Questions

    Q. Which region of the frontal lobe is responsible for initiating movement by directly connecting to cranial and spinal motor neurons?

    A. prefrontal cortex

    B. supplemental motor area

    C. premotor cortex

    D. primary motor cortex

    Answer

    D

    Q. Which extrapyramidal tract incorporates equilibrium sensations with motor commands to aid in posture and movement?

    A. tectospinal tract

    B. vestibulospinal tract

    C. reticulospinal tract

    D. corticospinal tract

    Answer

    B

    Q. Which region of gray matter in the spinal cord contains motor neurons that innervate skeletal muscles?

    A. ventral horn

    B. dorsal horn

    C. lateral horn

    D. lateral column

    Answer

    A

    Glossary

    anterior corticospinal tract
    division of the corticospinal pathway that travels through the ventral (anterior) column of the spinal cord and controls axial musculature through the medial motor neurons in the ventral (anterior) horn
    Betz cells
    output cells of the primary motor cortex that cause musculature to move through synapses on cranial and spinal motor neurons
    Broca’s area
    region of the frontal lobe associated with the motor commands necessary for speech production
    cerebral peduncles
    segments of the descending motor pathway that make up the white matter of the ventral midbrain
    cervical enlargement
    region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of and finer control of muscles of the upper limb
    corneal reflex
    protective response to stimulation of the cornea causing contraction of the orbicularis oculi muscle resulting in blinking of the eye
    corticobulbar tract
    connection between the cortex and the brainstem responsible for generating movement
    corticospinal tract
    connection between the cortex and the spinal cord responsible for generating movement
    executive functions
    cognitive processes of the prefrontal cortex that lead to directing goal-directed behavior, which is a precursor to executing motor commands
    extrapyramidal system
    pathways between the brain and spinal cord that are separate from the corticospinal tract and are responsible for modulating the movements generated through that primary pathway
    frontal eye fields
    area of the prefrontal cortex responsible for moving the eyes to attend to visual stimuli
    internal capsule
    segment of the descending motor pathway that passes between the caudate nucleus and the putamen
    lateral corticospinal tract
    division of the corticospinal pathway that travels through the lateral column of the spinal cord and controls appendicular musculature through the lateral motor neurons in the ventral (anterior) horn
    lumbar enlargement
    region of the ventral (anterior) horn of the spinal cord that has a larger population of motor neurons for the greater number of muscles of the lower limb
    premotor cortex
    cortical area anterior to the primary motor cortex that is responsible for planning movements
    pyramidal decussation
    location at which corticospinal tract fibers cross the midline and segregate into the anterior and lateral divisions of the pathway
    pyramids
    segment of the descending motor pathway that travels in the anterior position of the medulla
    reticulospinal tract
    extrapyramidal connections between the brainstem and spinal cord that modulate movement, contribute to posture, and regulate muscle tone
    stretch reflex
    response to activation of the muscle spindle stretch receptor that causes contraction of the muscle to maintain a constant length
    supplemental motor area
    cortical area anterior to the primary motor cortex that is responsible for planning movements
    tectospinal tract
    extrapyramidal connections between the superior colliculus and spinal cord
    vestibulospinal tract
    extrapyramidal connections between the vestibular nuclei in the brainstem and spinal cord that modulate movement and contribute to balance on the basis of the sense of equilibrium
    withdrawal reflex
    automatic withdrawal of an extremity (e.g. a hand) from a painful stimulus
    working memory
    function of the prefrontal cortex to maintain a representation of information that is not in the immediate environment

    Contributors and Attributions

    OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology