So far we have discussed the types and distribution of somesthetic receptors, now let’s consider what the central nervous system might do with the information it is receiving from these receptors. One option is to make conscious decisions about how to react to the information, such as deciding how to change our behavior if we are cold or in pain.  Here we will consider using sensory information to inform somatic reflexes, where automatic motor responses occur as a result of the sensory stimuli.

Figure 1. Too Hot to Touch. When high temperature is sensed in the skin, a reflexive withdrawal is initiated by the muscles of the arm. Sensory neurons are activated by a stimulus, which is sent to the central nervous system, and a motor response is sent out to the skeletal muscles that control this movement.

The somatic nervous system is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our voluntary responses to that perception by means of skeletal muscles. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system.

The distinction between the structures (i.e., anatomy) of the peripheral and central nervous systems and functions (i.e., physiology) of the somatic and autonomic systems can most easily be demonstrated through a simple reflex action. When you touch a hot stove, you pull your hand away. Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. This triggers an action potential, which travels along the sensory fiber from the skin, through the dorsal spinal root to the spinal cord, and directly activates a ventral horn motor neuron. That neuron sends a signal along its axon to excite the biceps brachii, causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove. The withdrawal reflex has more components, such as inhibiting the opposing muscle and balancing posture while the arm is forcefully withdrawn, which will be further explored at the end of this chapter.

The basic withdrawal reflex explained above includes sensory input (the painful stimulus), central processing (the synapse in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the biceps brachii). Expanding the explanation of the withdrawal reflex can include inhibition of the opposing muscle, or cross extension, either of which increase the complexity of the example by involving more central neurons. A collateral branch of the sensory axon would inhibit another ventral horn motor neuron so that the triceps brachii do not contract and slow the withdrawal down. The cross extensor reflex provides a counterbalancing movement on the other side of the body, which requires another collateral of the sensory axon to activate contraction of the extensor muscles in the contralateral limb.

A more complex example of somatic function is conscious muscle movement. For example, reading of this text starts with visual sensory input to the retina, which then projects to the thalamus, and on to the cerebral cortex. A sequence of regions of the cerebral cortex process the visual information, starting in the primary visual cortex of the occipital lobe, and resulting in the conscious perception of these letters. Subsequent cognitive processing results in understanding of the content. As you continue reading, regions of the cerebral cortex in the frontal lobe plan how to move the eyes to follow the lines of text. The output from the cortex causes activity in motor neurons in the brain stem that cause movement of the extraocular muscles through the third, fourth, and sixth cranial nerves. This example also includes sensory input (the retinal projection to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brain stem that lead to coordinated contraction of extraocular muscles).

Either the brain or the spinal cord can act as the integrating center to generate a reflex response.  If the brain is the integrating center, the reaction is a cranial reflex, if its the spinal cord is the integrating center then it is a spinal reflex. No matter which type of CNS mediated reflex or response we are referring to, the general model is the same. Sensory information activates a receptor that sends information to the CNS via an afferent neuron, some level of synaptic or higher level processing occurs, and, if a response is necessary or appropriate, the response is initiated through efferent neurons. You might recognize this as the same model used to maintain homeostasis. Reflexes are a unique category of responses because they do not require the higher centers used for conscious or voluntary responses. Instead reflexes are involuntary, stereotyped (they are repeatable under the same stimulus conditions) responses that occur quickly.

Categories of Reflexes

As mentioned, reflexes can either be visceral or somatic. Visceral reflexes involve a glandular or non-skeletal muscular response carried out in internal organs such as the heart, blood vessels, or structures of the GI tract. They utilize neurons of the autonomic nervous system to elicit their actions. Visceral reflexes have been more fully discussed in the section on the autonomic nervous system. In contrast, somatic reflexes involve unconscious skeletal muscle motor responses. In doing so, these reflexes utilize some of the same lower motor neurons (alpha motor neurons) used to control skeletal muscle during conscious movement. Because reflexes are quick, it makes sense that somatic reflexes are often meant to protect us from injury. As examples, reflexes contribute to the maintenance of balance and rapid withdrawal of the hand or foot from damaging stimuli.

Somatic reflexes can either be intrinsic (present at birth) or learned. We will be focusing on intrinsic reflexes, which occur as the result of normal human development. Learned reflexes are much more complicated in their anatomical structure and result from repetitive actions, such as athletic training. Reflexes can also be categorized by the number of synapses they involve (monosynaptic reflex versus polysynaptic reflex) or the relative position of the sensory receptors to the responding muscles (ipsilateral = same side of the body, contralateral = opposite sides of the body).

Somatic Reflexes

Flexor (Withdrawal) Reflex

Recall from the beginning of this unit that when you touch a hot stove, you reflexively pull your hand away. Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage. To avoid further damage, information travels along the sensory fibers from the skin and into the posterior (dorsal) horn of the spinal cord. Once in the spinal cord, the sensory fibers synapse with a variety of interneurons that mediate the responses of the reflex. These responses included a strong initial withdrawal of the flexor muscle (caused by activation of the alpha motor neurons), inhibition of the extensor muscle (mediated through inhibitory interneurons), and sustained contraction of the flexor (mediated by a spinal cord neuronal circuit).  Without the antagonistic contraction, withdrawal from the hot stove is faster and keeps further tissue damage from occurring.  And as already discussed, the sensory information will also travel to the brain to develop a conscious awareness of the situation such that conscious decision-making can take over immediately after the reflex occurs.

Another example of a withdrawal reflex occurs when you step on a painful stimulus, like a tack or a sharp rock. The nociceptors that are activated by the painful stimulus activate the motor neurons responsible for contraction of the tibialis anterior muscle. This causes dorsiflexion of the foot. An inhibitory interneuron, activated by a collateral branch of the nociceptor fiber, will inhibit the motor neurons of the gastrocnemius and soleus muscles to cancel plantar flexion. An important difference in this reflex is that plantar flexion is most likely in progress as the foot is pressing down onto the tack. Contraction of the tibialis anterior is not the most important aspect of the reflex, as continuation of plantar flexion will result in further damage from stepping onto the tack.

Crossed-Extensor Reflex

Imagine what would happen if, when you stepped on a sharp object, it elicited a strong withdrawal reflex of your leg. If you weren’t ready to put all of your weight on your other leg, you would likely topple over. In order to prevent this from happening, as the flexor (withdrawal) reflex involving the injured leg happens, an extension reflex of the opposite (contralateral) leg occurs at the same time, creating a crossed-extensor reflex. In this case, the ipsilateral limb reacts with a withdrawal reflex (stimulating flexor muscles and inhibiting extensor muscles on same side), but the contralateral extensor muscles contract so that the person can appropriately shift balance to the opposite foot during the reflex.

The Stretch Reflex

Another type of reflex is a stretch reflex. 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. The reflex helps to maintain muscles at a constant length. 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.

In this reflex, when a skeletal muscle is stretched, a muscle spindle in the belly of the muscle is activated. The axon from this receptor travels to the spinal cord where it synapses with the motor neuron controlling the muscle, stimulating it to contract. This is a rapid, monosynaptic, ipsilateral reflex that helps to maintain the length of muscles and contributes to joint stabilization. A common example of this reflex is the knee jerk reflex that is elicited by a rubber hammer striking against the patellar tendon, such as during a physical exam. When the hammer strikes, it stretches the tendon, which pulls on the quadriceps femoris muscle. Because bones and tendons do not typically pull muscles, the muscle “thinks” it is stretching very rapidly, and the reflex acts to counteract this stretch. In doing so, the “knee jerk” occurs.

Along with the monosynaptic activation of the alpha motor neuron, this reflex also includes the activation of an interneuron that inhibits the alpha motor neuron of the antagonistic muscle. This aspect of the reflex ensures that contraction of the agonist muscle occurs unopposed.

The Corneal Reflex

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.