6.5: Nervous System Pathways

Reflexes

The individual components of the nervous system work together to regulate the operations of parts of the body in order to maintain homeostasis. The simplest level of regulation involves a reflex, which is an involuntary response to a stimulus. Reflexes that use somatic neurons include blinking when something moves close to the eyes, coughing when something gets caught in the throat, and withdrawing from something that is painful. All activities controlled by autonomic neurons are reflex responses.

Many reflexes are built into the nervous system as it develops before birth. Others are acquired reflexes which develop when a person repeats the response every time a certain stimulus occurs. These reflexes involve the use of unconscious remembering.

A reflex occurs in basically the same way every time a particular stimulus occurs because the nervous system pathway that causes it is firmly established. Sensory neurons detect the stimulus and communicate through synapses with specific neurons in the CNS, and the CNS neurons quickly communicate with specific motor neurons. In a few reflex pathways, such as the one for the knee jerk, sensory neurons synapse directly with motor neurons. In either case the motor neurons complete the pathway by sending impulses to a muscle or gland, causing it to make the response.

Note that reflex pathways involve monitoring, communicating, and stimulating (or inhibiting). In many reflexes the adjustment caused by the response prevents or reverses the situation created by the stimulus. For example, the cough reflex removes material that enters the airways. These reflexes therefore are negative feedback systems that help maintain homeostasis. The responses produced by other reflexes contribute to homeostasis by improving conditions for the body. For example, the sight and smell of appetizing food cause a reflex that increases the secretion of saliva, which will be useful when the person begins to eat because it makes swallowing easier.

Some reflexes simultaneously use sensory impulses from several types of sense organs, such as the eyes, ears, skin receptors, and proprioceptors. Proprioceptors detect motion and tension in muscles and at joints. Some reflexes require a considerable amount of coordination by both brain and spinal cord interneurons and synapses. Some are influenced by voluntary motor impulses or by higher brain activities such as emotions and thinking, which send modifying impulses into the reflex synapses.

Reflex Pathways

The specific parts and activities in a reflex pathway must be understood to appreciate the effects of aging on reflexes. The withdrawal reflex that occurs when a sharp object jabs the bottom of the foot provides a good example (Figure 6.4).

Figure 6.4 Reflex pathways involving skeletal muscle (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

When sensory neurons in the skin of the left foot detect the intense pressure caused by stepping on a sharp object, their dendrites carry out (1) reception. This causes the dendrites to (2) conduct impulses up through the nerve in the leg. These impulses reach and enter the gray matter in the back of the spinal cord via the sensory neuron axons, which (3) transmit them through synapses to other neurons in the spinal cord gray matter. Since these next neurons extend from one neuron to another, they are called interneurons. The interneurons (4) transmit the impulses to somatic motor neurons in the front part of the gray matter of the spinal cord. The impulses are then (5) conducted down the motor axons in the nerves in the left leg to certain muscles in the thigh and calf. Neurotransmitters from the motor axons (6) stimulate these muscles to contract, causing the response of lifting the foot and thus relieving the intense pressure and protecting the foot from harm.

Proper reflex responses may require coordination in addition to monitoring, communicating, stimulating, and unconscious remembering. For example, to prevent loss of balance when lifting the foot, cooperation by a second reflex must occur. Branches of the sensory axons transmit impulses to other interneurons that cross over to the right side of the spinal cord. These crossing interneurons (7) transmit the impulses to other somatic motor neurons in the right side of the gray matter. Impulses in these motor neurons are (8) conducted down the nerves in the right leg. The impulses cause certain muscles in the right leg to contract, resulting in a straightening of the right leg at the same time that the left leg is bending and lifting the foot off the object. In this way, the right leg supports the weight of the body so that the person does not fall down.

Another aspect of coordination is shown by the withdrawal reflex. As the interneurons stimulate motor neurons to the muscles that will make the appropriate actions occur, the interneurons send (9) inhibitory impulses to motor neurons controlling leg muscles that would interfere with the proper movements. This prevents antagonism among the muscles.

The reflex pathway for the withdrawal reflex is a fairly simple one. Other reflex pathways may involve interneurons that extend up or down the spinal cord or through several areas of the brain. Countless synapses may become involved before the impulses are finally transmitted to the motor neurons. Autonomic reflexes are further complicated by the synapses in the PNS. This increased complexity permits more coordination and modulation in responses. However, more complicated reflex pathways operate in essentially the same manner as simple reflex pathways.

Conscious Sensation

Though a reflex is completely involuntary and requires no conscious awareness, a person may feel the stimulus. For example, a person feels a sharp object jabbing the foot because the sensory neurons may synapse with other interneurons extending up to the brain. These other neurons help form the conscious sensory pathways in the nervous system.

Information from perceived sensations is used to initiate and adjust voluntary actions so that people can respond properly to conditions in their bodies and the world around them. These sensations provide information necessary for learning. Finally, conscious sensation provides much of the enjoyment that makes life worthwhile.

All conscious sensory pathways begin in the same way as do reflex pathways. That is, sensory neurons that have carried out reception conduct impulses into the CNS (Figure 6.5). Sensory neurons that monitor regions below the head extend into the spinal cord, while those which monitor the head region pass into the brain. Once in the CNS, sensory impulses are passed to interneurons extending into the gray matter of the brain.

Figure 6.5 A conscious sensory pathway (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Impulses in each type of sensory neuron and from each part of the body are directed by synapses to the part of the brain designed to monitor that type of stimulus from that region. For example, impulses from the eyes are sent to vision centers, while impulses from the auditory parts of the ears are sent to hearing centers. The impulses are interpreted as perceived sensations when they reach the appropriate areas of the cerebral cortex, a layer of gray matter on the surface of the cerebral hemispheres. The postcentral gyrus is a raised area of the cortex on each cerebral hemisphere that is concerned mostly with conscious sensations from the integumentary, muscle, and skeletal systems (Figure 6.5). Other regions of the cortex are used for the special senses, such as vision, hearing, and smell.

Voluntary Movements

In many situations a person voluntarily chooses to move or not move in response to stimuli. One can also choose the type and degree of motion to make. For example, if someone calls, a person can choose to answer or not answer. If that person answers, the response may include a variety of motions or sounds. If the response is vocal, the sounds produced may be loud or soft, enunciated quickly or slowly, and projected with different intonations.

In addition to deciding whether to move in response to conscious stimuli, one can decide whether to take action on the basis of internal thought processes. Again, the type and degree of motion are usually up to the individual. Thus, one need not be called in order to decide to move or say something. A person may spontaneously decide to start a conversation or simply to sing.

Voluntary movements allow a person to take what he or she judges to be an appropriate action to optimize conditions in a given situation. Unlike reflex responses, voluntary movements allow freedom to select among many options rather than forcing a person to respond in a particular way.

Whether voluntary motion is initiated by stimuli or by thought processes in the brain, the nervous system pathway causing the motion is the same. It is called the somatic motor pathway because it controls voluntary muscles. The somatic motor pathway begins in a band of the cerebral cortex running down the side of each cerebral hemisphere. Each band is called a precentral gyrus (Figure 6.6).

Figure 6.6 A somatic (voluntary) motor pathway. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Each region of a precentral gyrus is designed to control the voluntary muscles in one area of the body. To move, a person (1) starts impulses from the area of the precentral gyrus that controls the muscles for the part of the body to be moved. Impulses from the precentral gyrus begin to (2) travel down the brain through white matter. As the impulses descend, they pass through areas of gray matter, where they are modified as they move through the gray matter synapses. In this way, the motion is performed at exactly the speed, strength, and distance chosen. Several important areas of gray matter that modify the motor impulses are called the (3) basal ganglia, located inside the cerebral hemispheres. In general, the basal ganglia dampen motor impulses so that motions are not exaggerated.

Descending motor neurons are also channeled through the gray matter of the (4) cerebellum, which lies behind and below the cerebral hemispheres. Its gray matter forms a wrinkled coating called the (5) cerebellar cortex. The synapses in the cerebellar cortex modify the impulses so that the resulting motion starts and stops smoothly, at the proper time, and within the desired distance. The cerebellar cortex also adds impulses to ensure that all muscles that can assist in the motion are stimulated appropriately. The additional impulses activate muscles that move in the same direction and muscles that hold other parts of the body still or prevent loss of balance. At the same time the cerebellar cortex blocks impulses that would cause muscle contractions antagonistic to the desired action.

The cerebellar cortex continues to work throughout the time during which the desired action is occurring. It monitors the motion that is occurring and, if the motion is not exactly what was intended, provides impulses to muscles that can correct the error. With practice, the cerebellar cortex improves its ability to adjust the action, leading to increasing skill at performing that action. Similar control activities occur in the part of the cerebral cortex in front of the precentral gyrus.

Other synapses in the descending somatic motor pathways modify impulses to a lesser degree. Finally, the impulses reach (6) synapses to the dendrites and cell bodies of the somatic motor neurons. These synapses are the last places where the impulses can be modified. Once within the somatic motor neurons, the impulses leave the CNS and travel along the (7) motor axons in the nerves.

Upon arriving at the ends of the motor axons, the impulses cause the (8) release of the neurotransmitter acetylcholine. Like neurotransmitters in synapses, acetylcholine binds to the receptor molecules on the cell membranes of muscle cells. Once enough acetylcholine is bound, the muscle cells initiate the steps that lead to contraction, producing the desired action, Enzymes from the muscle cells then destroy the acetylcholine, and the cells relax until the next nerve impulses arrive.

The brain improves the efficiency of this process by sending some impulses to somatic motor neurons just before the person attempts a motion. Some of these anticipatory impulses are sent to motor neurons controlling the muscles that will contract, making them more sensitive to the main impulses telling the muscle to contract. The result is that when the motion should occur, the correct muscle moves faster and stronger while muscles that oppose its motion are inactivated.

Higher‑Level Functions

The nervous system is also involved in activities that produce conscious remembering, thinking, interpretations, emotions, and personality traits. All these higher‑level functions take place in the brain.

The neuron pathways that produce these activities are poorly understood. There seem to be complicated interactions among several areas of the brain for each activity. Also, each activity seems to influence and interact with the others. However, many of the areas of the brain that are involved with these higher‑level functions have been identified, and some of the details of their operations have been discovered.