9.4: The Function of Nervous Tissue

Learning Objectives

By the end of this section, you will be able to:

• Distinguish the major functions of the nervous system: sensation, integration, and response
• List the sequence of events in a simple sensory receptor–motor response pathway

Having looked at the components of nervous tissue, and the basic anatomy of the nervous system, next comes an understanding of how nervous tissue is capable of communicating within the nervous system. The most basic pathway your nervous system consists of a sensory receptor sending a stimuli to interneurons which determine a response, and then motor neurons that conduct that response.

Before getting to the nuts and bolts of how this works, an illustration of how the components come together will be helpful. An example is summarized in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Testing the Water: Posterior View (1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) The graded potential from the sensory endings, if strong enough, will initiate an action potential at the initial segment of the axon (which is immediately adjacent to the sensory endings in the skin). (3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. The contact is a synapse where another graded potential is caused by the release of a chemical signal from the axon terminals. (4) An action potential is initiated at the initial segment of this neuron and travels up the sensory pathway to a region of the brain called the thalamus. Another synapse passes the information along to the next neuron. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent from the precentral gyrus of the frontal cortex. (7) The upper motor neuron sends an action potential down to the spinal cord. The target of the upper motor neuron is the dendrites of the lower motor neuron in the gray matter of the spinal cord. (8) The axon of the lower motor neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.

Imagine you are about to take a shower in the morning before going to school. You have turned on the faucet to start the water as you prepare to get in the shower. After a few minutes, you expect the water to be a temperature that will be comfortable to enter. So you put your hand out into the spray of water. What happens next depends on how your nervous system interacts with the stimulus of the water temperature and what you do in response to that stimulus.

Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a thermoreceptor. When you place your hand under the shower (Figure \(\PageIndex{2}\) ), the thermoreceptors send action potentials that transmit the signal to interneurons in your spinal cord. These spinal cord interneurons then pass the signal along to interneurons in your brain. Interneurons in a region of your brain called the thalamus then send the signal to the sensory regions of your cerebral cortex for the signal to be integrated and processed into a perception of the water temperature and your surroundings. The role of the thalamus will be discussed in the chapter on the CNS.

Figure \(\PageIndex{2}\): The Sensory Input Receptors in the skin sense the temperature of the water.

Within the cerebral cortex, information is processed among many neurons, integrating the stimulus of the water temperature with other sensory stimuli, with your emotional state (you just aren't ready to wake up; the bed is calling to you), memories (perhaps of the lab notes you have to study before a quiz). Finally, a plan is developed about what to do, whether that is to turn the temperature up, turn the whole shower off and go back to bed, or step into the shower. To do any of these things, the cerebral cortex has to send a command out to your body to move muscles (Figure \(\PageIndex{3}\) ).

Figure \(\PageIndex{3}\): The Motor Response On the basis of the sensory input and the integration in the CNS, a motor response is formulated and executed.

Just like your cerebral cortex has specialized sensory regions, it also has specialized motor regions which determine your motor response to any given situation. In this case, in response to the sensory information just received in your cerebral cortex, motor neurons in your cerebral cortex will send signals all the way down your spinal cord and from your spinal cord to the muscles that will contract for the appropriate reaction. In this situation with the water temperature, your response may be to use your muscles to adjust the temperature until it is to your liking. What has just been described is a typical sensory-motor pathway used by your nervous system.

Career Connection

Neurophysiologist

Understanding how the nervous system works could be a driving force in your career. Studying neurophysiology is a very rewarding path to follow. It means that there is a lot of work to do, but the rewards are worth the effort.

The career path of a research scientist can be straightforward: college, graduate school, postdoctoral research, academic research position at a university. A Bachelor’s degree in science will get you started, and for neurophysiology that might be in biology, psychology, computer science, engineering, or neuroscience. But the real specialization comes in graduate school. There are many different programs out there to study the nervous system, not just neuroscience itself. Most graduate programs are doctoral, meaning that a Master’s degree is not part of the work. These are usually considered five-year programs, with the first two years dedicated to course work and finding a research mentor, and the last three years dedicated to finding a research topic and pursuing that with a near single-mindedness. The research will usually result in a few publications in scientific journals, which will make up the bulk of a doctoral dissertation. After graduating with a Ph.D., researchers will go on to find specialized work called a postdoctoral fellowship within established labs. In this position, a researcher starts to establish their own research career with the hopes of finding an academic position at a research university.

Other options are available if you are interested in how the nervous system works. Especially for neurophysiology, a medical degree might be more suitable so you can learn about the clinical applications of neurophysiology and possibly work with human subjects. An academic career is not a necessity. Biotechnology firms are eager to find motivated scientists ready to tackle the tough questions about how the nervous system works so that therapeutic chemicals can be tested on some of the most challenging disorders such as Alzheimer’s disease or Parkinson’s disease, or spinal cord injury.

Others with a medical degree and a specialization in neuroscience go on to work directly with patients, diagnosing and treating mental disorders. You can do this as a psychiatrist, a neuropsychologist, a neuroscience nurse, or a neurodiagnostic technician, among other possible career paths.

Basic Reflex Pathways

Reflexes are automated responses by your nervous system to stimuli, especially pain. For example, lifting your foot up after stepping on something sharp or closing your eyes when being sprayed with water are responses due to reflexes. Reflexes occur before you have a chance to consciously react to the stimuli. Many reflexes occur without even reaching your brain, instead causing a reaction only by reaching the spinal cord. This is one of the reasons that reflex responses occur so quickly. A basic reflex pathway or reflex arc will be described in the following paragraphs.

We will use an example of the withdrawal reflex, where a limb is withdrawn in response to pain stimuli. When you place your finger on a hot flame, sensory receptors in your finger respond to the exceedingly hot temperature and pain by sending signals to your spinal cord. An interneuron in your spinal cord immediately processes this harmful stimuli by signaling a motor neuron in the spinal cord to cause the limb muscles to pull the hand away. The motor neuron immediately contacts your arm muscles and you pull your finger away from the hot flame. Note how the stimuli did not need to reach your brain for your body to have a response. The basic steps of this reflex arc can be seen in Figure \(\PageIndex{4}\) below.

Figure \(\PageIndex{4}\) A basic reflex arc: The withdrawal reflex can be seen, with sensory stimuli triggering a withdrawal reflex, all without contacting the brain allowing for a rapid response. (Image credit: "Imgnotraçat arc reflex eng" by Jmarchn, CC BY-SA 3.0, via Wikimedia Commons) / Labels modified by Harmony Folse