Chapter 9: Apply knowledge of the nervous system special senses and their functions

Understanding the nervous system and its special senses is vital for nutrition and dietetics professionals because these systems regulate appetite, digestion, and metabolic processes. The nervous system controls sensory inputs such as taste and smell, which directly influence food preferences and nutritional status. In Kenya, dietitians working in hospitals or community health settings must appreciate how nervous system functions affect patient nutrition, including conditions like neuropathies or sensory impairments that alter dietary intake.

9.1 Neurons

Neurons (nerve cells) are the working cells of the nervous system. They receive information, process it and pass it on as electrical and chemical signals, which allows the body to sense its surroundings, move and regulate its internal organs. Several of these signals control feeding: they carry the taste and smell of food to the brain, produce the feelings of hunger and fullness, start the swallowing reflex and drive the movements and secretions of the digestive tract. A dietitian who meets a client with a nerve injury, a stroke or a long-standing nerve disease therefore has to understand how neurons work in order to judge whether the client can chew, swallow and digest safely. Neurons are not repaired easily once damaged, so they depend on a steady supply of glucose, oxygen and nutrients such as the B vitamins. The way these cells are grouped into the brain, spinal cord and nerves is taught in the topic Divisions of the nervous system.

9.1.1 Parts of the neuron and their functions

A typical neuron has a cell body with two kinds of projections: short branching dendrites that receive signals, and one long axon that carries signals away. Each part has a specific task, and a signal moves through them in order: dendrites, cell body, axon and then the axon terminals.

Cell body (soma)

The cell body contains the nucleus, which holds the genetic material, and the organelles that make proteins and energy for the cell. It also contains clusters of rough endoplasmic reticulum, called Nissl bodies, which make the proteins needed to repair the cell and to make neurotransmitters. The cell body adds together the signals that arrive from the dendrites and decides whether to send an impulse along the axon. Because neurons use large amounts of energy, they depend on a continuous supply of glucose and oxygen from the blood. A client whose blood sugar falls very low can become confused, shaky or drowsy because neurons cannot work without glucose.

Dendrites

Dendrites are short, branched extensions of the cell body that look like the branches of a tree. They carry the signals that arrive from other neurons or from sensory receptors towards the cell body. Their branching gives a large surface for many connections, so one neuron may receive information from thousands of others. Taste and smell signals, for instance, reach the brain through chains of neurons whose dendrites collect the messages from the sensory cells of the tongue and nose.

Axon

The axon is a single, long, thin projection that starts at the cell body at a region called the axon hillock and carries the nerve impulse away from the cell body to other neurons, muscles or glands. It may be very short in the brain, or more than a metre long in the nerves that run from the spinal cord to the foot. The impulse is an electrical change that travels along the axon. In the digestive tract, axons carry the impulses that make the gut muscles contract and that stimulate the glands to secrete digestive juices.

Myelin sheath

The myelin sheath is a fatty, white, insulating layer that wraps around the axons of many neurons. In the nerves outside the brain and spinal cord it is formed by Schwann cells, and within the brain and spinal cord by cells called oligodendrocytes. The sheath has small gaps, the nodes of Ranvier, and the impulse jumps from one gap to the next, which makes conduction much faster than in an axon without myelin. Where myelin is damaged, as in some nerve diseases, signals slow down or fail. A long-standing deficiency of vitamin B12 can damage myelin and cause numbness, tingling and weakness, and a dietitian who finds a client with poor intake of animal-source foods or with absorption problems should recognise this risk and refer the client for medical assessment.

Synaptic terminals

The axon divides at its end into several small branches that finish in swollen synaptic terminals (axon terminals). A synapse is the tiny gap between a terminal and the next neuron, muscle or gland. When the impulse arrives, the terminal releases chemical messengers called neurotransmitters, which cross the gap and attach to receptors on the next cell, either exciting it or slowing it down. Acetylcholine, for example, makes gut and skeletal muscle contract, and serotonin is involved in mood and appetite. The body makes neurotransmitters from nutrients: choline is needed for acetylcholine, and the amino acid tryptophan is needed for serotonin, which is one reason that protein intake matters for the nervous system.

9.1.2 Types of the neuron

Neurons are grouped according to the direction in which they carry the signal and the job they do. The three functional types are sensory neurons, motor neurons and interneurons. They work together in chains. For example, when a person smells a meal, sensory neurons send the message to the brain, interneurons process it, and motor neurons make the salivary glands secrete saliva.

9.1.2.1 Sensory neurons

Sensory neurons (afferent neurons) carry signals from sensory receptors towards the brain and spinal cord. The receptors may lie in the skin, the eye, the ear, the tongue, the nose or the internal organs. Many sensory neurons have a single process that divides into two branches near the cell body (a unipolar shape), while the olfactory receptor cells of the nose are bipolar, with one dendrite and one axon.

Sensory neurons give the brain the information it needs to control eating. Taste and smell receptors tell the brain about the flavour of food. Stretch receptors in the wall of the stomach tell the brain how full it is, and receptors in the gut and liver respond to nutrients and hormones and help to signal fullness. A client who has lost taste or smell may eat less because food has no flavour, and these clients are monitored for weight loss.

9.1.2.2 Motor neurons

Motor neurons (efferent neurons) carry signals from the brain and spinal cord to muscles and glands, the effectors that carry out the response. They have many dendrites and one long axon (a multipolar shape). Somatic motor neurons supply skeletal muscle, which is under voluntary control and is used for chewing, closing the lips, moving the tongue and starting the swallow. Autonomic motor neurons supply smooth muscle, the heart and glands, and they control the contractions of the stomach and intestines and the secretion of saliva and digestive juices without conscious effort.

When motor neurons are damaged, the muscles they supply become weak or paralysed. After a stroke, for instance, a client may have a weak tongue or cheek on one side, may pocket food in the mouth or may cough when swallowing. The dietitian works with the doctor and speech therapist to choose a safe food texture and feeding position.

9.1.2.3 Interneurons

Interneurons (association neurons) lie wholly inside the brain and spinal cord. They connect sensory neurons to motor neurons and to each other, and they greatly outnumber the sensory and motor neurons. They process and integrate information, so they make possible reflexes, learning, memory and decision making. Simple reflexes such as pulling the hand away from a hot pot involve a sensory neuron, an interneuron in the spinal cord and a motor neuron. Interneurons in the hypothalamus, a region at the base of the brain, combine signals about nutrient levels, stomach fullness and hormones to produce feelings of hunger and fullness. A dietitian who counsels a client about poor or excessive appetite is working with the output of these circuits.

Structural types of neurons

Neurons can also be grouped by the number of processes that leave the cell body. Multipolar neurons have one axon and several dendrites; they include the motor neurons and many interneurons. Bipolar neurons have one dendrite and one axon and are found in the retina of the eye and in the lining of the nose. Unipolar neurons have a single short process that divides into two branches, and many sensory neurons that carry touch and pain signals to the spinal cord are of this kind.

Chapter Summary

This chapter provided a comprehensive understanding of the nervous system, beginning with the structure and functions of neurons, including the distinct roles of sensory, motor, and interneurons. It then explored the two main divisions of the nervous system: the central nervous system, which encompasses the brain and spinal cord, and the peripheral nervous system that connects the central system to limbs and organs. The discussion on nervous reflexes and impulses highlighted the differences between voluntary and involuntary reflex actions and introduced common disorders affecting the nervous system. Attention was given to the structure and functions of the sensory organs such as the eye, ear, tongue, nose, and skin, emphasizing their roles in detecting and processing stimuli. Finally, the chapter addressed various disorders that impair sensory organ function, underscoring the importance of these organs in overall human physiology.