The nervous system sends messages from nerve endings to the brain and from the brain to cells, tissues, and organs. Cells of the nervous system sometimes secrete chemical messengers instead of neurotransmitters. These specialized nervous system cells are called neurosecretory cells, and they produce neurosecretions.
Neurosecretions, which are classified as hormones because they carry information from sensor cells to target cells, can be released directly into the bloodstream or transported to storage cells, from which they are later released into the bloodstream.
One purpose of neurosecretions is to carry information to target cells that are not near the nerve cells that produce them. The hypothalamus, which is deep within the brain, detects conditions in the external environment of an organism as well as the internal environment of the organism.
In attempts to maintain homeostasis, the hypothalamus produces neurosecretions that are released into capillaries in the hypothalamus. Blood vessels then carry the secretions to the pituitary gland, which lies at the base of the hypothalamus, and the pituitary gland controls the secretion of many important hormones.
The nervous system contains two types of cells: neurons and neuroglial cells. Neurons are the cells that receive and transmit signals. The neuroglial cells are the support systems for the neurons — the neuroglial cells protect and nourish the neurons.
Each neuron contains a nerve cell body with a nucleus and organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Branching off the nerve cell body are the dendrites, which act like tiny antennae picking up signals from other cells.
At the opposite end of the nerve cell body is the axon, which is a long, thin fiber with branches at the end that sends signals. The axon is insulated by a myelin sheath made up of segments called Schwann cells. Nerve impulses are received by the dendrites, travel down the branches of the dendrites to the nerve cell body, and are carried along the axon.
When the impulse reaches the branches at the end of the axon, it is transmitted to the next neuron. Impulses continue to be carried in this way until they reach their final destination. The final destination depends on what type of neurons they are.
The three types of neurons
There are three types of neurons, each with different functions. The function of the neuron determines where those neurons transmit their impulses.
Sensory neurons: These neurons are also called afferent neurons. (Think of them as being affected by a sight, sound, smell, touch, or taste.) Their function is to receive initial stimuli from sense organs — eyes, ears, tongue, skin, and nose — as well as by impulses generated within the body in response to adjustments that are necessary to maintain homeostasis.
For example, if your internal body temperature is rising because of high heat outside, sensory organs will transmit an impulse carrying the message that action needs to be taken to cool down the body. Or, if you touch the tip of a knife, the sensory neurons in your finger will transmit impulses to other sensory neurons until the impulse reaches an interneuron.
Interneurons: These types of neurons are also called connector neurons or association neurons. What they do is “read” impulses received from sensory neurons. Interneurons are found in the spinal cord or brain.
When an interneuron receives an impulse from a sensory neuron, the interneuron determines what response should be generated. If a response is required, the interneuron passes the impulse on to motor neurons.
Motor neurons: These neurons are also called efferent neurons, and their function is to stimulate effector cells. When the motor neurons receive a signal from the interneurons, the motor neurons work to stimulate an effect. When the effector cells are stimulated, they generate reactions.
For example, motor neurons may carry impulses to the muscles in your hand to stimulate the movement of muscles to pull your hand away from the sharp knife. Or, in an effort to maintain homeostasis when your body temperature is rising, the motor neurons may stimulate the sweat glands to produce sweat in an attempt to release some heat to the outside, thereby decreasing your internal temperature.