The functional unit of the nervous system is a highly specialized cell called the neuron (Gr. "nerve"). Neurons are specialized to produce signals that can be communicated over short to relatively long distances, from one part of an animal's body to another. Neurons have two important properties: (1) excitability, the abil ity to respond to stimuli, and (2) conductivity, the ability to conduct a signal. The three functional types of neurons are sensory neurons, interneurons, and motor neurons. Sensory (receptor or afferent) neurons either act as receptors of stimuli themselves or are activated by receptors . Changes in the internal or external environments stimulate sensory neurons, which respond by sending signals to the major integrating centers in the brain where information is processed. Interneurons receive signals from the sensory neurons and transmit them to motor neurons. Interneurons are located entirely within the central nervous system. Motor neurons send the processed information via a signal to the body's effectors (eg, muscles), causing them to contract, or to glands, causing them to secrete. summarizes the flow of information in the nervous system.
Types of Vertebrate Neurons.
The brain and spinal cord make up the central nervous system (CNS) of vertebrates, and sensory and motor neurons for the peripheral nervous system (PNS). The sensory neurons of the peripheral nervous system carry information about the environment to the CNS. Within the CNS, interneurons provide the links between sensory and motor neurons. Motor neurons of the PSS carry impulses or "commands to the muscles and glands (effectors) of a vertebrate.
Generalized Pathway for the Flow of Information within the Nervous System.
An input stimulus initiates impulses wahin some sensory structure (the receptor the impulses are then transfemed via sensory neurons to intemeurons. After response selection, nerve impulses are generated and transferred along motor neurons to an effector (eg, a muscle or gland), which elicts the appropriate output response.
Neuron Structure: The Key to Function
Most neurons contain three principal parts: a cell body, den- drites, and an axon. The cell body has a large, central nucleus. The motor neuron has many short, threadlike branches called dendrites
which are actually extensions of the cell body and conduct signals toward the cell body. The axon is a relatively long cylindrical process that conducts signals (information) away from the cell body
The neurons of hydras and sea anemones do not have a sheath covering the axon of the neuron. Other invertebrates and all vertebrates have mostly sheathed neurons. When present, the laminated lipid sheath is called myelin. In some neurons, a neurolemmocyte (formerly known as a Schwann cell) wraps the myelin sheath in layers. In these neurons, gaps called neurofibril nodes (formerly nodes of Ranvier) seg ment the myelin sheath at regular intervals. The neurolem mocyte also assists in the regeneration of injured myelinated neuron
The nervous system receives data (input stimulus), inte grates it, and effects a change (output response) in the ani mal's physiology, In a given neuron, the dendrites are the receptors, the cell body is the integrator, and the ends of the axon are the effectors.
