Neurons, also known as nerve cells, are the basic units of structure and function in the nervous system. Their fundamental role is to receive and transmit information.
1. Composition
Neurons are composed of a cell body (including the nucleus and cytoplasm), dendrites, and an axon. The cell body's function is to integrate nerve impulses. Dendrites are shorter and their function is to receive nerve impulses and transmit them to the cell body. Axons are longer and each neuron has only one axon, which can transmit nerve impulses from the cell body to other neurons.
2. Classification
(1) Based on the number of processes, neurons can be classified as unipolar, bipolar, or multipolar.
(2) Based on function, neurons can be classified as:
① Sensory (afferent) neurons: They collect and transmit stimuli from inside and outside the body to the spinal cord and brain.
② Motor (efferent) neurons: They transmit information from the spinal cord and brain to muscles and glands, controlling their activities.
③ Interneurons (association neurons): They act as intermediaries between sensory and motor neurons. The connections between these interneurons form the microcircuits of the central nervous system, which is the main site for information processing in the brain.
3. Neuroglial Cells
There are a large number of neuroglial cells between neurons, with a total number exceeding 100 billion. The functions of neuroglial cells are:
(1) Providing pathways and support for the growth of neurons.
(2) Forming an insulating layer (myelin sheath) around neurons, allowing rapid transmission of nerve impulses.
(3) Supplying nutrients to neurons.
(4) Removing excess neurotransmitters between neurons.
4. Nerve Impulses
Excitability is an important characteristic of nerves and other excitable tissues, such as muscles and glands. When any stimulus acts on a neuron, the neuron transitions from a relatively resting state to a more active state, which is known as a nerve impulse.
Resting potential: The electrical potential change measured when a neuron is in a resting state. In the resting state, neurons are spontaneously electrically active.
Action potential: When a neuron is stimulated, the permeability of the cell membrane rapidly changes, causing a rapid increase in intracellular positive charge, which exceeds the extracellular potential. This process of potential change is called an action potential. It represents the excitatory state of the neuron.
Nerve impulse conduction refers to the conduction of nerve impulses within the same neuron and is closely related to action potentials. The conduction of nerve impulses follows the "all-or-none" law. The strength of the neuron's response does not change with the strength of external stimuli, which ensures that the information does not weaken during transmission.