The nervous system consists of nerve cells (neurons) and glia. Neuron (nerve cell) Neuron is a highly specialized cell, which is the basic structure and functional unit of the nervous system. It has the functions of feeling stimulation and conducting excitement. Neurons consist of cell bodies and processes. There is a nucleus in the center of the cell body, surrounded by cytoplasm. The nucleus contains not only mitochondria, endoplasmic reticulum and other organelles, but also unique neurofibrils and Nissl bodies. According to the shape and function, the processes of neurons can be divided into dendrites and axons. Dendrites are short, but there are many branches. They receive pulses and transmit them to the cell body. Dendrites of various neurons vary in number and shape. Each neuron only sends out one axon with different lengths, and the impulses generated by the cell body are transmitted along the axon. According to the number of processes, neurons can be morphologically divided into three types: pseudounipolar neurons, bipolar neuron neurons and multipolar neurons. 1) Pseudounipolar neuron: The cell body is in the brain ganglion or spinal ganglion. A protrusion from the cell body is divided into two branches not far away, and one branch reaches the receptors in the skin, motor system or viscera, which is called peripheral protrusion; Another branch enters the brain or spinal cord, called the central process. 2) bipolar neuron: There is a protuberance at each end of the cell body, one is a dendrite and the other is an axon. 3) Multipolar neurons: There are many dendrites and one axon. Cell bodies mainly exist in the brain and spinal cord, and some exist in visceral ganglion. According to the function of neurons, they can be divided into sensory neurons, motor neurons and contact neurons. Sensory neurons, also known as afferent neurons, are generally located in peripheral sensory ganglia and are pseudounipolar or bipolar neuron. The peripheral processes of sensory neurons receive various stimuli from the internal and external environment, and transmit impulses to the center through the cell body and central processes. Motor neurons, also known as efferent neurons, are generally located in the motor nuclei of the brain and spinal cord or in the plant ganglia around them. They are multipolar neurons that transmit pulses from the center to muscles or glands and other effectors. Contact neurons, also known as intermediate neurons, are located between sensory neurons and motor neurons, which play a role in contact and integration and are multipolar neurons. The longer processes (mainly axons) and sheath structures of nerve fiber neurons are called nerve fibers. The sheath structure in the central nervous system is composed of oligodendrocytes, while the sheath structure in the peripheral nervous system is composed of neuromembranous cells (also called Schwann cells). The contact between synaptic neurons is mutual contact, not cytoplasmic communication. The structural specialization of this contact site is called synapse, which is usually the functional connection between the axon of one neuron and the dendrite or cell body of another neuron through synapse, and the nerve impulse is transmitted from one neuron to another through synapse. The number of glial cells is 10~50 times that of neurons. There are no dendrites and axons in the process, and the cell body is small. There are no neurofibrils and Nissl bodies in the cytoplasm, and there is no function of conducting impulses. Glial cells play a role in supporting, isolating, nourishing and protecting neurons, and participate in the formation of blood-brain barrier.
The nervous system regulates and controls the functional activities of other systems, making the body a complete unity. For example, when participating in sports, with the contraction of skeletal muscle, there are a series of changes such as accelerated breathing, accelerated heartbeat and sweating. 2. The nervous system adapts the body to the changing external environment and maintains the balance between the body and the external environment by regulating the functional activities of the body. For example, when the temperature is low, through the adjustment of the nervous system, the surrounding small blood vessels contract, reducing heat dissipation in the body; When the temperature is high, the surrounding small blood vessels expand, increasing the heat dissipation in the body to maintain the body temperature at a normal level. In the long-term evolution and development of human beings, the nervous system, especially the cerebral cortex, has been highly developed, resulting in language and thinking. Humans can not only passively adapt to the changes in the external environment, but also actively understand and transform the objective world, so that nature can serve mankind. This is the most important feature of the human nervous system.