2 The Nervous System
Learn how nervous tissue detects and processes information, how neurons communicate, and how the nervous system coordinates sensory input, movement, reflexes, and body regulation.
Organization and role
The nervous system detects changes inside and outside the body, processes that information, and coordinates responses. Its fast electrical and chemical signals help control movement and perception, as well as many processes that maintain homeostasis.
The two major anatomical divisions are the —the brain and spinal cord—and the , which includes nerves and ganglia outside the CNS. These anatomical divisions describe where structures are; functional categories describe what pathways do.
Neurons and supporting cells
Neurons are electrically excitable cells that receive and transmit information. A typical has a cell body containing the nucleus, branching dendrites that receive input, and an axon that carries signals toward other cells.
Many axons are covered by , an insulating layer made by glial cells. Gaps in the , called nodes of Ranvier, help signals travel rapidly along the axon. is produced by oligodendrocytes in the CNS and by Schwann cells in the PNS. Other glial cells support and protect neurons and maintain the conditions they need to function.
Neurons can also be grouped by function:
Sensory neurons carry information from receptors toward the CNS.
Interneurons connect and process signals within the CNS.
Motor neurons carry commands from the CNS to muscles or glands.
How neurons communicate
A 's membrane maintains an electrical difference between the inside and outside of the cell, called the resting membrane potential. When incoming signals bring the membrane at the axon's trigger zone to threshold, voltage-gated ion channels open and generate an . This brief electrical impulse propagates along the axon.
In myelinated axons, the is regenerated at successive nodes of Ranvier, making conduction faster.
At most -to- connections, called chemical synapses, an arriving causes the axon terminal to release neurotransmitter molecules. The neurotransmitters cross the narrow synaptic gap and bind to receptors on a target cell, changing its activity. Depending on the transmitter and receptor, the input may make the target cell more or less likely to fire. A target cell may be another , a muscle cell, or a gland cell.
Sensory input and motor responses
A carries information toward the CNS, while a carries commands away from it. Sensory receptors detect stimuli such as pressure, temperature, light, or a change in blood chemistry, and convert them into signals that can travel through sensory neurons. The CNS processes and combines sensory input and may then produce a motor response.
Touching a hot surface illustrates this sequence: temperature and pain receptors are activated, and sensory signals travel through a peripheral to the spinal cord. The CNS processes the information and sends motor commands to muscles, which withdraw the hand.
Some rapid protective responses are organized as reflexes. Sensory input enters the spinal cord, connects with one or more neurons, and activates a motor output before the information is fully processed for conscious perception. The brain can also receive the sensory information and shape subsequent action.
Central and peripheral functions
The CNS integrates information and coordinates responses. The brain supports perception, thought, memory, and voluntary movement, and helps regulate bodily functions. The spinal cord carries signals between the brain and much of the body and also organizes many reflexes.
A collection of cell bodies in the CNS is called a nucleus; a bundle of axons there is called a . In the PNS, a bundle of axons is a , and a collection of cell bodies is a .
The PNS links the CNS with receptors, muscles, and organs. Its functional pathways include:
The carries sensory information from the body and controls skeletal muscles, including voluntary movement and some reflexes.
The regulates involuntary functions by acting on smooth muscle, cardiac muscle, and glands. Its sympathetic and parasympathetic divisions often have complementary effects that help adjust body activity to changing conditions.
The is a network of neurons in the digestive that helps coordinate digestive activity. It can operate with substantial independence while also communicating with the CNS.
These functional categories are not separate from the anatomical divisions. For example, a pathway can begin with a PNS sensory , be processed in the CNS, and end with a PNS motor .
How the system works together
Neurons communicate through electrical impulses along their membranes and, usually, chemical signals across synapses. Sensory pathways carry information toward the CNS, while motor pathways carry commands to muscles and glands. By integrating information and coordinating responses, the CNS and PNS together support sensation, action, reflexes, and regulation of the body.