5 The Nervous System
Learn how nervous tissue is organized, how signals travel through the nervous system, and how sensory input leads to responses.
How the nervous system is organized
The nervous system receives information, processes it, and coordinates responses. It includes neurons that carry signals and that support nervous tissue. A basic pathway connects sensory input to processing and then to a motor response.
Neurons and
A is specialized for communication. Its cell body contains the nucleus, dendrites receive many incoming signals, and an axon carries signals toward other cells. Axons communicate at junctions called synapses, often by releasing chemical messengers called neurotransmitters.
Some axons are wrapped in , an insulating covering that helps signals travel quickly. maintain and support nervous tissue.
In the CNS, astrocytes help regulate the chemical environment around neurons, oligodendrocytes form , microglia provide immune defense, and ependymal cells line fluid-filled spaces in the brain and spinal cord. In the PNS, Schwann cells form around peripheral axons, while satellite cells support cell bodies in ganglia. One oligodendrocyte can myelinate portions of multiple axons; a Schwann cell typically myelinates one segment of one axon.
Central and peripheral divisions
The consists of the brain and spinal cord. It integrates information and helps select responses. The includes nerves and ganglia outside the brain and spinal cord, carrying signals between the CNS and the rest of the body.
A is a cluster of cell bodies in the PNS; a bundle of axons in the PNS is a nerve.
Sensory and motor pathways
The PNS has sensory and motor components. carry information from receptors toward the CNS. Receptors detect stimuli such as pressure, temperature, light, or changes inside the body. carry commands away from the CNS to effectors, such as muscles and glands.
The controls skeletal muscle. The regulates smooth muscle, cardiac muscle, and glands, generally without conscious control. The direction words are useful: afferent arrives at the CNS, while efferent exits it.
From stimulus to response
A simple sensory-to-motor pathway has three stages:
Reception: A receptor detects a stimulus and converts it into a nerve signal.
Integration: A sensory carries the signal to the CNS, where neurons process it. Interneurons connect and coordinate signals within the CNS.
Response: A motor carries a command from the CNS to an effector, which produces a response.
Touching a hot surface activates temperature and pain receptors in the skin. Sensory neurons carry signals toward the spinal cord, which can quickly activate motor neurons that make arm muscles contract and pull the hand away. This can begin before the brain has fully processed the sensation; information also travels to the brain, where the stimulus can be perceived. More deliberate movements involve further processing by the brain before motor commands reach muscles.