Science · Body foundations
Nervous System and Neural Signaling
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Start here: key ideas
- The CNS processes information; the PNS carries signals to and from it.
- Neurons receive, conduct, and transmit signals.
- Neurotransmitters carry a signal across a synapse.
- A reflex follows a short sensory-to-motor pathway.
What you’ll be able to do
- Compare nervous system divisions.
- Explain neuron and synapse signaling.
- Trace sensory, reflex, and motor paths.
Follow information from a receptor to the CNS and back to an effector.
A receptor detects change, and an effector carries out a response.
A hierarchy by location and control
The central nervous system (CNS) is the brain and spinal cord. It processes information. The peripheral nervous system (PNS) is the nerves outside the CNS. Sensory, or afferent, pathways carry information toward the CNS. Motor, or efferent, pathways carry commands away. Somatic motor control targets skeletal muscle. Autonomic control targets smooth muscle, cardiac muscle, and glands. Sympathetic activity supports action; parasympathetic activity supports rest and digestion.
The autonomic system has two main branches. The sympathetic branch raises heart rate and redirects energy during challenge. The parasympathetic branch slows the heart and supports digestion during rest. They often have opposite effects on the same organ, but both are involuntary motor pathways.
From graded input to propagated signal
A neuron receives input at dendrites and the cell body. If input reaches threshold, an action potential begins. This electrical signal travels along the axon without becoming weaker. Myelin insulates many axons and lets the signal move quickly between gaps called nodes of Ranvier. (This gap-to-gap propagation is also called saltatory conduction.) Stronger stimuli usually increase action-potential frequency, not the size of each action potential.
Supporting explanation: ion movement
At rest, ion pumps and leak channels help keep the inside relatively negative. Reaching threshold opens voltage-gated sodium channels, making the inside less negative. This change is called depolarization. Potassium channels then help restore the negative charge, a process called repolarization. A refractory period prevents immediate backward travel, so the signal moves toward the axon terminals.
OpenStax College, Anatomy & Physiology (2014 CC BY archive) · CC BY 3.0 · https://github.com/philschatz/anatomy-book/blob/eadaa6b2992f4e4010f45d58ad4db065e398093e/resources/1206_The_Neuron.jpg · https://creativecommons.org/licenses/by/3.0/ · Original source image reproduced unchanged; explanatory caption added.
Chemical transmission crosses a gap
At a chemical synapse, an action potential reaches an axon terminal. Calcium enters the terminal. Vesicles release a neurotransmitter into the small gap. The chemical binds receptors on the next cell and changes its activity. The transmitter is then removed by breakdown, reuptake, or diffusion.
A neurotransmitter may excite or inhibit the next cell. Excitatory input makes threshold more likely; inhibitory input makes it less likely. The postsynaptic neuron adds many inputs over space and time. If the total reaches threshold at the trigger zone near the axon hillock (where the axon begins), it produces its own action potential.
OpenStax College, Anatomy & Physiology (2014 CC BY archive) · CC BY 3.0 · https://github.com/philschatz/anatomy-book/blob/eadaa6b2992f4e4010f45d58ad4db065e398093e/resources/1225_Chemical_Synapse.jpg · https://creativecommons.org/licenses/by/3.0/ · Original source image reproduced unchanged; explanatory caption added.
Reflexes show the complete information loop
A reflex shows the full path: receptor → sensory neuron → integration in the spinal cord → motor neuron → effector. For example, touching a hot surface activates pain receptors and quickly causes arm muscles to pull away. The brain also receives information, but the spinal pathway can start the response first.
Worked example: A pin activates a pain receptor in the foot. A sensory neuron carries the signal through a dorsal root into the spinal cord. Interneurons activate motor neurons to flexor muscles and inhibit opposing extensors. The foot withdraws. Signals also travel upward so the brain becomes aware of pain.
Trace before naming. Afferent pathways carry sensory information toward the CNS; efferent pathways carry motor commands away from the CNS. “Autonomic” identifies an efferent control division for smooth muscle, cardiac muscle, and glands—it does not mean sensory. A reflex can be integrated in the spinal cord before the brain creates conscious awareness, although information may still ascend to the brain.
Key terms
- CNS: The brain and spinal cord.
- PNS: All nerves outside the brain and spinal cord.
- neuron: A cell specialized to send information.
- action potential: An all-or-none electrical signal along an axon.
- synapse: The junction where one cell signals another.
- neurotransmitter: A chemical messenger released at a synapse.
- reflex: A rapid, automatic response to a stimulus.
Summary
- Sensory input enters the CNS; motor output leaves it.
- An action potential travels along an axon without fading.
- At a chemical synapse, neurotransmitter binds receptors on the next cell.
Afferent arrives at the CNS; efferent exits it.
Sources
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