Neurons and Nerves
Neurons are cells that form the core of nervous systems because they have the ability to receive and transmit signals. Neurons have a unique elongated shape and consist of three main parts:
Nerve cell body: The rounded part of the neuron. It contains typical eukaryotic cell components like the nucleus, organelles, and the endomembrane system.
Dendrites: Tiny projections that branch off the nerve cell body at the neuron's receiving end. The dendrites act like tiny antennae that pick up signals from other cells.
Axon: A long, thin fiber that extends off the nerve cell body and branches at its tips to end in synaptic terminals that are marked by swellings called synaptic knobs. Just like some copper wire has plastic insulation, many axons are insulated by a fatty myelin sheath, which is formed by cells called Schwann cells.
Between the Schwann cells are small gaps in insulation called nodes of Ranvier. Nerve signals travel rapidly along the axons of myelinated nerves because the electrical signals hop along the axon from gap to gap, rather than having to flow along the whole axon. Scientists call this type of nerve conduction saltatory conduction.
Nerve impulses enter a neuron through the dendrites. They then travel down the dendrite's branches to the nerve cell body before being carried along the axon. When the impulses reach the synaptic terminal, the neuron releases neurotransmitters from its synaptic terminals.
The neurotransmitters cross a small gap called a synapse to travel to the next neuron's dendrites. Impulses continue to be carried in this fashion until they reach their final destination.
The three major functions of a nervous system are to collect, interpret, and respond to signals. Different types of neurons carry out each of these functions.
Sensory neurons, also called afferent neurons, collect sensory information from sense organs and bring it to the CNS. Sensory neurons also receive internally generated impulses regarding adjustments that are necessary for the maintenance of homeostasis.
Interneurons within the CNS integrate the sensory information and send out responding signals. Interneurons, also called connector neurons or association neurons, "read" impulses received from sensory neurons. When an interneuron receives an impulse from a sensory neuron, the interneuron determines what (if any) response to generate. If a response is required, the interneuron passes the impulse on to motor neurons.
Motor neurons, also called efferent neurons, carry the responding signals from the CNS to the cells that are to carry out the response. Motor neurons stimulate effector cells that generate reactions.
Sometimes the nervous system can work without the brain, as in a reflex arc. A reflex arc gives sensory nerves direct access to motor nerves so information can be transmitted immediately.