One of the two main divisions of the autonomic nervous system, the other being the parasympathetic nervous system. It system functions to regulate the body's unconscious actions.

This knob is adjacent to a tiny cleft or synapse (s). When a nerve impulse reaches this knob, a drug called a neurotransmitter is released from vesicles into the synapse.

The somewhat enlarged, often club-shaped endings by which axons make synaptic contacts with other nerve cells or with effector cells (muscle or gland cells).

The nerve tissues that controls the activities of the body. In vertebrates it comprises the brain and spinal cord.

The voltage (charge) difference across the cell membrane when the cell is at rest.

The neuron to which the electrical impulse is transmitted across a synaptic cleft by the release of a chemical neurotransmitter from the axon terminal of a presynaptic neuron.

The part of the vertebrate nervous system that innervates smooth and cardiac muscle and glandular tissues and governs involuntary actions and that consists of the sympathetic nervous system and the parasympathetic nervous system.

The part of the peripheral nervous system associated with skeletal muscle voluntary control of body movements. It consists of afferent nerves and efferent nerves

The insulating covering that surrounds an axon with multiple spiral layers of myelin, that is discontinuous at the Nodes of Ranvier. It increases the speed at which a nerve impulse can travel along an axon.

The part of the nervous system that consists of the nerves and ganglia on the outside of the brain and spinal cord.

An action or movement not controlled by conscious thought. It may be anything from a hiccup to the involuntary response of a body part, such as the action that occurs in the knee-jerk reflex.

A short branched extension of a nerve cell, along which impulses received from other cells at synapses are transmitted to the cell body.

A short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory.

The long threadlike part of a nerve cell along which impulses are conducted from the cell body to other cells.

A gap in the myelin sheath of a nerve, between adjacent Schwann cells.

A class of more than 600 naturally occurring pigments synthesized by plants, algae, and photosynthetic bacteria. These richly colored molecules are the sources of the yellow, orange, and red colors of many plants.

A specialized cell transmitting nerve impulses; a nerve cell.

During this process, the negative internal charge of the cell becomes positive for a very brief period of time. This shift from a negative to a positive internal cellular environment allows for the transmission of electrical impulses both within a cell and, in certain instances, between cells.

This refers to the change in membrane potential that returns the membrane to a negative value. It occurs just after the depolarization phase of an action potential has changed the membrane potential to a positive value. It returns the membrane potential back to the resting membrane potential.

A structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron.

An organic compound and a vital nutrient that an organism requires in limited amounts.

This is the space between neurons at a nerve synapse across which a nerve impulse is transmitted by a neurotransmitter—called also synaptic gap.

Endogenous chemicals that enable neurotransmission. They transmit signals across a chemical synapse from one neuron (nerve cell) to another "target" neuron, muscle cell, or gland cell.

These store various neurotransmitters that are released at the synapse.

Pore-forming membrane proteins whose functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ions across the cell membrane.