The Nervous System

Acetylcholine

A chemical messenger that nerves use to communicate, including activating muscles at the neuromuscular junction and supporting parasympathetic nervous system signaling.

Afferent

Sensory nerve fibers that carry information such as touch, pain, temperature, and position toward the brain or spinal cord.

Autonomic nervous system

The division of the nervous system that regulates involuntary functions such as heart rate, digestion, breathing patterns, and pupil size through sympathetic and parasympathetic branches.

Axon

The long extension of a neuron that conducts action potentials away from the cell body to other neurons, muscles, or glands.

Blood-brain barrier

A selective barrier formed by tightly joined blood vessel cells that limits movement of substances from the bloodstream into the brain.

Brainstem

The region connecting the brain and spinal cord that contains centers for breathing, heart rate, vomiting, and consciousness.

Central nervous system (CNS)

The brain and spinal cord, which integrate sensory information and coordinate voluntary and involuntary responses.

Cerebellum

A brain structure that regulates balance, posture, coordination, and movement accuracy.

Cerebrospinal fluid (CSF)

A clear fluid that cushions the brain and spinal cord and circulates nutrients and waste products around CNS tissue.

Cranial nerves

Twelve paired nerves that emerge from the brain and control sensation and movement in the head and neck, plus some internal organ functions.

Efferent

Motor nerve fibers that carry commands away from the brain or spinal cord to muscles or glands.

Epinephrine

A sympathetic neurotransmitter and hormone that increases alertness and supports fight or flight responses.

Gamma-aminobutyric acid (GABA)

A major inhibitory neurotransmitter in the brain that reduces neuronal excitability.

Neurotransmitter

A chemical messenger released at synapses that transmits signals between neurons or from neurons to muscles.

Parasympathetic nervous system

The autonomic branch associated with rest and digest functions, including decreased heart rate and support of digestion.

Peripheral nervous system (PNS)

All nervous tissue outside the brain and spinal cord, including cranial nerves, spinal nerves, and autonomic nerves that connect the body to the CNS.

Pupillary light reflex (PLR)

A reflex that causes pupil constriction in response to light through pathways involving the retina, optic nerve, brainstem, and oculomotor nerve.

Reflex arc

A fast, automatic response pathway that follows a five step circuit: receptor to sensory neuron to spinal cord or brainstem to motor neuron to effector.

Sympathetic nervous system

The autonomic branch associated with fight or flight responses, including increased heart rate, blood pressure, and alertness.

Synapse

The junction between neurons where chemical signaling occurs when neurotransmitters cross a small gap to bind receptors on the next cell.