Chemical and Neural Signaling

This set covers the vocabulary and core concepts of chemical signaling, including ligands, secondary messengers, various hormone types, and the mechanics of neural signaling like action potentials and synapses.

Ligand

A chemical that binds to another specific molecule in cells; it act as the chemical signal (e.g., hormones, neurotransmitters, or cytokines).

Quorum Sensing

A form of cell signaling in bacteria based on the number of cells which allows communication and a common decision on a specific action.

Autoinducers

Signaling molecules that help bacteria perceive a quorum.

Hormones

Powerful chemical signals secreted from glands (endocrine or exocrine) that travel through the bloodstream to target distant cells.

Neurotransmitters

Chemicals that transmit signals across a synapse between adjacent neurons.

Cytokines

Small signaling proteins that bind to receptors on the receiving cell membrane; they can be systemic or local.

Reception

The process by which a cell detects a signal in the environment.

Transduction

The process of activating a change within a cell by converting a signal into another form, often involving a signaling cascade.

Secondary Messengers

Small molecules, such as cyclic AMP (cAMP), released by a cell in response to extracellular signaling molecules to trigger a cascade.

G-protein-coupled Receptors (GPCR)

Transmembrane receptors embedded in the plasma membrane composed of a single polypeptide folded into a globular shape; they associate with G-proteins consisting of alpha ($\alpha$), beta ($\beta$), and gamma ($\gamma$) subunits.

Epinephrine (Adrenaline)

An amine hormone produced by adrenal glands used for the "flight or fight" response; it binds to adrenergic receptors.

Insulin

A protein hormone secreted by the pancreas that causes cells to uptake glucose and maintain homeostasis; it binds to tyrosine-kinase receptors.

Steroid Hormones

Lipids derived from cholesterol (e.g., testosterone, estradiol, progesterone) that diffuse through the cell membrane and bind to intracellular receptors to act as transcription factors.

Positive Feedback

A regulation pathway that amplifies a signal to drive a response further (e.g., cyclin-CDK complex in the cell cycle).

Negative Feedback

A regulation pathway where the product inhibits a component at the start of the pathway to maintain homeostasis (e.g., testosterone production).

Sensory Neuron

A type of neuron that carries information towards the central nervous system.

Myelin Sheath

Layers of Schwann cells that act as an insulator and allow for faster signal transmission along the axon.

Nodes of Ranvier

Gaps between Schwann cells on a myelinated axon where ions are concentrated for saltatory conduction.

Resting Potential

The membrane potential when a neuron is not sending an impulse, typically maintained at $-70\,mV$ by the sodium-potassium pump.

Threshold

The minimum membrane potential value, typically $-55\,mV$, required to initiate an action potential.

Depolarization

The process of the membrane potential becoming more positive relative to the outside, occurring when voltage-gated sodium channels open and $Na^+$ flows in.

Repolarization

The process of the membrane potential becoming more negative, occurring when voltage-gated potassium channels open and $K^+$ exits the cell at approximately $+30\,mV$.

Refractory Period

A brief period after an action potential during which the membrane cannot repeat an impulse, preventing the signal from propagating backwards.

Saltatory Conduction

The process by which nerve impulses jump from one Node of Ranvier to another down a myelinated axon, speeding up transmission.

Synapse

The junction that connects neurons, where chemical signals are sent from a presynaptic cell to a postsynaptic cell across a synaptic gap.

Acetylcholine (Ach)

A neurotransmitter formed by combining choline and an acetyl group that triggers muscle contraction at neuromuscular junctions.

Sodium-Potassium Pump

An integral membrane protein that uses ATP to actively transport $3\,Na^+$ ions out of the cell and $2\,K^+$ ions into the cell against their concentration gradients.

Local Current

The movement of $Na^+$ ions within a neuron that depolarizes neighboring sites to propagate an action potential forward.

GABA (Gamma-Aminobutyric Acid)

An inhibitory neurotransmitter that opens ligand-gated chloride ($Cl^-$) channels, causing hyperpolarization and reducing the chance of an action potential.

Summation

The combined effect of all excitatory postsynaptic potentials (EPSP) and inhibitory postsynaptic potentials (IPSP) to determine if a neuron reaches threshold.