Chemical Signaling

Chemical Signaling: Cell Communication

Introduction

• Chemical signaling is the process by which cells communicate with each other.

• This is a foundational concept for understanding how different organ systems coordinate their activities in the body.

• Builds upon knowledge from biology 226 (nervous system and neuron communication).

Basics of Chemical Signaling

• Chemicals act as messaging substances, similar to letters carrying information.

• The message must physically reach the recipient cell.

• This topic covers various subtopics related to cell-to-cell communication and involves conceptual physiology.

Pathways of Chemical Communication

Direct Communication

• Chemical messenger molecules flow directly from one cell to another.

• Cells must be physically connected via protein structures called gap junctions.

• Gap junctions create a pore or tunnel allowing substances to pass between cells.

• The direction of flow is determined by the concentration gradient of the chemical.

• Substances shared include ions (sodium, calcium, potassium, chloride) and small molecules (glucose, amino acids).

• Gap junctions consist of multiple proteins forming a ring (connexon).

• Pore is much larger than channel proteins which only allow molecules as big as water through.

Indirect Communication

• Chemical messenger molecules are released from a secretory cell into the extracellular fluid (ECF).

• The chemical then travels through the ECF to reach the target cell.

• Target cells possess receptor molecules, usually proteins, that bind to the chemical messenger.

• Binding of the messenger changes the receptor's shape (conformation) and activates it.

• Activation triggers a chemical chain reaction inside the target cell, leading to a desired response.

• To stop the response, the messenger must be removed from the receptor, reverting it to its inactive form.

Specificity of Receptors

• Receptors are highly specific to particular messenger molecules based on their shapes.

• The messenger's shape must complement the binding site on the receptor.

• A given receptor is typically activated by only one or two different messengers due to the precise structural requirements.

• Computer-generated models illustrate the complex shapes of hormones like human growth hormone and the corresponding receptor, emphasizing the near-perfect match required for binding.

Termination of Response

• The messenger must be removed from the receptor to deactivate it.

• The most common method is enzymatic breakdown of the messenger.

• Example: Acetylcholine (ACH) in a chemical synapse is broken down by an enzyme.

• The enzyme is often associated with the target cell's membrane, with its active site facing the extracellular fluid (e.g., synaptic cleft).

• For messengers like hormones that travel long distances through the bloodstream, enzymes in the liver and kidneys break them down.

Control of Chemical Signal Release

• The body uses three methods to control when a chemical signal is released: humoral, neural, and hormonal.

Humoral Control

• A substance in the extracellular fluid (a "humor") influences the release of a chemical signal.

• Example: Parathyroid glands monitor blood calcium levels. If calcium is too low, they release parathyroid hormone (PTH).

Neural Control

• A neuron tells the secretory cell what to do.

• Example: Sympathetic neurons stimulate the adrenal medulla to release catecholamines (hormones involved in the fight-or-flight response).

Hormonal Control

• A hormone tells the secretory cell what chemical signal to release.

• Example: The pituitary gland releases hormones that tell other glands (e.g., thyroid, adrenal cortex) to release their hormones.

Patterns of Secretion

• Chemical signals exhibit different patterns of secretion over time: chronic, acute, and cyclic.

Chronic Secretion

• Fairly steady maintenance of chemical signal levels over time.

• The level hovers around a set point, with only small deviations.

• Example: Thyroid hormones, which maintain basal metabolic rate.

Acute Secretion

• Responds to a particular stimulus or change in the body and are secreted to make a correction.

• Short-lived, ceasing when the stimulus is removed.

• Example: Epinephrine (adrenaline) is released in response to stress (physical or psychological) and stops when the stressor is removed.

Cyclic Secretion

• Rises and falls in a fairly predictable pattern over time.

• Example: Female reproductive hormones, such as estrogen, fluctuate across the monthly menstrual cycle.