8.7 - Nervous and hormonal coordination

Nervous signals and connected effectors

Coordination links information about a change to appropriate responses in different parts of an animal.

A stimulus is a detectable change, inside or outside the body. A sensory receptor detects a stimulus; an effector, such as a muscle or gland, carries out a response. The central nervous system, comprising the brain and spinal cord, receives information and coordinates nervous output. Some responses are conscious, while reflexes and many internal adjustments occur without conscious decisions.

For example, when a mammal steps on a sharp object, sensory receptors detect the damaging stimulus. Signals travel along sensory neurones to spinal circuits, where connections through relay neurones coordinate motor output. Motor neurones stimulate particular muscles so that the limb withdraws. Activity in opposing muscles is coordinated so that they do not simply pull equally against one another. This is a directed response: the relevant nerve pathways connect the stimulus to the appropriate effectors.

Follow the signal at two scales. Action potentials, electrical changes in the neurone membrane, propagate along axons. At chemical synapses and neuromuscular junctions, released neurotransmitter crosses a small gap and binds receptor proteins on the next cell, changing its activity. Acetylcholine at a skeletal-muscle junction is one example. The electrical signal does not flow through the blood, and the neurotransmitter does not need to circulate around the whole animal to reach that muscle.

The word receptor therefore has two related uses: a sensory receptor detects a stimulus, while a receptor protein binds a chemical messenger. A connected target cell must still possess suitable receptor proteins and the machinery needed to respond to its transmitter. Both the connections and the molecular response matter.

This route suits rapid, precisely timed movement. A particular muscle can be activated while a neighbouring muscle receives different output. Nervous coordination can also control many organs at once by activating several pathways; it is not limited to a single effector.

Hormones and responsive target cells

Hormonal coordination distributes a chemical signal through the circulation rather than following a chain of neurones all the way to each target organ.

In endocrine signalling, hormone-secreting cells release a hormone, a chemical messenger, into the fluid around them and then into the blood. Endocrine glands are ductless: their hormone secretion enters the circulation rather than passing along a duct to a body surface. Blood carries the hormone to many tissues, and hormone molecules leave capillaries to reach cells.

Widespread delivery does not mean that every cell responds. A target cell has a functional receptor compatible with the hormone and suitable internal machinery for the response. A cell lacking that receptor cannot detect that hormone through this route, even if hormone reaches it. Receptor binding changes cell activity; the hormone is a signal, rather than a fuel that supplies the energy for the response.

Two examples show why hormonal control is more varied than one rule about gene expression:

• Adrenaline, also called epinephrine, is released from the adrenal medulla, the inner part of an adrenal gland. It binds cell-surface receptors. Intracellular signalling can change the activity of existing proteins, so effects such as increased heart activity or increased glycogen breakdown can begin rapidly. In the liver, glycogen breakdown helps supply glucose for release into the blood.
• Cortisol is a lipid-soluble steroid hormone released from the adrenal cortex, the outer part of the gland. It can enter target cells and bind intracellular receptors. As studied in hormone-controlled transcription, the activated receptor can regulate transcription; altered mRNA and protein production then change cell function. Such a response needs time for the relevant gene-expression steps.

The same hormone can produce different responses in different target tissues because receptor types and the processes coupled to them differ. Adrenaline can increase the rate of the heart while stimulating glycogen breakdown in liver cells: the coordinated outcome does not require every target to perform the same action. Nor does endocrine signalling require a direct nerve ending on every cell that responds to the circulating hormone.