Revise Control and coordination in mammals for Cambridge 9700 (15.1)

Mammals rely on two primary communication and control systems to coordinate their internal environment and respond to external stimuli: the nervous system and the endocrine system. While both work towards the same goal of maintaining homeostasis and ensuring survival, they employ distinct strategies.

The mammalian nervous system is a highly organised network. It's broadly divided into the Central Nervous System (CNS), comprising the brain and spinal cord, and the Peripheral Nervous System (PNS), which consists of all other nerves extending throughout the body. The PNS further subdivides into the sensory nervous system (transmitting information to the CNS) and the motor nervous system (transmitting information from the CNS).

The motor nervous system is then split into the somatic nervous system (voluntary control of skeletal muscles) and the autonomic nervous system (involuntary control of smooth muscle, cardiac muscle, and glands).

The fundamental unit of the nervous system is the neurone. Different types exist, each adapted for its specific role.

A typical motor neurone has several key features:

• Cell Body: Contains the nucleus, cytoplasm, and organelles like mitochondria and rough endoplasmic reticulum (for protein synthesis, e.g., neurotransmitters).
• Dendrites: Highly branched extensions from the cell body that receive signals from other neurones.
• Axon: A single, long fibre that conducts nerve impulses away from the cell body towards an effector or another neurone.
• Myelin Sheath: A fatty layer formed by Schwann cells that wraps around the axon. It acts as an electrical insulator.
• Nodes of Ranvier: Gaps in the myelin sheath between adjacent Schwann cells. Action potentials are generated at these nodes.
• Axon Terminals: The branched endings of an axon, which form synapses with other cells. They contain vesicles filled with neurotransmitters.

Nerve impulses, or action potentials, are rapid, transient changes in the electrical potential across the neurone membrane. This transmission relies on the movement of ions (primarily sodium and potassium) across the axon membrane via voltage-gated ion channels.

At rest, a neurone maintains a resting potential (typically -70mV) due to the action of the sodium-potassium pump (3 Na+ out for 2 K+ in) and the higher permeability of the membrane to K+ ions (via leak channels). The inside is more negative than the outside.

When a stimulus reaches the threshold potential, depolarisation occurs: voltage-gated Na+ channels open, Na+ ions rush into the axon, making the inside positive (+30mV to +40mV).

Immediately after, repolarisation begins: Na+ channels inactivate, and voltage-gated K+ channels open, allowing K+ ions to rush out of the axon, restoring the negative potential inside.

There's a brief period of hyperpolarisation (undershoot) before the resting potential is fully re-established by the sodium-potassium pump. During the refractory period, the neurone cannot transmit another impulse, ensuring unidirectional flow and distinct impulses.

In myelinated neurones, the myelin sheath prevents ion flow, so action potentials only occur at the nodes of Ranvier. The impulse 'jumps' from node to node, a process called saltatory conduction, which is much faster than continuous conduction in unmyelinated neurones.

Neurones communicate with each other, or with effector cells, at specialised junctions called synapses. At a chemical synapse, there's a small gap, the synaptic cleft, between the presynaptic neurone and the postsynaptic neurone.

When an action potential arrives at the presynaptic terminal, it triggers the opening of voltage-gated Ca2+ channels. Ca2+ ions rush in, causing synaptic vesicles (containing neurotransmitters like acetylcholine or noradrenaline) to fuse with the presynaptic membrane and release their contents into the synaptic cleft by exocytosis.

The neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic membrane. This binding causes ion channels on the postsynaptic membrane to open, leading to a change in its potential. If a threshold potential is reached, a new action potential is generated in the postsynaptic neurone.

To prevent continuous stimulation, neurotransmitters are rapidly removed from the synaptic cleft. For acetylcholine, the enzyme cholinesterase breaks it down, and the components are reabsorbed by the presynaptic neurone. Mitochondria in the presynaptic terminal provide ATP for neurotransmitter synthesis and reabsorption.

The endocrine system uses hormones for broader, slower, and longer-lasting control. Several key glands and hormones are vital for mammalian homeostasis.

The hypothalamus, part of the brain, is crucial for linking the nervous and endocrine systems. It plays a central role in thermoregulation (maintaining core body temperature) and osmoregulation (controlling blood water potential). It acts as a sensor and coordinates responses, often through the pituitary gland.

The pituitary gland is often called the 'master gland' because it secretes hormones that control the activity of many other endocrine glands throughout the body.

Negative feedback is a fundamental principle in hormonal control. When a regulated variable deviates from its set point, a response is initiated to counteract the change and bring the variable back to normal. A classic example is the control of blood glucose by insulin and glucagon.

When blood glucose rises (e.g., after a meal), the pancreas releases insulin, which promotes glucose uptake by cells and its conversion to glycogen in the liver, lowering blood glucose. Conversely, when blood glucose falls, the pancreas releases glucagon, which stimulates the breakdown of glycogen into glucose in the liver, raising blood glucose. These opposing actions maintain glucose within a narrow range.

Other important examples include adrenaline (epinephrine), a hormone released from the adrenal glands in response to stress or danger. It triggers the 'fight or flight' response, increasing heart rate, blood pressure, and diverting blood to muscles, preparing the body for immediate action.

Beyond homeostasis, specific brain regions also control vital functions. The cerebellum coordinates voluntary movements, balance, and posture. The medulla oblongata controls essential automatic functions like breathing, heart rate, and blood pressure.