Nervous & Hormonal Control

Structural Divisions: The system is split into the Central Nervous System (CNS), comprising the brain and spinal cord, and the Peripheral Nervous System (PNS), consisting of nerves that connect the CNS to the rest of the body. The CNS acts as the master coordinator, processing sensory data and initiating motor responses.

Neuronal Pathway: A typical response follows a specific sequence: Stimulus $\rightarrow$ Receptor $\rightarrow$ Sensory Neurone $\rightarrow$ CNS $\rightarrow$ Motor Neurone $\rightarrow$ Effector $\rightarrow$ Response. This linear flow ensures that sensory information is correctly translated into physical action or chemical secretion.

Synaptic Transmission: Neurones are separated by tiny gaps called synapses. When an electrical impulse reaches a terminal, it triggers the release of neurotransmitters, which diffuse across the gap and bind to receptors on the next neurone, regenerating the electrical signal.

The Reflex Arc: Reflexes are automatic, involuntary responses that bypass the conscious brain to minimize injury. In a reflex arc, the signal passes through a relay neurone in the spinal cord, allowing for a rapid 'shortcut' between detection and action.

Chemical Messengers: Hormones are specific chemicals that alter the activity of target organs. Because they are transported in the blood, they can affect multiple distant sites simultaneously, such as when adrenaline prepares the body for physical exertion.

Blood Glucose Control: Insulin, produced by the pancreas, is critical for lowering blood sugar levels. When glucose is high, insulin stimulates the liver and muscles to convert excess glucose into glycogen for storage, preventing osmotic damage to cells.

Water Balance (Osmoregulation): Antidiuretic Hormone (ADH) regulates the permeability of the kidney's collecting ducts. If blood water levels are low, more ADH is released, making the ducts more permeable so that more water is reabsorbed into the blood, resulting in concentrated urine.

Reproductive Hormones: Hormones like Oestrogen, Progesterone, FSH, and LH coordinate the menstrual cycle and secondary sexual characteristics. FSH triggers egg maturation, while LH induces ovulation and progesterone maintains the uterine lining.

Structure and Function: The eye is a complex sense organ where the retina contains light-sensitive receptors. Light enters through the pupil, is refracted by the cornea and lens, and is focused onto the fovea for high-detail vision.

Accommodation: This process describes how the eye adjusts to focus on objects at varying distances. To focus on a near object, the ciliary muscles contract, causing the suspensory ligaments to loosen, which allows the lens to become thicker (more convex) and refract light more strongly.

Distant Vision: When viewing far objects, the ciliary muscles relax and the suspensory ligaments tighten, pulling the lens into a thinner shape to reduce refraction.

Pupil Reflex: In bright light, circular muscles in the iris contract while radial muscles relax, constricting the pupil to protect the retina. In dim light, the radial muscles contract and circular muscles relax to dilate the pupil and maximize light entry.

Temperature Monitoring: The brain's thermoregulatory centre monitors blood temperature and receives input from skin receptors. It coordinates responses to maintain the body at $37^{\circ}\text{C}$, ensuring optimal enzyme kinetics.

Cooling Mechanisms: When the body is too hot, vasodilation occurs—arterioles near the skin surface widen, increasing blood flow to capillaries so more heat is lost by radiation. Additionally, sweating facilitates evaporative cooling.

Warming Mechanisms: In cold environments, vasoconstriction narrows arterioles to reduce blood flow to the skin, minimizing heat loss. Shivering generates metabolic heat through rapid muscle contractions, while hair erection traps an insulating layer of air.

Vasodilation vs. Vasoconstriction: Vasodilation involves the widening of arterioles to increase heat loss, whereas vasoconstriction involves the narrowing of arterioles to minimize heat loss. Note that vasoconstriction does not 'create' heat; it only conserves it.

FSH vs. LH: FSH (Follicle-Stimulating Hormone) stimulates the maturation of the egg cell and the release of oestrogen, whereas LH (Luteinising Hormone) triggers ovulation (release of the egg) and the production of progesterone.

Terminology Precision: Never state that suspensory ligaments 'contract' or 'relax'. They are not muscles; they only become taut (tight) or slack (loose) based on the action of the ciliary muscles.

Mechanism of Cooling: When describing vasodilation, specify that the arterioles widen, not the capillaries themselves. Capillaries do not have muscular walls to dilate; the control happens at the arteriole level.

Hormonal Feedback: Understand the relationship between hormones. For example, oestrogen inhibits FSH and stimulates LH. This 'cross-talk' is a common target for exam questions regarding the menstrual cycle.

Units and Variables: In calculations involving nerve impulses, ensure units are consistent (e.g., $m/s$ vs $cm/s$). Always check if a question asks for a 'reflex' (automatic) or 'voluntary' pathway, as the components differ.