Organism level systems Revision Notes

Central Nervous System (CNS)

The Central Nervous System (CNS) is the main processing unit of the body. It comprises two vital components: the brain and the spinal cord. The brain is responsible for receiving and interpreting sensory information, initiating motor responses, and controlling higher-level functions such as thought, memory, and emotion. It is protected by the skull. The spinal cord extends from the brain down the back, enclosed within the vertebral column, and acts as a major pathway for nerve impulses between the brain and the rest of the body. It also coordinates some simple reflexes independently of the brain.

Peripheral Nervous System (PNS)

The Peripheral Nervous System (PNS) consists of all the nerves that extend beyond the Central Nervous System, connecting it to every other part of the body. These nerves act like communication cables, transmitting sensory information from receptors (e.g., in the skin, eyes, ears) to the CNS and carrying motor commands from the CNS to muscles and glands. This allows the body to react to internal and external stimuli. The PNS essentially collects information from the environment and executes instructions from the CNS.

Key Functions of Nervous System Components

Each part of the nervous system has specific key functions. The brain coordinates conscious actions, processes sensory input, and controls involuntary functions like breathing and heart rate. The spinal cord transmits signals between the brain and the body and mediates reflex actions. Nerves within the PNS are responsible for relaying sensory information (e.g., touch, temperature, pain) to the CNS and carrying motor commands from the CNS to muscles, thereby enabling movement and glandular secretions. Together, these components ensure rapid communication and coordinated responses throughout the organism.

The Role of the Pancreas

The pancreas is an endocrine gland that plays a crucial role in regulating blood glucose levels. It contains specialized cells called beta cells, which produce insulin, and alpha cells, which produce glucagon. These hormones are released directly into the bloodstream.

Insulin's Action: Lowering Blood Glucose

When blood glucose levels are too high (e.g., after a meal), the pancreas releases insulin. Insulin circulates to target cells, primarily in the liver and muscles. It increases the permeability of these cells to glucose, allowing them to take up glucose from the blood. Inside these cells, glucose is converted into glycogen, a storage polysaccharide, through a process called glycogenesis. This effectively removes glucose from the bloodstream, lowering blood glucose concentrations.

Glucagon's Action: Raising Blood Glucose

When blood glucose levels are too low (e.g., during fasting), the pancreas releases glucagon. Glucagon primarily targets the liver, stimulating the breakdown of stored glycogen into glucose. This process is called glycogenolysis. The released glucose is then diffused into the bloodstream, increasing blood glucose concentrations back to a normal range.

Antagonistic Control and Homeostasis

Insulin and glucagon work in an antagonistic manner, meaning they have opposite effects, to maintain blood glucose homeostasis. This negative feedback loop ensures that blood glucose levels remain stable, providing a constant energy supply for cellular respiration while preventing the damaging effects of excessively high or low blood sugar on the body's cells and organs.

Detecting High Blood Glucose

After consuming carbohydrates, the process of digestion breaks them down into glucose. This glucose is then absorbed into the bloodstream from the small intestine, leading to an elevation in blood glucose concentration. This increase is a signal detected by specialised cells within the pancreas. The pancreas acts as both an endocrine gland (producing hormones) and an exocrine gland (producing digestive enzymes). Its role in blood sugar regulation is crucial for maintaining homeostasis.

Insulin Release and Action

When high blood glucose levels are detected, the pancreas responds by secreting the hormone insulin. Insulin is released directly into the bloodstream and circulates throughout the body. Its primary function is to lower blood glucose by acting on target cells, especially liver and muscle cells. It increases the permeability of these cells to glucose, allowing them to absorb more glucose from the blood.

Glucose Storage and Utilisation

Upon insulin's signal, liver cells take up excess glucose from the blood and convert it into glycogen, a storage polysaccharide. This glycogen is stored in the liver. Muscle cells also absorb glucose for their energy needs and convert any excess into glycogen for local storage. Additionally, insulin promotes the uptake of glucose by other body cells to be used in cellular respiration, providing energy. These actions collectively reduce the concentration of glucose in the blood.

Negative Feedback Loop

As blood glucose levels fall back to a normal range due to insulin's actions, this change is detected by the pancreas. This acts as a negative feedback mechanism, which then inhibits further insulin release. This regulatory loop prevents blood glucose from falling too low. If levels were to drop excessively, the pancreas would release glucagon, which has the opposite effect to insulin, raising blood glucose by stimulating the liver to release stored glucose.

The Endocrine System

The endocrine system is a control system that uses chemical messengers called hormones. Hormones are produced by specialised glands and are secreted directly into the bloodstream. They travel through the blood to specific target cells or organs, where they produce a response. This process is slower than nervous coordination but generally results in longer-lasting effects.

Hormones as Chemical Messengers

Hormones are chemical substances that act as messengers. Each hormone has a specific shape that binds to complementary receptor proteins on its target cells. Only cells with the correct receptors for a particular hormone will respond to it. This specificity ensures that hormones exert their effects on the correct parts of the body.

Key Features of Hormonal Control

Hormonal control is characterized by several key features: production in glands, transport via the bloodstream, action at specific target cells, a slower but long-lasting response compared to the nervous system, and regulation through feedback mechanisms, primarily negative feedback. Negative feedback helps to maintain stable hormone levels and thus a stable internal environment (homeostasis).

Examples of Endocrine Glands

Examples of important endocrine glands include the pituitary gland (often called the 'master gland' as it controls other glands), the thyroid gland (regulates metabolism), the adrenal glands (involved in stress response), the pancreas (controls blood sugar), and the ovaries or testes (involved in reproduction and secondary sexual characteristics).