Organism level systems Revision Notes

The Role of FSH and Oestrogen

The pituitary gland releases Follicle-Stimulating Hormone (FSH) at the start of the menstrual cycle. FSH stimulates the growth and development of follicles in the ovaries. As these follicles mature, they produce oestrogen. Rising oestrogen levels cause the uterine lining to thicken and repair, preparing it for pregnancy. Oestrogen also initially inhibits FSH production (negative feedback) but then triggers an LH surge.

The Role of LH and Ovulation

When oestrogen levels reach a peak around the middle of the cycle, they stimulate a sudden and significant increase in Luteinizing Hormone (LH) from the pituitary gland. This 'LH surge' is the primary trigger for ovulation. Ovulation is the release of a mature egg from the ovary, typically occurring around day 14 of a 28-day cycle. After ovulation, LH also causes the ruptured follicle to develop into the corpus luteum.

The Role of Progesterone and Uterine Maintenance

Following ovulation, the corpus luteum, formed from the ruptured follicle, starts to produce large amounts of progesterone, along with some oestrogen. Progesterone is essential for maintaining the thickened uterine lining, making it ready to receive a fertilised egg. High levels of progesterone, along with oestrogen, inhibit the production of FSH and LH, preventing further follicle development and ovulation during this phase.

End of Cycle and Menstruation

If fertilisation and implantation do not occur, the corpus luteum degenerates after about 10-14 days. This degeneration leads to a rapid decrease in the production of progesterone and oestrogen. The sharp drop in these hormones causes the uterine lining, which was maintained by progesterone, to break down and shed. This shedding of the uterine lining is known as menstruation. The low levels of progesterone and oestrogen then allow FSH to rise again, initiating a new cycle.

Thyroxine: Regulating Metabolism and Growth

Thyroxine is a hormone secreted by the thyroid gland. Its main function is to control the basal metabolic rate, which is the speed at which your body carries out its essential chemical processes at rest. This regulation directly impacts how quickly energy is released from food and used by cells. Thyroxine also plays a crucial role in normal growth and development, particularly during childhood and adolescence. An imbalance in thyroxine levels can lead to significant health issues, such as hypothyroidism (too little thyroxine, causing slow metabolism, weight gain, fatigue) or hyperthyroidism (too much thyroxine, causing fast metabolism, weight loss, anxiety).

Adrenaline: The 'Fight or Flight' Hormone

Adrenaline is a hormone produced by the adrenal glands, which are located above the kidneys. It is a key component of the body's rapid response to stress, danger, or excitement, often referred to as the 'fight or flight' response. When a threat is detected, adrenaline is quickly released into the bloodstream, triggering a cascade of physiological changes. These changes prepare the body for immediate action. Its effects are short-term but powerful, allowing for quick reactions in emergency situations.

Physiological Effects of Adrenaline

The release of adrenaline causes several immediate physiological changes. It increases heart rate and blood pressure, which improves blood flow to essential organs, particularly muscles and the brain. Adrenaline also raises the breathing rate, leading to increased oxygen intake. Furthermore, it stimulates the liver to convert stored glycogen into glucose, providing a rapid source of energy for muscle contraction. Blood flow is diverted away from less critical functions like digestion and towards the muscles, enhancing physical performance crucial for confronting or escaping danger.

Comparing Thyroxine and Adrenaline

While both thyroxine and adrenaline are hormones that regulate body functions, they have distinct roles. Thyroxine provides long-term control over metabolic rate, growth, and development, influencing energy use over extended periods. Its regulation is part of a slower, sustained process. Adrenaline, in contrast, provides an immediate, short-term response to acute stress or danger, preparing the body for rapid action. Its effects are quickly felt and subside relatively quickly once the threat is gone. Thyroxine levels are homeostatically controlled via a negative feedback loop involving TSH.

Components of a Reflex Arc

A reflex arc is the neural pathway that mediates a reflex action. It consists of five main components working in sequence: a receptor to detect the stimulus, a sensory neuron to transmit the impulse from the receptor to the central nervous system (CNS), a relay neuron within the CNS to connect the sensory neuron to the motor neuron, a motor neuron to transmit the impulse from the CNS to the effector, and an effector (usually a muscle or gland) to produce the response. This direct pathway allows for rapid responses without conscious brain involvement, which is crucial for protection.

Pathway of Nerve Impulses

When a stimulus is detected by a receptor, it generates an electrical impulse. This impulse travels along the sensory neuron towards the spinal cord. In the spinal cord, the sensory neuron synapses with a relay neuron. The relay neuron then transmits the impulse to a motor neuron. The motor neuron carries the impulse away from the spinal cord to an effector, which then produces the appropriate response. Crucially, this arc bypasses the brain for initial processing, allowing for extremely fast, involuntary actions.

Role of Each Neuron Type

The sensory neuron is responsible for carrying nerve impulses from receptors in the periphery towards the CNS. Its cell body is located outside the spinal cord. The relay neuron is found entirely within the CNS, specifically the spinal cord, and serves to link the sensory and motor neurons. The motor neuron carries impulses from the CNS to an effector organ, such as a muscle, prompting it to contract. This division of labour among neuron types ensures efficient and directed signal transmission for rapid reflex actions.

Efficiency and Protection

The structural arrangement of the reflex arc, particularly the involvement of a relay neuron in the spinal cord that bypasses higher brain centres, is fundamental to its rapid function. This shortcut allows for immediate, involuntary responses, which are vital for protecting the body from harm, such as withdrawing a hand from a hot object. While the brain eventually receives information about the stimulus, the reflex action occurs before conscious awareness, highlighting the protective efficacy of this pathway.

Key Hormones and Their Roles

The menstrual cycle is controlled by four main hormones: Follicle-Stimulating Hormone (FSH), Luteinising Hormone (LH), oestrogen, and progesterone. FSH, released by the pituitary gland, stimulates the development of follicles in the ovary and oestrogen production. LH, also from the pituitary, triggers ovulation and the formation of the corpus luteum. Oestrogen, produced by the developing follicle, repairs and thickens the uterine lining and inhibits FSH. Progesterone, produced by the corpus luteum, maintains the uterine lining for pregnancy and inhibits FSH and LH.

Stages of the Menstrual Cycle

The menstrual cycle can be divided into several stages. It begins with menstruation (a period), where the uterine lining is shed due to low hormone levels. Following this, rising FSH levels stimulate follicle development and oestrogen production. Oestrogen rebuilds the uterine lining. Around day 14, an LH surge causes ovulation, releasing an egg. After ovulation, the corpus luteum forms, producing progesterone, which maintains the uterine lining. If pregnancy does not occur, the corpus luteum breaks down, hormone levels drop, and the cycle restarts.

Hormonal Interactions (Negative and Positive Feedback)

Hormones in the menstrual cycle interact through feedback mechanisms. Rising oestrogen levels initially inhibit FSH (negative feedback), but then a peak in oestrogen triggers an LH surge (positive feedback), leading to ovulation. After ovulation, high levels of progesterone and oestrogen from the corpus luteum exert negative feedback on FSH and LH, preventing further follicle development and ovulation. This coordinated control ensures only one egg typically matures and is released per cycle and maintains pregnancy if it occurs.