Organism level systems Study Guide

The reflex arc is a crucial neural pathway that mediates rapid, involuntary responses to stimuli, protecting the body from harm. Its structure is finely tuned to its function, enabling quick reactions without conscious thought. The pathway begins with a receptor cell, typically located in a sense organ like the skin, which detects a specific stimulus (e.g., heat, pressure, pain). The receptor converts this stimulus energy into an electrical signal, or nerve impulse. This impulse is then transmitted by a sensory neuron (also called an afferent neuron) towards the central nervous system (CNS). The cell body of the sensory neuron is located just outside the spinal cord in the dorsal root ganglion. Upon reaching the spinal cord, the sensory neuron synapses with an relay neuron (also known as an interneuron or connector neuron). The relay neuron is situated entirely within the grey matter of the spinal cord and acts as a connector, transmitting the impulse from the sensory neuron to the motor neuron. This direct connection in the spinal cord bypasses the brain, which is key to the rapid nature of a reflex. Finally, the relay neuron synapses with a motor neuron (efferent neuron), which carries the nerve impulse away from the CNS to an effector organ, typically a muscle or gland. The effector then produces the response (e.g., muscle contraction to withdraw a hand from a hot object). The speed of this pathway is enhanced by the short distances impulses travel within the CNS and the minimal number of synapses involved, ensuring a swift protective action before the brain even fully registers the sensation. This structural arrangement ensures an automatic, protective response, vital for survival.

The human body systems are coordinated and controlled by two main systems: the nervous system and the endocrine system. The endocrine system uses hormones, which are chemical messengers transported in the blood to target organs. This explanation will focus on two key hormones: thyroxine and adrenaline.

Thyroxine is produced by the thyroid gland, located in the neck. Its primary role is to regulate the body's basal metabolic rate. This means it controls how quickly the body uses energy at rest. A higher metabolic rate leads to faster chemical reactions in cells, increased energy production, and consequently, a rise in body temperature. Thyroxine is crucial for growth and development, especially in children, ensuring normal physical and mental maturation. The release of thyroxine is controlled by a negative feedback loop: when thyroxine levels are low, the pituitary gland releases Thyroid-Stimulating Hormone (TSH), which stimulates the thyroid to produce more thyroxine. When thyroxine levels are high, TSH release is inhibited, reducing thyroxine production.

Adrenaline, also known as epinephrine, is produced by the adrenal glands, which sit on top of the kidneys. Adrenaline plays a vital role in preparing the body for stressful or dangerous situations, often referred to as the 'fight or flight' response. When a person perceives a threat, the nervous system rapidly stimulates the adrenal glands to release adrenaline into the bloodstream. Adrenaline has widespread effects: it increases heart rate and blood pressure, diverting blood flow to muscles and away from digestive organs. It also increases the breakdown of stored glycogen into glucose in the liver, providing a rapid energy supply to muscles. Breathing rate also increases, supplying more oxygen. These physiological changes enable the body to react quickly, either to confront the danger or to escape from it. Unlike thyroxine, adrenaline's effects are typically short-lived and immediate.