Database results:
examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Stimulus-Receptor-Effector
Our bodies need to respond quickly to changes in our environment to stay alive and healthy. This is called coordination. Whether it's pulling your hand away from something hot or adjusting your body temperature on a cold day, your body needs to detect changes and respond to them appropriately.
Key Definitions:
The nervous system provides fast, short-term responses to stimuli. It uses electrical impulses (action potentials) that travel along nerve cells called neurons. The nervous system includes the brain, spinal cord and nerves throughout the body.
The endocrine system provides slower, longer-lasting responses to stimuli. It uses chemical messengers called hormones that travel in the bloodstream. The endocrine system includes various glands like the pituitary, thyroid and adrenal glands.
The stimulus-receptor-effector model explains how our body coordinates responses to changes in our environment. This model has three main components that work together:
A change in the environment that can be detected, such as light, sound, temperature, pressure, or chemicals. For example, touching a hot object is a temperature stimulus.
Specialised cells that detect specific stimuli and convert them into nerve impulses. For example, temperature receptors in your skin detect heat from a hot object.
Muscles or glands that carry out responses. For example, muscles in your arm contract to pull your hand away from a hot object.
Our bodies have many different types of receptors, each specialised to detect specific stimuli:
Detect mechanical pressure or distortion. Examples include touch receptors in the skin and hair cells in the ear that detect sound vibrations.
Detect changes in temperature. Some respond to heat, others to cold. They're found throughout the skin and in the hypothalamus of the brain.
Detect chemicals. Examples include taste buds on the tongue, olfactory cells in the nose and receptors that monitor blood CO₂ levels.
Detect pain or damage to tissues. They respond to extreme pressure, temperature, or harmful chemicals.
Detect light. Rod cells (for dim light) and cone cells (for colour vision) in the retina of the eye are photoreceptors.
When a receptor detects a stimulus, it converts it into an electrical signal called a nerve impulse or action potential. This impulse travels along neurons to the central nervous system (brain and spinal cord) and then to effectors.
An electrical signal that travels along the neuron. It's an all-or-nothing event - either the impulse happens at full strength or not at all.
Neurons have dendrites (receive signals), a cell body, an axon (conducts impulses) and synaptic terminals (pass signals to other cells).
The junction between two neurons where signals pass from one to another using chemical neurotransmitters.
Effectors are the parts of the body that carry out responses to stimuli. The two main types of effectors are:
When muscles receive nerve impulses, they contract to produce movement. There are three types of muscle tissue:
Glands release substances when stimulated by nerve impulses or hormones. Examples include:
Reflex actions are rapid, automatic responses to stimuli that don't involve conscious thought. They're important for protecting the body from harm.
When a doctor taps just below your kneecap with a small hammer, your lower leg kicks forward automatically. This is called the knee-jerk or patellar reflex.
The tap stretches the tendon, which stimulates stretch receptors. These send impulses to the spinal cord, where they connect directly to motor neurons that cause the thigh muscle to contract. This makes your lower leg kick forward.
This reflex helps doctors check if your nervous system is working properly. It's a monosynaptic reflex, meaning it only involves one synapse in the spinal cord, making it one of the fastest reflexes in your body.
A reflex arc is the neural pathway that controls a reflex action. It has five components:
Examples of reflex actions include:
While the nervous system provides fast responses, the endocrine system uses hormones for slower, longer-lasting coordination.
Hormones are chemical messengers released by endocrine glands directly into the bloodstream. They travel throughout the body but only affect target cells that have specific receptors for that hormone.
Unlike nerve impulses, hormones:
Adrenaline (epinephrine) is a hormone released during stressful situations - the "fight or flight" response. It prepares the body for action by:
Both systems work together to maintain homeostasis (internal balance) and respond to changes, but they have important differences:
Imagine you're walking in the woods and suddenly see a snake on the path. Your body responds in multiple ways using both coordination systems:
Nervous system responses (immediate): You jump back (skeletal muscles), your eyes widen (eye muscles) and you might gasp (respiratory muscles).
Hormonal responses (slightly delayed): Adrenaline is released, increasing your heart rate, blood pressure and blood glucose levels to prepare for running away or defending yourself.
This dual-system response gives you both immediate protection and sustained readiness to deal with the threat.
Log in to track your progress and mark lessons as complete!
Login NowDon't have an account? Sign up here.