Coordination, Control & Homeostasis

• Receptors are groups of specialised cells. They detect changes in the environment stimulus. In the nervous system, this leads to an electrical impulse being made in response to the stimulus. Sense organs contain groups of receptors that respond to specific stimuli.
• An external stimulus is a change in the environment that causes a living organism to respond.
• Homeostasis is the maintenance of a constant internal environment despite changes in the external environment.
• Body water content and body temperature are both examples of homeostasis.
• A coordinated response requires a stimulus, a receptor and an effector.
• Transmission by the nervous system is short-lived but quick, whereas transmission by the hormonal system is long-lasting but takes much longer. This means that communication is faster when using the nervous system. In the nervous system, responses are localised, whereas in the hormonal system they are widespread.
• The nervous system uses electrical impulses to send signals through neurones. The hormonal system uses chemical messengers transported into blood plasma to target cells.
• The CNS (Central Nervous System) consists of the brain and spinal cord and is linked to sense organs by nerves.
• The stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the CNS, resulting in rapid responses.
• Neurotransmitters carry chemical signals (messages) from one neuron to the next target cell. The next target cell can be another nerve cell, a muscle or a gland.
• A simple reflex happens when we touch something hot. The receptors in the skin detect a stimulus (the change in temperature). Sensory neurons send electrical impulses to relay neurons, which is located in the spinal cord of the CNS. Relay neurons connect sensory neurons to motor neurons. The motor neurons send electrical impulses to an effector. The effector produces a response (eg: your muscle contracts to move your hand away).
• General reflex sequence: Stimulus - Receptor - Sensory neurone - Relay neurone - Motor neurone - Effector - Response
• Light passes through the eyeball to the retina. There are two main types of light receptors - rods and cones. Rods are more sensitive to light than cones so they are useful for seeing in dim light. There are three different types of cone cells which produce colour vision.
• Accommodation is the process of changing the shape of the lens to focus on near or distant objects. To focus on a near object the lens becomes thicker which allows the light rays to refract more strongly. To focus on a distant object the lens is pulled thin which allows the light rays to refract slightly.
• Increasing the amount and speed of blood flowing to and within the skin by widening the blood vessels (vasodilation) allows more heat to be lost, reducing your body temperature. Narrowing the blood vessels (vasoconstriction) means that less heat will be lost this way, maintaining the core temperature of the body.
• When we get too hot, sweat glands in the skin release more sweat. The sweat evaporates, transferring heat energy from the skin to the environment.
• Adrenaline is produced by the adrenal glands in times of fear or stress. It targets vital organs, increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles. Preparing the body for "flight or fight". Adrenaline is not controlled by negative feedback.
• Insulin comes from the pancreas. It is made to ensure that sugar from nutrients in food is correctly used or stored in the body cells to make fat, sugar and protein. Insulin is controlled by negative feedback as the body will start/stop producing insulin if it is/isn't needed.
• Plants respond to stimuli
• The stem of a plant moves away from gravity in order to grow upwards in a negative geotropism, whereas the root of a plant moves towards gravity to move downwards (positive geotropism).
• Phototropism is when a plant grows towards the light. Shoots usually move towards the light (positive phototropism) and roots usually move away from it (negative phototropism).
• In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. It grows because the auxin causes the cells to elongate on the shaded side, causing that side to grow more.
• A stimulus is a change in an animal's surroundings and a response is a reaction to that change.
• Your eyes are an example of a receptor organ. Your muscles are an example of an effector organ.
• stimulus -> receptor -> coordination -> effector -> response
• Glands secrete chemical substances.
• The link between the stimulus and the response is the nervous system.
• The information in the nerve cells is transmitted in the form of tiny electrical signals called nerve impulses.
• The role of any receptor is to detect the stimulus by changing its energy into the electrical energy of the nerve impulses.
• Impulses from receptors pass along nerves containing sensory neurones until they reach the brain and spinal cord. These two organs together are known as the CNS (Central Nervous System).
• Other nerves contain motor neurones, transmitting impulses to the muscles and glands. Some nerves contain only sensory or motor cells, while other nerves contain both.
• The innermost layer of the back of the eye is the retina. This is the light-sensitive layer, the place where light energy is converted into electrical energy of nerve impulses. The retina contains receptor cells called rods and cones. These cells react to light, producing impulses in sensory neurones. The sensory neurones then pass the impulses to the brain through the optic nerve.
• To form an image on the retina, light needs to be bent or refracted.
• The role of the iris is to control the amount of light entering the eye, by changing the size of the pupil.
• A synapse is a gap between two nerve cells. Impulses arriving at a synapse cause the ends of the fine branches of the axon to secrete a chemical called a neurotransmitter.
• A gland is an organ that releases or secretes a substance. Endocrine glands produce hormones that are secreted into the blood vessels that pass through the gland.
• Nervous system: Works by nerve impulses transmitted through nerve cells. Nerve impulses travel fast and usually have an "instant" effect. The response is usually short-lived. Impulses act on individual cells such as muscle fibres, so have a very localised effect.
• Endocrine system: Works by hormones transmitted through the bloodstream. Hormones travel more slowly and generally take longer to act. The response is usually longer lasting. Hormones can have widespread effects on different organs.