#96 Summary of coordination and response

All organisms are able to sense changes in their environment, called stimuli, and respond to them. The part of the body that senses the stimulus is a receptor, and the part that responds is an effector.

• The human nervous system contains specialized cells called neurons. The brain and spinal cord make up the central nervous system (CNS), which coordinates responses to stimuli.

• Reflex actions are fast, automatic responses to a stimulus. They involve a series of neurons making up a reflex arc. A sensory neurone takes the impulse to the CNS and a motor neurone takes it from the CNS to an effector.

• Receptors are generally found within sense organs.

• The receptors in the eye are rod and cone cells, found in the retina. Rods respond to dim light and cones to bright light. Cones give colour vision.

• The cornea and lens focus light rays onto the fovea, the part of the eye where cone cells are most densely packed.

• The shape of the lens is changed by the contraction or relaxation of the ciliary muscle. When focusing on a distance subject , the muscle relaxes so that the suspensory ligament are pulled taut and the lens is pulled into a thin shape. When focusing on a near object, the muscle contracts and the lens falls into its natural, more rounded shape.

• Muscles can pull when they contracts, but they cannot push. A pair of muscles is therefore needed to pull in different directions, e.g. at the elbow joint. They are antagonistic muscles.

• Hormones are chemicals made in endocrine glands and carried in the blood plasma. Adrenaline is secreted by the adrenal glands, and bring about changes that supply the muscles with extra glucose. This gives the energy for contraction for ‘fight or flight’.

• Plant response to some stimuli by growing towards or away from them. These responses are tropism.

• Auxins are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the shoots or roots. It collects in the shady side of the shoot, making the side grow faster so the shoot bends towards the light. Auxins are used as selective weedkillers.