Key Words

• Taxis


  A simple response whose direction is determined by the direction of the stimulus
• Kinesis


  A non-directional response to a stimulus which brings about increased random movements
• Tropism


  A growth movement of a part of a plant in response to a directional stimulus
• IAA


  Indoleacetic acid (IAA) is an auxin – a plant growth factor which is produced in small quantities. It controls plant cell elongation. It causes elongation of shoot cells but inhibits elongation in root cells.
• Plant growth factors


  Hormone-like substances which control plant responses to external stimuli. They exert their influence by affecting growth and they may be made by cells located throughout the plant rather than in particular organs. They can affect the tissues that release them. eg indoleacetic acid (IAA)
• Photo-


  A prefix meaning light
• Geo-


  A prefix meaning gravity
• Chemo-


  A prefix meaning chemical
• Rheo-


  A prefix meaning movement
• CNS


  The brain and spinal cord
• Peripheral nervous system


  Pairs of nerves that originate from either the brain or spinal cord to the rest of the body. Divided into the sensory and motor nervous system.
• Neurone


  Specialised cells adapted to rapidly carrying electrochemical changes called nerve impulses from one part of the body to another eg sensory, motor and intermediate
• Sensory neurons


  Neurones which carry nerve impulses from receptors towards the CNS (an intermediate or motor neurone)
• Motor neurons


  Neurones which carry nerve impulses away from CNS (an intermediate or relay neurone) to an effector
• Intermediate or relay neurons


  A neurone which is a co-ordinator which transmit impulses between neurons, for example, from the sensory neurone to the motor neurone in the spinal cord.
• Stimulus


  A detectable change in the internal or external environment of an organism
• Response


  The result of a stimulus on an organism
• Receptor


  A cell or organ which detects a stimulus
• Effector


  A cell, tissue, organ or system which responds to a stimulus
• Coordinator


  The link between a sensory neurone and motor neurone in the spinal cord
• Reflex arc


  Pathway of neurones involved in a reflex: Stimulus 🡪 receptor �� sensory neurone 🡪 intermediate (relay) neurone 🡪 motor neurone 🡪 effector 🡪 response.
• Somatic/Voluntary Nervous System


  A division of the motor nervous system which carries nerve impulses to body muscles and is under voluntary (conscious) control
• Autonomic


  A division of the motor nervous system which carries nerve impulses to glands, smooth and cardiac muscle and is not under voluntary control (subconscious)
• Sympathetic


  An autonomic pathway which stimulates effectors and so speeds up an activity
• Parasympathetic


  An autonomic pathway which inhibits effectors and so slows down an activity (parachute…)
• Pacinian corpuscle


  A sensory receptor which responds to change in mechanical pressure
• Stretch-mediated Na+ channels


  A special type of sodium channel which is found at the sensory neurone ending at the centre of the Pacinian corpuscle. Their permeability to sodium changes when they are deformed by stretching.
• Photoreceptors


  Light receptor cells of the mammalian eye found on the innermost layer, the retina. Two types: rod and cone cells. They act as transducers by conserving light energy into the electrical energy of a nerve impulse.
• Retina


  The innermost layer of the mammalian eye containing photoreceptors.
• Rod cells


  Cells in the retina which transduce light energy into a generator potential, based on intensity of light received. They respond to low-intensity light and give low visual acuity. Absent in the fovea.
• Cone cells


  Cells in the retina which transduce light energy into a generator potential, based on wavelength of light received. There are three different types, each responding to a different range of wavelength of light. Mainly concentrated in the fovea. Not sensitive to low-intensity light but give good visual acuity.
• Retinal convergence


  Many rods connected to one bipolar cell
• Visual acuity


  Ability to resolve fine detail
• Visual sensitivity


  Ability to detect low light intensity
• Fovea


  The point receives the highest intensity of light because light is focused by the lens on the part of the retina opposite the pupil. Cone cells are found here.
• Blind spot


  The point of entry of the optic nerve on the retina which is insensitive to light.
• Optic nerve


  Nerve which transmits impulses to the brain from the retina at the back of the eye
• Nerve impulse


  A self-propagating wave of electrical disturbance theta travels along the surface of an axon membrane
• Na+ K+ Pump


  A carrier protein which actively transports 2 potassium ions into the axon and 3 sodium ions out of the axon.
• Resting potential


  A potential difference of -65mV found inside a resting neurone relative to its outside, which results in the axon being polarised.