PC - Topic 6

• CELL SIGNALLING


  The release of a substance by one cell which transmits information to another cell, either locally or over some distance.
• AUTONOMIC NERVOUS SYSTEM


  Part of the nervous system that controls automatic responses such as heart rate; peristalsis in the gut. It consists of the symapthetic and parasympathetic nervous systems.
• STIMULUS


  A detectable change in the internal or external environment of an organism that leads to a response.
• RECEPTOR


  A sensory cell that can detect a stimulus. Receptors are specific to one type of stimulus. Photoreceptors detect light; chemoreceptors detect chemicals; proprioreceptors detect body position; mechanoreceptors detect mechanical strain or stretching; baroreceptors detect blood pressure; osmoreceptors detect concentration of body fluids.
• EFFECTOR


  An organ that becomes active in response to a nerve impulse e.g. muscle; gland.
• SENSORY NEURONE


  This type of nerve cell carries nerve impulses [electrical signals] from receptors towards the central nervous system. The cell body is not at the end of the cell and does not have dendrites.
• MOTOR NEURONE


  This type of nerve cell carries nerve impulses [electrical signals] from the central nervous system to effectors [muscles and glands]. Cell body is at one end of the cell and has dendrites.
• RELAY NEURONE


  Sometimes referred to as interneurones. Relay neurones are nerve cells with short axons that transmit electrical impulses from one neurone to another in the CNS.
• MYELIN SHEATH


  This forms a covering around the axon and is made up of the membranes of the Schwann cells. These membranes are rich is lipid and protein known as myelin. Myelin has electrical insulating properties. Neurones with a myelin sheath are called myelinated neurones.
• SCHWANN CELL


  These cells produce the myelin sheath, which wrap around the axon of a neurone. Schwann cells provide electrical insulation around the axon of a neurone. They also carry out phagocytosis [removal of cell debris] and play a part in nerve regeneration.
• DENDRON


  An extension of the cell body of a neurone, which can subdivide into smaller branches called dendrites. Dendrons carry nerve impusles towards the cell body of a neurone.
• AXON


  A long single fibre that carries nerve impulses away from the the cell body of a neurone.
• NODE OF RANVIER


  A constriction between adjacent Schwann cells where there is no myelin sheath. The constriction is between 2-3μm long and occursevery 1-3mm in humans. Nodes of Ranvier play a key role in saltatory conduction of nerve impulses.
• TRANSDUCER


  Something that changes one form of energy into another. A Pacinian corpuscle will change mechanical energy of the stimulus into an electrical impulse known as a generator potential.
• TAXIS


  A simple response whose direction is determined by the direction of the stimulus. Taxes are classified as positive [movement towards the stimulus] or negative [movement away from the stimulus].
• KINESIS


  A response to a stimulus which involves a change in speed of an organism and the rate at which it changes direction.
• TROPISM


  A growth response in a plant to a directional stimulus e.g. phototropism is a response to light; gravitropism [geotropism] a response to gravity; hydrotropism a response to moisture; thigmotropism a response to touch.
• PLANT GROWTH FACTOR


  Hormone like substances produced in response to external stimuli. Produced in small quantities. Unlike animal hormones, plant growth factors are not produced in specific organs although they may be produced in a restricted plant region. They are produced by unpsecialised cells and their effects on the plant may vary in different circumstances. Responses include tropisms; stomatal closure; seed germination; fruit formation and ripening.
• PHOTOTROPISM


  The response of shoots to unilateral light. A young shoot will grow towards the light. This is known as positive phototropism.
• GRAVITROPISM/ GEOTROPISM


  A plant growth response to gravity. Shoots show negative geotropism whilst roots are positively geotropic. This ensures that when a seed germinates the shoots and roots grow in the right direction, whatever the orientation of the seed. As roots are positively geotropic, this promotes anchorage in the soil and effective uptake of water and mineral ions.
• INDOLEACETIC ACID [IAA]


  A plant growth substance which belongs to a group of substances called auxins. Auxins may 'switch' some genes on and others off. IAA is produced in the shoot tip and passes down to the zone of cell expansion, where it stimulates young cells to expand. Here it stimulates proteins called expansins, which make cell walls more flexible by loosening the hydrogen bonds between cellulose fbres.
• APICAL DOMINANCE


  Auxins produced at the apical/ shoot meristem not only cause the stem to grow upwards but they also inhibit the growth of lateral [side] buds. If the shoot tip/ apical meristem and the source of auxin is removed, lateral shoots start to grow and the plant becomes more bushy in appearance.
• GIBBERELLINS


  A group of plant growth factors produced in young leaves, seeds and root tips. Their functions include the stimulation of germination, flowering and increasing internode length leading to stem elongation. Like auxins, gibberellins stimulate cell elongation but also promote cell division.
• ABSICISIC ACID [ABA]


  A plant growth factor that has a role in the closure of stomata in the leaves by guard cells. ABA also inhibits the production of amylase and this inhibits germination and prolongs seed dormancy. ABA promotes senescence and leaf abscission [leaf fall].
• ETHENE


  A plant growth factor which stimulates fruit ripening. This can be useful in fruits that are delicate and liable to damage in transit e.g. bananas. The fruit can be picked hard and unripe, transported and then artifically ripened with ethene, at their destination.
• PARTHENOCARPY


  The production of seedless fruits without fertilisation e.g. grapes. Auxins and gibberellins can be used to treat unpollinated flowers, causing them the develop seedless fruit.
• CENTRAL NERVOUS SYSTEM [CNS]


  A division of the nervous system consisting of the brain and the spinal cord.
• PERIPHERAL NERVOUS SYSTEM [PNS]


  A division of the nervous system consisting of pairs of nerves running to and from the CNS.
• REFLEX


  A rapid involuntary response to a sensory stimulus which usually has some protective or survival value. e.g. blinking; pupil reflex; withdrawal reflex; knee jerk.
• REFLEX ARC


  The pathway of neurones involved in a reflex.
• PACINIAN CORPUSCLE


  A type of receptor which responds to mechanical stimuli such as pressure. Pacinian corpuscles occur deep in the skin and are most abundant on the fingers, soles of the feet and external genitalia. They also occur in joints, ligaments and tendons.
• GENERATOR POTENTIAL


  When pressure distorts the lamellae of a Pacinian corpuscle, stretch mediated sodium channels widen in the plasma membrane of the exposed axon of the sensory neurone at the centre of the Pacinian corpuscle. Sodium ions diffuse into the neurone and the membrane becomes depolarised to produce a generator potential.
• STRETCH MEDIATED SODIUM CHANNEL

  A special type of sodium channel in the plasma membrane of the sensory neurone ending at the centre of the Pacinian corpuscle.
• ROD CELL


  A light receptor cell found in the retina of the mammalian eye, which acts as a transducer. Rod cells are more numerous than cone cells and are sensitive to low intensity light, leading to images in black and white. Many rod cells link to a single bipolar cell and therefore give low visual acuity.
• CONE CELL


  A light receptor cell found in the retina of the mammalian eye, which acts as a transducer. Cone cells are fewer in number than rod cells and are concentrated at the fovea. There are three types of cone cells, each type responds to a specific range of wavelengths of light to produce images in full colour. Each cone cell has its own connection to a single bipolar neurone and gives high visual acuity.
• VISUAL ACUITY


  The ability to detect fine details or small distances with the eye.
• SYMPATHETIC NERVOUS SYSTEM


  Part of the autonomic [involuntary/subconscious] nervous system which carries nerve impulses to glands, smooth muscle and cardiac muscle. Sympathetic nervous system deals with 'flight or fight' responses.
• PARASYMPATHETIC NERVOUS SYSTEM


  Part of the autonomic [involuntary/subconscious] nervous system which carries nerve impulses to glands, smooth muscle and cardiac muscle. Parasympathetic nervous system deals with the 'rest and digest' system.
• SINOATRIAL NODE [SAN]


  Referred to as the 'pacemaker' of the heart. A distinct group of cells in the wall of the right atrium of the heart. The initial stimulus for contraction orginates here. The SAN controls and coordinates the contraction of the heart.
• ATRIOVENTRICULAR NODE [AVN]


  A group of cells which lie between the atria of the heart. After a short delay, the AVN conveys a wave of electrical excitation between the ventricles along Purkyne fibres.