Unit 6 response to stimuli

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.

PURKYNE TISSUE

A series of specialised muscle fibres located in the inner ventrticular walls [septum] which collectively make up the bundle of His. The bundle of His conducts the wave of excitation from the AVN, through the atrioventricular septum to the base of the ventricles. The ventricles then contract quickly from the apex upwards.

BUNDLE OF HIS

A collection of Purkyne fibres located in the inner ventricular walls, which convey a wave of electrical excitation from the AVN to the apex of the ventricles.

MEDULLA OBLONGATA

A region of the brain and part of the autonomic nervous system. It contains control centres for heart rate; breathing rate and blood flow rate.

CHEMORECEPTOR

A type of receptor which responds to chemicals e.g blood carbon dioxide concentration.

OSMORECEPTOR

A type of sensory receptor which detects concentration of body fluids.

BARORECEPTOR

A type of sensory receptor which detects blood pressure.

CAROTID ARTERIES

Main arteries which supply blood from the heart to the head. Chemoreceptors located in the walls of the carotid arteries are sensitive to the pH of the blood resulting from changes in blood carbon dioxide concentration.

NEUROTRANSMITTER

A chemical substance released in small amounts by the ending of a nerve fibre; either at a synapse when it excites the adjacent nerve cell or at a motor nerve ending when it excites the adjacent effector organ. Acetylcholine, adrenaline and noradrenaline are common neurotransmitters.

RESTING POTENTIAL

A form of potential energy resulting from behaviour of charged ions leading to their unequal distribution across a membrane. The inside of an axon is negatively charged relative to the outside and is usually around -70mV in humans. The sodium-potassium pump actively transports three sodium ions out of the axon into tissue fluid for every two potassium ions pumped in. Membrane bound sodium votlage gated channels are closed which prevent sodium ions diffusing down a concentration gradient, back into the axon. Membrane bound potassium channels are also closed but they are leaky so potassium ions have a tendency to diffuse out of the axon into tissue fluid. Consequently there are more positive ions on the outside of the membrane than inside.

ACTION POTENTIAL

A form of potential energy resulting from behaviour of charged ions leading to their unequal distribution across a membrane. The inside of an axon is positively charged relative to the outside and is usually around +40mV in humans. Provided a stimulus reaches threshold level, voltage gated sodium channels start to open and sodium ions move into the axon down a concentration and electrochemical gradient. The influx of sodium ions raises the membrane potential from its resting level of -70mV but does not immediately start an impulse. If the stimulus raises the membrane potential to -55mV [threshold potential], this causes whole sale opening of the voltage gated sodium channels and further sodium ions flood into the neurone.

POLARISED

The state of the axon when the inside of the membrane is more negatively charged [-70mV] in comparison to the tissue fluid outside.

DEPOLARISED

The state of part of an axon when the inside of the membrane is more positively charged [+40mV] in comparison to the tissue fluid outside. When an action potential occurs.

VOLTAGE GATED CHANNEL

Channel protein in the membrane whose opening is controlled by voltage.

THRESHOLD POTENTIAL

The value of approximately -55mV across the axon membrane which causes the opening of a large number of voltage gated sodium channels.

REPOLARISATION

The resting potential of -70mV is re-established across a section of the axon membrane. Voltage gated sodium channels close and voltage gated potassium channels open and potassium ions move out of the axon due to electrochemical and concentration gradients.

HYPERPOLARISATION

Occurs when potassium ions continue to exit the axon through voltage gated potassium channels. The voltage gated potassium channels do not close until the potential difference reaches -80mV. This is lower than the normal resting potential.

REFRACTORY PERIOD

A span of time [approx 1ms] during which a second stimulus is unable to cause an impulse and which results from hyperpolarisation of the axon membrane. [absolute refractory period] . Relative refractory period follows, lasting for 5-10ms, during which a high intensity stimulus may produce a depolarisation.

ALL OR NOTHING PRINCIPLE

For any given neurone, the amplitude of the action potential is constant, as long as the stimulus reaches the threshold value. Increasing the strength or number of stimuli has no effect on the amplitude of an action potential.#

SALTATORY CONDUCTION

In a myelinated axon, depolarisation is largely prevented in the part of the axon covered by myelin. Action potentials occur at the nodes of Ranvier which then appear to 'jump from node to node'. This increases the speed of conduction.

SYNAPSE

A junction between two nerve cells, consiting of a minute gap across which impulses pass either by an electrical current [electrical synapse] or more often by diffusion of a chemical neurotransmitter [chemical synapse].

SYNAPTIC CLEFT

A small gap between the presynaptic knob and the post synaptic membrane.

PRESYNAPTIC NEURONE

The neurone at a synapse which releases the neurotransmitter substance e.g. acetlycholine. The membrane of the presynaptic neurone has votage gated calcium channels.

POSTSYNAPTIC NEURONE

The neurone which has receptors for the neurotransmitter substance on its membrane and possesses voltage gated sodium channels.

CHOLINERGIC SYNAPSE

Synapses which release acetlycholine as a transmitter substance. Cholinergic synapses are common in vertebrates where they occur in the CNS and at neuromuscular junctions.

ACETYLCHOLINE

A common neurotranmitter substance relased at excitatory synapses. Acetlycholine is made up of two parts: acetly [ethanoic acid] and choline.

ACETYLCHOINESTERASE

An enzyme which breaks down the neurotransmitter, acetylcholine, the products of which are absorbed back into the presynaptic knob.

SPATIAL SUMMATION

Occurs when a number of different presynaptic neurones collectively release sufficient neurotransmitter to exceed the threshold level to trigger an action potential in the postsynaptic neurone.

TEMPORAL SUMMATION

Occurs when a single presynaptic neurone is activated several times in quick succession. The neurotransmitter released builds up in concentration to exceed the threshhold value. An action potential is triggered in the postsynaptic neurone.

ANTAGONISTIC

A description of structures or chemicals in the body which perform opposite actions to each other e.g biceps and triceps muscles.

STRIATED MUSCLE

Skeletal or voluntary muscle. Makes up the bulk of body muscle in vertebrates. Attached to bones by tendons. Under conscious control.

SMOOTH MUSCLE

Non striated or involuntary muscle. Not under conscious control. Found in the walls of blood vessels, the gut, the iris of the eye.

CARDIAC MUSCLE

Found exclusively in the heart. Myogenic. Cardiac muscle cells appear striated under the microscope with many mitochondria. Cardiac muscle cells are connected by intercalated discs.

MYOFIBRIL

Skeletal muscle consists of muscle fibres bundled together. Each muscle fibre contains many myofibrils, which are the contractile elements of the muscle. Each myofibril is made up of two types of protein filament: thin actin and thick myosin.

ACTIN

A thin type of protein filament found in a myofibril. Consists of two strands twisted around each other.

MYOSIN

A thick type of protein filament found in a myofibril. A molecule of myosin consists of a long rod shaped region with a bulbous head.

SARCOMERE

The section of a myofibril between two Z lines.

SARCOPLASM

The cytoplasm of a muscle fibre. The sarcoplasm contains a large concentration of mitchondria and endoplasmic reticulum.

SARCOLEMMA

The membrane which surrounds a muscle fibre.

SARCOPLASMIC RETICULUM

A membrane bound structure found within a muscle fibres, similar to the endoplasmic reticulum in other cells. The main function of the sarcoplasmic reticulum is to store calcium ions.

I BAND

Isotropic band. A light region consisting of thin actin filaments within a sarcomere. The I band becomes narrower when a myofibril contracts.

A BAND

Anisotropic band. A dark region consisting of overlapping thin actin and thicker myosin filaments. The A band remains the same width when a myofibril cotracts.

Z LINE

A dark line which traverses the middle of each I band. The section of the myofibril between two Z lines is known as a sarcomere.

H ZONE

A lighter region which occurs in the centre of the darker A band of a myofibril. The H zone consists only of myosin filament. The H zone becomes narrower during contraction of the myofibril.

TROPOMYOSIN

A protein which forms a fibrous strand wrapped around an actin filament.

TROPONIN

A globular protein which binds to tropomyosin and can reversibly attach to calcium ions.

SLOW TWITCH FIBRES

These type of muscle fibres contract less rapidly and fatigue slowly. They provide less powerful contractions but over a long period of time. These type of muscle fibres are adapted to endurance work. They possess a large store of myoglobin, a rich blood supply and numerous mitochondria to produce ATP.

FAST TWITCH FIBRES

These type of muscle fibres contract rapidly, fatigue rapidly and produce powerful contractions over a short period of time. They are adapted for intense exercise e.g. weight lifting. They possess thicker and more numerous myosin filaments; a high concentration of glycogen; high concencentrations of enzymes involved in anaerobic respiration to provide ATP quickly; a store of phosphocreatine.

NEUROMUSCULAR JUNCTION

A specialised synapse where the end of a motor neurone meets a skeletal muscle fibre. Unlike nerve synapses, the postsynaptic membrane at a neuromuscular junction is folded to increase the surface area to allow more postsynaptic receptors to be present. In a neuromuscular junction a single impulse will always cause a contraction, unlike a nerve synapse.

MYOGLOBIN

A pigment found in muscles which acts as an emergency store of oxygen for aerobic respiration. Myoglobin has a high affinity for oxygen and will only release oxygen when the surrounding concentration [partial pressure] of oxygen is very low.

SLIDING FILAMENT THEORY

A theory to explain how the thin actin and thick myosin filaments can slide past one another to shorten the myofibrils during muscle contraction.

PHOSPHOCREATINE [PCr]

Sometimes referred to a creatine phosphate. Phosphocreatine is a chemical stored in muscle fibres which acts as a reserve supply of phosphate to convert ADP to ATP. No lactate is produced in this process so it does not lead to muscle fatigue. Phosphocreatine stores in muscle fibres are limited and will run out in a few seconds. The system provides only enough extra ATP for short bursts of activity. The phosphocreatine store is replenished using phosphate from ATP when the muscle is relaxed.

TRANSVERSE [T] TUBULES

Extensions of the sarcolemma which traverse [run across ] each muscle fibre and between myofibrils. They are involved in the uptake and release of calcium ions in the sarcoplasm, which in turn controls ATPase activity.

HOMEOSTASIS

The maintenance of a constant internal environment within restricted limits in organisms e.g. blood pH; blood glucose concentration; core temperature; water potential.

ENDOTHERM

An animal that maintains its body temperature using heat generated within its body tissues e.g. mammals and birds.

ECTOTHERM

An animal that does not have physiological mechanims to maintain a constant core body temperature. These animals rely on behavioural adapatations to ensure that their core temperature does not sink too low or rise too high e.g. reptiles.

ENDOCRINE SYSTEM

A series of ductless glands which produce and secrete chemical messengers called hormones into the blood stream which affect target organs. Hormonal responses are generally slower than those initiated by electrical impulses carried by neurones but hormones control long lasting effects such as growth, metabolic rate and the oestrous cycle.

NEGATIVE FEEDBACK

Type of control mechanism in which a change from the normal value of a physiological factor is detected and a response is made that restores the value to the norm. e.g. control of blood glucose level

VASODILATION

The diameter of the arterioles near the surface of the skin become larger. This allows warm blood to pass close to the skin surface through the capillaries. Heat from this blood is then radiated away from the body surface.

VASOCONSTRICTION

The diameter of the arterioles near the surface of the skin become narrower. This reduces the volume of blood reaching the skin surface through the capillaries. Most of the blood entering the skin, passes beneath the insulating layer of fat and so little heat is lost to the environment.

SHUNT VESSEL

A blood vessel which directly links an artery to a vein. In thermoregulation, shunt vessels can control blood flow by constriction or dilation. When dilated, shunt vessels divert blood away from the capillaries close to the skin surface thereby reducing heat loss.