responding to changes definitions

Acetylcholine

A type of neurotransmitter that is used for communication between neurones

Actin

A type of protein filament found in myofibrils.

It forms thin filaments consisting of two long twisted chains.

Actinomyosin bridge

The cross-bridge formed when a myosin head attaches to the myosin binding site on an actin filament.

Action potential

The temporary change in electrical potential across the membrane of an axon in response to the transmission of a nerve impulse.

Adenylate cyclase

An enzyme that catalyses the conversion of ATP to cAMP.

Adrenaline

A hormone that is secreted by the adrenal glands under stressful conditions.

It increases blood glucose concentration by activating enzymes involved in glycogenolysis.

Afferent arteriole

The blood vessel that stems from the renal artery and supplies blood to the nephron.

It has a larger diameter than the efferent arteriole and divides into a complex system of capillaries, the glomerulus.

All-or-nothing

A principle that states that all stimuli above a certain threshold value will generate the same size of action potential, regardless of the strength of the stimulus.

A - Bands

The darker bands in a myofibril, which consist of overlapping actin and myosin filaments.

Antagonistic muscles

Pairs of muscles that work in opposite directions.

Antidiuretic hormone

A hormone made by the hypothalamus and secreted by the posterior pituitary gland in response to a fall in blood water potential.

It increases the permeability to water of the distal convoluted tubule and the collecting duct, allowing more water to be reabsorbed into the blood.

Ascending limb

The limb of the loop of Henle that rises into the cortex.

It is wider in diameter than the ascending limb and its walls are impermeable to water.

Sodium ions are moved out of the ascending limb by active transport.

Atrioventricular node (AVN)

A group of cells located between the atria that slow down the wave of excitation and pass it between the ventricles, along the bundle of His.

Atrioventricular septum

A layer of non-conductive tissue between the right atrium and left ventricle of the mammalian heart.

Autonomic nervous system

A branch of the motor nervous system that carries nerve impulses to muscles and glands. It controls involuntary activities and has two divisions: the sympathetic nervous system and the parasympathetic nervous system.

Auxins

A class of plant hormones that control cell elongation.

Axon

A long fibre that conducts nerve impulses away from the cell body.

Bundle of His

A collection of Purkyne fibres which run from the AVN down to the apex of the ventricles.

Cell body

The region of the neurone that contains the organelles, notably the nucleus and the rough endoplasmic reticulum.

Central nervous system (CNS)

The brain and spinal cord.

Chemoreceptors

A type of receptor found in the walls of the carotid arteries that detects changes in blood pH and transmits nerve impulses to the medulla oblongata.

For example, if blood pH decreases, chemoreceptors increase the frequency of nerve impulses to the medulla oblongata.

Cholinergic synapse

An excitatory or inhibitory synapse formed between neurones or neurones and other effector organs.

It uses the neurotransmitter, acetylcholine.

Collecting duct

The final region of the nephron that collects urine from the distal convoluted tubules and empties it into the renal pelvis.

Its permeability to water is altered by ADH.

Cone cells

A type of light receptor cell that transduces light energy into a generator potential.

Cone cells are concentrated in the fovea, detect light of high intensity, and lead to colour images.

One cone cell forms a synapse with a single bipolar cell, giving high visual acuity.

Control mechanism

A self-regulating system consisting of five features: optimum point, receptor, coordinator, effector, and feedback mechanism.

Coordinator

Coordinates information from the receptors and sends instructions to the effectors.

Cyclic AMP (cAMP)

A ‘second messenger’ involved in the action of adrenaline that activates protein kinase.

Dendrites

Short, branched extensions of the cell body that receive nerve impulses from other neurones

Dendrons

Extensions of the cell body which branch into smaller fibres, dendrites

Depolarisation

A sudden, temporary change in the membrane potential of a neurone in response to the transmission of a nerve impulse.

The inside of the axon is less negative than the outside.

Descending limb

The limb of the loop of Henle that dips down into the medulla. It is smaller in diameter than the ascending limb.

The walls of the descending limb are permeable to water, so the filtrate loses water as it moves down.

Diabetes

A disorder of metabolism in which blood glucose concentration is not regulated properly.

There are two forms: Type I and Type II diabetes.

Distal convoluted tubule

The twisted region of the nephron between the loop of Henle and the collecting duct.

It controls blood pH by reabsorbing ions and alters the concentration of water and salts reabsorbed. Its permeability to water is altered by ADH.

Effector

An organ, tissue, or cell that produces a response to a stimulus.

Efferent arteriole

The blood vessel that carries blood away from the glomerulus and sub-divides to form a network of capillaries.

Its diameter is smaller than the afferent arteriole, creating a build up of hydrostatic pressure in the glomerulus.

Excitatory synapse

A synapse that produces new action potentials when neurotransmitters bind with receptor proteins on the postsynaptic neurone.

Fast-twitch muscle fibres

A type of muscle fibre that contracts more rapidly, with more power, over a shorter period.

They are adapted for anaerobic respiration and intense activity.

Feedback mechanism

The mechanism by which the change to a system, brought about by the effector, is detected by the receptor.

Fovea

The point on the retina, opposite the pupil, that receives the highest intensity of light.

It contains the greatest concentration of cone cells but no rod cells.

Generator potential

Depolarisation of the membrane of a sensory receptor cell that occurs in response to a stimulus.

Glomerular filtrate

The fluid produced by ultrafiltration of the blood into the renal capsule.

It contains water, glucose, mineral ions and urea.

Glomerulus

A bundle of capillaries located in the renal capsule which are adapted for the filtration of blood.

They later merge to form the efferent arteriole.

Glucagon

A hormone that is produced by α cells of the islets of Langerhans.

It increases blood glucose concentration by activating enzymes involved in gluconeogenesis and the conversion of glycogen to glucose.

Gluconeogenesis

The formation of glucose from sources that are not carbohydrate, e.g. amino acids and glycerol.

Glycogenesis

The formation of glycogen from glucose in the liver.

Glycogenolysis

The breakdown of glycogen into glucose in the liver.

Gravitropism

A plant’s growth response to gravity.

Homeostasis

The maintenance of a constant internal environment in the body, despite fluctuations in internal and external conditions.

Hormones

Cell signalling molecules produced by endocrine glands and released into the blood.

They travel to target cells and bind to specific receptors, initiating a response. The effects of hormones are usually long-lasting.

Hyperpolarisation

A decrease in the membrane potential of an axon, so that it is even more negative than the resting potential.

Hypothalamus

The region of the brain close to the pituitary gland that serves as the control centre for the autonomic nervous system.

It is responsible for the regulation of body temperature and the water potential of body fluids.

H-Zone

The lighter region in the centre of each A band.

Indoleacetic acid (IAA)

A plant growth factor that is a type of auxim and controls cell elongation.

It stimulates elongation in shoots and inhibits elongation in roots.

Inhibitory synapse

A synapse that decreases the likelihood of an action potential in the postsynaptic neurone by causing potassium ions (K+ ) to leave the postsynaptic neurone and chloride ions (Cl- ) to enter. T

his results in hyperpolarisation of the postsynaptic neurone.

Insulin

A hormone that is produced by β cells of the islets of Langerhans.

It decreases blood glucose concentration by activating enzymes involved in the conversion of glucose to glycogen and increasing the number of glucose transport channels in the cell surface membranes of target cells.

Intermediate neurone

A neurone located in the spinal cord that links the sensory neurone to the motor neurone.

Iodopsin

The pigment found in cone cells.

Islets of Langerhans

Clusters of hormone-producing cells located in the pancreas.

They consist of α-cells that secrete glucagon, and β-cells that secrete insulin.

I-Bands

The lighter bands in a myofibril, which consist of non-overlapping actin and myosin filaments.

Kinesis

A response to a stimulus that is non-directional, changing the speed at which an organism moves and the rate at which its direction changes.

Loop of Henle

A loop consisting of a descending limb (dips into the medulla) and ascending limb (rises into the cortex) surrounded by blood capillaries.

It creates a low water potential in the medulla, enabling the reabsorption of water.

Medulla oblongata

The part of the brain that controls heart rate.

It is made up of two centres that are linked to the SAN.

One centre is linked by the sympathetic nervous system and increases heart rate.

The other is linked by the parasympathetic nervous system and decreases heart rate.

Motor neurone

A neurone that carries nerve impulses from the CNS to the effectors.

Myelin sheath

An electrically insulating layer consisting of the membranes of Schwann cells. It increases the speed of nerve impulses.

Myofibrils

Tiny contractile muscle fibres which group together. Numerous myofibril bundles constitute muscles.

Myofibrils consist of two protein filaments: actin and myosin.

Myogenic

Describes cardiac muscle tissue that initiates its own contraction without outside stimulation from nervous impulses.

Myosin

A type of protein filament found in myofibrils.

It forms thick filaments, consisting of long tails with bulbous heads, positioned to the side.

Myosin binding site

A site on actin that is normally blocked by tropomyosin.

During muscle contraction, it becomes exposed, allowing a myosin head to attach.

Negative feedback

A feedback mechanism that inhibits the original stimulus and reverses the change in conditions, restoring the optimum point.

Negative Trophism

The growth of a plant away from a stimulus.

Nephron

The functional unit of the mammalian kidney.

Nerve impulse

A wave of depolarisation that travels across an axon membrane.

It is self-propagating.

Neuromuscular junction

An excitatory synapse formed between a motor neurone and a muscle fibre that uses the neurotransmitter, acetylcholine.

Neurones

Nerve cells adapted to quickly transmit nerve impulses.

Neurotransmitters

Chemicals that are used for communication between neurones and their target cells.

Neurotransmitters are stored in synaptic vesicles in the presynaptic neurone and released into the synaptic cleft.

Nodes of ranvier

Gaps between adjacent Schwann cells in the myelin sheath at which action potentials can occur.

Optic nerve

A nerve that carries electrical impulses to the brain from the retina.

Optimum point

The point at which a system works most effectively.

Osmoreceptors

Sensory receptor cells located in the hypothalamus that detect a decrease in water potential.

Osmoregulation

The regulation of the water potential of the blood by the kidney.

Pacinian corpuscle

A sensory receptor that detects changes in mechanical pressure.

Parasympathetic nervous system

A branch of the autonomic nervous system that is active under normal, resting conditions.

It inhibits effectors, slowing down activity.

Peripheral nervous system (PNS)

Pairs of nerves that originate from the CNS and carry nerve impulses into and out of the CNS.

It is divided into the sensory nervous system and motor nervous system.

Phosphocreatine

A compound stored in muscles that serves as a phosphate reserve, enabling ATP regeneration.

Phototrophism

A plant’s growth response to light

Plant growth factors

Hormone-like substances (e.g. IAA) that control the growth of plants in response to external stimuli.

Polarisation

Describes the condition in which an axon has a membrane potential of -65mV (resting potential).

Positive feedback

A feedback mechanism that enhances the original stimulus and increases the change in conditions, deviating the system further from the optimum point.

Positive tropism

The growth of a plant towards a stimulus.

Posterior pituitary gland

The gland responsible for the secretion of ADH into the bloodstream.

Postsynaptic neurone

The neurone after the synapse which contains specific receptor proteins on its membrane, complementary to the neurotransmitter.

Pressure receptors

A type of receptor found in the walls of the carotid arteries and aorta which detects changes in blood pressure and transmits nerve impulses to the medulla oblongata.

For example, if blood pressure increases, pressure receptors increase the frequency of nerve impulses to the medulla oblongata.

Presynaptic neurone

The neurone before the synapse which releases neurotransmitters from synaptic vesicles into the synaptic cleft.

Protein kinase

An enzyme that catalyses the conversion of glycogen to glucose.

Proximal convoluted tubule

The twisted portion of the nephron between the renal capsule and the loop of Henle.

Its walls consist of epithelial cells that are adapted for the reabsorption of glucose and water into the blood.

Purkyne tissue

Specialised cardiac muscle fibres which conduct the wave of excitation from the AVN down to the apex of the ventricles.

Receptor

Specialised structure that detects a specific type of stimulus.

Reflex

A rapid, automatic response to a sensory stimulus by the body. It serves as a protective mechanism.

Reflex arc

The pathway of neurones involved in a reflex action:

Refractory period

The time period after an action potential during which further action potentials are prevented.

This ensures that action potentials can only be propagated in one direction.

It limits the frequency of action potentials and ensures nervous impulses are discrete.