Nervous coordination, receptors and muscles definitions

Organisation of the nervous system

Central nervous system

The brain and the spinal cord: the coordinating part of the nervous system, containing the relay neurones.

Peripheral nervous system

The neurones outside the brain and spinal cord, carrying impulses into and out of the CNS.

Somatic nervous system

The part of the peripheral system carrying impulses to skeletal muscle, under voluntary control.

Autonomic nervous system

The part of the peripheral system carrying impulses to glands, smooth muscle and cardiac muscle, not under conscious control.

Circular muscles

Rings of smooth muscle round the pupil in the iris; their contraction constricts the pupil.

Radial muscles

Smooth muscle fibres running outwards from the pupil in the iris; their contraction dilates the pupil.

Pupil reflex

The automatic change in pupil diameter with light intensity: constriction in bright light, dilation in dim light.

Cerebrum

The paired, folded hemispheres responsible for conscious thought, learning, memory and the initiation of voluntary movement.

Cerebellum

The region below and behind the cerebrum that coordinates balance and the timing of movement, without conscious involvement.

Medulla oblongata

The base of the brainstem, containing the autonomic centres controlling heart rate, vessel diameter and breathing.

Hypothalamus

The region beneath the thalamus that monitors the blood, controls thermoregulation and osmoregulation, and directs the pituitary.

Pituitary gland

The endocrine gland hanging beneath the hypothalamus, whose two lobes release hormones under its control.

Dendron

A fibre of a neurone that carries impulses towards the cell body.

Axon

A fibre of a neurone that carries impulses away from the cell body.

Ganglion

A swelling containing the cell bodies of many neurones, outside the central nervous system.

Grey matter

The central region of the spinal cord, containing cell bodies, dendrites, relay neurones and synapses.

White matter

The outer region of the spinal cord, made of myelinated axons carrying impulses up and down the cord.

Central canal

The narrow channel through the centre of the grey matter of the spinal cord, containing cerebrospinal fluid.

Dorsal root

The root by which sensory neurones enter the spinal cord, at the back, bearing the dorsal root ganglion.

Ventral root

The root by which motor neurones leave the spinal cord, at the front.

Receptors: transduction and the reflex arc

Stimulus

A detectable change in the internal or external environment.

Sensory receptor

A cell or structure that detects one kind of stimulus and converts it into a change in membrane potential.

Effector

A muscle or gland that brings about a response.

Reflex

A rapid, automatic response to a stimulus, involving a fixed pathway of neurones and no conscious decision.

Reflex arc

The pathway of neurones taken by a reflex. In the examined withdrawal reflex: receptor, sensory neurone, relay neurone, motor neurone, effector.

Taxis

A directional movement in response to a directional stimulus, towards it (positive) or away from it (negative).

Kinesis

A non-directional response in which the speed of movement or the rate of turning changes with the intensity of a stimulus.

Transducer

Something that converts energy from one form into another; a receptor converts the energy of a stimulus into a change in membrane potential.

Generator potential

The graded depolarisation produced in a receptor by a stimulus, whose size depends on the strength of that stimulus.

Threshold

The membrane potential, around −55 mV, that must be reached before an action potential is triggered.

Bipolar neurone

A neurone in the retina that carries the signal from rods or cones to a ganglion cell.

Ganglion cell

A neurone in the retina whose axon forms part of the optic nerve, carrying action potentials to the brain.

Blind spot

The point where the optic nerve leaves the retina; it has no rods or cones, so light falling there is not detected.

Fovea

The small region of the retina directly behind the lens, packed with cones and containing no rods, where acuity is greatest.

Rhodopsin

The light-sensitive pigment in rod cells, broken down by low light intensities into retinal and opsin.

Iodopsin

The light-sensitive pigment in cone cells, in three types, broken down only by higher light intensities.

Retinal convergence

Several rods connecting to one bipolar neurone, so their generator potentials are summed.

Visual acuity

The ability to distinguish two points close together as separate.

The action potential and its propagation

Resting potential

The potential difference across the membrane of a neurone that is not conducting an impulse, about −70 mV inside relative to outside.

Polarised

Describes a membrane with a potential difference across it; at rest the axon membrane is polarised with the inside negative.

Voltage-gated channel

A channel protein that opens or closes in response to a change in the potential difference across the membrane.

Sodium-potassium pump

A carrier protein in the axon membrane that uses ATP to move three sodium ions out for every two potassium ions in.

Action potential

A brief reversal of the potential difference across an axon membrane, from about −70 mV to about +40 mV, caused by sodium ions entering through voltage-gated channels and passed along the axon as a nerve impulse.

Depolarisation

A change in membrane potential that makes the inside of the cell less negative.

Repolarisation

The return of the membrane potential towards the resting value after depolarisation, as potassium ions leave.

Hyperpolarisation

A change in membrane potential that makes the inside more negative than the resting potential.

Local currents

The movement of ions along the inside of an axon from a depolarised region to the resting membrane next to it.

Lidocaine

A local anaesthetic that blocks voltage-gated sodium channels, so no action potentials are produced in the neurones it reaches.

All-or-nothing principle

Once threshold is reached an action potential of full size is produced; below threshold none is produced at all.

Refractory period

The short time after an action potential during which the membrane cannot produce another, or can do so only with a larger stimulus.

Myelin sheath

Layers of Schwann cell membrane wrapped round an axon, insulating it so that ions cannot cross the membrane beneath.

Node of Ranvier

A gap in the myelin sheath between adjacent Schwann cells, where the axon membrane can depolarise.

Saltatory conduction

Conduction along a myelinated axon in which depolarisation occurs only at the nodes of Ranvier, so the impulse jumps from node to node.

The synapse: transmission, and why it runs one way

Synapse

The junction between the end of one neurone and another neurone or an effector cell, across which chemical transmission occurs.

Neurotransmitter

A chemical released from a presynaptic neurone that diffuses across the cleft and binds to receptors on the postsynaptic membrane.

Cholinergic synapse

A synapse that uses acetylcholine as its neurotransmitter.

Excitatory postsynaptic potential

A depolarisation of the postsynaptic membrane that makes an action potential more likely.

Spatial summation

Summation of inputs arriving at the same time from several presynaptic neurones.

Temporal summation

Summation of impulses arriving in quick succession from one presynaptic neurone.

Inhibitory synapse

A synapse whose transmitter hyperpolarises the postsynaptic membrane, making an action potential less likely.

Neuromuscular junction

The synapse between a motor neurone and a skeletal muscle fibre.

Acetylcholinesterase

The enzyme in the synaptic cleft that hydrolyses acetylcholine into choline and ethanoic acid.

Dopamine

A neurotransmitter in the brain; too little where it controls movement contributes to Parkinson's disease.

Serotonin

A neurotransmitter in the brain involved in mood, sleep and appetite; an imbalance of it contributes to depression.

L-Dopa

The precursor of dopamine, which crosses the blood-brain barrier and is converted to dopamine in the brain; used to treat Parkinson's disease.

SSRI

Selective serotonin reuptake inhibitor: an antidepressant that blocks the reuptake of serotonin from the synaptic cleft.

MDMA

The active substance in Ecstasy, which blocks serotonin reuptake and causes serotonin to be released into the cleft.

Muscles and movement: the sliding-filament model

Sarcomere

The functional unit of a myofibril, running from one Z line to the next.

A band

The dark band, the full length of the thick filaments, including any overlap with thin filaments.

I band

The light band, containing thin filaments only, spanning a Z line.

H zone

The lighter region at the centre of the A band, containing thick filaments only.

Sarcoplasmic reticulum

The membrane system inside a muscle fibre that stores and releases calcium ions.

Brain imaging, development and learning

Computed tomography (CT)

A scan that builds cross-sectional images of the body from the absorption of X-rays passed through it from many angles.

Magnetic resonance imaging (MRI)

A scan that uses a strong magnetic field and radio waves to image soft tissue from the signals given out by hydrogen nuclei.

Functional magnetic resonance imaging (fMRI)

A series of MRI scans that detects increases in oxygenated blood flow, showing which brain regions are active during a task.

Positron emission tomography (PET)

A scan that locates a radioactive tracer, usually labelled glucose, showing where cells are most metabolically active.

Critical period

A window of time early in development when the nervous system needs a particular stimulus, such as light entering both eyes, for its connections to develop fully.

Visual cortex

The region at the back of the cerebrum that receives and processes impulses from the eyes.

Ocular dominance columns

Alternating bands of the visual cortex, each responding mainly to the left eye or mainly to the right eye.

Animal model

A species studied in place of humans because part of its biology is similar enough for the results to apply to humans.

Utilitarianism

The ethical standpoint that the right action is the one with the best overall consequences, the greatest good or the least harm for all affected.

Rights-based ethics

The ethical standpoint that some actions are wrong whatever their consequences, because they violate a right or break a duty.

Three Rs

Replacement, reduction and refinement: the principles for minimising the use and suffering of animals in research.

Habituation

A reduced response to a repeated stimulus that is neither harmful nor rewarding; a simple form of learning.

Sensory adaptation

A fall in the response of a receptor to a constant stimulus, so that fewer impulses are sent along the sensory neurone.

Nature

The contribution of an organism's genes to a characteristic, such as an ability of the brain.

Nurture

The contribution of the environment and experience, including conditions in the womb, to a characteristic.

Cross-cultural study

A study comparing people brought up in different cultures, to see whether an ability depends on upbringing.