Nerves

Central nervous system (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

Sensory

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

Motor

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

Relay/Intermediate

A neurone which is a co-ordinator which transmit impulses between neurons

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

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

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

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

Generator potential

A nervous impulse produced by a sensory receptor following transduction (or conversion) of one form of energy into electrical energy

Threshold value

The minimum level of stimulus needed to trigger an action potential

Action potental

A temporary reversal of the charges across the axon membrane which increase from -65mV to +40mV

Polarised

Condition used to describe the axon when the inside of an axon is negatively charged relative to the outside (at the resting potential usually around 65mV). This is established because sodium ions are actively transported out of the axon and potassium ions actively transported into the axon by the sodium-potassium pump. The outward movement of sodium ions is greater than inward movement of potassium ions which means there are more sodium ions in the tissue fluid surrounding the axon than in the cytoplasm and more potassium ions in the cytoplasm than in the tissue fluid which creates an electrochemical gradient. Most of the gates in the channels that allow the potassium ions to diffuse back out of the axon are open

Voltage gated channels

Channels in the axon membrane which change shape

depolarised

Condition used to describe the part of the axon membrane when the inside of the membrane has a positive charge of around +40mV (when an action potential is happening). This occurs because of a stimulus which causes some sodium voltage-gated channels in the axon membrane opening and sodium ions diffusing into the axon along their electrochemical gradient. More sodium channels open

Hyperpolarisation

When the inside of the axon is more negative (relative to the outside) than the usual. Caused because voltage-gated potassium channels open and the electrical gradient that prevented further outward movement of potassium ions is now reversed causing more potassium ion channels to open. The outward diffusion of these potassium ions causes a temporary overshoot of the electrical gradient.

Repolarisation

When the resting potential of -65mV is re-established the axon is described as this. This happens because the closable gates on the potassium ion channels now close and the activities of the sodium-potassium pumps once again cause sodium ions to be pumped out and potassium ions in.

Refractory period

Time period after an action potential when it is impossible for a further action potential to be generated because inward movement of sodium ions is prevented because the sodium voltage-gated channels are closed. Means that impulses are propagated in one direction only

All or nothing principle

An action potential is exactly the same size

dendrons

Extensions of the cell body which subdivide into smaller branched fibres

axon

A single long fibre that carries nerve impulses away from the cell body

Cell body

Contains all of the usual cell organelles

Schwann cells

wrap themselves around the axon many times

Node of ranvier

Constrictions (gaps) between adjacent Schwann Cells where there is no myelin sheath (2-3µm long and every 1-3mm in humans).

Myelin sheath

Covering to the axon and is made up of the membranes of the Schwann cells wrapped around a neurone which helps speed up impulse transmission

Synapse

The point where the axon of one neurone connects with the dendrite of another or an effector: they help coordinate activities and are unidirectional.

Cholinergic synapse

A synapse which links neurones to neurones or neurones to other effector organs

Adrenergic synapse

A synapse in which the neurotransmitter is noradrenaline (epinephrine in the US)

Neurotransmitter

A chemical which is secreted by a neurone within the nervous system to stimulate a target cell

hormonal/endocrine system

A communication system which transports hormones via the plasma to produce a slow

Chemical mediators

Chemicals eg histamine

Acetlycholine

A neurotransmitter used at a cholinergic synapse. Released from synaptic vesicles in presynaptic neurones when an action potential reaches the synaptic knob. This neurotransmitter diffuses across the synaptic cleft and binds to specific receptor proteins on the postsynaptic neurone which leads to a new action potential in the postsynaptic neurone.

Presynpatic neurone

Neurone that releases the neurotransmitter from synaptic vesicles when an action potential arrives at the end of it and causes calcium ion protein channels to open and calcium ions to influx in.

Postsynaptic neurone

Neurone which has receptor sites that the neurotransmitter binds to after diffusing across the synaptic cleft.

Presynaptic knob

The swollen portion of the presynaptic neurone.

Synaptic cleft

The 20-30nm wide gap which is found between the presynaptic and postsynaptic neurone.

Synaptic vesicle

Contain the neurotransmitter in the presynaptic neurone. The influx of calcium ions causes them to fuse with the presynaptic membrane

ACh Receptors

Found on the postsynaptic neurone of a cholinergic synapse where acetyl choline is the neurotransmitter. As acetylcholine binds to receptor sites on sodium ion protein channels in the membrane it causes the sodium ion protein channels to open

Acetylcholine esterase enzyme

The enzyme which breaks down acetylcholine into choline and ethanoic acid (acetyl)

Excitory synapse

Synapses that produce new action potentials when the neurotransmitter binds with the receptor proteins. The neurotransmitter binds to and causes sodium ion channels on the postsynaptic neurone to open (Na+ ions move in) causing a new action potential.

Inhibitory synapses

Synapses which make it less likely that a new action potential will be created on the postsynaptic neurone. The neurotransmitter binds to and causes chloride ion channels on the postsynaptic neurone to open (Cl- ions move in) and causes potassium channels to open (K+ ions move out) making the inside of the postsynaptic membrane more negative and therefore less likely that a new impulse will be created.

Summation

Can either be spatial or temporal. Allows a build up of neurotransmitter which enables low-frequency action potentials to trigger a new action potential in the postsynaptic neurone.

Spatial summation

a number of presynaptic neurones together release enough neurotransmitter to exceed the threshold value of the postsynaptic neurone and therefore trigger a new action potential

Temporal summation

one presynaptic neurone releases neurotransmitter many times over a short period- if the concentration of neurotransmitter exceeds the threshold of the postsynaptic neurone a new action potential will be triggered.

Neuromuscular junction

The point at which a motor neurone meets a skeletal muscle fibre. Only excitatory synapses.