The Nerve Impulse

See Interactive Tutorial for Activity 8.2 The nerve impulse This is the electrical signal which is transmitted by the neurones around the nervous system. See Interactive Tutorial for Activity 8.2

Distribution of ions and charge across the surface of a neurone. + ve Tissue fluid outside neurone [Na+] high [K+] low Neurone cell membrane ______________________________ Cytoplasm inside neurone [Na+] low [K+] high + proteins- - ve - ve The difference in charge results in a resting potential of –70 mV across the membrane.

The resting potential is the result of: the action of an active transport system called the ‘sodium pump’: i.e. involves a transporter protein + the use of energy from respiration this ‘pumps 3 Na+ ions out and 2 K+ ions in

Also the differential permeability of the neurone membrane to Na+ and K+ ions these ions can only cross the membrane via specific ion channel proteins that allow facilitated diffusion: (there are also so special ‘gated ions channels which only allow ions to diffuse through them when they are ‘opened’ but they are not relevant here.) sodium pump ion channel protein

The membrane is very impermeable to Na+ ions so these cannot diffuse back in. The membrane is slightly more permeable to K+ ions so these diffuse out slowly (down their concentration gradient). This establishes electrochemical diffusion gradients for the ions across the membrane. The net result is that relatively more +ve ions end up outside the neurone than remain inside to balance the – ve charged proteins so the outside is more positive with respect to the inside; this charge difference is responsible for the resting potential. At this point the neurone is said to be polarised.

The nerve impulse. When a nerve impulse is transmitted along a neurone a wave of electrical activity passes along it (NB: it is NOT a flow of electrons so it is not an electrical current) This can be detected (e.g. by using a cathode ray oscilloscope with electrodes placed inside and outside the neurone) as a transient change in electrical charge on the membrane surface – this is called an ACTION POTENTIAL and is the basis of the nerve impulse.

An action potential

Events during an action potential

Action potential/ion channels http://www.blackwellpublishing.com/matthews/channel.html

The action potential [1] Resting potential: Na+ and K+ special ‘voltage gated’ ion channels closed: neurone is polarised. [2] Na+ gated channel proteins open (due to a change in shape of the protein itself: makes the membrane more permeable to Na+ ions now) which allows Na+ ions to diffuse into the neurone down the electrochemical diffusion gradient. [3] This causes the inside of the neurone to become increasingly more + ve: this is depolarisation and neurone has become depolarised

[4] Na+ channels close and K+ gated channels now open [5] K+ ions diffuse out of the neurone down the electrochemical diffusion gradient, so making the inside of the neurone less positive (= more negative) again: this is repolarisation and the neurone has become repolarised [6] The neurone has its resting potential restored. During this time the Na+ and K+ ions which have diffused in/out of the cell are redistributed by active transport (sodium pump) [NB: the ion movements during the action potential occur by facilitated diffusion; active transport is not involved]

Action potential/ion channels http://www.blackwellpublishing.com/matthews/channel.html

Events during an action potential

http://www.mrothery.co.uk/images/nerveimpulse.swf

Properties of the action potential. Action potentials have a threshold: this is the minimum level of stimulus necessary to cause depolarisation (i.e. open the ion channels) are all or nothing: all action potentials are the same size, irrespective of the intensity of the stimulus consequently information about the intensity of a stimulus is coded in the number of impulses - strong stimulus  many action potentials etc

have a refractory period (about 1 ms); the ion channels remain closed and cannot be made to reopen so no further depolarisation is possible; the consequences are it separates impulses from one another by a fraction of a msec, and so imposes a maximum on the number of impulses which can be transmitted along a neurone in a given time it ensures impulses can only pass in one direction along an axon, i.e. forward into the next resting region but not back into a region still in its refractory period.

Role of action potentials in the transmission of a nerve impulse. A STMULUS causes the Na+ gated channel proteins to open (effectively causes a change in shape of the protein itself) which allows Na+ ions to diffuse down the concentration gradient across the membrane into the cell and so set off an action potential. [see handout]

How action potentials are transmitted along a neurone                                          Sodium gates open                                          Sodium ions flow into neurone

Sodium gates close Potassium gates open Potassium ions flow                                          Sodium gates close Potassium gates open Potassium ions flow out of neurone Sodium gates in next bit of membrane open and sodium ions flow in                                                                                   Sodium pump redistributes sodium and potassium ions after impulse has passed

Speed of conduction of nerve impulses. Transmission speeds can range from 0.5 m per sec to over 100 m per sec. Impulse transmission can be speeded up by:

increasing the diameter of the axon

TS Nerve Connective tissue coat of nerve Myelin sheath Bundle of nerve fibres but this imposes constraints on packing a large number of large axons into a nerve

having a myelin sheath; the myelin sheath is composed of tightly packed layers of the cell membrane of the Schwann cells which are wrapped around the axon

this is rich in lipid (called myelin) which makes the axon impermeable to ions so they are unable to diffuse between the tissue fluid and the neurone so action potentials cannot be generated by the myelinated regions (it acts as an insulator);

action potentials can only be generated at the nodes of Ranvier so the local currents involved in nerve impulse transmission flow over longer distances: thus action potential seem to ‘jump’ from node to node (this is called salutatory conduction):

Saltatory conduction

since the intervening parts of the axon membrane do not have to be successively depolarised it takes less time for the action potentials to pass from node to node this results in nerve impulse transmission that is much faster, the consequence of which is that smaller myelinated nerves can transmit impulses much faster than larger non-myelinated ones which alleviates the ‘packing problem’.

temperature which affects the rate of diffusion and the rate of energy release by respiration for active transport (since it is controlled by enzymes) the consequence is that nerve impulse transmission is faster in endothermic animals which maintain a high body temperature.

Synapses

Nerve endings

synaptic knobs (neurone endings)

Motor end plate Synapses between motor nerve and muscle

Neurones are connected together (normally via axons and dendrites) at synapses

N.B. this is not a physical junction, there is actually a small gap of approx 20 nm between the cells so there is no membrane continuity so nerve impulses cannot cross directly. synaptic vesicles synaptic bulb pre-synaptic membrane post-synaptic membrane

Instead transmission is by chemicals called neurotransmitters These made in the Golgi body (synthesis requires energy from respiration) stored in vesicles in the synaptic bulb

There are several different types of neurotransmitters, of which two are Acetyl choline (used in the voluntary [motor neurones  muscles] and parasympathetic nervous systems) Noradrenaline (used in the sympathetic nervous system)

How nerve impulses are transmitted across a synapse by neurotransmitters

Interactive tutorial: Activity 8.4 Crossing a synapse

neurotransmitter molecules diffuse across the synaptic cleft neurotransmitter receptors are membrane protein with binding sites with a shape complementary to the neurotransmitter neurotransmitters broken down by enzymes so the ion channels to close and the receptors are available again and so the resting potential can be re-established this sequence takes 0.5 msec which results in a synaptic delay in the transmission of impulses.

Types of synapse. Excitory synapses Binding of neurotransmitter to postsynaptic neurone opens Na+ gated channels  Na+ diffuses IN depolarisation  action potentials so nerve impulses can continue around the nerve circuit.

Inhibitory synapses Binding of neurotransmitter to postsynaptic neurone opens K+ gated channels  K+ diffuses OUT  inside of neurone becomes even more – ve and so impossible to depolarise  no action potentials so nerve impulses cannot continue around the nerve circuit.

Functions of synapses. Enables impulses to be transmitted from one neurone to another, so enables nerve circuits to function. Neurotransmitter only released from the presynaptic neurone, so nerve impulses can only be transmitted IN ONE DIRECTION around a nerve circuit

Sometimes neurotransmitter released by a single neurone is insufficient to depolarise the postsynaptic neurone; however simultaneous release of neurotransmitter from the synapses several neurones (sometimes from other nerve circuits – known as integration) will be sufficient to cause sufficient depolarisation to generate nerve impulses this is called SUMMATION.

impulses arrive at the same time

impulses very close together

Summative effect of excitory and inhibitory synapses also determines whether nerve impulses will be generated in the postsynaptic neurone.

Nerve impulses are only transmitted if threshold of postsynaptic neurone is exceeded, so ensures only significant information is passed around the circuit (so insignificant, low level stimuli get “filtered out”).

animations of how drugs work http://www.pbs.org/wnet/closetohome/science/html/animations.html http://learn.genetics.utah.edu/units/addiction/drugs/mouse.cfm ‘mouse party – fun!