Neuron structure Neurons all have same basic structure, a cell body with a number of dendrites and one long axon.

Neuron Axons carry information from the cell body to the axon terminals. Axon terminals communicate with their target cells at synapses.

* Divisions of the nervous system

* Signal transmission in neurons

* Membrane Potential electrochemically electrically excitable cells  Neurones send messages electrochemically; this means that chemicals (ions) cause an electrical impulse. Neurones are electrically excitable cells, which means that they can transmit electrical nerve impulses. electrical potential difference  An electrical potential difference, or membrane potential, can be recorded across the plasma membrane of living cells resting potential  The potential of unstimulated nerve cells, or resting potential, amounts to – 50 to – 100mV (cell interior is negative) action potential  All living cells have a (resting) membrane potential, but only excitable nerve cells are able to greatly change the ion conductance of their membrane in response to a stimulus, as in an action potential

 Definition: influences other neurons  Definition: an action potential (also known as a nerve impulse) is a pulse-like wave of voltage that passes on through an axon that influences other neurons  An action potential has 2 main phases called depolarisation and repolarisation:  During depolarization:  Opening of sodium channels and influx of sodium ions  is usually associated with cell stimulation  During repolarization:  Inactivation of sodium channels and repolarizing efflux of potassium ions  is usually associated with cell inhibition  The normal ratio of ion concentrations across the membrane is maintained by the continual action of the sodium–potassium pump, which transports three sodium ions out of the cell and two potassium ions in  The action potential stops at the end of the neuron, but usually causes the secretion of neurotransmitters at the synapses that are found there  These neurotransmitters bind to receptors on adjacent cells Depolarization Repolarization

The Sodium-Potassium Pump (Na+K+ATPase) Three sodium ions from inside the cell first bind to the transport protein. Then a phosphate group is transferred from ATP to the transport protein causing it to change shape and release the sodium ions outside the cell. Two potassium ions from outside the cell then bind to the transport protein and as the phospate is removed, the protein assumes its original shape and releases the potassium ions inside the cell.

If the pump was to continue unchecked there would be no sodium or potassium ions left to pump, but there are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they “leak”, allowing sodium ions to leak in and potassium ions to leak out, down their respective concentration gradients.

At rest, the inside of the neuron is slightly negative due to a higher concentration of positively charged sodium ions outside the neuron. When stimulated past threshold (about –30mV in humans), sodium channels open and sodium rushes into the axon, causing a region of positive charge within the axon. This is called depolarisation The region of positive charge causes nearby voltage gated sodium channels to close. Just after the sodium channels close, the potassium channels open wide, and potassium exits the axon, so the charge across the membrane is brought back to its resting potential. This is called repolarisation. This process continues as a chain-reaction along the axon. The influx of sodium depolarises the axon, and the outflow of potassium repolarises the axon. The sodium/potassium pump restores the resting concentrations of sodium and potassium ions

* Resting Membrane Potential

* ACTION POTENTIAL

* Action Potential Propagation

* ACTION POTENTIALS * CONDUCTION ALONG A MYELINATED AXON

* Characteristics of action potentials * Refractory period lasts from time action potential begins until normal resting potential returns * Continuous propagation * Spread of action potential across entire membrane in series of small steps * Salutatory propagation * Action potential spreads from node to node, skipping internodal membrane

* Synaptic transmission * The definition of synaptic transmission is simply the communication between two nerve cells. Communication believed to involve specialized structures termed "synapses". * Charles Sherrington (1897) : named ‘Synapse’

* Principles of Synaptic Transmission * Basic Steps * Neurotransmitter synthesis * Load neurotransmitter into synaptic vesicles * Vesicles fuse to presynaptic terminal * Neurotransmitter spills into synaptic cleft * Binds to postsynaptic receptors * Biochemical/Electrical response elicited in postsynaptic cell * Removal of neurotransmitter from synaptic cleft * Must happen RAPIDLY!

* Neurotransmitter Synthesis and Storage

Release of Neurotransmitter (NT) Molecules : * Exocytosis – the process of NT release * A nerve impulse reaches the terminal knob of a neuron, causing the pre-synaptic membrane to depolarize. * The depolarization of the pre-synaptic membrane causes voltage gated-calcium-channels to open. * The entry of Ca 2+ causes vesicles to fuse with the terminal membrane and release their contents

* Neurotransmitter Reuptake, Enzymatic Degradation, and Recycling * As long as NT is in the synapse, it is active – activity must somehow be turned off * Clearing of neurotransmitter is necessary for the next round of synaptic transmission * Simple Diffusion * Reuptake aids the diffusion * Neurotransmitter re-enters presynaptic axon terminal or enters glial cells through transporter proteins * The transporters are to be distinguished from the vesicular forms * Enzymatic destruction * In the synaptic cleft * Acetylcholinesterase (AchE)

* Synaptic transmission