Nervous System - Nerve Impulses

Nerve impulse

Messages in the form of an electrochemical change that travel along the nerve fibre

Charges

When unlike charges are separated, an electrical force pulls them together which can be measured (increases as charges get closer). When they come together, energy is released

Potential difference in cell membrane

Extracellular fluid - high concentration of Na+ and Cl- (sodium chloride)

Intracellular fluid - low in concentration of Na+ and Cl-, high in concentration of K+ and other anions

Different concentrations make a difference in net charge, therefore has potential to come together and release energy

Transport proteins in cell membrane

Leakage channels - specific to an ion/particle, open all the time, more K+ leakage than Na+ leakage

Na+/K+ pump - always working to send 3 Na+ out of cell for every 2 K+ brought in, requires energy as they are sent against concentration gradient

Ligand-gated channels - open when stimulated by neurotransmitter/sensory receptors

Voltage-gated channels - open when potential difference reaches -55mV

Resting membrane potential

Potential inside is 70mV less than outside, therefore -70mV membrane potential, fluid inside cell is more negatively charged (polarised) than fluid outside of cell

Distribution of ions

Ions are unable to diffuse across phospholipid bilayer, have to go through channels.

Factors maintaining resting membrane potential

Concentration of Na+ being 10x higher outside

Concentration of K+ being 30x higher inside

Cell membrane is highly permeable to K+ and Cl-, slightly permeable to Na+ and impermeable to large negatively charged ions.

Parts of nerve impulse transmission

Depolarisation - occurs if stimulus exceeds the threshold

Repolarisation - membrane goes back to its resting potential

Hyperpolarisation - membrane potential temporarily drops lower than the resting potential

Refractory period - period where membrane cannot be stimulated again

Depolarisation

If strong stimulus is applied to nerve fibre, membrane becomes more permeable to Na+ by opening ligand-gated Na+ channels, causing Na+ moving into cell, becoming more positive. If voltage reaches -55mV, action potential is met

If action potential is met: voltage gated Na+ channels open as well, depolarising inside of membrane

Repolarisation

Due to movement of Na+, polarity of membrane is 0, inside is positively charged. When inside is 40mV, voltage gated Na+ channels close and voltage gated K+ channels open, K+ moves inside cell, causing repolarisation

Hyperpolarisation

Initially, too much K+ moves out of cell because K+ voltage gated channels are slow to close, causes inside of cell to become more negative than resting potential. Once K+ channels close, Na/K pumps restore resting potential

Refractory period

For a brief time during and afterwards action potential, part of the fibre cannot be stimulated again, which prevents nerve impulse from going backwards. 2 stages: absolute and relative

Absolute refractory period

Na+ channels are shut and Na+ can't move in = no action potential

Relative refractory period

Some Na+ channels open, requires a much larger stimulus to create action potential

Speed of nerve impulses

Depends on whether axon is myelinated/unmyelinated, diameter of nerve fibre. (myelinated 18m/s - 140 m/s, unmyelinated 2m/s)

saltatory conduction

Rapid transmission of a nerve impulse along an axon, resulting from the action potential jumping from one node of Ranvier to another, skipping the myelin-sheathed regions of membrane.

All or none response

Size of response is not related to strength of stimulus - once threshold is reached, the strength of the impulse won't change, regardeless of strength of stimulus. However: Strong stimulus causes depolarisation of more nerve fibres, and produces more nerve impulses due to being able to overcome relative refractory period

Process of transmission across a synpase

1. Wave of action potential travels along the pre-synaptic neuron to the synaptic end bulb

2. Depolarisation of membrane leads to voltage-gated Ca2+ channels opening

3. Ca2+ flows into pre-synaptic neuron across concentration gradient, activates vesicles containing neurotransmitters

4. Vesicles containing neurotransmitters bind to surface of cell membrane

5. Neurotransmitters are released from vesicles into synaptic cleft (via exocytosis)

6. Neurotransmitters diffuse across synaptic cleft until they reach neurotransmitter receptors on post-synaptic neuron

7. Neurotransmitters bind to receptors on post-synaptic neuron, stimulates opening of ligand-gated Na+ channels, depolarising membrane for action potential in postsynaptic neuron or stimulus for effector (muscle/gland)

8. Neurotransmitter breaks free from neurotransmitter receptor and diffuses across synaptic cleft once again to travel back to pre-synaptic neuron to be re-used

9. Enzymes in synaptic cleft deactivate neurotransmitters and is reabsorbed by pre-synaptic neuron to be re-used (endocytosis)