4- Transmission of excitation and inhibition between cells. Synapses-types of synapses. Chemical synapses. Neurotransmitters and neuromodulators. Post synaptic potentials. Summation of the postsynaptic potentials.

section

excitation

action potential

excitation transition

types of synapses

types of chemical synapses

excitation

• Excitation of sensory and nerve cells is dependent upon the action of sodium/potassium ion channels in response to a stimulus.

• Can lead to a large momentary change in the membrane potential

• Excitation is conducted in one direction only because one Na+ channel has opened it will remain open for some time after a successful action potential. This state is known as the refractory period.

action potential

• In neurones, when the cell membrane has been stimulated and reaches the threshold potential

• Voltage dependent Na+ channels open simultaneously allowing MP to reach a positive value.( influx)

• This causes depolarisation

• when MP reaches equilibrium at +40mv, Na+ ion passage into the cell is stopped → overshoot

• voltage gated K+ channels open , K+ goes out, MP decreases to original state and Na+ channels close → hyperpolarization then RMP

• The momentary rapid change in membrane potential to +40mv generates an action potential.

excitation transition

• sensory cells → neurons

• between neurons

• from neurons → muscle cells

• via the synapse.

types of synapses

1. electrical synapses - transmission speed faster, direct transmission of electrical signals through gap junctions between neurons, can be bidirectional

2. chemical synapses - transmits excitation neurotransmitters, slower because neurotranmsitter must diffuse across synaptic cleft, it is unidirectional

types of chemical synapses

Two types of chemical synapses, excitatory and inhibitory

• excitatory chemical synapses:

  • the neurotransmitters acetylcholine, glutamic acid can be used to cause the opening of sodium ion channels in the post synaptic membrane

  • Causing increase in membrane potential due to depolarisation and so the cell is excited

• Inhibitory chemical synapses

  • chemical such as GABA and glycine are released, which opens the Cl- and K+ channels in the post synaptic membrane so membrane potential decreases and cell excitation does not occur.

• Spatial- multiple simultaneous stimuli

• Temporal- repeated stimuli

neurotransmitters and neuromodulators

while neurotransmitters (like glutamate or GABA) act quickly and locally at synapses, neuromodulators work more slowly and broadly, often affecting entire networks of neurons. They can enhance or dampen the effects of neurotransmitters, adjusting the sensitivity or responsiveness of neurons over time.

summation of post synaptic potentials

Summation of postsynaptic potentials is how a neuron adds up signals to decide whether to fire an action potential.

There are two types:

• Spatial summation: Signals from different places on the neuron arrive at the same time and add up.

• Temporal summation: Signals from the same place arrive quickly one after another and build up.

If the total signal is strong enough, the neuron fires. If not, it stays quiet.