- The response to change in membrane potential allows passage of ions.
- Depolarisation occurs when mem. pot. changes from -70mV to 40mV. Na+ channels open. Influx of Na+ triggers K+ channels to open and Na+ channels to close.
- After this the cell begins to repolarise although often it will overshoot to -90mV causing hyperpolarisation. This is useful so that the neuron does not receive another stimulus and keeps the signal going in one direction.
- The Na+/K+ pump restores the balance back to -70mV.
- Absolute refractory period - 2nd AP cannot be produced
- Relative refractory period - another AP can be started by applying higher charge
Cause of Membrane Potential
Diffusion, where molecules move from high conc. to low conc, and electrostatic pressure, where ions are moved from place to place.
Uneven distribution occurs when diffusion pushes in and electrostatic pressure pushes out which leaves uncompensated negative charges (as positive charges leave cell) and this separation of charges causes the membrane potential.
Adenosine Triphosphate = energy storing molecules to fuel pumps
- Place 1 neuron in saline and 1 micro electrode in neuron, with another neuron outside
- The difference is 70mV as inside is -70mV
- Apply small pos charge (depolarisation)
- Sufficient charge - threshold of excitation and reversal of mem. pot.=> action potential
- Returns to resting but becomes hyperpolarised
Chemical synapse - fast transmission, precise direction, amplified signal (more in human brain)
Electrical synapse - extremely rapid transmission
- Presynaptic membrane = membrane of terminal button
- Postsynaptic membrane = membrane of receiving neuron
- Synpatic cleft-gap = contains extracellular fluid
- Synaptic vesicles = contain transmitters
Neurones communicate across synapses - most are axodendritic (axon on dendrites) while some are axosomatic (axon on cell bodies). Rarely they can be dendodendritic (capable of transmission in opposite direction) and axoaxenic (pre-synaptic inhibition).
1. Neurotransmitters are released at short distance
2. Neuromodulators are released in large amounts at longer distance
3. Hormones are released by endocrine glands, widespread action to target cells
Synaptic Transmission Cont
Neurotransmitters are synthesised in terminal buttons and packaged in vesicles
Neuromodulators are assembled in the cell body and taken to axon terminal (axoplasmic transport)
Some synpatic vesicles are docked against membrane to form a fusion pore - opens and releases neurotransmitter (exocytosis)
When AP reaches terminal button, it depolarises the membrane and opens the voltage dependent calcium channels. The junction between the axon and terminal buttons = buds of membrane pinch off (pinocytosis)
The buds fuse with cisternae to create new vesicles to fill with neurotransmitter
The NT diffuses and attaches to post-synpatic receptors to open channels either directly (ionotrophic receptor) or indirectly (metabotrophic receptor)
Synaptic Transmission Fin
Neural integration = interaction between effects of excitatory and inhibitory synapes -> produces AP or may cancel out
Reuptake = rapid removal of neurotransmitter back into cytoplasm
Enzymatic deactivation = NT destroyed by enzymes -> only for acetycholine