A synapse is the gap between the axon of one nerve and the dendirte of the next one- the average neuron has 1,000 synapses with other neurons. Synapses can be either an electrical or chemical type however the majority are the chemical type- where there is no continuity between two neurons. Neurotransmission at most chemical synapses is a one way conduction. At electrical synapses, gap junctions are found between pre and post synaptic neurons and new synapses can always occur throughout our life and is the basis of learning and memory. The majority of synapses are axo-dendritic type where the axon of one neuron ends on the dendrites of another neuron. Some neurons end on soma or axon of the other neuron.
The general sequence of events during nerve impulse transmission involve:
Steps in neurotransmission include:
There are dozens of different neurotransmitters in teh nervous system and they can be either excitatory or inhibitory and each neuron generally synthesizes and releases a single type of neurotransmitters.
Neurotransmitters
There are 2 types of neurotransmitters: non-peptide and peptides. Non-peptides include monoamines (noradrenaline, dopamine), amino acids (GABA) and other transmitters such as acetylcholine. Peptides include enkephalin, substance p and B-endorphin.
Neurotransmitters are often localised in specific brain regions. Dopamine is in teh basal ganglia, serotonin in the hypothalamus and brain stem, acetylcholine in the basal ganglia and cortex, glutamate is widespread, noradrenaline is in the cortex and the limbic system and glycine is in teh spinal cord in interneurons.
Synaptic Potentials
In thepost synaptic membrane there are receptors to which synaptic transmitters can bind. This neurotransmitter receptor complex alters the conductance of the postsynaptic plasma membrane to one or more ions and accordingly Exciatory post synaptic potential (EPSP) or inhibitory post synaptic potential (IPSP) develops.
Because depolarizations can lead to the excitation and activation of a neuron, thy are commonly called excitatory postsynaptic potentials. In contrast, hyperpolarizations of the membrane preent the cell from becoming activated and are called inhibitory postsynaptic potentials. These membrane potential changes are caused by the influx or efflux of specific ions.
During the development of an EPSP the neurotransmitter receptor complex causes the opening of the ligand gated sodium channels. Sodium ions from the ECF enter into the postsynaptic neuron which causes the development of the local potential (EPSP).
During the development of an IPSP, the neurotransmitter receptor complex results in the influx of chlorine ions (Cl-) or efflux of potassium ions (K+). This in turn increases the negativity inside the postsynaptic membrane- known as hyperpolarization (IPSP). This makes it harder to excite the membrane when it is at resting potential.
Slow neurotransmission occurs when receptor are linked to second messenger system resulting in slow post synaptic changes and is used to perform modulatory functions. Fast transmission occurs when receptors are linked to ion channels resulting in fast excitatory post synaptic potentials which increases the flow of sodium ions into and potassium out of the cells which leads to membrane depolarization.
Summation is a single excitatory post synaptic potential but does not always depolarize the membrane. A large excitatory potential initiates action potential at post synaptic membranes. Individual EPSP's added together can result in a large depolarization. The number of action potential received can be increased by temporal summation (many action potentials received close together over time) or spatial summation (many action potentials received from many different axons).
Learn More
Crash Course: Synapses!
CCSI: Function of Neurons
Questions
The general sequence of events during nerve impulse transmission involve:
- action potential in presynaptic nerve terminal
- calcium ion influx into presynaptic terminal
- release of a neurotransmitter from the presynaptic terminal
- binding to specific receptor in teh post synaptic membrane
- change in membrane potential in teh post synaptic membrane.
Steps in neurotransmission include:
- synthesis
- storage
- release
- receptor interaction
- inactivation
- reuptake
- degradation
There are dozens of different neurotransmitters in teh nervous system and they can be either excitatory or inhibitory and each neuron generally synthesizes and releases a single type of neurotransmitters.
Neurotransmitters
- synthesized in a neuron
- stored in presynaptic terminals
- stored in vesicles
- release is calcium ion depenent
- exogenously used as a drug (acts on same receptors)
- inactivated in the synaptic cleft
There are 2 types of neurotransmitters: non-peptide and peptides. Non-peptides include monoamines (noradrenaline, dopamine), amino acids (GABA) and other transmitters such as acetylcholine. Peptides include enkephalin, substance p and B-endorphin.
Neurotransmitters are often localised in specific brain regions. Dopamine is in teh basal ganglia, serotonin in the hypothalamus and brain stem, acetylcholine in the basal ganglia and cortex, glutamate is widespread, noradrenaline is in the cortex and the limbic system and glycine is in teh spinal cord in interneurons.
Synaptic Potentials
In thepost synaptic membrane there are receptors to which synaptic transmitters can bind. This neurotransmitter receptor complex alters the conductance of the postsynaptic plasma membrane to one or more ions and accordingly Exciatory post synaptic potential (EPSP) or inhibitory post synaptic potential (IPSP) develops.
Because depolarizations can lead to the excitation and activation of a neuron, thy are commonly called excitatory postsynaptic potentials. In contrast, hyperpolarizations of the membrane preent the cell from becoming activated and are called inhibitory postsynaptic potentials. These membrane potential changes are caused by the influx or efflux of specific ions.
During the development of an EPSP the neurotransmitter receptor complex causes the opening of the ligand gated sodium channels. Sodium ions from the ECF enter into the postsynaptic neuron which causes the development of the local potential (EPSP).
During the development of an IPSP, the neurotransmitter receptor complex results in the influx of chlorine ions (Cl-) or efflux of potassium ions (K+). This in turn increases the negativity inside the postsynaptic membrane- known as hyperpolarization (IPSP). This makes it harder to excite the membrane when it is at resting potential.
Slow neurotransmission occurs when receptor are linked to second messenger system resulting in slow post synaptic changes and is used to perform modulatory functions. Fast transmission occurs when receptors are linked to ion channels resulting in fast excitatory post synaptic potentials which increases the flow of sodium ions into and potassium out of the cells which leads to membrane depolarization.
Summation is a single excitatory post synaptic potential but does not always depolarize the membrane. A large excitatory potential initiates action potential at post synaptic membranes. Individual EPSP's added together can result in a large depolarization. The number of action potential received can be increased by temporal summation (many action potentials received close together over time) or spatial summation (many action potentials received from many different axons).
Learn More
Crash Course: Synapses!
CCSI: Function of Neurons
Questions
- what is the primary mechanism that resotres/maintains the Na+ and K+ concentration gradients across the nerve cell membrane?
- what do you think might hapen to the resting membrane potential if Na+/K+ ATPase stopped working?
- There will be change in the Membrane potential in response to receptor-neurotransmitter binding at the post synaptic membranes. Explain the cause for EPSP and IPSP.
- What is the effectof higher frequency of action potentials generated in a neuron on the neurotransmitter release?