6.0 Nervous System 4: Electrophysiology of the Nerve Flashcards
Describe the cells of the nervous system and their roles
Neurons: specialized for the transmission of signals and communication with each other through chemical synapses.
Glial (glia, glue) cells: surround and support neurons
- In the central nervous system (CNS) glial cells include – astrocytes, oligodendrocytes and microglia.
- In the PNS there are 3 types – the Schwann cells, enteric glial cells and satellite cells.
Identify the different parts of the neuron and explain their function
Dendrites
= processes that receive incoming signals from neighbouring cells and increase the surface area of a neuron
Axon
= process that sends outgoing electrical signals that results in the release of a neurotransmitter, neuromodulator, or neurohormone
Varicosities
= enlarged region on axon that store and release neurotransmitters
Cell body (cell soma)
= part of neuron in which the nucleus is situated (position of cell body can vary depending on neuron)
Synapse
= region where axon terminal meets its target cell
Pre-synaptic axon terminal
= terminal end of axon that delivers a signal to the synapse. The terminal has the transmitter vesicles and the mitochondria which supply ATP (energy) for synthesizing new transmitter for release
Post-synaptic neuron
= neuron that receives signal from pre-synaptic neuron
Synaptic cleft
= narrow space between two neurons filled with extracellular matrix fibers that hold the pre- and post-synaptic cells in position
Define what is meant by axonal transport and describe the different types and their function
Proteins required by the axon are synthesized in the rough endoplasmic reticulum in the cell body and then moved down the axon by a process known as axonal transport. The process is either slow or fast.
Slow axonal transport - transports enzymes and cytoskeleton proteins down the axon of the neuron
Fast axonal transport - moves organelles such as vesicles, which contain neurotransmitter for release from the presynaptic terminal into the synaptic cleft, and mitochondria, along stationary tracks of microtubules. The transport can move either forward (anterograde) or backward (retrograde) along the axon.
Define what is meant by resting membrane potential and how it is determined
All excitable cells, including muscle cells and neurons, have a voltage difference (or potential) across their cell membranes. This results in the cell interior being negative compared to the cell exterior which is known as the resting membrane potential – in a healthy neuron this is usually -50 to -70 mV
The resting membrane potential is due to uneven distribution of ions across the cell membrane, largely potassium (K+), which are intracellular, and sodium (Na+) which are largely extracellular
As K+ permeability is dominant in the resting membrane, the potential reached will be largely determined by its concentration gradient, but modified slightly by that of Na.
Recognise the ion concentrations in a mammalian neuron and how this determines ionic movement across the cell membrane
K+
intracellular = 140 mM
extracellular= 5mM
Na+
intracellular = 5-15mM
extracellular= 145 mM
Cl-
intracellular = 4-30 mM
extracellular= 110 mM
Ca2+
intracellular = 0.0001 mM
extracellular= 1-2mM
Describe action potentials and the related ion permeability changes
Nerve action potentials are the result of changes in Na+ and K+ ion permabilities. From the beginning of the action potential at the axon hillock, to the restoration of the resting membrane potential, the neuron is in a refractory period.
An action potential occurs when specialized voltage-sensitive sodium (Na+) ion channels are activated in the cell membrane of an excitable cell like a neuron or muscle fiber.
Action potentials are “all or none” events. When an action potential begins, it propagates down the length of the axon. When the action potential reaches the end of the axon, a neurotransmitter is released into the synapse.
Describe the roles of ions and ion channels and their
gating mechanisms during the rising phase, the falling phase, and refractory periods of an action potential
The four major types of selective ion channels in the neuron are
Na+ channels
K+ channels
Ca2+ channels
Cl- channels
The rising phase is a rapid depolarization followed by the overshoot, when the membrane potential becomes positive.
The rising phase is caused by the opening of voltage-gated sodium channels. These ion channels are activated once the cell’s membrane potential reaches threshold and open immediately. The electrochemical gradients drive sodium into the cell causing the depolarization.
The falling phase is a rapid repolarization followed by the undershoot, when the membrane potential hyperpolarizes past rest.
The falling phase of the action potential is caused by the inactivation of the sodium channels and the opening of the potassium channels. After approximately 1 msec, the sodium channels inactivate. The channel becomes blocked, preventing ion flow. At the same time, the voltage-gated potassium channels open. This allows potassium to rush out of the cell because of the electrochemical gradients, taking its positive charge out of the cell, and repolarizing the membrane potential, returning the cell’s membrane potential back near rest.
Like the voltage-gated sodium channels, the voltage trigger for the potassium channel is when the cell’s membrane potential reaches threshold. The difference is that the sodium channels open immediately, whereas the potassium channels open after a delay.
Finally, the membrane potential will return to the resting membrane potential.
Describe the phases of the refractory period
From the beginning of the action potential to the restoration of the resting membrane potentail, the neuron is in a refractory period. This can be divided into two phases:
In the absolute refractory period it is impossible to initiate a second action potential.
the membrane is completely resistant to further stimulation, i.e. no matter how strongly the membrane is stimulated, another action potential will not fire. This is due to the characteristics of voltage-gated Na+ channels in the membrane. The “voltage-gated” property of these channels means that they are in different states (that is, open, closed, or inactivated) at different voltages. In the absolute refractory period a large number of these channels are voltage inactivated, and will only open again when the cell enters the relative refractory period.
In the relative refractory period a stimulus of greater than normal intensity can elicit a response.
the membrane is more resistant to stimulation than usual. In this period some voltage-gated Na+ channels are still inactivated. As a result, a stronger stimulus than usual is required to open a sufficient number of these channels for another action potential to fire. The cell membrane is more permeable to K+ during this period. This further opposes depolarization of the membrane.
Describe the mechanism of nerve conduction
Conduction of an action potential along a nerve fiber is greatly enhanced by the myelin sheaths which surround larger diameter fibers.
Sheaths of myelin that are about 20–300 layers thick surround the axon, and are separated every 1–2 mm by gaps called nodes of Ranvier. These effectively insulate areas of the membrane so that the changes in membrane permeability during an action potential only result in current flow at the nodes of Ranvier. The action potential is described as “jumping” between the nodes of Ranvier; this process is called saltatory conduction.
Explain compound action potentials and how they differ from action potentials
Action potentials are generated in individual axons, whereas a compound action potential (CAP) is the sum of action potentials recorded from a whole nerve. When a peripheral nerve is externally stimulated, the total electrical activity that results (the CAP) can be recorded between two external electrodes.
Identify the factors affecting the rate of propagation of action potentials along a nerve fiber
Myelin:
- Myelin accelerates nerve conduction velocity significantly
- decreases membrane capacitance
- increases membrane resistance
Diameter:
- Axons with a larger diameter have a lower internal resistance. As a result, conduction of an action potential is faster along axons with larger diameter.
Internode distance:
The distance between nodes of Ranvier, referred to as internode length, positively correlates with axon diameter, and is optimized during development to ensure maximal neuronal conduction velocity.
