Ions, nerves, the action potential and muscles
In order to understand how muscles are activated it is necessary to understand how neurones transmit messages.
In order to understand how neurones transmit messages it is essential to understand how ions move across membranes
Cardiac muscle does not require nervous stimulation. Cardiac muscle is self-excitatory but the process of myogenic stimulation is still explained by the movement of ions across membranes.
Structure of a neurone
A nerve is a bundle of many neurones
Each neurone is a single cell that carries an impulse from one end to the other
The neurone that will be considered on this course is the motor neurone. This is a neurone that carries an impulse to a muscle causing the muscle to contract.
The motor neurone has three main regions:
- the cell body which contains the
nucleus
- processes from the cell body called dendrites receive signals from other neurones
- the axon - the long thin process which carries the impulse to....
- the motor end plate - this forms the neuromuscular junction and transmits the message to the muscle
The diagram below (from http://www.botany.uwc.ac.za/sci_ed/grade10/mammal/nervous.htm) shows these major regions
The impulse thus: arrives at a dendrite, passes across the cell body, passes along the axon and finally reaches the motor end plate causing stimulation of the muscle
We will look first at the events at one point in the axon and, using that knowledge, will develop an understanding of all of the events occurring in the neurone.
An unstimulated neurone shows a characteristic arrangement of ions either side of the cell membrane
This creates a potential across the membrane called the resting potential of -70mV.
This means that the inside of the cell is negative with respect to the outside because of the higher concentration of negative ions inside compared to the outside
Stimulation of the membrane leads to an action potential
An action potential begins with a small depolarization (change in potential towards zero).
The initial depolarization occurs because of local movements of ions within the cell
If the potential depolarizes to -50mV the membrane is said to have reached threshold and an action potential occurs. If threshold is not reached the membrane returns to its resting state.
Some ion channels in the membrane are sensitive to voltage and are called voltage-gated ion channels. These open and close according to the voltage.
Sodium and potassium gates both open in response to the threshold potential BUT sodium gates open more quickly and for a very short time while potassium gates open relatively slowly but remain open for longer
Hence, if the threshold value of -50mV is reached, sodium gates open allowing the movement of positively charged sodium ions across the membrane.
This causes the potential across the membrane to become positive.
By the time the membrane potential has become positive the sodium gates close and the potassium gates open
Because potassium is at a higher concentration inside the cell they move out across the membrane.
So the membrane potential becomes negative again.
These changes together are known as an action potential and can be visualized by monitoring changes in an axon using an electrode.
The diagram below shows the potential changes across a section of axon membrane with time and shows the gates that are open at particular times. It comes from http://virtual.yosemite.cc.ca.us/dward/physo101/lab_act_modeling.htm
Once the action potential is complete the ion concentrations are returned to their original values by the action of sodium-potassium exchange pumps
The threshold potential is generated by the lateral movement of sodium ions from a region of the axon that is being stimulated. Hence the stimulus is propagated along the axon with each stimulated region "providing" sodium ions to the adjacent region and, thus, stimulating it.
Note that once an ion channel has been through an open/close cycle it is unable to begin another cycle for a period of time. This non-responsive time is known as the refractory period
Synapses and neuromuscular junctions
The junction between adjacent neurones is called a synapse
The junction between a neurone and a muscle is called a neuromuscular junction
At both of these chemical transmitters are released
In the synapse, transmitters are released by the pre-synaptic neurone and pass across to the post-synaptic neurone. This leads to the opening of ligand-gated ion channels (i.e. ion channels controlled by the presence of chemicals). An influx of sodium ions then sets up an action potential i.e. it depolarizes the membrane sufficiently to cause a threshold potential.
In the neuromuscular junction, transmitters are released by the neurone and pass across to the muscle. This causes the release of calcium ions from the sarcoplasmic reticulum into the myofilaments and thus the contraction of the myofilament.