BME355 Lab Listing: Neuromuscular
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Neuromuscular System


Preparation

Lab Outline

Procedure

Learning more


GOALS

Understand the neuromuscular system

Understand requirements for biomedical instrumentation of neuromuscular signal acquisition

Perform data analysis

Preparation

Introduction

Nerve conduction studies are typically performed for the evaluation of neuromuscular disease to provide a quantitative measure of physiologic changes. Electrophysiologic studies measure peripheral nerve dysfunction such as peripheral neuropathy, carpal tunnel syndrome and Guillain-Barre' syndrome.

Neuromuscular junction

Skeletal muscle fibers are innervated by nerve fibers originating in the spinal cord. The neuromuscular junction joins the axon terminal of the nerve ending with the midpoint of the muscle fiber. The nerve fiber branch terminals enter into the muscle fiber at the motor end plate. Schwann cells support and surround the nerve axons and create the myelin sheath. The myelin sheath allows the action potential to propagate along the nerve.

Excitable tissue

Cells have a steady potential across the membrane where the inner part is typically -80mV relative to the external part. Nerve and muscle cells have the ability to temporarily change this condition.


Resting potential

The resting potential is due to the selective permeability of the membrane and the ionic composition at each side of the membrane. The electrical resting potential is reached when the diffusion (Fick's law) and electrical fields are equilibrated. This is a dynamic equilibrium; there is particle movement but no change in the potential.

Action potential

Changes in concentrations in one of the compartments (intracellular, extracellular) or changes in the permeability to one or more ions (K, Na, Cl) bring about changes in Vm, that is, changes in the polarization.


These changes could occur due to thermal, mechanical, chemical or electrical factors.

If Vm becomes more positive than the threshold, a chain effect starts developing an action potential (AP). If not, the membrane will recover to the resting potential. The muscle fiber action potential is a depolarization wave

The action potential is an all or nothing phenomena. If the stimulus is bigger than the threshold, the AP will develop. At the threshold level, the probability is 50% (that is the definition for the threshold), and if it is smaller the membrane will recover without further change.

The threshold can change from cell to cell. Also, if the membrane depolarizes slowly enough to allow time for the sodium channels to accommodate, the threshold for the axon to develop an action potential will be increased.

Acetylcholine (ACh) is released by the nerve action potential at the presynaptic terminal.

Propagation of the excitation

If one point in the membrane depolarizes (becomes positive), it will affect the neighborhood. If the membrane depolarizes, the permeability for sodium increases and the sodium concentration changes. This change in concentration when no changes in the vicinity occur will cause an ionic flux, and this will produce a change in the membrane potential at that point. This change will shift and propagate the action potential. This propagation will be in one direction since in the other direction the membrane will be in the refractory period.

Conduction velocity

Conduction velocity is related with the diameter of the fiber. Empirically, velocity is equal to the square root of the diameter (m/s) for non-myelinated nerve fibers and is 6 times the diameter (m/s) for myelinated nerve fibers

The increase in velocity for myelinated nerve fibers is due to the descreased loss of current in the internodal space, so the propagation jumps from one Ranvier node to the next.

In general, average velocity will be lower in motor studies due to the time delay at the end plate. Pathology can lead to a decrease in conduction velocity.

Motor units

The basic unit of force in a muscle is a motor unit. This is the set of muscle fibers innervated by one motor neuron. The number of fibers vary according to the performance. For fine activities, the number is small and for muscles requiring less precision it is large.
The terminal branches are unmyelinated fibers with a diameter of 1.5um while the muscle fibers have a typical diameter of 50um. Under this condition, stimulus from the nerve will never reach the muscular fibers by electrical propagation, since the current flux will never be strong enough to depolarize. That is why chemical mediators are present at the neuromuscular junction (synapses). The excitatory neurotransmitter is Acetylcholine (Ach).

Muscle contraction


The response of a single nerve stimulus is a twitch. The time to reach the maximum is about 200msec, and other 600msec are needed to recover to the original state. Under normal conditions, muscles shorten when they develop force (tension). Under experimental conditions, the contraction could be isotonic (keeping force as a constant) or isometric (keeping length as a constant).

To study isometric contraction, the muscle is kept in a fixed position and the force is measured using a force transducer.

Summation

Summation occurs when a second stimulus occurs before the end of recovery. The muscle will contract again, and the developed tension will be increased. This allows a summation or build up of force. Increasing the repetition rate, the strength will increase (up to approximately 4 or 5 times the twitch max). until tetanus is reached. There will not be sufficient chemical mediators (calcium) to allow further contractions.


Summation with increasing stimulus frequency



Electrical Stimulation

Electrical stimulation of a nerve starts an action potential that propagates in both directions.

Two electrodes are placed in the surface of the body: an anode and a cathode between which a current is made to flow. The axoplasm within the nerves and the extracellular fluid surrounding the fibers is an aqueous solution of ionized salts and proteins and the current is carried by the ions. In the region of the cathode, current will flow out of the nerve and thereby depolarize the membrane. If the current is big enough, it will initiate an action potential.

A peripheral nerve is composed of many axons of different diameters, some sensory and some motor, resulting in a complex pattern of electrical activity.



Recruitment

You get more force if you stimulate more motor units and the more motor units you stimulate, the more muscle fibers you stimulate. Recruitment means that you bring more and more motor units into action. Movements start with only small numbers of motor units and build up force production by adding more and more active motor units.

EMG

The EMG (electromyographic signal) is the electrical manifestation of the activation of a contracting muscle. The evoked motor response is called a compound muscle action potential; it is the sum of action potential of the individual fibers.

Electrode placement

The current density needed to stimulate a nerve will be lower than the value needed to propagate the stimulus directly to the muscle. If the electrodes are on the surface, the resistance will be increased compared to if they are subcutaneous. The increase in resistance requires an increase in voltage to keep the current constant.

Electrode recordings (particularly signal latency and amplitude) are affected by skin temperature. Thus, skin temperature controlling units are used as a part of clinical instruments to control for this variability.

History

Luigi Galvani applied currents to frog nerves in the late 18th century.
Guillaume Duchenne studied the neuromuscular system in the mid-19th century.

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