Electrophysiology: A possible cure for Paralysis?

Electrophysiology is the study of electrical activity of living neurons and investigates the molecular and cellular process that govern the signaling. Neurons communicate via electrical and chemical signals and electrophysiology studies these signals by measuring the electrical activity allowing us to decode the underlying message. This branch of neuroscience can be a potential cure to paralysis but for that we need to understand how nerve cells communicate and what is paralysis.


How nerve cells communicate?



Outline of a neuron

The major parts of a neuron are dendrites, nucleus, myelin sheaths and axon terminals. Dendrite is the receiving part of the neuron. Dendrites receive synaptic inputs from axons, with the sum total of dendritic inputs determining whether the neuron will fire an action potential. Axons are the long, thin structure in which action potentials are generated and are the transmitting part of the neuron. After initiation, action potentials travel down axons to cause release of neurotransmitter.


Neurons are essentially electrical devices. There are many channels sitting in the cell membrane (the boundary between a cell’s inside and outside) that allow positive or negative ions to flow into and out of the cell.


Normally, the inside of the cell is more negative than the outside; neuroscientists say that the inside is around -70 mV with respect to the outside, or that the cell’s resting membrane potential is -70 mV.


This membrane potential isn’t static. It’s constantly going up and down, depending mostly on the inputs coming from the axons of other neurons. Some inputs make the neuron’s membrane potential become more positive (or less negative, e.g. from -70 mV to -65 mV), and others do the opposite.


These are respectively termed excitatory and inhibitory inputs, as they promote or inhibit the generation of action potentials (the reason some inputs are excitatory and others inhibitory is that different types of neuron release different neurotransmitters; the neurotransmitter used by a neuron determines its effect).


Action potentials are the fundamental units of communication between neurons and occur when the sum total of all of the excitatory and inhibitory inputs makes the neuron’s membrane potential reach around -50 mV, a value called the action potential threshold.

Neuroscientists often refer to action potentials as ‘spikes’, or say a neuron has ‘fired a spike’ or ‘spiked’. The term is a reference to the shape of an action potential as recorded using sensitive electrical equipment.


A Voltage vs Time graph showing the "spikes". Image on the right is that of a mouse.

An electrical signal travels from a dendrite to axon of a nerve. The gaps between axon of one neuron and dendrite of another neuron is called as synapse. Neurons talk to each other across synapses. When an action potential reaches the presynaptic terminal, it causes neurotransmitter to be released from the neuron into the synaptic cleft, a 20–40nm gap between the presynaptic axon terminal and the postsynaptic dendrite (often a spine).


After travelling across the synaptic cleft, the transmitter will attach to neurotransmitter receptors on the postsynaptic side, and depending on the neurotransmitter released (which is dependent on the type of neuron releasing it), particular positive (e.g. Na+, K+, Ca+) or negative ions (e.g. Cl-) will travel through channels that span the membrane.


Synapses can be thought of as converting an electrical signal (the action potential) into a chemical signal in the form of neurotransmitter release, and then, upon binding of the transmitter to the postsynaptic receptor, switching the signal back again into an electrical form, as charged ions flow into or out of the postsynaptic neuron.


Image showing how does an action potential travel

Or to be more simple, the steps occurring are

1.An electrical signal travels down the axon.

2.Chemical neurotransmitter molecules are released into the synapse.

3.The neurotransmitter molecules bind to receptor sites on the releasing neuron and the second neuron.

4.The signal is picked up by the second neuron and is either passed along or halted.

5.The signal is also picked up by the first neuron, causing reuptake, the process by which the cell that released the neurotransmitter takes back some of the remaining molecules.


Keywords:

Action potential – Brief (~1 ms) electrical event typically generated in the axon that signals the neuron as 'active'. An action potential travels the length of the axon and causes release of neurotransmitter into the synapse. The action potential and consequent transmitter release allow the neuron to communicate with other neurons.

Neurotransmitter – A chemical released from a neuron following an action potential. The neurotransmitter travels across the synapse to excite or inhibit the target neuron. Different types of neurons use different neurotransmitters and therefore have different effects on their targets.


What is paralysis?


Paralysis is the loss of muscle function in part of your body. It happens when something goes wrong with the way messages pass between your brain and muscles. Paralysis can be complete or partial. It can occur on one or both sides of your body. It can also occur in just one area, or it can be widespread. Paralysis of the lower half of your body, including both legs, is called paraplegia. Paralysis of the arms and legs is quadriplegia.


Our point of interest is the certain type of paralysis wherein the nerve cells in hands remain intact but there is a loss in communication in between the Spinal Cord and the nerves in hand. To put it in layman terms, the hand is perfect but it is not receiving any signals from the Spinal Cord and hence it is not able to move. Consider a restaurant having a fully functional kitchen but with no orders. In spite of a having a fully functional kitchen they can't do anything as they are not getting any orders.


How to use to Electrophysiology to overcome Paralysis?



Nerves in our arm

Our hand has three major nerves namely the median nerve, the ulnar nerve, the radial nerve. Our nerve of interest is the ulnar nerve which can be easily located and is known to give painful sensations when we bump our elbow. It enables us to grasp objects.

The nerves in the motor cortex of the brain send signals via the spinal cord to the nerves in hands which enables to do different things. We place micro needle electrodes on one's arm which picks up the electrical signals and quantify them (Hand A). After noting such signals, we connect the apparatus to the hand affected by paralysis as well (Hand B). Now we have the apparatus connected to both a fully functional hand (Hand A) and the one affected by paralysis (Hand B). Once we move the fully functional hand, the electrodes in the other hand receive similar electrical signals and hence moves the other hand as well. The electrical signals from Hand A act as input signals to Hand B. So essentially what we are doing is to send electrical signals from one hand to the other using electrodes.


A screenshot from the video. Left- Hand A, Right - Hand B


PS:- This idea of mine has been inspired from a TEDx video on How to control someone else's arm with your brain by Greg Gage.

For a better understanding you can watch the video here:- https://www.youtube.com/watch?v=rSQNi5sAwuc

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