Motor end plate-Definition, Structure, Function & Diseases

The motor end plate is a part of the neuromuscular junction which occurs between the motor neuron and the muscle fiber. Neuromuscular junction plays a vital role in the transmission of the nerve signal or impulse from the motor neuron to the muscle fiber that leads to the contraction of the muscle.

What is motor end plate:

We already discussed that the neuromuscular junction is present between motor neuron or nerve cell and the muscle fiber. The part of the muscle fiber that is connected or interacts with the motor neuron in the neuromuscular junction is called the motor end plate.

Parts of the Neuromuscular junction:

There are 3 parts of the neuromuscular junction. They are the presynaptic neuron (motor neuron), synaptic cleft and the motor end plate. Let us discuss all of them.

Presynaptic neuron or Presynaptic bulb:

This is the part of the motor neuron that ends in a bulb-like structure. Each nerve cell has a cell body and certain branches. The branches that convey a signal to the nerve cell are called dendrites and they are many in number.

The branch that carries message or signal away from the nerve cell or neuron is called the axon. The axon is only one. At the end of the axon, there is a bulb-like swelling. This swelling is called the presynaptic end bulb.

Inside the Presynaptic bulb, there are many vesicles. These vesicles are filled with a chemical substance called Acetylcholine.

When the presynaptic neuron becomes excited or depolarized due to the entry of the sodium ions, the vesicles fuse with the membrane of the presynaptic bulb and release the Acetylcholine into the synaptic cleft. This process is called Exocytosis.

Synaptic cleft:

It is also known as the synapse. Synaptic cleft is the space or region that is present between the presynaptic bulb and the motor end plate. The neurotransmitters like Acetylcholine are released into the synaptic cleft. They later act on the motor end plate.

Motor end plate:

The motor end plate is the part of the muscle fiber where the neurotransmitters released from the presynaptic bulb act on. The muscle fiber is also a cell and it has a cell membrane called the sarcolemma. Motor end plate also has few parts. Let us look at them.

Synaptic trough:

The motor end plate has a membrane that is depressed inside or invaginated. This is called the synaptic trough. It is also known with the name of the synaptic gutter. The synaptic gutter is the regions which come into contact with the neurotransmitters in the synaptic cleft.

Subneural clefts:

Subneural clefts are also known as palisades or sub neutral apparatus or secondary synaptic clefts. They are the many folding of the sarcolemma membrane. They increase the surface area of the motor end plate.

Mitochondria:

There are a large number of mitochondria in the motor end plate. There is a high density of mitochondria in motor end plate as the skeletal muscle cells are very active and require a lot of energy.

Receptors:

There are many receptors located on the Motor end plate. Do you know what are these receptors celled? They are the Nicotinic receptors. The Acetylcholine that is released from the Presynaptic bulb attaches to the Nicotinic receptors.

There are about 10,000 Nicotinic receptors per every square millimeter in the motor end plate. When Acetylcholine binds to these Nicotinic receptors, the Sodium ions and Calcium ions present in the synaptic cleft start entering the motor end plate or the muscle fiber.

Then this positively charged sodium and calcium ions depolarize the muscle fiber or motor end plate. There is also a structure called sarcoplasmic reticulum in the motor end plate.

When the motor end plate is depolarized, the sarcoplasmic reticulum is activated and it also releases the stored calcium ions in it.

Then the calcium ions depolarize the entire muscle fiber. In this way, the nerve impulse rapidly spreads across the entire muscle fiber and then the entire muscle and the muscle contracts.

Diseases which affect the Neuromuscular junction and the motor end plate:

There are many diseases which affect the neuromuscular junction. In simple terms, these diseases are caused due to some defect at the neuromuscular junction. Some of these diseases affect the motor end plate too. Let us explore such diseases.

Myasthenia gravis:

Myasthenia gravis is an autoimmune disorder, a disease in which our body produces antibodies against our own body parts. In Myasthenia gravis, there is the production of autoantibodies against the Nicotinic receptors of the Motor end plate which bind the Acetylcholine neurotransmitter.

Due to blockage of the Nicotinic receptors, there are very few receptors left for the action of the Acetylcholine on them. This leads to reduced activity of the Acetylcholine on skeletal muscles and also reduced action or functioning of the skeletal muscles.

The muscles controlling eye movements, swallowing, head and neck, respiration is all skeletal muscles. So, they don’t function properly in Myasthenia gravis.

So, there is the weakness of muscles of the face, head, and neck. There is drooping of eyelids or ptosis and also double vision or diplopia due to impaired eye muscles.

These people suffer from difficulty swallowing or dysphagia due to affected swallowing muscles. They also experience voice changes or problems like dysarthria and hypophonia due to impaired muscles of the larynx.

If the condition becomes severe they might also suffer from respiratory distress and death due to respiratory failure. This is because of respiratory muscles area also skeletal muscles.

Neonatal Myasthenia gravis:

Neonatal Myasthenia gravis is seen in neonates born to mothers having Myasthenia gravis disease. This condition is caused when the antibodies blocking the Nicotinic receptors are transferred from the mother to the fetus through the placenta.

The neonates also have a weak body and they are unable to move due to blocked Nicotinic receptors. They need to be given drugs to treat Myasthenia Gravis. Otherwise, it can even lead to the death of the fetus.

Some other diseases of the neuromuscular junction:

• Lambert-Eaton syndrome
• Neuromyotonia
• Duchenne muscular dystrophy
• Bulbospinal muscular atrophy

You, people, have seen all the structures involved in the formation of the motor end plate and neuromuscular junction. We have also discussed how all the structures in the neuromuscular junction act collectively to cause skeletal muscle contraction.

Now, we also know the diseases caused due to defects in the motor end plate. Let us learn about another disease of the nervous system in our next article.