How are messages transmitted from the spinal cord to the muscles in your body?

By Elizabeth A. Murray, Mount St. Joseph University

The nervous system is at the root of all muscle movements, whether smooth, cardiac, or skeletal muscle. Let’s say one reaches out and pushes an elevator button. What are the motor pathways that allow us to push that elevator button using the muscles in our upper limb? That conscious decision begins in the motor portion of our brain’s cerebral cortex.

Spinal Nerves and the Brachial Plexus

Beginning in the motor portion of our cerebral cortex, the impulse to act reaches and travels down descending tracts of the spinal cord, and then that message travels in a mixed spinal nerve, which exits an intervertebral foramen, and then uses the ventral ramus to reach our brachial plexus—since only ventral rami can become part of a plexus.

From there, the pathway will pass along nerves in our upper limb and control the muscles that lets us complete an action such as pushing the elevator button. Now, that’s being very simplistic, because numerous muscles would be necessary for us to flex at the shoulder extend at the elbow, extend our wrist and digit, and push the elevator button. So, in reality several spinal nerves that all braid together in the brachial plexus would be involved.

Now, how about the sensation that will allow us to feel that we’ve pushed the elevator button? Receptors in our finger send a signal through our hand using sensory nerve cell.

This sensation travels along nerves that become part of the brachial plexus of the upper limb, and from there travels along the ventral ramus of a specific spinal nerve, since only ventral rami make up the nerve plexuses that govern the limbs. From the tip of our finger, the electrical activity will most likely wind up in the seventh cervical nerve—but that’s assuming we are a part of the 70% club that has the cutaneous distribution from our index finger in the typical textbook pattern.

This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.

The Sensory Portion of the Cerebral Cortex

Next, that pathway needs to travel to one of the roots of that spinal nerve. Since we are discussing sensation from the skin, it will pass through the posterior root, which is the sensory root of a spinal nerve. From there, that sensation will travel up ascending tracts within our spinal cord, until it reaches the sensory portion of our brain’s cerebral cortex, where we will actually feel the touch of the elevator button on our skin.

In addition, as some of us may know that the right side of the brain controls the left side of the body and vice versa, so when we talk about sensation from our right finger it is perceived by the left side of our brain, and the decision to push that button with our right finger began on the left side of our brain.

There is a part of the brainstem where these messages cross over to the opposite side of the body. That’s the reason a person who has a major stroke on the left side of his brain will lose control of right-side body muscles and will also not be able to sense things on the right side.

Receiving Messages from the Nervous System

Muscles do not contract without first receiving a message from the nervous system. These signals from nerve cells—called neurons—have both electrical and chemical components.

Electrical currents are generated within the human body—that’s the principle behind diagnostic procedures like electrocardiography or electromyography. Those currents are generated by the flow and differences in chemical ion concentrations, similar to how current is conducted in a battery. Ions are charged compounds within body fluids and body cells.

Other chemicals, called neurotransmitters, can convey the message between one nerve cell and another, or between a neuron and a muscle. And while how all this works is very interesting, much of it takes place at the microscopic level.

Muscles and Movement

When it comes to muscles and movement, of course, it is the brain that is the site of initiation for voluntary control over skeletal muscle. Specific regions within the brain’s surface, called the cerebral cortex, control particular parts of the body.

Signals sent from the brain to muscles of the neck, trunk, and limbs travel to skeletal muscle groups in those regions by passing inferiorly down the spinal cord, and then out of the anterior roots, the motor roots of spinal nerves. From there, those motor pathways will travel to their target muscles using either the ventral ramus or dorsal ramus of that spinal nerve, depending on precisely where in the body those target muscles are located.

If the skeletal muscles are in the head and neck, they may go straight from the brain out of a cranial nerve to reach their target, and thus, the spinal cord will then not be involved.

Common Questions about the Role of the Nervous System in Muscle Movement

Q: When do the muscles contract?

Musclesdo not contract without first receiving a message from the nervous system. These signals from nerve cells—called neurons—have both electrical and chemical components.

Q: What is the principle behind diagnostic procedures like electrocardiography?

Electrical currents are generated within the human body—that’s the principle behind diagnostic procedures like electrocardiographyor electromyography. Those currents are generated by the flow and differences in chemical ion concentrations, similar to how current is conducted in a battery.

Q: How do signals sent from the brain to muscles travel?

Signals sent from the brain to musclesof the neck, trunk, and limbs travel to skeletal muscle groups in those regions by passing inferiorly down the spinal cord, and then out of the anterior roots, the motor roots of spinal nerves.

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