How does the spinal cord work?

Author: SCIRE Community Team | Reviewer: Riley Louie | Published: 21 September 2017 | Updated: ~

The spinal cord is part of the nervous system. The nervous system is the body’s main communication system. It allows messages to be passed from one area of the body to another. This is possible because of special cells called neurons.

Neurons are cells that transmit nerve signals in the body.⁵

Neurons are the main cells of the nervous system. Neurons generate, conduct, and pass along nerve signals within the nerves, spinal cord, and brain.

Neurons communicate with other cells (such as other neurons, muscle cells and sensory receptors) through connections called synapses. These connections allow the cells to pass electrical and chemical nerve signals to other cells.

The brain and spinal cord together make up the central nervous system. The central nervous system is the main control system of the body.

• The brain is the command center of the nervous system. It sends commands to the rest of the body which control movement, breathing, and other functions. The brain also receives signals about sensations from the whole body, which it interprets to help manage bodily functions.
• The spinal cord is the main pathway for information travelling between the brain and the rest of the body. It acts like a highway along which nerve signals can travel between distant areas of the body. This includes both commands sent from the brain to the body and sensations from the body to the brain. The spinal cord is also the center for reflexes (a reflex is a muscle response to a touch stimulus).

The main functions of the spinal cord are to pass along information related to movement, sensation, reflexes, and organ function.

The spinal cord provides a pathway for movement commands to travel from the brain to the muscles. This is called

motor function

. Neurons that send movement commands are called

motor neurons

.

Movement begins in special movement (motor) areas of the brain that plan and generate nerve signals to create movements. Upper motor neurons from this area of the brain have long nerve fibers (axons) that project down the spinal cord, where they pass their signals to lower motor neurons that travel out of the spinal cord and into the body. These lower motor neurons connect with muscle cells to pass along movement commands that tell the muscles to contract.

The spinal cord is also a pathway for sensations traveling from the body to the brain. This is called sensory function. Neurons that send sensory information are called sensory neurons.

The body’s tissues (like the skin, muscles, and organs) contain special sensory receptors. Sensory receptors detect sensations such as touch, pressure, vibration, or temperature. When a sensation is detected, a signal is sent along the nerve fiber (axon) of a sensory neuron to the spinal cord.

The signal then travels up the spinal cord where it is passed along to other neurons in the brainstem and brain. When these signals reach the sensation centers in the brain, they are interpreted and the person feels the sensation.

Pain signals from touching something hot travels to the spinal cord and back to the muscles without going to the brain first.⁷

Reflexes are automatic responses that happen in the spinal cord and do not travel to the brain first. For example, when the tendon below the knee cap is tapped, it causes the knee jerk reflex.

Spinal reflexes involve neurons for both sensation and movement. When certain sensory receptors are activated, a nerve signal is sent through sensory neurons to the spinal cord. In the spinal cord, the signal is passed on to lower motor neurons involved in the reflex movement. The motor neurons then send a signal out of the spinal cord to the muscles, causing an immediate muscular response.

The spinal cord also plays a role in controlling some of the functions of the internal organs through the autonomic nervous system.

The autonomic nervous system controls largely unconscious bodily processes such as blood pressure, heart rate, breathing rate, body temperature, digestion, bladder, bowel, and sexual function.

It has two divisions:

• The sympathetic nervous system prepares the body for stressful or emergency situations. It is often called the ‘fight or flight’ system, because it prepares the body for action. For example, it increases heart rate and slows digestion.
• The parasympathetic nervous system prepares the body for normal, non-emergency situations. It is often called the ‘rest and digest’ system, because it allows the body to restore itself. For example, it slows heart rate and increases digestion.

The sympathetic and parasympathetic systems have different (and often opposite) effects on the organs and work together to control bodily functions according to the situation.

The autonomic nervous system controls various body functions.^8-11

Neurons that control the function of the autonomic nervous system begin in the brain or brainstem. Some of these neurons leave in the cranial nerves (nerves that arise directly from the brain and brainstem), and the rest travel down the spinal cord, where they branch from certain areas:

• The nerves of the sympathetic nervous system arise from the thoracic and lumbar spinal cord from the levels of T1 to L2.
• Some of the nerves of parasympathetic nervous system arise from the sacral spinal cord, from S2 to S4 (the others leave from the brainstem).

After leaving the spinal cord, the neurons connect (synapse) with other neurons in clusters of nerve cells called ganglia. From these ganglia, motor neurons project out to the organs and signal changes to their function.