Sympathetic motor neurons are the nerve cells that execute the body’s “fight or flight” response. These neurons are part of the autonomic nervous system, which manages involuntary bodily functions. Their primary role is to prepare the body for perceived threats or intense physical activity. This system regulates processes from heart rate to pupil diameter without conscious thought, resulting in a coordinated reaction that mobilizes energy for survival.
Anatomical Pathway of Sympathetic Signals
The journey of a sympathetic signal begins in the central nervous system, within the intermediolateral cell column of the spinal cord. This region, in the thoracic and upper lumbar segments (T1 to L2), houses the cell bodies of the initial neurons. These first neurons are called preganglionic neurons, and they have a relatively short axon that exits the spinal cord.
From the spinal cord, the preganglionic axon travels to a sympathetic ganglion. These ganglia are clusters of nerve cell bodies arranged in a chain running parallel to the spinal column. Within the ganglion, the preganglionic neuron makes a connection, or synapse, with a second neuron.
The second neuron in this sequence is the postganglionic neuron. Its axon is much longer than the preganglionic neuron’s and extends from the ganglion to the target tissue or organ. This two-neuron structure, with a short preganglionic and long postganglionic fiber, is a hallmark of the sympathetic pathway. This arrangement allows a single signal to be distributed to multiple postganglionic neurons for a widespread response.
Neurotransmitters and Signal Transmission
The transmission of signals through the sympathetic pathway relies on chemical messengers called neurotransmitters. At the junction within the sympathetic ganglia, the preganglionic neuron releases acetylcholine (ACh). This chemical crosses the synapse and binds to receptors on the postganglionic neuron, passing the signal along.
Once the signal reaches the end of the postganglionic neuron, a different neurotransmitter is released to act on the target organ. In most cases, this chemical is norepinephrine, also known as noradrenaline. Norepinephrine generates the physiological response in tissues like the heart, blood vessels, and lungs. It binds to specific sites on these cells, called adrenergic receptors, initiating the “fight or flight” changes.
A notable exception to this process involves the adrenal glands. In this pathway, preganglionic neurons travel directly from the spinal cord to the adrenal medulla without synapsing in a ganglion. These neurons stimulate cells in the adrenal medulla to release signaling molecules directly into the bloodstream. These molecules, primarily epinephrine (adrenaline) and some norepinephrine, then circulate throughout the body, producing a broad, systemic response.
Physiological Effects of Sympathetic Activation
Activation of sympathetic motor neurons triggers physiological adjustments designed to enhance survival. In the cardiovascular system, the heart rate accelerates and the force of its contractions increases, boosting the output of oxygenated blood. Simultaneously, blood vessels in areas like the digestive system constrict, while those in skeletal muscles dilate, redirecting blood flow for physical exertion.
The respiratory system responds by relaxing the smooth muscles around the bronchial passages in the lungs. This action, known as bronchodilation, widens the airways, allowing more air to be inhaled. In the eyes, the pupils dilate (a state called mydriasis) as the radial muscles of the iris contract. This allows more light to enter the eye, enhancing vision.
Metabolic processes are also affected to supply energy, while non-essential functions are suppressed. The sympathetic system stimulates several changes:
- The liver is stimulated to break down glycogen and release glucose into the bloodstream, providing immediate fuel.
- Digestion slows as the motility of the stomach and intestines decreases and their sphincters contract.
- Sweat gland activity increases to help cool the body during physical exertion.
- Piloerection, or goosebumps, may occur on the skin.
Sympathetic Nervous System Dysfunction
Imbalances in the sympathetic nervous system can lead to significant health issues. Chronic overactivity, where the “fight or flight” response is persistently engaged, contributes to high blood pressure (hypertension) as blood vessels remain constricted. It is also closely linked with chronic stress and anxiety disorders, where a perceived threat keeps the system on high alert.
Conversely, an underactive or damaged sympathetic system prevents the body from responding appropriately to challenges. One example is Horner’s syndrome, which results from damage to the sympathetic nerve supply to one side of the face. This condition is characterized by a drooping eyelid, a constricted pupil, and reduced sweating on the affected side. Insufficient sympathetic tone can also cause orthostatic hypotension, a form of low blood pressure that occurs upon standing, leading to dizziness or fainting.
The function of this system is a target for various medical treatments. For instance, beta-blockers are commonly prescribed for hypertension and anxiety. These medications work by blocking the adrenergic receptors that norepinephrine and epinephrine bind to on target organs like the heart. They dampen the effects of sympathetic activation, slowing the heart rate and reducing the force of its contractions to lower blood pressure.