The human nervous system processes sensory input received from the sensory organs. Topographically, it is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The following is an overview of the structure and function as well as possible diseases of the peripheral nervous system.
What is the peripheral nervous system?
The peripheral nervous system is composed of those parts of the nervous system that lie outside the brain and spinal cord (the CNS). It connects the brain to the periphery of the body, and thus acts as a delivery and execution organ of the central nervous system. Functionally, the two systems cannot be separated. Through the interaction of the central and peripheral nervous systems, stimulus processing and muscle and glandular activity of the body are controlled. The PNS consists mainly of nerve cell processes (axons), which are sheathed by glial cells.
Anatomy and structure
Nerves, also called neurons, are the “conduits” that link the peripheral nervous system to the central nervous system. Nerves are made up of bundled nerve fibers. These, in turn, are composed of nerve cell processes and glial cells. Glial cells occur in nerve tissue in ten times greater numbers than nerve cells. In the PNS, these include Schwann cells (which form the myelin sheaths) and mantle cells (which envelop the cell bodies of peripheral neurons). In the peripheral nervous system, a distinction must be made between two types of nerves: Cranial nerves (Nn. craniales) are connected to the brain. The spinal nerves (Nn. spinales), on the other hand, are connected to the spinal cord. There are 12 pairs of cranial nerves and 31-33 pairs of spinal nerves. In addition, afferent (lat. afferens = leading in) and efferent (lat. efferens = leading away) neurons exist. The PNS is further divided into somatic (voluntary) and vegetative (autonomic) nervous system. The autonomic nervous system can in turn be subdivided into sympathetic, parasympathetic and enteric nervous systems. In addition to the cranial and spinal nerves, other autonomic nerves of the autonomic nervous system exist in the PNS, as well as sensory and motor ganglia. The cell bodies (perikarya) belonging to the axons are located either in the CNS or in the ganglia of the PNS.
Functions and tasks
The peripheral nervous system has central roles in the perception of sensory signals from the environment and in involuntary and voluntary motor activity. Afferent (sensory) neurons transmit sensory input received via receptors to the CNS. Efferent (motor) neurons transmit the commands from the CNS via the axons to the effector organs and thus trigger their movement. Effector organs are, for example, the skeletal muscles or the smooth muscles of the viscera. The somatic system is responsible for the voluntary, i.e. consciously controlled, movement of the musculature. The autonomic system mostly unconsciously controls the function of the vital internal organs, for example breathing or digestion. Afferent or efferent neurons that are part of the somatic nervous system are also called somatoafferent or -efferent. If they are part of the autonomic nervous system, they are called visceroafferent or -efferent.
Diseases, ailments, and disorders
Diseases of the peripheral nervous system may be manifested by various symptoms. The classification of possible PNS nerve lesions is roughly into radicular lesions, plexus lesions, and (poly- and mono-) neuropathies. Nerve lesions can, for example, be the trigger for herniated discs (radicular lesion) or various paralysis symptoms (paresis) on the body. Sensory disorders, such as impairments of the sense of touch, can also have their cause in a disorder of the PNS. In the thoracic, cervical, and lumbar regions, there are bundled nerve roots (plexuses) that are divided among various nerves. Severing a peripheral nerve can result in paralysis of a muscle belonging to that area. Each peripheral nerve is responsible for a narrowly defined region or function of the body. Disease of a single peripheral nerve (mononeuropathy) can therefore result in sensory or motor deficits in that region of the body. There are multiple possibilities of underlying diseases that can damage a single nerve.For example, diabetes mellitus or some diseases related to rheumatism are associated with neuropathies, because they often cause circulatory disorders. However, neuritis can also be triggered by a herpes zoster infection (through initial infection with the varicella zoster virus). This disease, also known as shingles, is often accompanied by severe nerve pain.
Typical and common nerve diseases
- Nerve pain
- Nerve inflammation
- Polyneuropathy
- Epilepsy
