What is a nerve?
Nerves are the functional and structural units of the peripheral nervous system (PNS). They are composed of groups of individual specialized cells called neurons (or nerve cells), which transmit motor and sensory information back and forth between the PNS and central nervous system (CNS). Transmission is initiated via electrochemical impulses known as action potentials.
By definition, nerves are bundles of axons (or nerve fibers) found in the PNS. Within the CNS, the axons form entities are known as tracts.
|Definition||Bundles of axons (nerve fibers) in the peripheral nervous system|
|Types||Afferent (sensory), efferent (motor), mixed|
While the structure of a nerve is simple, their functions, innervations and nomenclature can be complex. In this article, we break down the different types of nerves, as well as their morphological features and functions.
- Basic structure
- Endoneurium, perineurium and epineurium
- Types of nerves
- Nerves of the body
- Neuropathy (Clinical note)
We'll begin by discussing the basic structure of the nerves. An individual nerve cell (neuron) is made up of small branching extensions called dendrites, a cell body (soma), and an axon which is one single, long branch. At the end of the axon, we find the axon terminals. Axon terminals meet the dendrites of adjoining neurons at the synaptic cleft. Here, neurotransmitters are passed from one neuron to another, initiating action potentials that allow for the transmission of electrical signals to various target cells around the body. The axons of a neuron (also known as nerve fibers) come together to form a bundle of axons which is finally called “nerve”. The difference between nerves vs neurons presents a common source of confusion. Are they the same thing? Simply put, no. Recall that nerves are found in the PNS, and are composed of bundles of nerve fibers (axons). The neurons on the other hand are just the cells of the nervous system.
Some nerve fibers (axons) and are enveloped in myelin sheath, a lipid and protein rich layer that aids in increasing the velocity of electrical impulse conduction. Those fibers are called myelinated fibers whereas the fibers without a myelin sheath are unmyelinated.
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Endoneurium, perineurium and epineurium
The nerve fibers are organized in nerves with the help of three layers of connective tissue: endoneurium, perineum and epineurium. The endoneurium is a thin sleeve of loose connective tissue surrounding each axon. It allows for the presence of tissue fluid required for the creation of electrical impulses in between each axon, as well as space for capillaries to provide them with oxygen and organic nutrients. The neurons are bundled together into fascicles, which are then encased in another, thicker, layer of connective tissue called perineurium. This layer provides further access for blood vessels, as well as an effective barrier against foreign substances. The final, strongest and densest layer, the epineurium, envelopes several groups of fascicles. It provides reinforcement to the nerve, helping to avoid tearing.
Types of nerves
There are several ways to categorize the nerves. The first and most common categorization of the nerves is based on the way they transmit information.
Afferent (sensory) nerves
The nerves that conduct signals from the periphery to the central nervous system are called afferent or sensory nerves. Those nerves can be further divided into somatic sensory (carrying information from the skin, skeletal muscle and joints) and visceral sensory (from visceral organs, such as the liver or kidneys). All those sensory impulses are sent to the CNS via ascending tracts.
Efferent (motor) nerves
On the other hand, the nerves that conduct signals from the central nervous system to the periphery are called efferent or motor nerves. Efferent nerves convey motor impulses away from the CNS, to effector organs such as muscles and glands. They can be divided into two types: somatic motor nerves (they provide innervation to the skeletal muscles) and visceral motor nerves (they are part of the autonomic nervous system and they innervate smooth muscle, cardiac muscle and glands). All motor impulses are sent to the PNS via descending tracts
An easy way to remember this terminology is that the signals from the efferent nerves exit the central nervous system and from the afferent nerves arrive to the central nervous system. Finally there are the mixed nerves that contain both sensory and motor fibers.
We’ve now talked about afferent sensory and efferent motor nerves, and their somatic and visceral divisions. Nerves that have both afferent sensory and efferent motor fibers are called mixed nerves. They transmit messages in both directions at once.
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Nerves of the body
Another way to categorize the nerves is based on their location in the body.
The nerves of the body are either cranial nerves, spinal nerves, or derivatives of them. Nerves exiting the cranium (skull) are cranial nerves; those exiting the spinal column are the spinal nerves. Altogether, there are 43 paired nerves in the human body. Thirty-one of these are in the spinal cord (spinal nerves), and 12 of them are in the brain (cranial nerves).
The spinal nerves are part of the PNS. Through them, the CNS receives information and controls the actions of the trunk and limbs. All spinal nerves are composed of both sensory and motor fibers; thus, they are mixed nerves.
They exit in bilateral pairs from specific segments of the spinal cord, through the union of dorsal and ventral roots. Preceding the roots are approximately 8 nerve rootlets.
Passing from nerve cell bodies in the ventral horn of the spinal cord gray matter, efferent (motor) fibers emerge at the ventral root and move out to peripheral effector organs.
Afferent (sensory) fibers emerge from the posteior root, passing through nerve cell bodies in the dorsal root ganglion, which return sensory information from the trunk and limbs to the CNS.
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There are 31 bilateral pairs of spinal nerves:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
While the cranial nerves are so named because they emerge through foramina and fissures in the cranium, only 11 of the 12 pairs of cranial nerves arise from the brain; the spinal accessory nerve (CN XI) arises from the superior part of the spinal cord. Moreover, the olfactory and optic nerves originate from the forebrain. They are individually named (e.g. “trochlear nerve”) and numbered (I to XII) in a rostrocaudal sequence (from top to bottom).
Their functions primarily concern the head and neck, as well as the thoracic and abdominal viscera to a lesser extent. While all spinal nerves are mixed, cranial nerves can be mixed, purely motor, or purely sensory.
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Neuropathy (Clinical note)
Peripheral neuropathy is a disease or injury affecting the function of the peripheral nerves. The condition can be acute with a rapid onset and progression, or chronic, with a slower onset and progression.
Signs and symptoms depend on the type of nerve (motor, sensory, autonomic or mixed) and number of nerve fibres involved. Damage to one nerve is referred to as mononeuropathy, while damage to several nerves is known as polyneuropathy. The causes of peripheral neuropathy are numerous, but some of the most common are diabetes type I and II, alcoholism, severe nutrient deficiency, trauma, and autoimmune diseases.
Damage to sensory nerves can give rise to “negative” symptoms like numbness and imbalance, or “positive” symptoms like burning, itching, tingling, pins and needles. Motor involvement might involve symptoms like weakness, gait impairment and muscle atrophy.
Some general examples are discussed below.
Radiculopathy, also known as a pinched nerve, refers to damage to a nerve root as opposed to a nerve fibre.
Mononeuropathy refers to damage to one single peripheral nerve. In this case, trauma is generally the most common cause. Entrapment and compression syndromes are also common causes.
Polyneuropathies refer to situations in which multiple peripheral nerves are damaged. The most common cause of this type of neuropathy is diabetes mellitus.