Get To Know All About Nerves!
A nerve is the basic unit of the nervous system. It provides a path for action potentials to travel from the periphery to the CNS as in the case of sensory neurons, or from the CNS to the peripheral organs as in the case of motor neurons.
In the peripheral nervous system, it is defined as a collection of axons from individual neurons along with Schwann cells for myelination and connective tissue for support. In the central nervous system, the same structure, is referred to as tracts.
Each axon is protected by an endoneurium consisting of an outer coat of collagen fibers and an inner layer of glycocalyx proteins. This forms a sleeve, protecting the axon from its start to the end. This sleeve is filled with endoneurial fluid which acts as a blood-nerve barrier, similar in function to the cerebrospinal fluid. During nerve injury, there is an increase in endoneurial fluid (neural edema) which can be visualized on MR neurography and thus is a good marker for the injury.
Each axon with its endoneurium is then further bundled together with other such axons to form a collection of axons called as fascicles. These fascicles are then covered by perineurium, a smooth, transparent, tubular membrane. The perineurium consists of flat epithelioid myofibroblasts cells and lamellar arranged connective tissue.
Several fascicles are then grouped together with other important structures. These include blood vessels which support energy requirements and, lymphocytes and fibroblasts which play a role in the repair and maintenance of the tissue. All of these structures are then covered by the epineurium, a dense connective tissue layer that constitutes the outer most layer.
They make up the nervous system, which allows cell across the body to communicate with each other and thus helps coordinate functions across the body. The nervous system is divided into:
- Central Nervous System: this consists of tracts, the brain and spinal cord
- Peripheral Nervous System: this is made up mainly of nerves, which connect the CNS to the periphery. The PNS is further divided into:
- Somatic : controls primarily voluntary activities
- Automatic: controls primarily involuntary activities
- Sympathetic Nervous System: functions in emergencies to prepare for flight-or-fight response
- Parasympathetic Nervous System: activated in relaxed, less energy consumption state
- Enteric: controls gastrointestinal motility and secretions
They help conduct electrical impulses in the form of action potentials, to synapses or other organs. Action potentials are changes in the polarity across the membrane, which help play a major role in cell-to-cell communication. If it is myelinated by Schwann cells, impulses travel very fast aided by saltatory conduction.
Synapses occur between two neurons, and in order to communicate across a synapse, there is a change in electrical conduction to chemical conduction. This is where the first neuron secretes a substance called a neurotransmitter which travels across the synaptic cleft, to the other neuron where it helps cause a change in the charges across the cell membrane, and thus elicit an action potential.
They can be divided based on the direction of travel of the impulses. They would be grouped as:
- Afferent: from the sensory neurons in the periphery to the CNS
- Efferent: from the motor neurons in the CNS to organs/glands/muscles in the periphery
- Mixed: They carry fibers from both sensory and motor neurons
They can also be divided according to their anatomical position in the nervous system.
- Cranial Nerves: 12 nerves that innervate parts of the head and connect directly to the brain or brain stem
- Spinal Nerves: 31 pairs that innervate the rest of the body and are connected to the spinal cord. They are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
When the axon is damaged, as long as the cell body remains intact, the nerve can regenerate and reconnect with other neurons. This process is known as neuroregeneration. Neuroregeneration is seen in the peripheral nervous system where neurons are able to repair and restore its axons. It is not seen in the central nervous system.
When there is nerve damage, the axon can be divided into two parts. The part still attached to the cell body is called the proximal segment while the other part is called the distal segment.
- Within 24-36 hours hours of injury, the distal part undergoes Wallerian degeneration, where the axonal skeleton degrades and the axonal membrane breaks down. This is followed by the degradation of the myelin sheath.
- The proximal segment swells and there is retrograde degeneration.
- Phagocytes and macrophages migrate to the area of lesion to clear away debris.
- Schwann cells and basal lamina help produce a regeneration tube.
- Schwann cells and macrophages also secrete neuro-trophic factors; these help produce new axon growth spurts from the cell body of the neuron.
- These spurts are guided by the regeneration tube so that it can make connections and lengthen.
Neuroregeneration is a very slow process and can take months. Axon growth can be 2mm/day in small nerves and 5mm/day in larger nerves. The end result is usually not perfect, as damage cannot be fully repaired.
Nerve damage as a result of physical trauma or any disease is a very serious condition because of their important role in coordinating movement and other physiological activities of the body.
Common conditions include carpal tunnel syndrome where the median nerve gets compressed as it passes through the wrist at the carpal tunnel; this causes a pinched nerve due to pressure being placed on the nerve. Repetitive strain injuries caused by sedentary lifestyle can also affect nerves.
Autoimmune diseases can lead to the body attacking its own nervous system as in the case of Guillain-Barre Syndrome where there is attack on the peripheral nervous system. This causes rapid-onset muscle weakness beginning in the hands and feet. In Multiple Sclerosis macrophages attack myelin sheath. Symptoms include muscle weakness, double vision and uncoordinated movements.
Other factors that can lead to damage are diabetes, infection and inflammation. Cancers of the prostate, head and neck can metastasize to the areas surrounding nerves causing their destruction.
The damage can be assessed first by a physical examination that tests out reflexes, coordination, weakness and proprioception. These are followed by lab investigations such as nerve conduction studies, electromyography and computed tomography to diagnose the degree and severity of damage.