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Structure and Classification of Nerves
- Nerves are enclosed, cable-like bundles of nerve fibers in the peripheral nervous system.
- Each nerve contains axons, which are extensions of individual neurons.
- Nerves have connective tissue layers called endoneurium, perineurium, and epineurium.
- Nerve cells are classified as sensory, motor, or mixed nerves.
- Nerves can be categorised based on the direction of signal conduction: afferent, efferent, or mixed.
- Afferent nerves transmit signals from sensory neurons to the central nervous system.
- Efferent nerves transmit signals from the central nervous system to muscles and glands.
- Mixed nerves contain both afferent and efferent axons.
- Nerves can also be categorised based on their connection to the central nervous system: spinal nerves and cranial nerves.
Terminology and Development of Nerves
- Specific terms are used to describe nerves and their actions.
- Nerves that supply information to the brain from a body area or control body actions are said to 'innervate' that section.
- Terms like ipsilateral and contralateral describe whether the nerve affects the same or opposite side of the body.
- Some terms relate to the part of the brain that supplies the nerve.
- Anatomical terms of neuroanatomy are used to describe nerves.
- Nerve growth typically ends in adolescence but can be re-stimulated through Notch signaling.
- Notch signaling is a molecular mechanism that can promote nerve growth.
- Nerve growth is an important process in the development of the nervous system.
- The development of nerves involves the formation of axons, synapses, and connections.
- Nerve development is crucial for proper functioning of the nervous system.
Function of Nerves
- Nerves convey information through electrochemical impulses called action potentials.
- Impulses travel from one neuron to another by crossing synapses.
- Nerves can be categorised based on function: afferent and efferent.
- Afferent nerves conduct sensory information to the central nervous system.
- Efferent nerves transmit signals from the central nervous system to muscles.
Clinical Significance of Nerves
- Cancer can spread by invading the spaces around nerves, especially in head and neck cancer, prostate cancer, and colorectal cancer.
- Nerve damage can be caused by physical injury, conditions like carpal tunnel syndrome and repetitive strain injury, autoimmune diseases, neurodegenerative diseases, polyneuropathy, infection, neuritis, diabetes, or failure of the blood vessels surrounding the nerve.
- Multiple sclerosis is a disease associated with extensive nerve damage due to the immune system damaging the myelin sheaths.
- Pinched nerves occur when pressure is placed on a nerve, leading to pain, weakness, numbness, or paralysis.
- Neurologists diagnose nerve disorders through physical examinations, reflex testing, muscle weakness assessment, proprioception evaluation, and sense of touch examination, followed by tests such as nerve conduction study, electromyography (EMG), and computed tomography (CT).
Other Animals and History
- Identified neurons in vertebrate nervous systems have unique properties such as location, neurotransmitter, gene expression pattern, and connectivity.
- Vertebrates have few identified neurons, with the gigantic Mauthner cells being the best-known example in fish.
- Mauthner cells are located in the brainstem and have powerful synapses that trigger a fast escape response in fish.
- Command neurons, like Mauthner cells, can drive specific behaviors by themselves.
- Nerve nets serve as the nervous system in organisms with radial symmetry, such as Cnidaria, Ctenophora, and Echinodermata.
- Herophilos (335–280 BC) described the functions of the optic nerve in sight and the oculomotor nerve in eye movement.
- William Cullen's 1785 hypothesis associating mental states with physical nerves has not been confirmed by modern research.
- Lay medicine may still use the concept of nerves to diagnose or blame psychological worry or hesitancy.
- The term 'neurosis' was coined by William Cullen to describe certain mental disorders resulting from unknown physical changes in the nerves.
- The meaning of 'neurosis' has evolved and now refers to a nervous state rather than a state of the nerves.
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
| Nerve | |
|---|---|
 Nerves (yellow) in the arm | |
| Details | |
| System | Nervous system |
| Identifiers | |
| Latin | nervus |
| TA98 | A14.2.00.013 |
| TA2 | 6154 |
| FMA | 65132 |
| Anatomical terms of neuroanatomy | |
Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system. Each axon, within the nerve, is an extension of an individual neuron, along with other supportive cells such as some Schwann cells that coat the axons in myelin.
Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium. Nerve cells (often called neurons) are further classified as sensory, motor, or mixed nerves.
In the central nervous system, the analogous structures are known as nerve tracts.
