The nervous system allows us to sense, understand, and respond to our environment. It is composed of two main types of cells: nerve cells (neurones), which are the functional basis of the nervous system and responsible for transmitting signals through electrical or chemical means, and glial cells, which offer supportive and structural assistance to neurones. In this article, we will focus on the ultrastructure of nerves, exploring the general structure of nerves and their layers of connective tissue, as well as the conditions that can arise when the typical structure is disrupted.
All nerve cells, regardless of their function, contain the same basic structural components: the cell body, dendrites, the axon, and the axon terminals. The cell body houses the nucleus, and it is the site of protein synthesis, which takes place via small granules of rough endoplasmic reticulum called nissl substance. When many neuronal cell bodies group together, a distinct structure is formed. In the Central Nervous System (CNS), it is known as a nucleus, and in the Peripheral Nervous System (PNS) as a ganglion.
Dendrites are elongated parts of the cell body that extend outward, receiving input from the environment and other neurones. The axon is a long, thin structure down which action potentials—the nerve impulse—are conducted. Neurones have many dendrites, whereas every neuron typically possesses only one axon.
To further assist in efficient nerve signal transmission, each axon is coated in myelin—a lipid layer. Myelin is created by cells wrapped around the nerve axon. In the CNS, this is performed by oligodendrocyte cells, whereas in the PNS, Schwann cells are responsible for this job. Between the myelin sheaths formed by different cells, there are gaps known as nodes of Ranvier, which permit saltatory conduction of signals.
The axon terminal is the farthest reach of the axon and is from where the neurone communicates chemical signals to other cells, usually via neurotransmitter release. To facilitate neurotransmitter secretion, the axon terminals contain numerous mitochondria.
When neurones in the PNS are grouped together, they are encapsulated by several connective tissue layers. The endoneurium surrounds the axon of an individual neurone, the perineurium encompasses a fascicle—a collection of neurones—and the epineurium holds the entire nerve, composed of a number of fascicles, together. Notably, the epineurium contains numerous small blood vessels that supply the nerve fibres. It is formed by the merger of the arachnoid and pia mater, two layers of the meninges, as the nerve leave the intervertebral foramen.
Neurones can be classified by both structure and function, and the structure of a neuron is visible through histology. Structural classification includes unipolar, pseudounipolar, bipolar, and multipolar neurones. Unipolar neurones have a cell body at one end of a single unbranched axon, and lack dendrites. These are found in the cochlear nucleus of the brain. Pseudounipolar neurones possess one axon that is divided into two branches by the presence of the cell body, and sensory neurones are all pseudounipolar. Bipolar neurones possess two processes that arise from a central cell body, usually an axon and a dendrite, and these cells are found in the retina.
Neurons are specialized cells that are found in the central nervous system (CNS) and peripheral nervous system (PNS) of the human body. They are responsible for the synthesis of proteins and play a vital role in relaying information between different parts of the body. Each neuron has four key components - the cell body, dendrites, axon, and axon terminals - all of which are essential for their functioning.
The cell body is the location of the nucleus and the site of protein synthesis, which occurs on small granules of rough endoplasmic reticulum called nissl substance. In the nervous system, large numbers of neuronal cell bodies can cluster together to form either a nucleus in the CNS or a ganglion in the PNS.
Dendrites are the elongated projections that extend outwards from the cell body and are responsible for receiving input from the environment and from other neurons.
The axon is a long and thin structure that conducts nerve impulses known as action potentials. While neurons have many dendrites, they usually only have one axon. Each axon has a layer of insulating lipid known as myelin which is formed by cells wrapping around the nerve axon. In the CNS this is performed by cells called oligodendrocytes, and in the PNS, by Schwann cells. There are small gaps between the myelin sheaths which are known as nodes of Ranvier and allow for saltatory conduction of impulses.
The axon terminals are the most distal parts of the axon and are responsible for transmitting chemical signals usually via the release of neurotransmitters. This occurs when the axon terminals contain a large number of mitochondria. In the peripheral nervous system, these axons are grouped together to form nerves. These nerves have several layers of connective tissue - endoneurium which surrounds individual axons, perineurium which circles multiple neurones, and epineurium which covers the entire nerve.
Neurons can be classified into two categories - structure and function. According to structural classification, neurons can be categorized as unipolar, pseudounipolar, bipolar, and multipolar. Unipolar neurons have a single unbranched axon and no dendrites, which can be found in the cochlear nucleus of the brain. Pseudounipolar neurons have one axon which is divided into two branches with the cell body located in the middle, and these are typically found in sensory neurons. Bipolar neurons have two branched axons - one axon and one dendrite, which can be found in the retina. Multipolar neurons have one axon and multiple dendrites, and these are found in motor neurons.
According to functional classification, neurons can be divided into sensory (afferent), intermediate, and motor (efferent). Sensory nerves have small axons and a pseudounipolar structure, while motor nerves have larger axons and a multipolar structure. Intermediate neurons have a central cell body and many dendrites, and these are found in the CNS. Sensory and motor nerves can be found in the PNS, whereas intermediate nerves are found in the CNS.
Neurons play an important role in the functioning of the human body. Clinical relevance can be seen in neurological diseases such as multiple sclerosis and motor neurone disease, both of which are caused by damage to the nerve cells or tissue. Thus, understanding the structure and classification of neurons can provide key insights into the treatment and prevention of such conditions.
