The branchlike extensions of a neuron that receive impulses and conduct them toward the cell body

This article, the first of a six-part series on the nervous system, describes the anatomy of the central nervous system and how it coordinates sensory information and bodily actions

Abstract

This article, the first in a six-part series, provides an introduction to the nervous system. In particular, it explores the different cell types that act as the building blocks and the key functional units of the nervous system.

Citation: Bayram-Weston Z et al (2022) Nervous system 1: introduction to the nervous system. Nursing Times [online]; 118: 3.

Authors: Zubeyde Bayram-Weston is a fellow of the Higher Education Academy and a senior lecturer in biomedical science; Maria Andrade-Sienz is honorary associate professor in biomedical science; John Knight is associate professor in biomedical science. All are at the College of Human and Health Sciences, Swansea University.

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Introduction

The key functions of the nervous system are to detect, analyse and convey information. Raw information is collected by sensory organs before being relayed to the brain for processing. A variety of brain centres then initiate signals along motor and autonomic pathways to control physical movement and regulate internal physiology. These actions are primarily mediated by neurons that carry electrochemical signals termed action potentials (nerve impulses). As well as neurons, the nervous system contains neuroglial cells, which are responsible for a variety of immunologic and support functions and facilitate the activity of neurons.

Understanding the pathophysiology of the nervous system requires knowledge of neuronal and neuroglial cells’ structure and functions. This article, the first in a six-part series, reviews several basic aspects of cellular biology, anatomy and physiology of the nervous system.

Anatomically, the nervous system can be subdivided into two:

• Central nervous system (CNS) – brain and the spinal cord;
• Peripheral nervous system (PNS) – spinal nerves, cranial nerves and ganglia.

Functionally, the nervous system consists of the following:

• Somatic nervous system (SNS) – under direct conscious control;
• Autonomic nervous system (ANS) – as implied by its name, the ANS is involuntary and not under conscious control. The ANS is further divided into the sympathetic and parasympathetic systems.

These divisions of the ANS will be explored in detail in further articles in the series. In this first article, we primarily explore the different cell types that act as the building blocks and key functional units of the nervous system.

The cells of the nervous system are conveniently split into two major categories – neurons and neuroglial cells, which are often called glia.

Neurons

Neurons are the basic functional units of the nervous system. Their main function is to receive, integrate and transmit information to other cells. Unlike most other cell types, neurons do not usually retain the capacity to divide. However, in certain regions – for example, the olfactory apparatus and hippocampus – neuronal stem cells have been shown to be capable of dividing to produce limited numbers of new neurons (Mtui et al, 2016).

Each neuron consists of three major regions: a cell body, axon and several dendrites. The cell body contains the nucleus, prominent regions of granular rough endoplasmic reticulum termed Nissl bodies, and organelles, such as mitochondria and Golgi complexes, which are common to other cells in the body. Dendrites – from the Greek word for ‘tree’ – are small, shorter cytoplasmic branches that extend from the cell body. These branches receive information from the environment or from other neurons via connections termed synapses.

The axon is usually the longest portion of the neuron and extends away from the cell body, where it functions much like an electric cable to convey action potentials away from the cell body. Axons in human adults vary greatly in length, from less than 1mm to more than 1m, and are supported by a variety of neuroglial cells (Pawlina and Ross, 2015).

Structural types

Neurons are classified according to their structure and function. Structurally, there are three types (Fig 1).

Multipolar neurons
Most neurons are multipolar, containing one axon and several dendrites (Fig 1a). Multipolar neurons function as motor neurons that carry information from the CNS to effector organs, such as muscles and glands. Since the presence of multiple dendrites allows many synaptic connections to be established, most of the neurons in the CNS – brain and spinal cord – are multipolar. In the brain, many of them are given specific names, such as pyramidal cells and basket cells, which reflect their physical appearance under a microscope (Thibodeau, 2019).

Bipolar neurons
These contain two processes emanating from their cell bodies; one dendrite and one axon (Fig 1b). They are primarily found in sensory organs, including the cochlear (inner ear), retina (photosensitive area of the eye) and olfactory mucosa (responsible for giving a sense of smell). Here, the bipolar neurons function primarily as sensory neurons, which relay information from receptors to the CNS (Van Hook et al, 2019; Marieb and Hoehn 2018).

Pseudo-unipolar neurons (unipolar neurons)
These have a single process that emerges from the cell body before dividing into two. One of these extensions connects to the spinal cord, while the other extends outwards to the periphery (Fig 1c). These two processes collectively form the axon of the neuron, which conducts action potentials from the periphery towards the CNS. Pseudo-unipolar neurons are always sensory neurons and are restricted to the PNS (Muzio and Cascella, 2021, Kiernan et al, 2014).

Functional types

Neurons can be subdivided into three groups according to their function:

• Efferent (motor) neurons – carry information away from the CNS towards muscles and glands to achieve actions such as muscle contraction or secretion from a gland;
• Afferent (sensory) neurons – carry information towards the CNS for processing;
• Interneurons (association neurons) – multipolar neurons located extensively in the brain and spinal cord (CNS). They are not direct motor neurons or sensory neurons but act as a link between sensory and motor neurons (Marieb and Hoehn, 2018).

Neuroglial cells

Neuroglial cells were once thought of as basic support cells for neurons, but they are now recognised as playing multiple roles in neurophysiological processes.

Astrocytes

These cells take their name from the Greek astron, meaning star. These star-shaped cells are the most abundant neuroglial cells and have several key roles. They guide the migration of immature progenitor cells, which then differentiate into a variety of cell types that collectively build the complex architecture of the CNS. Astrocytes also help facilitate the formation and functioning of synaptic connections in the brain. How this is achieved is poorly understood, but it is known they modulate the release of a variety of neurotransmitter substances in chemical synapses (Kimelberg, 2010).

Astrocytes help regulate the metabolic activity of the CNS, for example, by storing glucose in the form of glycogen (also known as animal starch), thereby functioning as an energy reserve buffer that can be used when required. A key role of these cells in metabolism is to ensure that neurons are supplied with a steady stream of nutrients while helping to remove the potentially damaging waste products generated during cellular respiration.

Probably their most famous role is in helping to form and maintain the integrity of the blood–brain barrier (BBB): the cytoplasmic extensions of astrocytes form ‘foot processes’, which wrap tightly around the capillaries of the brain, forming a physical barrier between the circulating blood and delicate neurons (Fig 2). This BBB tightly regulates the movement of materials from the blood and is effective in preventing many toxins – including those derived from pathogens such as bacteria – from inflicting neural damage.

In health, most lipid-soluble molecules – for example oxygen and carbon dioxide, together with small molecules such as water and alcohol – are afforded free passage across the BBB, while larger molecules, such as proteins, toxins and many drugs, are not able to cross it. This can create major challenges for health professionals in the selection and administration of appropriate pharmaceutical agents. For example, Parkinson’s disease is usually associated with depleted levels of the neurotransmitter dopamine in the brain, but dopamine cannot be used to treat Parkinson’s disease directly because it does not cross the BBB. Instead, the dopamine precursor levodopa (L-DOPA) is used as this crosses the BBB relatively freely and is then converted to dopamine in the brain. This makes L-DOPA a suitable treatment in some patients (Pardridge, 2012).

Microglial cells

Microglial cells (Fig 2) are the local immune cells of the CNS. They enter the CNS during embryonic development and then become the resident macrophages. They are highly phagocytic, naturally engulfing and destroy microorganisms and cellular debris on contact. During the process of inflammation that is usually associated with damage or infection of brain tissue, microglial cells become activated and can divide, increase in number, size and become more mobile. This allows them to effectively patrol damaged, inflamed or infected brain tissue in search of material to phagocytose (Ginhoux, 2010). This resident population of phagocytic cells are essential because the presence of the BBB makes it difficult for most circulating leukocytes (white blood cells) to cross over into the tissues of the CNS.

Ependymal cells

Ependymal cells are ciliated cuboidal to columnar epithelial cells which line the brain ventricles. Functionally, these cells are responsible for producing, monitoring and facilitating the circulation of the cerebrospinal fluid (CSF). The brain has four connected cavities called ventricles and is surrounded by three membranes called the meninges, which provide protection to the brain and spinal cord. The innermost of the three meninges is called the pia mater; this undergoes invagination (folding inwards) in some parts of the ventricles. These invaginations are vascularised and lined by a population of resident ependymal cells and are referred to as the choroid plexuses. The major role of the choroid plexuses is to produce CSF (Javed et al, 2021). This will be discussed in more detail in the next article in this series.

Satellite glial cells

Satellite glial cells are flat cells that surround and envelope the neuronal cell bodies of ganglia in the PNS (ganglia are clusters of neurons located outside of the CNS). The role of satellite glial cells is poorly understood, but they are thought to play a similar role to astrocytes in the CNS, being responsible for providing nutrient support and protection to the neurons of ganglia of the PNS (Tortora and Derrickson, 2014).

Myelin-containing neuroglial cells

Most neurons in the human body have axons that are insulated with a myelin sheath. Myelin has a high lipid content – typically around 70% – which includes galactosphingolipids, phospholipids (such as sphingomyelin), saturated long-chain fatty acids and cholesterol (Jäkel and Dimou, 2017; Salzer et al, 2016).

There are two major types of myelin-containing neuroglial cells.

Oligodendrocytes

Oligodendrocytes are the myelin-containing neuroglial cells of the CNS. The name oligodendrocyte is from the Greek word meaning ‘a cell with a few branches’. These branch-like extensions are wrapped around the axons of the neurons in the CNS in a spiral manner, like a Swiss roll. However, the cell body and the nucleus of the oligodendrocytes do not wrap around the axon and remain separate from the myelin sheath (Fig 2). Unlike Schwann cells, oligodendrocytes are capable of sending out branches that wrap around multiple axons. This makes them ideally suited to the myelination of neurons in the CNS, where neurons are packed together at high density.

Oligodendrocytes also act as a framework to hold nerve fibres together. Combined, these myelin-containing cells are thought to account for around 50% of the weight of the human brain (Salzer and Zalc, 2016).

Schwann cells

In addition to the oligodendrocytes of the CNS, myelin is also produced by the Schwann cells of the PNS, with each Schwann cell wrapping around only a single axon.

With both oligodendrocytes and Schwann cells, the myelin sheath is not continuous. There are small gaps of unmyelinated axon in between the adjacent myelin-containing cells. These gaps are referred to as the nodes of Ranvier and play a crucial role in speeding up the conduction of action potentials (see ‘Myelin sheath and saltatory conduction’).

Although myelination provides the clear advantage of more rapid nerve conduction, not all axons are myelinated. In the PNS, small fibres, such as those involved in transmitting pain and temperature stimuli, remain unmyelinated (VaPutte et al, 2017; Kiernan and Rajakumar, 2014).

Electrochemical signalling

Neurons communicate with each other and other tissues using electrochemical signalling. They can move charged ions across their membrane to generate small electric currents called action potentials. Mental processes and the ability to coordinate the complex physiological responses necessary to maintain life depend on the ability of neurons to generate these signals and transmit them efficiently.

As with other cells, neurons maintain their size, osmolarity and electrochemical balance mainly by maintaining the correct distribution of electrolytes (ions) across their cell membranes.

Establishing the resting potential

The sodium potassium pump is common to most cells in the human body and plays a vital part in neural physiology. This mechanism actively pumps sodium (Na+) ions out of cells while pumping potassium (K+) ions in. For this reason, most K+ in the body is found located intracellularly (within the cells), while most Na+ is found in the extracellular fluids of the body, such as in interstitial fluid (tissue fluid) and blood. As a result of the sodium potassium pump, neurons can establish a potential of around -70mV (millivolts; a millivolt is one thousandth of a volt). Since this potential of -70mV is present when the neuron is not conducting an electrochemical signal, it is referred to as the ‘resting potential’.

Generation of an action potential

Along the length of a neuron’s axon are tiny openings called sodium channels. These remain closed while the neuron is resting, maintaining its resting potential. If the neuron is excited (stimulated), the sodium channels rapidly flip open and the Na+ ions accumulated outside the neuron (because of the sodium potassium pump) rapidly flood inside. As each Na+ ion has a positive charge associated with it, the resting potential of -70mV rapidly transitions to an action potential of around +30/40mV. Since the polarity of the neuron has now changed from negative (-70mV) to positive (+30mV), this process of generating an action potential is often referred to as depolarisation.

The depolarisation and generation of an action potential is a fleeting event, typically lasting around one thousandth of a second (1ms). The activity of the sodium potassium pump rapidly pumps Na+ ions back out of the neuron and the resting potential is once again quickly re-established.

Simplistically, a nerve impulse is a wave of depolarisation which cannot go backwards, but which travels continuously in one direction along the axon (Fig 3). It can be visualised as like a ‘Mexican wave’, with the first person putting up their hands and everyone else following behind until the end of the line (Knight et al, 2020; Thibodeau, 2019; Mtui et al 2016).

Myelin sheath and saltatory conduction

The speed of a nerve impulse varies depending on the type of neuron involved, with the larger diameter axons able to transmit impulses faster than thinner axons. Another key factor influencing the rate of transmission is the myelin sheath formed by oligodendrocytes in the CNS or by Schwann cells in the PNS (see ‘Neuroglial cells’).

In neurons with unmyelinated axons, the opening and closing of sodium channels required to generate an action potential has to occur along the entire length of the axon and, therefore, nerve conduction occurs relatively slowly. In a myelinated axon, the gaps between each adjacent myelin-containing cell (nodes of Ranvier) allow the action potential to ‘leapfrog’ along the length of the axon, by jumping from one node of Ranvier to the next. This type of nerve transmission is referred to as saltatory conduction (from the Latin word saltare, meaning ‘leap’). It allows myelinated neurons to conduct action potentials much faster than their unmyelinated counterparts.

Demyelination in multiple sclerosis

Oligodendrocytes are typically damaged and eventually killed in the neurodegenerative disease multiple sclerosis (MS). MS is a chronic autoimmune disease, which is characterised by profound inflammation and subsequent progressive demyelination (Dendrou et al, 2015). Patients may display severe symptoms, such as neuropsychological problems, cognitive decline, weakness, poor motor coordination and sensory disturbances. These symptoms reflect the damage to axonal connections between brain and spinal cord regions. As the myelin sheath is damaged, nerve conduction slows, since saltatory conduction may no longer be possible (Patrikios et al, 2006).

Synapses

Action potentials that reach the branching ends of an axon arrive at the bulb-like pre-synaptic terminals (Fig 4). These are usually closely positioned to the dendrite of a neighbouring neuron or an effector structure, such as a muscle or gland. These specialised junctions are called synapses, and it has been estimated that there are more than 100 trillion synapses in the brain alone. In nervous tissue, synapses allow the relaying of action potentials from one neuron to another. Synapses can be broadly divided into electrical synapses and chemical synapses (Pereda, 2014).

Electrical synapses

In some synapses, the synaptic cleft between two neurons is directly connected by a narrow gap junction to its neighbour. This allows the action potential to flow as an electrical current directly between neurons without the need for neurotransmitter involvement. It was originally thought that electrical synapses were more abundant in invertebrates and less prevalent in mammals, but recent research suggests that electrical synapses are abundant in certain regions of mammalian brains and sensory organs (Curti et al, 2022). For example, electrical synapses are present in the respiratory centres of the medulla oblongata (lowest portion of the brain stem) and are also found in the retina and olfactory bulb (Pereda, 2014; Connors and Long, 2004).

Chemical synapses

If the communication between neurons is mediated by a chemical neurotransmitter substance, the synapse is referred to as a chemical synapse (Fig 4). A chemical synapse has three elements:

• Pre-synaptic element (the neuron before the gap);
• Synaptic gap (the synaptic cleft);
• Post-synaptic element (neuron after the gap).

The pre-synaptic terminals have vesicles containing neurotransmitter substances which function as chemical signals. When an action potential arrives at the pre-synaptic terminal it triggers the opening of calcium channels, allowing calcium ions (Ca++) to flood inside. This Ca++ influx triggers the secretory vesicles to fuse with the pre-synaptic membrane and release their chemical neurotransmitter substance into the synaptic cleft (Fig 4).

The neurotransmitter rapidly diffuses across this tiny space before binding to specific receptors on the post-synaptic membrane. This binding is highly specific, similar to a key fitting into a lock. The binding of the neurotransmitter to its receptors may stimulate or inhibit generation of an action potential in the next cell.

There are various neurotransmitters in the nervous system. The first identified and most common neurotransmitter in the CNS and PNS is acetylcholine (Ach) (Ferreira-Vieira et al, 2016). It is important to note that the chemical structure of a neurotransmitter determines if it exerts either an excitatory (for example Ach or glutamine) or inhibitory (such as gamma-aminobutyric acid) response on the post-synaptic neuron. As their name suggests, excitatory neurotransmitters stimulate post-synaptic neurons to generate an action potential, while inhibitory neurotransmitters prevent action potentials from being generated.

Conclusion

This article has explored the anatomy and physiology of the nervous tissue, including neurons and glia cells, which are the building blocks of the nervous system. The next article in this series will focus on the structures of the CNS and PNS.

Key points

• Neurons and neuroglial cells (glia) are the basic building blocks of the nervous system
• Neurons communicate with each other and other tissues using electrochemical signalling
• Glia are support cells for neurons, but also play multiple roles in neurophysiological processes
• Specialised junctions between neurons are called synapses
• Synapses can be electrical or chemical and allow the relaying of action potentials from one neuron to another

Also in this series

• Nervous system 2: the central and peripheral nervous system I
• Nervous system 3: the central and peripheral nervous system II
• Nervous system 4: the peripheral nervous system – spinal nerves
• Nervous system 5: the peripheral nervous system – cranial nerves
• Nervous system 6: the autonomic nervous system – anatomy and function

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Dendrou CA et al (2015) Immunopathology of multiple sclerosis. Nature Reviews Immunology; 15: 9, 545-558.

Ferreira-Vieira TH et al (2016) Alzheimer’s disease: targeting the cholinergic system. Current Neuropharmacology; 14: 1, 101-115.

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What region of the neuron are Branchlike extensions that receive impulses and carry them toward the cell body?

What are the extensions on a neuron that carry nerve impulses toward the cell body?

What are the extensions of the cell body of a neuron?

What conducts impulses toward from the cell body?