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The anatomy and function of neurons and glial cells

Neurons

Parts of a neuron

Cell body (soma)

•contains the nucleus & organelles

•responsible for maintaining the neurons structure

Dendrites

•receives signals from other neurons through neurotransmitters

•the word “dendrite” comes from the Greek word “déndron” which translates to tree

•dendrites may also play a role in long-term potentiation (LTP) which is important for learning and memory

•dendrites receive signals from other neurons through neurotransmitters, If strong enough the signal can lead to an action potential

Cell membrane

•mainly composed of a double layer of phospholipids

•the lipid bilayer consists of hydrophobic and hydrophilic regions

•maintains a resting membrane function of 70mV

Axon hillock

•the place on a cell body where the axon originates

•is cone shaped

•contains fascicles of microtubules (a bundle of microtubules that help with structural support and specialised cellular functions)

•where action potentials originate

Axon

•also called nerve fibers

•covered by a membrane known as axolemma

•varies in length from a millimetre up to a meter

•the white matter in the brain is mostly made up of long myelinated axons, the myelin gives the white appearance to to the tissue

Myelin sheath

•is an insulation layer of the axon

•composed mainly of fat and proteins

•multiple sclerosis is caused by the immune system attacking the myelin sheath in the CNS

•allows electrical impulses to transmit efficiently and speedy among nerve cells

Node of ranvier

•these are small (1um) gaps that are not insulated by myelin

•although not coated by myelin the axon is in direct contact with the Schwann cells in the PNS or with processes of astrocytes in the CNS

•they are crucial for saltatory conduction (allowing electrical impulses to jump from node to node)

Axon terminal

•stores small neurotransmitters in little sacs named synaptic vesicles

•located on the ends of axons and look like small branches

Synaptic end bulbs

•also known as boutons

•the bulbs contain neurotransmitters that make it easier for communication with other cells

•located in the tips of axon terminals

•according to new research there’s a specific type of synaptic end bulb called the "endbulb of Held" found in the auditory system and plays a vital role in processing auditory information

•where neurotransmitters are stored before being released into the synaptic cleft

Types of neurons

Motor neurons

•transmits signals from the CNS to muscles and glands causing muscle contractions and gland secretions

Interneurons

•transmits impulses between other neurons, especially as part of a reflex arc

Sensory neurons

•transmits signals from sensory receptors to the CNS, detects changes in the external and internal environment and relay this information allowing us to experience sensations

Structural types of neurons

As pictured below there are five structural types of neurons, these are

Unipolar

Has a single extension that splits into two branches, is often found in sensory neurons

Pyramidal

Shaped like a pyramid with one long axon and many dendrites, is common in the cerebral cortex

Multipolar

Has one axon and many dendrites, is most common type in the CNS

Bipolar

Has one axon and one dendrite, is found in sensory systems like the retina

Purkinje

Large, branched neurons found in the cerebellum, helps coordinate movement

Glial cells

Glial cells are support cells that help neurons maintain proper function

Glial cells in the central nervous system (CNS)

Astrocytes

•gives nutrients to the nervous tissue and neurons

•repairs the brain and spinal cord after injury and/or infection

•regulates cerebral blood flow

•astrocytes in white matter are called fibrous whilst astrocytes in grey matter are called protoplasmic

•they express plasma membrane transporters for several neurotransmitters such as glutamate, ATP, and GABA

•astrocytes form glial scars upon injury to nerve cells within the CNS

•astrocytes are the most abundant glial cells in the brain

Oligodendrocytes

• produces myelin to insulate axons in order for axons to send faster signals

•a single oligodendrocyte can myelinate up to 40 axons

•oligodendrocyte dysfunctions may play a role in schizophrenia and bipolar disorder

•the chemotherapy drug 5-FU can damage oligodendrocytes in mice, causing both immediate and delayed brain damage

•oligodendrocyte dysfunction may also be implicated in the pathophysiology of schizophrenia and bipolar disorder

•JC virus can infect oligodendrocytes causing PML which is a serious white matter disease in people with weak immune systems

•myelin damage may worsen amyloid plaque buildup, making myelin loss a possible early risk factor in Alzheimer’s disease

•oligodendrogliomas are slow-growing tumors from neuronal satellite cells in the brain’s grey matter, often found beneath the pia mater.

Microglial cell

•maintains brain health by clearing debris and supporting other glial cells

•acts as the primary immune cells of the CNS by initiating neuroinflammatory responses

•transforms into an activated state during injury or disease although effects can be protective or harmful

• dysfunctional microglia are linked toneurodegenerative diseases (e.g Alzheimers & Parkinsons)

• engulfs and digest pathogens (phagocytosis) debris and dead cells

•helps refine neural circuits by removing unnecessary synapses

•interacts with neurons and glial cells via complex signaling pathways

Ependymal cells

•ciliated epithelial cells lining the brain ventricles and central canal of the spinal cord

•contributes to the blood-cerebrospinal fluid barrier

•involved in CSF production and circulation

•can be activated after injury, potentially functioning as endogenous stem-like cells

•lines brain ventricles and the central canal of the spinal cord

•has a single epithelial layer which is cuboidal or columnar in shape

•possess cilia and microvilli on apical surfaces to help circulate CSF

•connected by tight junctions, forming a selective barrier

•research shows they may act as reservoir cells post-stroke or injury

•can behave as in vivo/in vitro stem cells in the spinal cord

•does not self-renew and are eventually depleted, limiting long-term stem cell functionality

•evidence suggests they may serve as a cell source for cochlear repair

•ependymoma is a tumor derived from ependymal cells, often found in the fourth ventricle

Glial cells in the Peripheral nervous system (PNS)

Schwann cells

•wraps around axons to form the myelin sheath thus increasing nerve impulse speed

•supplies nutrients and support to both myelinated and unmyelinated axons

•aids in nerve repair by removing damaged axon fragments and promoting regrowth

•involved in modulating immune responses in the PNS

•myelinating Schwann Cells form myelin sheaths around individual axons

•non-Myelinating Schwann Cells (Satellite Cells) enclose unmyelinated axons and neuronal cell bodies

•each myelinating Schwann cell wraps around one axon segment

•action potentials jump between nodes increasing conduction speed up to 10x

•myelination reduces membrane capacitance and conserves energy

•shaped like a rolled-up sheet, the inner layers form the myelin and outer layer forms the neurilemma

Satellite cells

•found in the sensory, sympathetic, and parasympathetic ganglia of the PNS

•completely surround neuron cell bodies, forming a "neuron-glial unit"

•provides structural support and essential nutrients to neurons

•maintain chemical balance, including calcium levels and neurotransmitter control

•promote neuronal survival and differentiation, especially after injury or during inflammation

•activates and proliferate following nerve injury or inflammation although it may contribute to chronic pain and pathological states

•similar in function to astrocytes in the central nervous system

•include subtypes with varying gene expression and specialized functions

•derived from neural crest cells just like Schwann cells

•previously referred to as amphicytes

•is implicated in chronic pain and herpes simplex virus pathogenesis due to ion channel activity

photo credit

Healthline. (n.d.). Structure of a neuron. Retrieved April 26, 2025, from https://www.healthline.com/health/structure-of-a-neuron

Healthline. (n.d.). Types of neurons. Healthline Media. Retrieved April 28, 2025, from https://www.healthline.com

Healthline. (n.d.). Glial cells. Healthline Media. Retrieved April 28, 2025, from https://www.healthline.com

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Types of vaccines

Inactivated vaccines

Contains bacteria or viruses that have been inactivated or killed, since the pathogen cannot duplicate within the body these vaccinations are safe for immunocompromised individuals, although the downside to this is that boosters are needed, an example of this vaccine type is the polio and rabies vaccines.

Live-attenuated vaccines

These contain live but weakened down pathogens that can duplicate within the body but not cause as much harm, these vaccinations are not suitable for immunocompromised individuals, examples of these vaccines are the MMR (measles, mumps and rubella) vaccination.

Messenger RNA (mRNA) vaccines

This vaccine contains genetic material from a pathogen that instructs the body to defend against it in future exposures, an example of these are the Pfizer-BioNTech and Moderna COVID-19 vaccinations.

Subunit, recombinant, polysaccharide, and conjugate vaccines

•Subunit contains specific parts of a pathogen, most commonly sugar molecules or proteins, examples include the HPV and hepatitis B vaccines.

•recombinant produced via genetic engineering, contains a gene from a harmful pathogen put into a harmless pathogen, examples are the hepatitis B vaccine.

•polysaccharides contains sugar molecules found in the surface of bacterias. examples are the pneumococcal vaccine (protects against 23 strains of pneumococcus bacteria) and the MPSV4 vaccine (protects against meningitis-causing bacteria).

•conjugate Links the polysaccharides vaccine to a carrier protein, and makes the immune system react more effectively, examples include the HiB vaccine (protects against bacterial meningitis in children).

Toxoid vaccines

Contains inactivated bacterial toxins as bacteria toxins can also cause disease, examples include the diphtheria and Tetanus shots, these vaccines need repeat boosters and does not provide lifelong immunity.

Viral vector vaccines

Uses a harmless virus to deliver genetic instructions to the body to support future immunity, examples include the Johnson & Johnson and AstraZeneca COVID-19 vaccines.

Vaccines have 3 classifications, these are live,inactivated and attenuated.

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The blood cells

Monocyte- these white blood cells (scientific term leukocytes) are the largest type and can split into macrophages and dendritic cells and then engulf pathogens (phagocytosis).

Lymphocyte- Includes multiple cells (such as natural killer cells & B cells) and makes up approximately 18-42% of circulating white blood cells and produces antibodies.

Neutrophil-Most common white blood cell in the body, they phagocytise external pathogens.

Eosinophil-Helps fight parasitic infections by releasing toxic granules and are also involved in allergic reactions, these cells make up around 1-3% of circulating white blood cells.

Basophil- These cells release histamine and are responsible for allergic reactions, they are an uncommon cell making up 0.5-1% of circulating white blood cells.

Macrophage-Large immune cell that engulfs microbes & pathogens (scientific term for this is phagocytosis).

Erythrocyte- the common red blood cell that everybody knows, responsible for the transportation of oxygen from the lungs into the bodies tissues.

Platelets- small cell fragments that aid in the clotting of blood and wound healing.

Pictured above is the process of blood stem cells being developed into various types of blood cells, this process is called hematopoiesis.

Photo credit (Healio (2018). Components of the Immune System. [online] Healio.com. Available at: https://www.healio.com/hematology-oncology/learn-immuno-oncology/the-immune-system/components-of-the-immune-system.)

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A brief guide to the 5 main immunoglobulin classes

IgG- Located on macrophages, natural killer cells and B cells. Involved in phagocytosis, inflammatory responses and antibody-dependant cellular toxicity.

IgA- Located on myeloid cells. Involved in opsonisation, phagocytosis and inflammatory responses. This immunoglobulin makes up 15% of all the immunoglobulins located throughout the body and is found in mucous membranes.

igD- Located on B cells, not as much information has been gathered on IgD compared to the other immunoglobulins unfortunately.

IgE- Located on mast cells, basophils and eosinophils. This immunoglobulin is responsible for allergic reactions and responds to parasitic infections.

IgM- Located on B cells, T cells and certain myeloid cells. IgM is the largest immunoglobulin and is the first to appear in the initial defence response to an antigen entering the body

Photo credit (Britannica (2020). Immune system - Classes of immunoglobulins | Britannica. In: Encyclopædia Britannica. [online] Available at: https://www.britannica.com/science/immune-system/Classes-of-immunoglobulins.) ‌