Structure and Functions of Neurons

Introduction

• Neurons are specialized cells that form the basic structural and functional units of the nervous system. Their unique organization enables reception, integration, and transmission of signals, while myelination enhances conduction speed and neurotrophic factors support neuronal growth and survival.
• A neuron is the structural and functional unit of the nervous system. It receives stimuli and transmits signals to other neurons and effector tissues.
• Neurons are excitable cells that generate and conduct action potentials.
• Signal transmission occurs through electrical and chemical processes.
• The human central nervous system contains approximately 10¹¹ neurons.
• Glial cells support neurons and are 10 to 30 times more numerous than neurons.

Structure Of A Neuron

• A neuron consists of cell body, dendrites, and an axon ending in terminals.

Cell Body

• The cell body is the central part that maintains cellular functions. It contains the nucleus and cytoplasm with essential organelles.
• The surrounding membrane, called plasmalemma, is a lipid–protein bilayer.
• The cell body integrates incoming signals before transmission.

Table 22.1: Differences between axons and dendrites.

Feature	Axon	Dendrites
Structure	Single, long, slender process extending away from soma	Multiple, short, tapering branches near soma
Branching	Limited; ends in axon terminals	Extensive branching forming dendritic tree
Cytology	Lacks Nissl substance	Contains Nissl substance
Function	Conducts impulses away from soma	Conducts impulses toward soma

Cell Organelles

• Neuronal cytoplasm contains abundant cell organelles required for metabolism and signal transmission.
• Nissl granules are rough endoplasmic reticulum with ribosomes, involved in protein synthesis. They are present in the cell body and dendrites but absent in the axon.
• Numerous mitochondria provide energy for active transport and impulse conduction.
• The Golgi apparatus participates in protein processing and vesicle formation.
• Lysosomes are involved in intracellular digestion and waste removal.
• Cytoskeleton includes neurofilaments, microtubules, and actin filaments, which maintain structure and support intracellular transport.

Nucleus

Functions of Soma

• The nucleus usually contains one prominent nucleolus, though occasionally two may be present.
• Centrioles are absent, which explains the limited regenerative ability of neurons.
• The soma stores genetic material and performs active protein synthesis. It maintains cellular metabolism and supports neuronal survival.
• The soma gives rise to dendrites at one end and the axon at the other.

Dendrites

• Dendrites are multiple short, branched processes arising from the cell body. They significantly increase the surface area available for synaptic input.
• Small projections called dendritic spines further enhance receptive capacity.
• Dendrites receive signals from other neurons and transmit them toward the soma. They may also participate in local protein synthesis in certain brain regions.
• A single neuron may possess thousands of dendritic branches in the central nervous system.

Axon

• The axon is a long process that conducts impulses away from the cell body. It forms the nerve fiber and transmits signals to target cells.
• The cytoplasm, called axoplasm, contains mitochondria, cytoskeletal elements, and limited organelles.
• The axon originates from the axon hillock, a specialized region of the soma.
• The initial segment follows the hillock and is the primary site of action potential generation in many neurons.
• In sensory neurons, impulses may begin at the first node of Ranvier.

Axon terminal:

• The distal end of the axon branches into fine processes called telodendria.
• These end in synaptic terminals, which contain vesicles filled with neurotransmitters.
• Neurotransmitters are released into synapses for communication with other cells.

Types of Axon

• Axons are of two types: myelinated and unmyelinated.

Myelinated Axons

• Myelinated axons are covered by a lipid-rich myelin sheath that increases conduction velocity.
• In the peripheral nervous system, myelin is formed by Schwann cells.
• In the central nervous system, it is produced by oligodendrocytes.

Role of Schwann Cell in Myelination

• Schwann cells are responsible for myelination of axons in the peripheral nervous system. Their plasma membrane wraps repeatedly around the axon, forming multiple compact layers of myelin sheath.
• This wrapping may occur many times, producing a thick insulating layer that enhances conduction.
• Adjacent membrane layers adhere tightly through specific proteins, leading to myelin compaction.
• Proper structure of these proteins is essential for normal nerve conduction.
• Genetic defects affecting these proteins can result in peripheral neuropathies with reduced conduction velocity.
• The nucleus of the Schwann cell remains in the outermost cytoplasmic layer.
• Each Schwann cell forms myelin over a short segment of the axon, and multiple cells cover the entire length.
• The gaps between adjacent cells are called nodes of Ranvier.
• These nodes expose the axonal membrane to extracellular fluid and are essential for saltatory conduction.
• This mechanism allows impulses to jump from node to node, increasing transmission speed.
• Damage to myelin, such as in demyelinating disorders, leads to impaired conduction in both sensory and motor pathways.

Myelinogenesis

• The myelin sheath is a lipid-rich insulating layer present outside the axonal membrane.
• In the peripheral nervous system, it is formed by Schwann cells, while in the central nervous system it is produced by oligodendrocytes.
• During myelination, the axon invaginates into the Schwann cell cytoplasm, forming a structure called the mesaxon.
• The mesaxon elongates and wraps repeatedly around the axon in a spiral manner.
• Successive membrane layers become closely packed with lipid deposition, forming compact myelin.
• A thin outer layer of Schwann cell cytoplasm persists and forms the neurilemma, which supports regeneration in peripheral nerves.
• Each Schwann cell myelinates a short segment of the axon, and multiple cells cover its entire length.
• Gaps between adjacent myelinated segments are called nodes of Ranvier.
• Each node is a small exposed region of the axonal membrane that plays a key role in impulse transmission.
• The segment of myelin between two nodes is termed the internode.
• Nerve impulses travel by saltatory conduction, jumping from one node to the next.
• This type of conduction significantly increases speed and reduces energy expenditure.
• In contrast, unmyelinated fibers conduct impulses continuously along the membrane, which is slower and less efficient.
• The structural organization of myelin is essential for rapid and efficient nerve impulse transmission.

Composition of Myelin Sheath

• The myelin sheath is composed of lipids, proteins, and water.
• Major lipids include cholesterol, phospholipids, and glycosphingolipids, which provide insulation and structural stability.

Objectives of Myelination

• Myelination increases the speed of nerve impulse conduction by enabling saltatory transmission. It reduces energy expenditure by limiting ion exchange to nodes of Ranvier.
• The myelin sheath provides mechanical protection and insulation to the axon. It contributes to the white matter appearance of the brain and spinal cord.

Timing of Myelination during Development

• Myelination occurs in a specific developmental sequence.
• Sensory fibers of the dorsal columns begin myelination during the fourth to fifth month of intrauterine life.
• Corticospinal tracts start myelination around two months after birth.
• This process is usually completed by about two years of age, coinciding with motor development such as walking.

Unmyelinated Axons

• Unmyelinated axons lack a myelin sheath and conduct impulses more slowly.
• Schwann cells are present, but their membranes do not form complete wrapping around the axon.
• Multiple axons may be enclosed within a single Schwann cell cytoplasm.
• These fibers are commonly seen in small diameter somatic nerves and postganglionic autonomic neurons.
• Impulse conduction occurs by continuous propagation, resulting in reduced conduction velocity compared to myelinated fibers.

Composition of Myelin Sheath

• The myelin sheath contains lipids, proteins, and water.
• Major lipids include cholesterol, phospholipids, and glycosphingolipids, which provide electrical insulation and structural stability.

Objectives of Myelination

• Myelination increases conduction velocity by enabling saltatory transmission along axons. It reduces energy consumption by limiting ionic exchange to nodes of Ranvier.
• The myelin sheath provides electrical insulation and protection to axons. It contributes to the characteristic white matter appearance of the central nervous system.

Timing of Myelination during Development

• Myelination follows a specific developmental sequence.
• Sensory pathways of the dorsal columns begin myelination during the fourth to fifth month of intrauterine life.
• Corticospinal tracts start myelination after birth, around two months of age.
• Completion by about two years correlates with development of coordinated motor functions such as walking.

Unmyelinated Axons

• Unmyelinated axons lack a myelin sheath and conduct impulses slowly.
• Schwann cells are associated but do not form complete membrane wrapping.
• Multiple axons may lie within a single Schwann cell cytoplasm.
• These fibers are common in small diameter somatic nerves and postganglionic autonomic neurons.
• Conduction occurs by continuous spread of depolarization along the membrane.
• Reduced conduction velocity reflects absence of saltatory conduction, making transmission less efficient.

Axoplasmic Transport

• Axoplasmic transport is the movement of substances between the cell body and axon terminal. It is essential for neuronal growth, maintenance, and function.
• Proteins, organelles, and vesicles are transported along microtubules within the axon.
• The process depends on cellular energy and intact cytoskeletal elements. It is impaired by agents that disrupt microtubules or inhibit oxidative metabolism.

Types of Axoplasmic Transport

• Transport occurs in different directions within the axon.

Anterograde Transport

• Anterograde transport moves materials from the cell body toward the axon terminal. It delivers neurotransmitters, enzymes, and membrane components to synaptic endings.
• Fast transport occurs at approximately 400 millimeters per day. It is mediated by the motor protein kinesin and carries vesicles and organelles.
• Slow transport occurs at about 0.5 to 2 millimeters per day. It carries structural proteins such as actin and neurofilaments.
• This slow transport is important for axon repair and regeneration.

Retrograde Transport

• Retrograde transport refers to movement of substances from the axon terminal toward the cell body. It occurs along microtubules and is mediated by the motor protein dynein.
• The transport rate is approximately 200 millimeters per day.
• This process provides feedback to the soma regarding the synaptic environment. It is important for neuronal survival, signaling, and recycling of cellular components.

Physiological and Clinical Examples

• Certain viruses enter nerve terminals and travel to the cell body via retrograde transport.
• Varicella zoster virus can remain latent in sensory ganglia and later reactivate.
• Toxins, such as tetanus toxin, are transported from neuromuscular junctions to the neuron cell body.
• This contributes to central nervous system involvement in toxic states.
• Neurotrophic factors, including nerve growth factor, are transported to the soma to support neuronal survival and function.
• Neurotransmitter reuptake occurs at synaptic terminals and supports efficient synaptic transmission.
• Recovered neurotransmitters may be recycled locally or transported back to the cell body.
• This mechanism helps regulate neurotransmitter synthesis and maintain synaptic balance.
• Proper functioning of retrograde transport is essential for neuronal communication and health.

Table 22.2: Difference between myelinated and unmyelinated nerves.

Feature	Myelinated Nerve Fibers	Unmyelinated Nerve Fibers
Axon size	Large diameter axons	Small diameter axons
Covering	Wrapped by multiple layers of myelin sheath	Enclosed within Schwann cell cytoplasm without myelin
Conduction type	Saltatory conduction between nodes of Ranvier	Continuous conduction along membrane
Sodium channels	High density at nodes and initial segment	Lower, evenly distributed density
Speed and energy	Rapid conduction with lower energy use	Slower conduction with higher energy demand
Examples	Preganglionic autonomic fibers, larger peripheral nerves	Postganglionic autonomic fibers, small peripheral nerves

Transneuronal Transport

• Transneuronal transport involves movement of trophic factors across synapses to adjacent neurons. It supports synaptic maintenance, neuronal survival, and functional connectivity between neurons.

Functions Of Neurons

• Neurons perform specialized roles in receiving, processing, and transmitting information.
• The cell body maintains structural integrity and supports metabolic functions of the neuron. It synthesizes proteins and neurotransmitter-related molecules in Nissl granules.
• These substances are transported to the axon terminal by axoplasmic transport.
• Dendrites act as the receptor region and receive signals from other neurons.
• They generate local potentials and integrate incoming information.
• These signals are transmitted toward the cell body for further processing.
• The axon hillock and initial segment are the sites of action potential initiation.
• The axon conducts impulses away from the cell body toward target cells.
• Rapid conduction ensures effective communication within the nervous system.
• Synaptic terminals release neurotransmitters when an action potential arrives. This enables transmission of signals across synapses to other neurons or effector organs.
• Overall, neurons ensure coordinated signal integration and communication throughout the body.

Metabolism And Growth Of Neurons

Metabolism

• Neurons are highly metabolically active cells with abundant mitochondria.
• Continuous activity is required to maintain membrane potential and cellular functions.
• About 70 percent of energy is used by the sodium–potassium pump to preserve ionic gradients.
• Neurons rely mainly on glucose metabolism for energy production.
• During reduced oxygen supply, pyruvate is converted to lactate, similar to muscle metabolism.
• Energy for impulse conduction is derived from adenosine triphosphate and creatine phosphate.
• Neurons contain high intracellular potassium ions, which support electrical activity.
• Vitamin B1 is essential for oxidative metabolism and prevents accumulation of metabolic byproducts.
• Despite brief tolerance, prolonged oxygen deprivation leads to irreversible neuronal damage.

Growth of Neurons

• Neuronal growth and survival depend on specific growth-promoting factors.
• Neurotrophins play a major role in differentiation, maintenance, and survival of neurons. They support synaptic formation and functional connectivity.
• Additional growth factors regulate development and repair processes.
• These include glial-derived factors, fibroblast growth factors, and insulin-like growth factors.
• Such factors influence neuronal proliferation, maturation, and response to injury.
• Proper regulation of these signals is essential for normal nervous system development and plasticity.

Neurotrophins

• Neurotrophins are proteins that promote neuronal growth, differentiation, and survival.
• They are produced by neurons, muscles, glands, and astrocytes.
• Major types include nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4.

Nerve Growth Factors

• Nerve growth factor supports survival and growth of sympathetic and selected sensory neurons. It reduces neuronal apoptosis by activating specific tyrosine kinase receptors. It consists of multiple subunits, among which the beta component has major biological activity.

Brain-derived Neurotrophic Factor (BDNF)

• Brain-derived neurotrophic factor promotes growth and maintenance of sensory neurons. It plays an important role in synaptic plasticity and neuronal survival.

Neurotrophin 3

• Neurotrophin-3 supports development of mechanoreceptors and peripheral neurons. It interacts with multiple receptor types to regulate neuronal differentiation.

Neurotrophins 4 and 5

• Neurotrophin-4 and neurotrophin-5 act mainly through specific tyrosine kinase receptors. They contribute to neuronal survival, although their exact physiological roles are still being studied.

Types Of Neurons

• Neurons are classified based on number of processes, axon length, function, and dendritic pattern.

According to the Arrangement of Axon

• According to process arrangement, neurons are grouped into unipolar, pseudounipolar, bipolar, and multipolar types.

Unipolar Neurons

• Unipolar neurons have a single process arising from the cell body.
• They are common in invertebrates and rare in vertebrates.

Pseudounipolar Neurons

• In pseudounipolar neurons, a single process divides into peripheral and central branches.
• They are typical of sensory neurons in dorsal root ganglia.
• They transmit impulses from peripheral receptors to the central nervous system.

Bipolar Neurons

• Bipolar neurons possess two processes, one axon and one dendrite.
• They are found in specialized sensory structures such as the retina.

Multipolar Neurons

• Multipolar neurons have one axon and multiple dendrites.
• They are the most common type in the nervous system.
• Examples include motor neurons of the spinal cord.

According to the Length of Axon

• Neurons are classified into Golgi type one and Golgi type two based on axon length.

Golgi Type 1

• Golgi type one neurons have long axons that extend over considerable distances.
• They are typically involved in projection pathways, such as corticospinal neurons.

Golgi Type 2

• Golgi type two neurons have short axons that terminate near the cell body.
• They function mainly as local interneurons within neural circuits.

According to Function

• Neurons are classified into sensory and motor neurons based on function.

Sensory Neurons

• Sensory neurons carry impulses from receptors to the central nervous system.
• They are also called afferent neurons and transmit information about external and internal stimuli.

Motor Neurons

• Motor neurons transmit impulses from the central nervous system to muscles or glands.
• They are known as efferent neurons and produce appropriate responses in target organs.

According to Dendritic Pattern

• Neurons are classified into pyramidal cells and stellate cells based on dendritic arrangement.

Pyramidal Cells

• Pyramidal cells have dendrites arranged in a pyramid-like pattern and are common in the hippocampus.

Stellate Cells

• Stellate cells show radial dendritic branching and are typically found in the cerebral cortex.

Arrangement Of Neurons And Neroglia

Arrangement of Neurons in Nerve Fibers

Basic Structure of Peripheral Nerves

• Peripheral nerves are organized into layers called endoneurium, perineurium, and epineurium.
• These layers provide structural support and protect nerve fibers.

Endoneurium

• Endoneurium surrounds individual nerve fibers along with their Schwann cells. It contains collagen, fibroblasts, endothelial cells, and macrophages.
• This layer maintains the microenvironment and supports metabolic exchange. It also helps group nerve fibers into small bundles called fascicles.

Perineurium

• Perineurium encloses each fascicle and forms a protective sheath. It consists of layers of flattened cells and collagen fibers. It acts as a diffusion barrier, regulating movement of substances into and out of nerve fibers.
• Most peripheral nerves contain multiple fascicles within this layer.

Epineurium

• Epineurium is the outermost dense connective tissue layer surrounding the entire nerve. It binds multiple fascicles together into a single nerve trunk. It provides mechanical strength and protects nerves from external injury.
• This organized arrangement ensures efficient conduction and protection of peripheral nerve fibers.

Neuroglia

• Neuroglia are supporting cells of the nervous system that assist neurons.
• They are abundant in the central nervous system and maintain the neuronal environment.
• Neuroglia provide structural support, insulation, and metabolic assistance to neurons.