Neuron Structure and Function

The Nervous System: Neurons and Neuroglia

The nervous system is composed of two primary cell types: neurons, which are responsible for rapid communication, and neuroglia, which provide support and protection to neurons.

Santiago Ramón y Cajal: Father of Modern Neuroscience

Santiago Ramón y Cajal is widely recognized as the father of modern neuroscience. His significant contributions include the discovery of the axonal growth cone and providing crucial evidence for the neuron theory, which established the neuron as the fundamental structural and functional unit of the nervous system. He was awarded the Nobel Prize in Medicine in 1906, shared with Camillo Golgi.

Neurons

Neurons are the structural and functional unit of the nervous system. They are highly specialized cells that are excitable, meaning they can respond to stimuli and generate action potentials.

• Electrical excitability: Neurons convert stimuli into electrical signals (action potentials).
• Mitosis: Mature neurons do not undergo mitosis. However, neural stem cells in specific brain regions (e.g., subventricular zone) can generate new neurons.
• Degeneration and Regeneration: If an axon is severed, the part connected to the cell body (soma) can regenerate over short distances.
• Energy Source: Neurons primarily use glucose for energy.

Structure and Function of Neurons

Cell Body or Soma

The cell body, or soma, is the main part of the neuron containing the nucleus and most organelles. It is the primary site of protein synthesis due to the presence of abundant ribosomes and rough endoplasmic reticulum (RER), visible as Nissl bodies. It also contains mitochondria.

Dendrites

• Specialized processes designed for the reception of signals through membrane receptors.
• Often highly branched, resembling a "tree" (from the Greek "dendron"), and covered by projections called dendritic spines.
• Contain Nissl bodies (RER) and mitochondria in their cytoplasm.

The Axon

The axon is a long, thin, and cylindrical projection responsible for the propagation of signals (nerve impulses) away from the cell body.

• Axoplasm: The cytoplasm within the axon, containing mitochondria, microtubules, and neurofibrils, but no protein synthesis occurs here.
• Axolemma: The cell membrane of the axon.
• Axon Hillock (AH): A cone-shaped region where the axon connects to the soma.
• Initial Segment (IS): The part of the axon closest to the axon hillock.
• Trigger Zone: The junction between the axon hillock and the initial segment, where action potentials are typically generated.
• Axon Collaterals: Side branches extending from the main axon.
• Axon Terminals (Telodendria): Highly branched endings of the axon containing synaptic vesicles filled with neurotransmitters.
• Synaptic End Bulbs: Swollen tips of axon terminals that form synapses.
• Varicosities: Bead-like swellings along some axons that release neurotransmitters.

Synapses

A synapse, or neuronal junction, is the site of communication between a neuron and its effector (another neuron, muscle, or gland).

• Types of Synapses:
  • Axi-somatic: Axon to cell body.
  • Axo-dendritic: Axon to dendrite.
  • Axo-axonic: Axon to axon.
• Neurotransmitter: The chemical substance released at the synapse to transmit signals.

"You are your synapses. They are who you are." - Joseph LeDoux, 2002 (in Synaptic Self)

Axonal Transport

Proteins synthesized in the soma, including neurotransmitters and repair proteins, need to be transported along the axon. This transport occurs via two main mechanisms:

Slow Axonal Flow

• Direction: Anterograde (away from the cell body).
• Rate: 0.2-4 mm per day.
• Cargo: Primarily cytosolic proteins.

Fast Axonal Flow

• Mechanism: Moves materials along the surface of microtubules, requiring ATP.
• Rate: 200-400 mm per day.
• Cargo: Primarily cargo vesicles.
• Directions:
  • Anterograde: From the soma to the axon terminal.
  • Retrograde: From the axon terminal back to the soma.

Classification of Neurons

Neurons can be classified based on their structure (number of processes) and function.

Structural Classification (Number of Neurites)

Functional Classification

• Sensory or Afferent Neurons:
  • Function: Receive signals from the external environment or internal body (skin, muscles, joints, sense organs, viscera).
  • Direction: Transmit signals towards the Central Nervous System (CNS).
  • Role: Receptor function.
• Interneurons (Association Neurons):
  • Function: Connect sensory neurons to motor neurons.
  • Location: Primarily found in the CNS, constituting about 90% of all neurons.
  • Role: Integrative or associative function.
• Motor or Efferent Neurons:
  • Function: Send signals from the integration or control center (CNS) to effectors (muscles and glands) via cranial or spinal nerves.
  • Role: Conducting function.

Specialized Neuron Shapes

Neurons can also be classified by their unique shapes, often named after the researchers who described them.

• Purkinje Cell: Large, highly branched neurons found in the cerebellum.
• Pyramidal Cell: Cone-shaped neurons found in the cerebral cortex and hippocampus.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system. They are generally smaller than neurons and about 25 times more numerous. Unlike neurons, they do not propagate action potentials and can divide.

Neuroglia of the Central Nervous System (CNS)

Astrocytes

Astrocytes are star-shaped cells, with variations based on location:

• Fibrous astrocytes: Found mainly in white matter.
• Protoplasmic astrocytes: Found mainly in gray matter.

Functions of Astrocytes

• Blood-Brain Barrier (BBB): Form part of the BBB by covering blood capillaries, regulating substance exchange.
• Structural Support: Provide physical support to neurons.
• Neuronal Protection: Guard against toxic blood products.
• Neurotransmitter Metabolism: Metabolize excess neurotransmitters.
• Chemical Environment Regulation: Maintain ion balance (e.g., Ca²⁺, K⁺).
• Memory and Learning: Play a role in modulating synapses.
• Secretory Function: Release chemical substances that regulate growth and cellular migration during development.
• Vascularization: Crucial for the development of CNS vascularization. The brain requires significantly more O₂ and nutrients than other organs.

Astrocytes are also increasingly recognized for their role in tripartite synapses, actively participating in synaptic function alongside pre- and post-synaptic neurons.

Oligodendrocytes

Oligodendrocytes are the most common type of glial cell in the CNS.

• Myelination: Their primary role is to form and maintain the myelin sheath around axons in the CNS.
• Myelin Layer: A multi-layered covering of lipids and proteins that insulates axons, increasing the speed of nerve impulse conduction.
• Analogy: Oligodendrocytes in the CNS are analogous to Schwann cells in the Peripheral Nervous System (PNS).

Microglia

Microglia are small cells with few projections, derived from cells of the immune system (mesoderm origin). They are often found near blood vessels.

• States: Exist in resting and activated states.
• Cytotoxic Role: Release reactive oxygen species (ROS) to kill bacteria or viruses.
• Phagocytic Role: Act as the immune cells of the brain, clearing away dead cells, bacteria, and biological debris.

Ependymal Cells

Ependymal cells are epithelial cells that form the ependyma, a simple cuboidal to columnar epithelium with microvilli and cilia.

• Cerebrospinal Fluid (CSF): Produce, monitor, and contribute to the circulation of CSF.
• Structural Function: Form the epithelial lining of the brain ventricles and the central canal of the spinal cord. They create a leaky barrier between CSF and interstitial fluid.
• Choroid Plexus: Form the epithelial layer of the choroid plexus in contact with blood vessels, contributing to a selectively permeable barrier.
• Stem Cell Properties: Can acquire neural stem cell properties.

Neuroglia of the Peripheral Nervous System (PNS)

Satellite Cells

Satellite cells are flat cells that surround neuronal cell bodies in peripheral ganglia.

• Support: Offer structural support to neurons within the PNS ganglia.
• Signaling: Participate in signal processing and transmission in sensory ganglia.
• Regulation: Regulate the exchange of substances between the neuron's soma and the interstitial fluid.

Schwann Cells

Schwann cells are anucleated cells in the PNS, responsible for axon myelination.

• Myelination: Each Schwann cell typically wraps around only one axon in the PNS, forming a myelin sheath.
• Regeneration: Schwann cells are crucial for nerve regeneration in the PNS, which is why PNS neurons regenerate better than CNS neurons.
• Non-Myelinating Schwann Cells: A single Schwann cell can enclose multiple unmyelinated axons without forming a myelin sheath.

Myelin Sheath

The myelin sheath is a layered covering composed of lipids and proteins. It acts as an electrical insulator around axons, significantly increasing the speed of nerve impulse conduction. It is formed by oligodendrocytes in the CNS and Schwann cells in the PNS.

Components of Nervous Tissue Grouped Together

Grey and White Matter

The nervous system is broadly divided into grey matter and white matter, distinguishable by their appearance and composition.

• White Matter: Consists primarily of myelinated processes (axons), which gives it a white appearance.
• Gray Matter: Composed mainly of neuron cell bodies, dendrites, axon terminals, bundles of unmyelinated axons, and neuroglia. It appears darker (grey).
  • Spinal Cord: Gray matter forms an H-shaped inner core surrounded by white matter.
  • Brain: A thin outer shell of gray matter covers the surface (cerebral cortex), and it is also found in clusters called nuclei deep within the CNS.

Groupings of Neuronal Structures

Nerves

A nerve is a bundle containing hundreds or thousands of axons, each following a specific path to serve a particular body region.

• Cranial Nerves: 12 pairs emerge from the base of the brain.
• Spinal Nerves: 31 pairs emerge from the spinal cord, each serving a specific body region.

Ganglia

Ganglia are small masses of nervous tissue located outside the brain and spinal cord, containing primarily the cell bodies of neurons.

Enteric Plexuses

Often referred to as the "brain of the gut," these plexuses are networks of neurons within the walls of the gastrointestinal tract.

Sensory Receptors

These are specialized parts of neurons or dedicated cells that monitor changes in either the internal or external environment of the body.