12,2 nervous system

Neuron Structure and Function

• Nervous tissue comprises two main cell types: neurons and glial cells (neuroglia). Neurons are the primary signaling cells; glial cells provide essential support. Ongoing research explores broader signaling roles for glia, but neurons remain the core for electrical signaling and neurotransmitter release.

• Neurons are responsible for:

  • Electrical signals that convey sensations and initiate movements

  • Thought processes in the brain

  • Release of chemical signals (neurotransmitters) to target cells

• The distinctive feature of neurons is their morphology, which enables vast connectivity in the nervous system.

Parts of a Neuron

• Soma (cell body): contains the nucleus and major organelles.

• Processes extending from the soma:

  • Dendrites: highly branched structures that receive information from other neurons at synapses.

  • Axon: typically a single, long projection that propagates nerve impulses to target cells; may branch to contact multiple targets.

• Information flow and polarity:

  • Information flows from dendrites/cell body toward the axon, establishing directional polarity.

  • The axon propagates the action potential to target cells via the axon terminals.

• Key regions:

  • Axon hillock: region where the axon emerges from the cell body; site where cytoplasm becomes axoplasm.

  • Initial segment: beginning portion of the axon after the axon hillock; action potentials are generated in the trigger zone, a combination of the axon hillock and initial segment.

  • Nodes of Ranvier: gaps in the myelin sheath along the axon; critical for rapid conduction (saltatory conduction).

  • Axon segment: the myelinated portion between two nodes of Ranvier.

  • Synaptic end bulb (terminal): enlarged endings at the axon terminals that form synapses with target cells.

• Myelin and insulation:

  • Many axons are wrapped in myelin, an insulating lipid-rich sheath produced by glial cells; acts like electrical insulation but with gaps (nodes of Ranvier) that optimize signal speed.

  • Myelin is not just a membrane; it includes specific proteins that help hold membrane layers together.

• Diagrammatic note: the major parts of the neuron can be observed in figures that label soma, dendrites, axon, axon hillock, initial segment, nodes of Ranvier, and synaptic terminals.

Neuron Classification by Polarity (Number of Processes)

• Unipolar (pseudounipolar in humans):

  • One process emerges from the cell body and bifurcates into peripheral and central branches.

  • True unipolar cells are found in invertebrates; human neurons of this type are better described as pseudo-unipolar.

  • Characteristics:

  • Dendrites often receive sensory information, sometimes directly from stimuli.

  • Cell bodies located in ganglia (peripheral locations); the axon projects from peripheral processes into the CNS.

  • Function: primarily sensory.

• Bipolar: two processes extending from opposite ends of the cell body (one axon, one dendrite).

  • Locations: olfactory epithelium (smell) and retina.

• Multipolar: one axon and two or more dendrites; the most common type in the CNS and PNS.

• Anaxonic: very small neurons with multiple processes but no clearly identifiable axon or dendrites under standard histology.

  • Some sources describe anaxonic neurons, but they are typically still multipolar; the axon/dendrite distinction may be difficult to discern.

• Named classifications (based on other criteria):

  • Purkinje cell: a well-known multipolar neuron in the cerebellum (named after Jan Evangelista Purkinje).

  • Pyramidal cell: multipolar neuron with a pyramidal-shaped soma.

  • Olfactory neurons: named for their functional role (olfaction).

Glial Cells: Overview and Roles

• Glial cells (neuroglia) are the other major cell type in nervous tissue; they provide support to neurons and influence signaling.

• Etymology: “glia” from Greek glüo, meaning glue; Virchow described glial tissue as the glue that supports nervous tissue.

• There are six principal glial cell types discussed here: four in the CNS and two in the PNS.

• Glial Cells of the CNS:

  • Astrocyte (astro- = star): star-shaped; interact with neurons, blood vessels, and pia mater; key support roles include:

  • Regulating extracellular chemical concentrations and buffering ions

  • Clearing excess signaling molecules

  • Responding to tissue damage

  • Contributing to the blood–brain barrier (BBB)

  • Oligodendrocyte (oligo- = few; dendro- = branches; -cyte = cell): insulates axons in the CNS by forming myelin wrapped around multiple axon segments; one oligodendrocyte can myelinate several axon segments.

  • Microglia: immune surveillance; CNS-resident macrophages; phagocytose damaged cells and pathogens; originate from white blood cells in some models.

  • Ependymal cell: line ventricles; filter blood to form cerebrospinal fluid (CSF) at the choroid plexus; resemble epithelial cells; tight junctions with cilia to move CSF; part of the CNS’s barrier and CSF production system.

• Glial Cells of the PNS:

  • Satellite cell: surround neuronal cell bodies in sensory and autonomic ganglia; provide peripheral support similar to astrocytes in the CNS (but do not form a BBB).

  • Schwann cell: myelinate axons in the PNS; unlike oligodendrocytes, each Schwann cell myelinates a single axon segment; the nucleus and cytoplasm of the Schwann cell reside on the outer edge of the myelin sheath.

• Myelin basics across CNS and PNS:

  • Myelin = lipid-rich sheath around axons that speeds electrical conduction.

  • Myelin is produced by glial cells: oligodendrocytes in the CNS and Schwann cells in the PNS.

  • Structure: multiple layers of glial cell membrane wrap around the axon with little cytoplasm between layers; for oligodendrocytes, residual cell body processes extend to insulate multiple segments; for Schwann cells, the wrapping is around a single axon segment with the nucleus/cytoplasm at the outer edge of the sheath.

  • Appearance metaphor: myelin is like the pastry around a hot dog (“pigs in a blanket”).

• Myelin specifics:

  • Nodes of Ranvier are gaps between myelin segments where the axon is exposed; critical for saltatory conduction.

  • The axon diameter ranges from axons as small as $1 \, ext{μm} \, ext{to} \, 20 \, ext{μm}$: d_{ ext{axon}} ounding 1 \, ext{μm} ext{ to } 20 \, ext{μm}.

  • Myelin segment length between nodes can extend to roughly $1 \, ext{mm} \, ext{to} \, 2 \, ext{mm}$: $1 \, ext{mm} \, <br>leq \, L_{ ext{myelin}} \, <br>leq \, 2 \, ext{mm}.$

  • The ratio of myelin segment length to axon diameter is approximately $rac{L<em>{ ext{myelin}}}{d</em>{ ext{axon}}} \, ext{in the range} \, 100 ext{ to } 1000.$

The Blood–Brain Barrier (BBB) and CNS Protection

• The CNS has a privileged, largely non-diffusive blood supply; most substances cross via active transport mechanisms, not simple diffusion.

• BBB consequences:

  • Only specific molecules cross from blood to CNS (e.g., glucose and certain amino acids) and some small particles like water and gases can pass; many immune cells (e.g., white blood cells) are restricted.

  • This barrier protects the CNS from toxins and pathogens but also limits drug delivery to treat CNS conditions.

• BBB and transport implications:

  • Drug development is challenged by the need to design compounds that can cross the BBB and reach their CNS targets.

• Astrocytes contribute to the BBB by interacting with blood vessels and supporting endothelial cell properties that restrict leakage.

• Ependymal cells and the choroid plexus participate in CSF production and maintenance, which is part of the CNS’s barrier and homeostasis system.

CNS vs PNS: Myelination and Neuron Support in Context

• Myelin’s role in speeding conduction:

  • Myelination increases conduction velocity of action potentials along axons.

  • Saltatory conduction occurs at nodes of Ranvier, where action potentials regenerate in a jump-like fashion between gaps.

• Differences between oligodendrocytes (CNS) and Schwann cells (PNS):

  • Oligodendrocyte: can myelinate multiple axon segments across different neurons.

  • Schwann cell: typically myelinates a single axon segment; a given Schwann cell wraps only one segment of one axon.

• Practical anatomy notes:

  • In development, myelination involves wrapping steps that progressively enclose the axon with membrane layers; the inner edge of wrapping encircles the axon while the outer edge remains as a layer of cytoplasm and nucleus at the sheath edge.

  • Demyelination disorders slow or disrupt nerve signaling and can produce characteristic neurological deficits.

Disorders of Myelination: Demyelinating Diseases

• Multiple sclerosis (MS):

  • An autoimmune disease; antibodies target myelin in the CNS, causing inflammation and demyelination.

  • Sclerosis means hardening of tissue due to scar formation; in MS, multiple scars appear in the white matter of the brain and spinal cord.

  • Symptoms span both somatic (motor control, sensation) and autonomic systems (bladder control, etc.).

• Guillain–Barré syndrome (GBS):

  • Autoimmune demyelination of peripheral nerves (PNS).

  • Often presents with sensory symptoms or motor deficits; autonomic disturbances can affect heart rate and blood pressure, especially on standing (dizziness).

• General note on demyelination:

  • The underlying causes vary (genetic, pathogens, autoimmune), but the unifying outcome is reduced conduction velocity and impaired signal transmission.

Key Terms and Etymology to Remember

• Soma: cell body of a neuron.

• Dendrite: input extensions that receive synaptic signals.

• Axon: output projection that conducts impulses toward targets.

• Axon hillock: region where axon begins, rich in voltage-gated channels for action potential initiation.

• Axoplasm: cytoplasm within the axon.

• Initial segment: early axial portion where action potentials are generated.

• Trigger zone: combination of axon hillock and initial segment where action potentials are initiated.

• Node of Ranvier: gaps in myelin that enable saltatory conduction.

• Synaptic end bulb: enlarged axon terminal ending at the synapse.

• Pseudo-unipolar neuron: a sensory neuron with a single process that bifurcates; cell bodies located in ganglia.

• Purkinje cell: a distinctive large neuron in the cerebellum.

• Pyramidal cell: a pyramidal-shaped multipolar neuron.

• BBB: Blood–Brain Barrier, a selective barrier restricting CNS entry of many substances.

• Choroid plexus: the site where CSF is formed in the ventricles via ependymal cells.

• Myelin: lipid-rich sheath around axons that increases conduction speed, formed by oligodendrocytes (CNS) or Schwann cells (PNS).

Connections to Foundational Concepts and Real-World Relevance

• Structure–function relationship: Neuron morphology (soma, axon, dendrites) directly supports directional signaling and integration of information.

• Signal transmission efficiency: Myelination and nodes of Ranvier enable fast, energy-efficient conduction, essential for rapid reflexes and complex processing.

• Glial support and neural health: Glia regulate the extracellular milieu, protect neurons, supply nutrients, and form barriers that protect CNS integrity. Disruption can have broad consequences for neural function.

• BBB implications for therapy: CNS drug delivery requires strategies to cross the BBB, influencing treatment design for neurological diseases.

• Disease context: Demyelinating diseases illustrate how immune dysregulation can derail neural communication, underscoring the importance of intact myelination for motor, sensory, and autonomic functions.

Quick Reference: Numerical and Quantitative Details

• Axon diameter range: $d_{ ext{axon}} ext{ in } [1, ext{ to } 20] \ \, \mu\mathrm{m}.$

• Myelin segment length between nodes: $1 \, ext{mm} \le L_{ ext{myelin}} \le 2 \, ext{mm}.$

• Myelin length-to-diameter ratio: $100 \le \frac{L<em>{ ext{myelin}}}{d</em>{ ext{axon}}} \le 1000.$

Summary of Core Concepts

• Neurons are the signaling units of the nervous system, with a defined polarity and specialized regions for signal reception and transmission.

• Glial cells provide essential support, insulation, immune defense, and barrier functions; they are integral to CNS and PNS health.

• Myelination accelerates conduction and is critical for rapid and synchronized neural signaling; disruptions lead to significant neurologic deficits.

• The BBB is a double-edged feature: protective, yet a major consideration for pharmacology and therapeutics in neurology.

• Demyelinating diseases like MS and GBS illustrate the consequences of disrupted myelin on nervous system function and the immune system’s role in neural pathology.