6.1-6.4 Cells of the Nervous System

6.1 Structure and Maintenance of Neurons

• Nervous System Divisions:

  • Central Nervous System (CNS): Brain and spinal cord; processes information, coordinates responses.

  • Peripheral Nervous System (PNS): Nerves connect CNS to body; carries sensory info to CNS, relays motor commands from CNS.

Neurons

• Neurons: Functional units; generate and transmit electrical signals.

• Generate action potentials: rapid changes in electrical potential across membrane.

• Electrical signals trigger neurotransmitter release for communication across synapse.

• Neurons integrate inputs from other neurons for complex decisions.

Glial Cells

• Glial cells: Non-neuronal cells supporting neurons with structure, insulation, and nutrition.

• Essential for neuron function, but don't directly participate in electrical communication.

Neuron Structure

• Neurons: cell body (soma), dendrites, and axons for receiving, integrating, and transmitting signals.

• Cell Body (Soma): Nucleus and ribosomes for protein synthesis, genetic information.

• Dendrites: Receive incoming information; increase surface area.

  • Dendritic spines: Increase surface area for receiving signals; form synapses.

• Axon: Carries outgoing signals.

  • Length: microns to over a meter.

  • Axon Hillock (Initial Segment): Generates electrical signals.

  • Collaterals: Branches of the axon.

  • Axon Terminal: Releases neurotransmitters; forms synapse.

  • Varicosities: bulging areas that release chemical messengers along axon.

Myelin Sheath

• Myelin sheaths: Insulate axons (20-200 layers of modified plasma membrane); speeds up signal conduction.

  • Oligodendrocytes: Glial cells in CNS; myelinate up to 40 axons.

  • Schwann Cells: Glial cells in PNS; myelinate 1-1.5 mm segments.

  • Nodes of Ranvier: Spaces between myelin sections; axon plasma membrane exposed.

• Myelin speeds up signal conduction and conserves energy; saltatory conduction.

Axonal Transport

• Axonal transport moves materials between cell body and axon terminals (up to 1 meter).

  • Microtubules: "Rails" for transport.

  • Kinesins: Anterograde transport (cell body to axon terminals).

  • Transports nutrients, enzymes, mitochondria, neurotransmitter vesicles.

  • Dyneins: Retrograde transport (axon terminals to cell body).

  • Transports recycled vesicles, growth factors, chemical signals.

  • Retrograde transport: Route for harmful agents (tetanus, herpes, rabies, polio) to invade CNS.

6.2 Functional Classes of Neurons

• Neuron Classes:

  • Afferent Neurons

  • Efferent Neurons

  • Interneurons

• For each afferent neuron, ~10 efferent neurons and ~200,000 interneurons.

  • Most neurons are interneurons.

Afferent Neurons

• Afferent neurons: Transmit information from tissues/organs to CNS; carry sensory information.

• Sensory receptors respond to physical/chemical changes.

• Receptor region: specialized membrane or separate cell.

• Single process divides into peripheral and central processes.

Efferent Neurons

• Efferent neurons: Convey information from CNS to effector cells; carry motor commands.

• Effector cells: muscle, gland, or other cell types.

• Cell bodies/dendrites in CNS, axons extend to periphery.

Interneurons

• Interneurons: Connect neurons within CNS; form complex circuits.

• 99% of all neurons; critical for higher-level processing.

• Number varies with action complexity; knee-jerk vs. memory recall.

Nerves

• Nerves: Afferent/efferent neuron axons in PNS; myelin, connective tissue, and blood vessels.

Table 6.1 Characteristics of Three Classes of Neurons

I. Afferent Neurons

• Transmit information into CNS from receptors.

• Single process splits: long peripheral (PNS axon), short central (CNS axon).

II. Efferent Neurons

• Transmit information out of CNS to effector cells.

• Cell body in CNS; most of axon in PNS.

III. Interneurons

• Function as integrators and signal changers.

• Integrate afferent/efferent neurons into reflex circuits.

• Lie entirely within CNS.

• > 99% of all neurons.

Synapses

• Synapse: Junction where one neuron alters electrical/chemical activity of another; communication sites.

  • Signal via neurotransmitters.

  • Neurotransmitters bind to receptors.

• Presynaptic Neuron: Conducts signal toward synapse.

• Postsynaptic Neuron: Conducts signal away from synapse.

  • Could have thousands of synaptic junctions on surface.

    • many pre synaptic neurons can affect it

6.3 Glial Cells

• Glial cells: > half of cells in human CNS; essential for nervous system function.

• Provide physical/metabolic support to neurons.

• Can divide throughout life.

Primary Types of Glial Cells

1. Oligodendrocytes: Form myelin sheath of CNS axons.

1. Astrocytes:

   • Regulate extracellular fluid composition.

     • removing potassiom ions + neurotransmitters around synapses

   • Stimulate tight junctions for blood-brain barrier.

   • Provide metabolic support.

     • provide glucose, remove secreted metabolic waste (ammonia)

   • Guide CNS neurons in embryos; secrete growth factors to stimulate nuronal growth.

   • Have neuron-like characteristics; participate in signaling.

     • ion channels

     • receptors for certain neurotransmitters + enzymes for processing them

     • capability of generating weak electrical responses

     • possible information signaling role

2. Microglia:

   • Macrophage-like

   • Perform immune functions in CNS.

   • Contribute to synapse remodeling and plasticity.

3. Ependymal Cells:

   • Line fluid-filled cavities in brain/spinal cord.

   • Regulate production/flow of cerebrospinal fluid.

4. Schwann Cells:

   • PNS glial cells; produce myelin sheath.

   • Similar properties to CNS glia.

6.4 Neural Growth and Regeneration

Growth and Development of Neurons

• Nervous system development: Begins with stem cell division into neurons/glia.

• Neuronal daughter cells: Differentiate, migrate, and extend axons/dendrites.

• Growth cone: Tip of extending axon; finds correct route and target.

• Axon guidance: Influenced by attracting, supporting, deflecting molecules.

  • Cell adhesion molecules: On glia and embryonic neurons.

  • Neurotrophic factors: Soluble growth factors.

• Synapse formation: Occurs upon reaching target.

• Early neural development: Vulnerable to alcohol, drugs, radiation, malnutrition, viruses.

  • Zika virus in moms → microcephaly in babies

• Neuron and synapse degeneration: 50-70% undergo apoptosis.

  • Function: Refines connectivity.

  • Memory: May explain lack of early childhood memories (before age 4).

• Plasticity: Brain's ability to modify structure and function.

  • Stimuli: Exercise and cognitive activities.

  • Mechanism: Remodeling synaptic connections; new neurons.

• Critical time windows: Vary with neural system.

  • Visual pathways: Impaired if no visual stimulation between 1-2 years.

  • Language learning: Easier in youth, slower after adolescence.

• Mature CNS: Basic circuits remain, but synaptic contacts change throughout life.

• Neurogenesis: Production of new neurons continues in some brain regions.

  • Stimuli: Cognitive stimulation and exercise.

  • Antidepressants: Effectiveness may depend on new neuron production.

Regeneration of Axons

• Axon repair: Possible outside CNS if cell body is unaffected.

• Process: Axon segment separates and degenerates, growth cone grows from cell body

• Rate: 1 mm per day.

  • Example: Thumb sensation may take 2 years to restore after shoulder injury.

• Spinal injuries: Typically crush tissue, causing oligodendrocyte apoptosis and demyelination.

• CNS axon regeneration: Limited; no significant return of function.

  • Reasons: Differences in CNS neurons, inhibitory factors from glia.

  • Evolutionary pressure: Limits growth to maintain network architecture.