Chapter 7: The Nervous System: Neurons and Synapses

Nervous System (NS)

• Is divided into:

  • Central nervous system (CNS)

    • = brain & spinal cord

  • Peripheral nervous system (PNS)

    • = cranial & spinal nerves

• Consists of 2 kinds of cells:

  • Neurons & supporting cells (= glial cells)

    • Neurons are functional units of NS.

    • Supporting cells maintain homeostasis.

      • Are 5X more common than neurons.

Neurons

• Gather & transmit information by:

  • Responding to stimuli.

  • Sending electrochemical impulses.

  • Releasing chemical messages.

• Have a cell body, dendrites, & axon.

  • Cell body contains nucleus.

• Cell body makes macromolecules.

• Groups of cell bodies in CNS are called nuclei; in PNS are called ganglia.

• Dendrites receive information, convey it to cell body.

• Axons conduct impulses away from cell body.

  

• Axon length necessitates special transport systems:

  • Axoplasmic flow moves soluble compounds toward nerve endings.

    • Via rhythmic contractions of axon.

  • Axonal transport moves large & insoluble compounds bidirectionally.

    • Along microtubules; very fast.

    • Viruses & toxins enter CNS this way.

Functional Classification of Neurons

• Sensory/Afferent neurons conduct impulses into CNS.

• Motor/Efferent neurons carry impulses out of CNS.

• Association/Interneurons integrate NS activity.

  • Located entirely inside CNS.

  

Structural Classification of Neurons

• Pseudounipolar:

  • Cell body sits along side of single process.

  • e.g. sensory neurons

• Bipolar:

  • Dendrite and axons arise from opposite ends of cell body.

  • e.g. retinal neurons/

• Multipolar:

  • Have many dendrites and one axon.

  • e.g. motor neurons.

  

Supporting/Glial Cells

• PNS has Schwann and satellite cells.

  • Schwann cells myelinate PNS axons.

  

• CNS has oligodendrocytes, microglia, astrocytes, and ependymal cells.

  

• Each oligodendrocyte myelinates several CNS axons.

• Ependymal cells are neural stem cells.

• Other glial cells are involved in NS maintenance.

  

Myelination

• In PNS each Schwann cell myelinates 1mm of 1 axon by wrapping round and round axon.

  • Electrically insulated axon.

  

• Uninsulated gap between adjacent Schwann cells is called node of Ranvier.

  

Nerve Regeneration

• Occurs much more readily in PNS than CNS.

  • Oligodendrocytes produce proteins that inhibit regrowth.

  • Neurotrophins.

• Promote fetal nerve growth.

• Required for survival of many adult neurons.

• Important in regeneration.

• When axon in PNS is severed:

  • Distal part of axon degenerates.

  • Schwann cells survive; form regeneration tube.

    • Tube releases chemicals that attract growing axon.

    • Tube guides regrowing axon to synaptic site.

  

Astrocytes

• Most common glial cell.

• Involved in:

  • Inducing capillaries to form blood-brain barrier.

  • Buffering K+ levels.

  • recycling neurotransmitters.

  • Regulating adult neurogenesis.

  • Maintain interstitial fluid.

  

Blood-Brain Barrier

• Allows only certain compounds to enter brain.

• Formed by capillary specializations in brain.

  • Capillaries are not as leaky as those in body.

    • Do not have gaps between adjacent cells.

      • Closed by tight junctions.

Resting Membrane Potential (RMP)

• At rest, all cells have a negative internal charge and unequal distribution of ions:

  • Results from:

    • Large anions being trapped inside cell.

    • Na+/K+ pump and limited permeability keep Na+ high outside cell.

    • K+ is very permeable and is high inside cell.

      • Attracted by negative charges inside.

Excitability

• Excitable cells can discharge their RMP quickly.

  • By rapid changes in permeability to ions.

  • Neurons and muscles do this to generate and conduct impulses.

Membrane Potential (MP) Changes

• Measured by placing 1 electrode inside cell and 1 outside.

• Depolarization occurs when MP becomes more positive.

• Hyperpolarization: MP becomes more negative than RMP.

• Repolarization: MP returns to RMP.

  

Membrane Ion Channels

• MP changes occur by ion flow through membrane channels.

  • Some channels are normally open; some closed.

  • Closed channels have molecular gates that can be opened.

    • Voltage-gated (VG) channels are opened by depolarization.

    • 1 type of K+ channel is always open; other type is VG and is closed in resting cell.

    • Na+ channels are VG; closed in resting cells.

The Action Potential (AP)

• Is a wave of MP change that sweeps along the axon from soma to synapse.

• Wave is formed by rapid depolarization of the membrane by Na+ influx; followed by rapid repolarization by K+ efflux.

• Depolarization causes more channels to open (positive feedback loop)

Mechanism of Action Potential

• Depolarization and repolarization occur via diffusion.

  • Do not require active transport.

  • After an Ap, Na+/K+ pump extrudes Na+, recovers K+.

APs Are All-or-None

• When MP reaches threshold, an AP is irreversibly fired.

  • Because positive feedback opens more and more Na+ channels.

  • Shortly after opening, Na+ channels close and become inactivated until repolarization.

Refractory Periods

• Absolute refractory period:

  • Membrane cannot produce another AP because Na+ channels are inactivated.

• Relative refractory period occurs when VG K+ channels are open, making it harder to depolarize to threshold.

  

Cable Properties

• Refers to ability of axon to conduct current.

• Axon cable properties are poor because:

  • Cytoplasm has high resistance.

  • Though resistance decreases as axon diameter increases.