Study Guide - The Nervous System

Study Guide - The Nervous System

The Synapse

  • A junction that mediates information transfer from one neuron:
    • To another neuron
    • To an effector cell

Terminology of Synapse

  • Presynaptic Neuron: Conducts impulses toward the synapse.
  • Postsynaptic Neuron: Transmits impulses away from the synapse.

Types of Synapses

  • Axodendritic: Between the axon of one neuron and the dendrite of another.
  • Axosomatic: Between the axon of one neuron and the soma of another.
  • Less Common Types:
    • Axoaxonic: Axon to axon.
    • Dendrodendritic: Dendrite to dendrite.
    • Dendrosomatic: Dendrite to soma.

Electrical Synapses

  • Prevalence: Less common than chemical synapses.
  • Mechanism: Neurons are electrically coupled (joined by gap junctions).
  • Communication: Very rapid, can be either unidirectional or bidirectional.
  • Importance: Significant in embryonic nervous tissue and certain brain regions.

Chemical Synapses

  • Specialized for the release and reception of neurotransmitters.
  • Composition: Typically composed of two parts:
    • Axon Terminal: Of the presynaptic neuron containing synaptic vesicles.
    • Receptor region: On the postsynaptic neuron.

Synaptic Cleft

  • Definition: Fluid-filled space separating the presynaptic and postsynaptic neurons.
  • Function: Prevents direct nerve impulse transmission from one neuron to the next.
  • Transmission:
    • Is a chemical event (as opposed to an electrical one).
    • Involves release, diffusion, and binding of neurotransmitters.
    • Ensures unidirectional communication between neurons.

Information Transfer

  • Event Sequence:
    • Action Potential (AP) arrives at the axon terminal of the presynaptic neuron, opening voltage-gated Ca2+ channels.
    • Synaptotagmin Protein: Binds Ca2+, promoting fusion of synaptic vesicles with the axon membrane.
    • Exocytosis: Occurs for neurotransmitter release.
  • Neurotransmitter Action:
    • Diffuses and binds to receptors on the postsynaptic neuron (often chemically gated ion channels).
    • Ion channels open, triggering excitatory or inhibitory events (graded potential).

Termination of Neurotransmitter Effects

  • Occurs within a few milliseconds via:
    • Degradation by enzymes.
    • Reuptake by astrocytes or axon terminal.
    • Diffusion away from the synaptic cleft.

Synaptic Delay

  • Definition: Time needed for neurotransmitter release, diffusion, and receptor binding (0.3-5.0 ms).
  • Significance: Synaptic delay is the rate-limiting step of neural transmission.

Postsynaptic Potentials

  • Definition: Graded potentials whose strength is determined by:
    • Amount of neurotransmitter released.
    • Time the neurotransmitter remains in the synaptic area.
  • Types of Postsynaptic Potentials:
    • EPSP (Excitatory Postsynaptic Potentials).
    • IPSP (Inhibitory Postsynaptic Potentials).

Excitatory Synapses and EPSPs

  • Mechanism: Neurotransmitter binds to and opens chemically gated channels allowing simultaneous flow of Na+ (influx) and K+ (efflux) in opposite directions.
  • Net Effect:
    • Na+ influx predominates over K+ efflux, resulting in net depolarization.
    • EPSP can help trigger AP at the axon hillock when threshold strength is reached.

Inhibitory Synapses and IPSPs

  • Mechanism: Neurotransmitter binds to channels for K+ or Cl-, causing hyperpolarization (the inner surface of the membrane becomes more negative).
  • Result: Reduces the postsynaptic neuron's ability to produce an action potential.

Integration: Summation

  • Definition: A single EPSP cannot induce an action potential.
  • Summation Types:
    • Temporal Summation: One or more presynaptic neurons transmit impulses in rapid-fire order.
    • Spatial Summation: Postsynaptic neuron is stimulated by a large number of terminals simultaneously.

Integration: Synaptic Potentiation

  • Process: Repeated use increases efficiency of neurotransmission.
  • Mechanism: Increased Ca2+ concentration in both presynaptic and postsynaptic terminals.
  • Effect: Brief high-frequency stimulation partially depolarizes postsynaptic neuron.
  • Chemically gated channels (e.g., NMDA receptors) allow Ca2+ entry which activates kinase enzymes, promoting better responses to subsequent stimuli.

Integration: Presynaptic Inhibition

  • Mechanism: Release of an excitatory neurotransmitter by one neuron may be inhibited by another neuron via an axoaxonic synapse, leading to reduced neurotransmitter release and smaller EPSPs.

Neurotransmitters

  • Production: Most neurons produce two or more neurotransmitters, released at different stimulation frequencies.
  • Classifications: Over 50 neurotransmitters identified, categorized by chemical structure and function.

Chemical Classes of Neurotransmitters

  • Acetylcholine (ACh):

    • Function: Released at neuromuscular junctions and some ANS neurons; synthesized by the enzyme choline acetyltransferase; degraded by acetylcholinesterase (AChE).

  • Biogenic Amines Include:

    • Catecholamines: Dopamine, norepinephrine (NE), and epinephrine.
    • Indolamines: Serotonin and histamine, widely distributed in the brain, involved in emotional behavior and biological rhythms.

  • Amino Acids Include:

    • GABA (Gamma-Aminobutyric Acid)
    • Glycine
    • Aspartate
    • Glutamate

  • Peptides (Neuropeptides) Include:

    • Substance P: Mediates pain signals.
    • Endorphins: Natural opiates reducing pain perception.
    • Gut-brain peptides (e.g., somatostatin and cholecystokinin).

  • Purines (such as ATP): Act in both CNS and PNS; cause fast or slow responses; induce Ca2+ influx in astrocytes and provoke pain sensation.

  • Gases and Lipids

    • Nitric Oxide (NO): Synthesized on demand; activates intracellular receptor guanylyl cyclase, involved in learning and memory.
    • Carbon Monoxide (CO): Regulates cGMP in the brain.
    • Endocannabinoids: Lipid-soluble, synthesized from membrane lipids; bind to G protein-coupled receptors in the brain; involved in learning and memory.

Functional Classification of Neurotransmitters

  • Effects may be excitatary (depolarizing) or inhibitory (hyperpolarizing);
    • Determined by the receptor type of the postsynaptic neuron.
    • Inhibitory Examples: GABA and Glycine.
    • Excitatory Example: Glutamate.
    • Acetylcholine: Excitatory in skeletal muscle, but inhibitory in cardiac muscle.

Neurotransmitter Actions

  • Direct Action:

    • Neurotransmitter binds to channel-linked receptor, opening ion channels for rapid responses (examples: ACh and amino acids).

  • Indirect Action:

    • Neurotransmitter binds to a G protein-linked receptor, induces long-lasting effects through intracellular second messengers (examples: biogenic amines, neuropeptides, and dissolved gases).

Neurotransmitter Receptors

  • Types:

    1. Channel-linked Receptors:
    • Ligand-gated ion channels.
    • Action is immediate and brief.
    • Excitatory receptors allow small cation influx, predominantly Na+.
    • Inhibitory receptors permit Cl- influx or K+ efflux, resulting in hyperpolarization.
    1. G Protein-linked Receptors:
    • Transmembrane protein complexes.
    • Responses are indirect, slow, complex, and often widespread.
    • Examples include muscarinic ACh receptors and those that bind biogenic amines and neuropeptides.

G Protein-Linked Receptors: Mechanism

  1. Neurotransmitter binds to G protein-linked receptor.
  2. G protein is activated.
  3. Activated G protein controls production of second messengers (e.g., cyclic AMP, cyclic GMP, diacylglycerol, Ca2+).
Second Messengers
  • Function:
    • Open or close ion channels.
    • Activate kinase enzymes.
    • Phosphorylate channel proteins.
    • Activate genes and induce protein synthesis.

Neural Integration: Neuronal Pools

  • Definition: Functional groups of neurons that integrate incoming information and forward the processed information to different destinations.

Types of Neuronal Pools

  • Simple Neuronal Pool: A single presynaptic fiber that branches and synapses with several neurons within the pool.
    • Discharge Zone: Neurons closely associated with incoming fiber.
    • Facilitated Zone: Neurons more distantly located from incoming fiber.
Types of Circuits in Neuronal Pools
  • Diverging Circuit: One incoming fiber stimulates an increasing number of fibers (common in sensory and motor systems).
  • Converging Circuit: Opposite of diverging circuits, leading to strong stimulation or inhibition (common in sensory and motor systems).
  • Reverberating Circuit: Chain of neurons with collateral synapses to previous members.
  • Parallel After-Discharge Circuit: Incoming fiber stimulates several neurons in parallel to a common output cell.

Patterns of Neural Processing

  • Serial Processing: Input travels along one pathway to a specific destination, produces specific responses in an all-or-none manner (e.g., reflexes).
  • Parallel Processing: Input travels along multiple pathways, one stimulus prompts multiple responses, important for higher-level functions (e.g., a smell reminding someone of associated experiences).

Axonal Growth

  • Mechanism: Growth cone at the axon's tip interacts with the environment via:
    • Cell surface adhesion proteins (laminin, integrin, and nerve cell adhesion molecules or N-CAMs).
    • Neurotropins which attract or repel the growth cone.
    • Example: Nerve Growth Factor (NGF) keeps the neuroblast alive.

Support for Axonal Growth

  • Role of Astrocytes: Provide physical support and cholesterol crucial for the construction of synapses.

Cell Death

  • About two-thirds of neurons die before birth due to:
    • Failure to make functional synaptic contacts.
    • Apoptosis (programmed cell death) during development.