12-6

Synapses and Their Function in Neuronal Communication

Introduction to Synapses

  • Synapses transmit signals among neurons or between neurons and other cells.
  • Learning Outcome: Describe the structure of a synapse and explain the mechanism involved in synaptic activity.
  • In the nervous system, messages are propagated in the form of action potentials along axons.
  • Transfer of messages occurs at the synapse, which is a specialized site for communication between neurons.
  • At synapses between two neurons, information passes from the presynaptic neuron to the postsynaptic neuron.
  • Synapses may also involve other types of postsynaptic cells (e.g., neuromuscular junction).

Types of Synapses

  • Two Main Types of Synapses:
    1. Electrical Synapses
    2. Chemical Synapses
Electrical Synapses
  • Definition: Direct physical contact occurs between cells; presynaptic and postsynaptic membranes are interconnected at gap junctions.
  • Structure:
    • The intercellular gap between the membranes is about 2 nanometers (0.002 micrometers).
    • Integral membrane proteins called connexins form pores that allow ions to move between cells.
  • Function:
    • Changes in membrane potential of one cell create local currents affecting the other cell, allowing for quick and efficient transmission of action potentials.
  • Occurrence:
    • Extremely rare in adult CNS and PNS, but present in certain brain areas (e.g., vestibular nuclei for balance) and specific ganglia (ciliary ganglia for pupil constriction).
Chemical Synapses
  • Definition: A communication occurs through chemical messengers (neurotransmitters) released by one neuron to another.
  • Structure:
    • Involves two cells: the presynaptic cell (axon terminal) and the postsynaptic cell.
    • A narrow synaptic cleft separates the two cells, averaging 20 nanometers (0.02 micrometers) in width.
  • Types of Chemical Synapses:
    • Axoaxonic Synapse: Between the axons of two neurons.
    • Axosomatic Synapse: Junction at the axon terminal of one neuron with the cell body of another.
    • Axodendritic Synapse: Contact between the axon terminal of one neuron and the dendrite of another neuron.
    • Neuromuscular Junction: Between a neuron and a skeletal muscle cell.
    • Neuroglandular Junction: A neuron regulating a secretory gland cell.

Mechanism of Chemical Synapse Activity

Neurotransmitter Release
  • Process:
    • The axon terminal of the presynaptic cell releases neurotransmitters into the synaptic cleft.
    • Neurotransmitters are stored in synaptic vesicles within the axon terminal.
  • Structure Variation:
    • Axon terminals differ based on the type of postsynaptic cell; for example, neuromuscular junctions have complex structures with many vesicles and mitochondria.
  • Function of Chemical Synapses:
    • Communication typically occurs in a single direction from presynaptic to postsynaptic cell.
Steps in Chemical Synaptic Transmission
  1. Action Potential Arrival:
    • Arrival of an action potential depolarizes the presynaptic membrane.
  2. Calcium Ion Influx:
    • Voltage-gated calcium channels open, allowing calcium ions (Ca²⁺) to enter the presynaptic terminal.
  3. Neurotransmitter Release:
    • Increased Ca²⁺ triggers exocytosis of synaptic vesicles containing neurotransmitters into the synaptic cleft.
  4. Diffusion and Binding:
    • Neurotransmitters diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.
    • This process is known as synaptic delay, lasting 0.2 to 0.5 milliseconds.
  5. Graded Potentials:
    • Binding changes postsynaptic membrane permeability, contributing to graded potentials.
  6. Signal Processing:
    • If a graded potential reaches the threshold, an action potential is generated in the postsynaptic neuron.
Specific Example: Cholinergic Synapses
  • Definition: Cholinergic synapses release acetylcholine (ACh), the most studied neurotransmitter.
  • Occurrence:
    • Present at all:
    1. Neuromuscular junctions with skeletal muscle.
    2. Many synapses in the CNS.
    3. Neuron-to-neuron synapses in the PNS.
    4. Neuromuscular and neuroglandular junctions in the parasympathetic division of the ANS.
  • Mechanism at Cholinergic Synapses:
    • Calcium Ion Channels: Voltage-gated channels allow Ca²⁺ influx.
    • Release Dynamics: Roughly 3000 ACh molecules are released from each vesicle during synaptic transmission.
    • Enzyme Role: Acetylcholinesterase (AChE) breaks down ACh into acetate and choline to terminate the signal.
    • Reuptake: Choline is reabsorbed into the axon terminal for ACh resynthesis.
Importance of Synaptic Delay and Fatigue
  • Synaptic Delay:
    • Refers to the time between arrival at the presynaptic terminal and effect on the postsynaptic membrane, primarily due to calcium influx and neurotransmitter release.
    • Important for understanding reflex responses; fewer synapses equal faster responses.
  • Synaptic Fatigue:
    • Occurs when synaptic activity is high and the demand for neurotransmitter exceeds synthesis and transport capacity.
    • The synaptic response diminishes until ACh pools are replenished.

Key Questions

  1. Describe the general structure of a synapse:
    • A presynaptic cell and a postsynaptic cell separated by a narrow synaptic cleft.
  2. Type of synapse with direct contact:
    • Electrical synapse.
  3. Type of synapse involving neurotransmitters:
    • Chemical synapse.
  4. Impact of blocking voltage-gated calcium channels:
    • Prevents Ca²⁺ entry, inhibiting ACh release and synaptic communication.
  5. Pathways with fewer neurons transmit impulses more rapidly due to:
    • Less synaptic delay; hence faster impulse transmission.