Nervous System Physiology Essentials 3

Nervous System Physiology Essentials 3

• The connection between two cells is called a synapse.
• In the central nervous system (CNS), both cells are neurons.
• In the peripheral nervous system, the second cell can be a neuron, a muscle cell, or a gland cell.
• The synapse between a neuron and a muscle cell is called a neuromuscular junction.
• The cell before the synapse is the presynaptic cell.
• The cell behind the synapse is the postsynaptic cell.

Synapse Types

• Synapses are connections that transmit information between neurons.
• They are categorized into different types based on function and structural features.
• The main types of synapses are electrical and chemical synapses.
• Synapses can also be categorized by their structural features: axodendritic, axosomatic, and axoaxonic synapses.
• Or by their functional features: excitatory and inhibitory synapses.

Electrical Synapse

• Electrical synapses establish a direct connection between neurons, allowing ions to flow.
• These provide a rapid transmission of information.
• Ions are supplied by proteins called gap junctions.
• Gap junctions create a small space between cells (24 nm), much shorter than a chemical synapse.
• A syncytium is formed between the cells, allowing for simultaneous stimulation.
• Nerve conduction is based on ion balance.
• Chemical substances are not needed to initiate conduction.
• These connections are common in smooth muscle and heart muscle.

Chemical Synapse

• Chemical synapses are more common than electrical synapses.
• A physical space, the synaptic cleft, separates the nerve endings.
• The synaptic cleft is 20-40 nm wide.
• Chemical transmission begins at the axon end of the presynaptic neuron.
• When an action potential arrives, neurotransmitters are released into the synaptic cleft by calcium entering the cell.
• These neurotransmitters bind to receptors on the postsynaptic neuron, triggering a response.
• The entire postsynaptic structure is called the motor endplate.

Neurotransmitters

• Neurotransmitters are chemical signals used by nerve cells to transmit information between each other.
• They are stored in synaptic vesicles and released when an action potential reaches the presynaptic neuron.
• They bind to receptors on the postsynaptic cell, triggering a response.
• Examples include glutamate (excitatory), GABA (inhibitory), and acetylcholine.

Conditions Affecting the Neuromuscular Junction

• Diseases and conditions can affect the neuromuscular junction, such as Lambert-Eaton syndrome, Botulism, Tetanus, Myasthenia Gravis, and Curare Effect.
• In these conditions there are problems in acetylcholine release or reception, or the structure of the connection.

Botulinum Toxin

• Botulinum toxin is a potent neurotoxin produced by Clostridium botulinum.
• It blocks the release of acetylcholine, which inhibits muscle contraction.
• It's used medically to reduce muscle spasms, migraines, and other conditions.

Common Ways to Be Infected with Botulinum Toxin

• Botulism can infect people through contaminated food (often homemade canned food), and less commonly through honey.

Tetanus

• Tetanus toxin destroys synaptobrevin.
• It's produced by Clostridium tetani.
• This causes a blockade of inhibitory interneurons.
• Resulting in muscle contraction and spastic paralysis.

Lambert-Eaton Syndrome

• This is a rare autoimmune disease.
• Autoantibodies destroy a specific subtype of calcium channels.
• This reduces calcium-dependent acetylcholine release.
• Leads to muscle weakness.

Myasthenia Gravis

• Autoantibodies form against acetylcholine receptors (AChR).
• This leads to the accumulation of acetylcholine in the synaptic cleft.
• Results in delayed neurotransmission.

Curare Effect

• Curare is a natural compound from tropical plants used as a muscle relaxant.
• It competes with acetylcholine, blocking receptors.
• This causes muscle paralysis.
• It often affects respiratory muscles leading to asphyxy.

EPSP and IPSP

• EPSPs (excitatory postsynaptic potentials) increase the likelihood of producing an action potential in the receiving neuron.
• IPSPs (inhibitory postsynaptic potentials) make the receiving neuron less likely to fire.
• Summation of EPSP and IPSP influences the firing of the receiving neuron.

Summation

• Temporal summation occurs when successive stimuli arrive rapidly at the synapse.
• Spatial summation occurs when stimuli from multiple synapses act on the same post-synaptic neuron to generate a response.

Convergence and Divergence

• Convergence is when multiple presynaptic neurons connect to a single postsynaptic neuron.
• Divergence is when one presynaptic neuron connects to multiple postsynaptic neurons.

V-SNARE and T-SNARE Proteins

• These are essential in the release of neurotransmitters.
• V-SNARE proteins are found on the vesicle membranes.
• T-SNARE proteins are found on the target cell membranes.