Neuroscience: Transmission Across a Synapse

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Questions and Answers

What is the primary function of norepinephrine in the body?

Inhibits brain synapse activity

Regulates sensory perception and temperature

Acts as a natural pain reliever

Prepares the body for stressful situations (correct)

Which statement correctly describes GABA's function in the central nervous system?

Produces a calming effect on the body (correct)

Regulates sensory perception

Enhances the ability of neurons to communicate

Acts as an excitatory neurotransmitter

What is the resting potential of a polarized neuron?

-50 mV

-90 mV

-70 mV (correct)

-60 mV

What happens to the Na+ and K+ ions during depolarization?

Na+ ions move inside and K+ ions move outside. Signup and view all the answers

What is the result of a neuron's hyperpolarization?

Inside becomes more negative than resting potential. Signup and view all the answers

During the refractory period, which ions are returned to their original positions?

K+ ions move outside and Na+ ions move back to their original positions. Signup and view all the answers

What initiates the release of neurotransmitters across a synapse?

Nerve impulses triggering chemical release. Signup and view all the answers

What is the function of the Na-K exchange pump?

To create resting potential by maintaining ion concentrations. Signup and view all the answers

In a synapse, what role do neurotransmitters play?

They bind with receptor proteins and carry signals. Signup and view all the answers

What effect does a synaptic cleft have on nerve impulses?

It allows neurotransmitters to diffuse between neurons. Signup and view all the answers

What is the primary role of cholinesterase in the synapse?

To break down acetylcholine Signup and view all the answers

What occurs during the process of summation in neurons?

Multiple impulses add together to reach threshold Signup and view all the answers

What effect do excitatory neurotransmitters have on the postsynaptic neuron?

They trigger Na+ channels to open Signup and view all the answers

What happens to the postsynaptic neuron after neurotransmitter signaling is complete?

It returns to resting potential Signup and view all the answers

How do inhibitory neurotransmitters affect the postsynaptic neuron?

They cause potassium ions to flow out Signup and view all the answers

Which statement about excitatory and inhibitory signals is true?

A neuron can receive both types of signals simultaneously Signup and view all the answers

What occurs if acetylcholine remains in the synapse without degradation?

Muscle cells continue to contract Signup and view all the answers

Which ion primarily causes depolarization in response to excitatory signals?

Sodium (Na+) Signup and view all the answers

Study Notes

Transmission Across a Synapse

Neurons are polarized, having a resting potential (-70mV), maintained by the Na-K exchange pump. This pump actively transports sodium ions out and potassium ions in.
Action potentials occur when a stimulus reaches a neuron, triggering a depolarization. sodium (Na+) channels open, and Na+ ions rush into the neuron if the threshold is reached. This is an "all-or-none" response.
The impulse travels down the axon, and myelinated axons transmit the impulse more quickly, jumping between nodes of Ranvier.
Depolarization causes Na+ channels to close and K+ channels to open, initiating repolarization. Potassium (K+) ions move out of the neuron, restoring the resting potential.
Hyperpolarization follows repolarization, the potential drops below resting potential briefly, as more K+ ions move out.
The refractory period is the recovery time for a neuron to return to its resting potential before another stimulus can trigger an action potential.
The Na-K exchange pump restores the neuron to its original resting potential.
Nerve impulses transmit via a domino effect, with each neuron passing the signal to the next. This involves chemical events where the signal is picked up by the dendrites and transmitted through the axon to the next neuron.

Synapse

Synapses (or synaptic clefts) are gaps between neurons or a neuron and another effector (like a muscle cell or gland).
Impulses travel from one end of a neuron to the opposite end (synaptic terminal).
Most synaptic terminals are not directly connected, but a gap (synaptic cleft) separates them.
Neurotransmitters bridge this gap, carrying signals across to the next neuron or effector cell.

Neurotransmitters

Neurotransmitters have an excitatory or inhibitory effect on the next neuron.
Excitatory neurotransmitters stimulate the brain, triggering receptor proteins to open sodium channels, allowing sodium to flow into the postsynaptic neuron, slightly depolarizing it. If depolarization reaches the threshold (-70mV), then an action potential is produced.
Inhibitory neurotransmitters trigger the flow of potassium out of the postsynaptic neuron, increasing its polarization (hyperpolarization).
A neuron can receive multiple signals simultaneously (both excitatory and inhibitory).
After producing their effects, neurotransmitters are broken down by enzymes in the cell or reabsorbed.

How Neurotransmitters function

Neurotransmitters are released from the synaptic vesicles after an impulse reaches the axon terminal.
Neurotransmitters bind to receptor proteins on the postsynaptic membrane, eliciting either an excitation or inhibition of the membrane.
After their effect, enzymes break down the neurotransmitter, removing it from the receptors and the synaptic cleft.
The ion channels then close returning the neuron to the resting potential.

Other Key Neurotransmitters

Acetylcholine is an excitatory neurotransmitter critical for neuromuscular junctions. It crosses the synapse, exciting the muscle cell membrane which causes depolarization. For a muscle to relax, acetylcholinesterase breaks down acetylcholine. If acetylcholine remains in the synapse the muscle cell would not repolarize. The ion channel would remain open, maintaining muscle contraction.
Norepinephrine: An excitatory hormone used by the brain and some autonomic neurons, working in conjunction with epinephrine to ready the body for stressful situations. Overproduction can lead to elevated blood pressure, anxiety and insomnia.
Dopamine: Affects brain synapses for controlling body movements. It is linked to sensations of pleasure. Excessive production can lead to schizophrenia, and insufficient levels can cause Parkinson's disease.
Serotonin: Regulates temperature and sensory perception and helps control mood. Inadequate amounts of serotonin are linked to depression.
Endorphins: Natural pain relievers that affect areas of the brain related to emotions. A deficit can increase the risk of alcoholism.
GABA (Gamma-aminobutyric acid): The most common inhibitory neurotransmitter in the central nervous system. It lessens the neuron's ability to send signals to other nerve cells and produces a calming effect.

How Drugs Affect the Brain

Drugs can interact with neurotransmitters by mimicking or blocking their actions, or affecting the way they are broken down or reabsorbed. These effects greatly impacts the brain and can have various consequences on the body.

Specific Example: Congenital Myasthenia Gravis

Congenital Myasthenia Gravis (MG) is a genetic disorder that causes a reduced number of acetylcholine receptors on muscle cells.
Treating congenital MG involves inhibiting the action of acetylcholinesterase– preventing the breakdown of acetylcholine. This allows acetylcholine to remain active for longer, enhancing transmission at the neuromuscular junction.

Additional Points

Summation is when multiple impulses add up to reach the threshold, triggering an action potential in a neuron.

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Description

Explore the intricacies of neuronal communication in this quiz about synaptic transmission. Delve into the processes of depolarization, repolarization, and the role of ion channels in action potentials. Test your knowledge of how neurons maintain their resting potential and the importance of the refractory period.