(10) Synapse

synapse

A junction between two neurons or between a neuron and an effector (muscle/gland) where neural signals are transmitted.

Types of synapses

Electrical synapses (fast, direct ion flow) and Chemical synapses (most common, use neurotransmitters).

electrical synapse

Ions pass directly between neurons through gap junctions, allowing for fast and synchronized activity.

Locations of electrical synapses.

Cardiac muscle, some brain areas (breathing rhythm), and reflex circuits.

neurotransmitters at a chemical synapse

An action potential reaches the presynaptic terminal, opening voltage-gated Ca²⁺ channels.

calcium (Ca²⁺) in neurotransmitter release

Ca²⁺ enters the presynaptic terminal, triggering synaptic vesicle fusion with the membrane, leading to exocytosis of neurotransmitters.

SNARE proteins

proteins (Synaptobrevin, Syntaxin, SNAP-25) that help vesicles fuse with the presynaptic membrane to release neurotransmitters.

blocked SNARE proteins

Neurotransmitter release is prevented, leading to paralysis (e.g., Botox blocks SNARE function).

steps of synaptic transmission

1. Action potential arrives at presynaptic terminal. 2. Voltage-gated Ca²⁺ channels open, Ca²⁺ enters. 3. SNARE proteins cause vesicle fusion, releasing neurotransmitters. 4. Neurotransmitters diffuse across synaptic cleft. 5. Bind to postsynaptic receptors, causing a response.

factors that determine whether the postsynaptic neuron is excited or inhibited

The type of neurotransmitter and receptor it binds to.

main excitatory neurotransmitter

Glutamate (causes depolarization via Na⁺ influx).

primary inhibitory neurotransmitter

GABA (causes hyperpolarization via Cl⁻ influx).

neurotransmitter involved in muscle contraction and memory

Acetylcholine (ACh).

the neurotransmitter associated with motivation, movement, and pleasure.

Dopamine.

neurotransmitter regulates mood and is linked to depression

Serotonin.

two main categories of postsynaptic receptors

Ionotropic (fast, direct ion flow) and Metabotropic (slow, uses G-proteins).

ionotropic receptors

Neurotransmitters directly open ion channels, leading to fast synaptic responses.

metabotropic receptors.

Neurotransmitters activate G-proteins, leading to slower, widespread effects (e.g., second messengers).

Excitatory postsynaptic potential

depolarizes neuron, increasing chance of action potential

Inhibitory postsynaptic potential

hyperpolarizes neuron, decreasing chance of action potential

three main methods for neurotransmitter clearance from synapse

1. Reuptake into the presynaptic neuron (e.g., serotonin reuptake). 2. Enzymatic degradation (e.g., acetylcholinesterase breaks down ACh). 3. Diffusion away from the synaptic cleft.

medication preventing serotonin reuptake

SSRIs (e.g., Prozac), used for depression.

Parkinson’s disease

Loss of dopamine-producing neurons, leading to tremors and movement difficulties.

neurotransmitter disrupted in schizophrenia

Excess dopamine activity leads to hallucinations.

disorder linked to low levels of serotonin

Depression.

benzodiazepines (e.g., Valium)

Enhance GABA activity, leading to sedation & reduced anxiety.

How nicotine impacts the nervous system

Stimulates nicotinic ACh receptors, increasing dopamine release (addictive).

opioids

Bind opioid receptors, reducing pain signals.