Synapses lect 18

Synapse

The gap between the presynaptic and postsynaptic cells where neurotransmitters are released. Includes neurons transferring and space between them

Presynaptic cell (2)

The neuron that sends the signal, signal comes from this cell.

Synthesizes and stores neurotransmitters.

Postsynaptic cell

The neuron that receives the signal and begins the action potential.

Excitatory Postsynaptic Potential (EPSP) (4)

• Channels open: Na⁺/K⁺

• Na⁺ ions flow in

• depolarization (more +)

• More likely to fire AP at axon hillock

Inhibitory Postsynaptic Potential (IPSP) (4)

• Channels Open: K⁺ or Cl⁻

• K⁺ out or Cl⁻ in

• Hyperpolarization (more -)

• Less likely to fire AP at axon hillock

Neurotransmitter

Small chemical signals that cross synapse, bind to specific receptors

Axon hillock (4)

The area of the neuron where the cell body connects to the axon.

Its job is to integrate (add up) all the EPSPs and IPSPs coming from the dendrites and cell body.

It "decides" whether the neuron should fire an action potential (AP) or not.
If the sum of EPSPs minus IPSPs is greater than a critical threshold voltage (usually around -55 mV),→ An action potential is triggered!

Long-term potentiation (LTP)

The strengthening of synapse transmission that occurs due to high-frequency action potentials and other conditions.

Ligand-gated ion channels

Ion channels that open in response to the binding of a neurotransmitter.

Glutamate

A common neurotransmitter that plays a vital role in neural activation and signaling.

Neurotransmitter diffusion

The process by which neurotransmitters cross the synaptic cleft and bind to receptor sites.

Membrane potential

The voltage difference across a cell membrane, influencing a neuron's excitability.

Synaptic Cleft

Small gap or space that physically separates the presynaptic and postsynaptic membranes within a synapse (~20nm)

1. Gene expression

2. Signal Transduction

How does the nervous system form what guides it? (2)

Gene Expression

• Specific genes are turned on or off in different cells.

• This controls what kind of neuron or glial cell (support cells) a particular cell will become.

Signal Transduction

• Cells respond to chemical signals from other cells or the environment.

• These signals tell cells to grow, move, differentiate, or connect.

Neuronal remodeling

Changes in neurons before birth.

Neuronal Plasticity

The brain's ability to change based on experience after birth.

"Use it or lose it" - reinforced through experiences

(ex. practicing a skill)

Short Term Memory (2)

Memory accessed by temporary links made between neurons, mainly in the hippocampus. Held for a short time

Long Term Memory

1. Information Stored Permanently in cerebral cortex bc new synapses(connections) were physically made

2. SLEEP consolidates memories

ex. remembering how to ride a bike

Long Term Potentiation

long-lasting strengthening of the connection (synapse) between two neurons. After strong repeated use, a synapse becomes stronger and faster → so neurons communicate more easily

Pre-Conditions of LTP in presynaptic neuron (2)

1. High frequency of action potentials (APs) — lots of firing close together.

   (Meaning the presynaptic neuron is very active!)

2. Depolarization from a second stimulus — the postsynaptic cell must already be partially depolarized when new signals arrive.

together these strengthen synapse

Before LTP - postsynaptic neuron ligand-gated ion channels + Function (2)

1. AMPA Receptors - Let Na⁺ into cell (depolarizes cell)

2. NMDA Receptors - Normally blocked by Mg²⁺

Before LTP (Process)

1. Glutamate (a neurotransmitter) is released from the presynaptic neuron.

2. It binds to: NMDA receptors (in the postsynaptic membrane).

   BUT! The NMDA receptor is blocked by Mg²⁺ ions, so no ion flow through NMDA yet.

Steps of LTP (6) + result

1. High-frequency stimulation causes lots of glutamate release.

2. Postsynaptic neuron already depolarized a bit (from AMPA activation).

3. Depolarization removes Mg²⁺ block from NMDA receptors.

4. NMDA channels open!

5. Na⁺ and Ca²⁺ ions rush into the postsynaptic neuron.

6. Ca²⁺ is especially important because it acts as a second messenger to trigger more changes inside the cell.

Result:

More AMPA receptors added to the membrane = stronger, faster depolarization next time.

Sensory Pathway (4) in order

1. Sensory Reception

2. Sensory Transduction

3. Transmission

4. Perception

1. Sensory Reception Function

Detect a stimulus

A stimulus (like light, sound waves, chemicals, or pressure) is detected by sensory receptor cells located in sense organs (eyes, ears, nose, skin, tongue, etc.).

Sensory Transduction Function

Converting stimulus into a receptor potential - (converting external signal into internal signal) (When the receptor is stimulated, its membrane potential changes this is called receptor potential)

3. Transmission

(Sending the signal)

If the receptor potential is big enough, it triggers an action potential (AP) in the sensory neuron.

Impt: Stronger stimulus → larger receptor potential → more frequent action potentials

4. Perception

Brain interprets the information

Types of Sensory Receptors (3) and what they detect

1. Chemoreceptors - taste and smell

2. Mechanoreceptors - touch, hearing, and equilibrium

3. Photoreceptors - light and seeing

Functions of the ear (2)

1. Hearing - sense changes in external vibrations

2. Equilibrium - balance controlled by sensing movement of fluid in ears

Hearing

Ability to sense changes in pressure. Sense changes in external vibrations

Sound Waves

Waves of air or water pressure.They are transmitted through the ear by a series of vibrating parts that change shape as they receive these.

Outer ear parts (3)

1. Pinna

2. Auditory Canal

3. Tympanic Membrane (ear drum)

Pinna

Collect sound waves

Auditory Canal

channels waves to tympanic membrane

Tympanic Membrane

Thin membrane - vibrates → transmits wave to middle ear. Separates outer ear from middle

Middle Ear + what its contains

Air Filled cavity

Contains: Ossicles (3 smalls bones)

Ossicles

Amplify sound, transmit to oval window→ Transmit vibration to inner ear

Inner ear

Cochlea - spiral tube where sound is detected and has 3 fluid filled chambers

Perilymph

Fluid in cochlea

Cochlea

Contains hair cells with cilia which are mechanoreceptors, they vibrate against membrane and trigger nerve signal

Converting Sound to Hearing

hair cells stimulated → depolarization → AP

Depolarization triggers the release of neurotransmitters onto sensory neurons

Neurotransmitters bind to receptors on the sensory neurons.

These neurons generate action potentials (APs).

Their axons bundle together to form the auditory nerve