Integration of Synaptic Potentials and Introduction to Neural Networks

Question 1

Definition of action potential

Answer 1

Carry signals from point to point along the axon

Question 2

Definition of synaptic potential

Answer 2

Depolarisation/hyperpolarisation of the post synaptic membrane by a NT

Question 3

Definition of convergence

Answer 3

Many presynaptic neurones send impulses to 1 postsynaptic neurone to receive an integrated signal

Question 4

Definition of divergence

Answer 4

1 neurone will send its input to many other neurones

Question 5

What is the

• role
• characteristics
• strengths
• weaknesess of action potentials

Answer 5

Role
-Carry signal from point to point along the axon

Characteristics

• Coordination of VGNC and VGKC
• All or nothing response
• 2-5ms

Strengths

• Signal size maintained over distance and axonal branches
• Versatility of info coding within AP

Weakness

• Hyperpolarisation needed before AP firing
• Refractory period limits rate

Question 6

Describe the pathway of an excitatory synaptic potential

Answer 6

Arrival of an AP in the presynaptic membrane
Fusion of vesicle and exocytosis of NT into synaptic cleft and diffuses
After delay, NT binds to cognate receptors
Non selective cation channels open => influx of Na
NT unbinds and removed, rate of ion influx decreases and intracellular mV decreases

Question 7

Why does Na diffuse into the post synaptic membrane when the non selective cation channels are open

Answer 7

Intracellular membrane potential is -70mV
Prevents K+ from leaving and making the membrane potential more negative

Na enters => depolarisation

Question 8

How is the rate of decay of synaptic potential determined

Answer 8

Mainly due to unbinding, diffusion and uptake by glial cells

Other mechanisms function

• Glutamate + AMPA receptor => EPSP
• decay due to AMPA receptor desensitisation

-decay in cholinergic synapses due to cholinesterase in Ach on post synaptic membrane

Depends on NT

Question 9

What does amplitude of EPSPs depend on

What does decay of EPSPs depend on

Answer 9

Amplitude

• amount of NT released
• no of receptors
• state of receptors

Decay

• dissociation of ligand
• diffusion and uptake
• can also depend on desensitisation and enzymatic destruction

Question 10

What are the properties of EPSPs

• role
• characteristic features
• advantages
• disadvantages

Answer 10

Role
-synaptic potentials that contribute to somatic depolarisation => AP generation at axon hillock

Characteristic features

• graded, NOT ALL OR NOTHING
• normally fast (slow ones do exist)
• fast EPSPs normally due to activation of ligand gated non specific cation channels

Advantages

• versatility, different NT act on same post synaptic cell with different receptors
• different receptors/ion channels controlled independently
• independent post/presynaptic control of synaptic strength

Disadvantages

• metabolically expensive
• vulnerable to chemical attack from drugs and toxins

Question 11

Describe the pathway for IPSPs

Answer 11

Arrival of an AP in the presynaptic membrane
Fusion of vesicle and exocytosis of NT into synaptic cleft and diffuses
After delay, NT binds to cognate receptors
Non selective anion channels open => influx of Cl
NT unbinds and removed, rate of ion influx decreases and intracellular mV increases

Question 12

What are the properties of the IPSPs

• role
• characteristic features
• advantages
• disadvantages

Answer 12

Role
-synaptic potentials that contribute to somatic depolarisation => AP generation at axon hillock

Characteristic features

• graded, NOT ALL OR NOTHING
• normally fast (slow ones do exist)
• fast IPSPs normally due to activation of ligand gated non specific anion channels

Advantages

• versatility, different NT act on same post synaptic cell with different receptors
• different receptors/ion channels controlled independently
• independent post/presynaptic control of synaptic strength

Disadvantages

• metabolically expensive
• vulnerable to chemical attack from drugs and toxins

Question 13

Describe how EPSPs and IPSPs converge

Answer 13

Different presynaptic neurones send inputs to 1 postsynaptic neurone

Neurone will fire when the EPSPs and IPSPs add up to cause sufficient depolarisation of axon hillock => AP
Receptor dictates if synaptic potential is an EPSP or IPSP

Question 14

Describe convergence and divergence of neurones within nuclei

Answer 14

Nuclei within nervous structures receive a variety of diverging fibres
Each nuclei can integrate signals from multiple inputs
Integrated output signals diverge to provide inputs to the next level of the heiracrchy of nuclei

Question 15

Describe the importance of interneurones

Answer 15

Acuity of sensation compromised due to the divergence of sensory signals at higher levels
Acuity restored by surround inhibition by negative feedback mediated by inhibitory interneurons

CONTROLS DIVERGENCE

Question 16

What are the 3 type of interneurone configurations

Answer 16

In complex networks, configurations of interneurones control the amount of divergence within a neuronal circuit

1. feed forward inhibition
   - prevent divergence in other 2nd order neurones from sensory afferent
2. feed back inhibition
   - prevents other 2nd order neurones from sending sensory info
3. recurrent inhibition
   - inhibits further inputs from being sent on itself (self inhibition)