Neural circuits basics

Question 1

What are the ingredients for circuit function?

Answer 1

1. Electrical activity of individual neurones
2. Synaptic connections
3. Intrinsic / endogenous properties
4. Simple ‘building blocks’ of larger circuits based on different types of synaptic connectivity
5. Different postsynaptic receptor types in different neurones for the same neurotransmitter.

Question 2

What is variation of the electrical activity of individual neurones?

Answer 2

Generation of action potentials.

Question 3

Outline the myotatic reflex (knee jerk).

Answer 3

1. Hammer tap stretches tendon, stretching sensory receptors in leg extensor muscle.
2. Sensory neurone excites motor neurone for extensor muscle.
3. Sensory neurone also excites interneurone in spinal cord.
4. Interneurone inhibits a motor neurone for flexor muscle.

Question 4

How can we prove how the myotatic reflex works through intracellular recordings?

Answer 4

• Micro electrode used to measure membrane potential of sensory neurone, interneurone and the 2 motor neurones.
• The sensory neurone action potential triggers subsequent excitatory post synaptic potentials (epsp) in interneurone and motor neurone that synapses on to the extensor muscle, and an ipsp on the motor neurone synapsing onto the flexor muscle.

Question 5

How can we prove how the myotatic reflex works through extracellular reco

Answer 5

• bipolar nerve recording electrodes measuring each neurone.
• measuring the relative frequency of APs in different the components.
• All neurones but the motor neurone synapsing onto the flexor muscle display an acceleration in spikes after sensory neurone innervates them.

Question 6

What is variation of the synaptic connectivity?

Answer 6

• electrical v chemical

Question 7

What is the importance of synapses?

Answer 7

• connect neurones to form circuits
• act as information filter based on individual properties
• can change properties (memory)
• sites for nervous dysfunction and toxins.

Question 8

Outline electrical synapse transmission.

Answer 8

• current always flows along path of least resistance
• faithful ‘graded’ transmission of sub-threshold input (generates output even when not spiking).
• high speed
• bidirectional
• faithful so limited flexibility. Can not amplify signal for example.
• e synapses are not used for the purpose of speeding up synaptic transmission as delay is negligible for chemical synapses in warm blooded vertebrates.
• direct transport of ions, metabolites and 2nd messengers. In chemical synapses there is no direct transport.

Question 9

Describe the function of chemical and electrical synapses together.

Answer 9

Both types of synapses may exist between the same pre and post synaptic neurones. Can work synergistically or antagonistically.

Question 10

What are the types of intrinsic / endogenous properties?Describe them too.

Answer 10

1. Endogenous bursting - pacemaker like activity. Periods of high frequency APs followed by a period of inactivity.
2. Plateau potentials - a persistent depolarised state and then once triggered, fires APs without continuous excitation.
3. Post inhibitory rebound - increased neuronal excitability after periods of hyperpolarisation (A depolarisation above normal resting state).
4. Spike adaptation - frequency of APs decreases during maintained depolarisation.