02. Principles of circuit organisation Flashcards
What is a myotatic reflex?
A muscle stretch reflex
What neurons are involved in the knee-jerk response (and all other myotatic reflexes)?
3 neurons: motor neuron (extensor), interneuron (communicates between the extensor and flexor) and motor neuron (flexor)
What are the steps of the knee-jerk response?
- Hammer tap stretches tendon which stretches sensory receptors in leg extensor muscles
- Sensory neuron excites motor neuron for the extensor muscles in the spinal cord & spinal interneuron -> the interneuron synapse inhibits the motor neuron for the flexor muscles
- The motor neuron conducts action potential to synapses on extensor muscle fibres, causing contraction -> Flexor muscle relaxes because the activity of its motor neurons has been inhibited
- Leg extends
What is happening electrically during a knee-jerk reflex?
Action potentials (‘spikes’) are recorded in all the involved neurons. The reflex triggers an EPSP in the extensor and an IPSP in the flexor
Relative frequency of APs in the different neurons
- The hammer tap occurs
- Activity (extracellular spikes) increases in the sensory (afferent) neuron, then decreases as the stimulus is no longer present
- After a delay, the motor neuron (extensor) and interneuron increase activity
- After a delay, activity ceases in the motor neuron (flexor)
Why are synapses important?
*Connect neurons, forming circuits
*Act as information filter
*Can change their properties (Long-Term Potentiation / Depression)
*Major sites for nervous dysfunction/targets for toxins
What are some properties of synapses
They provide a faithful graded transmission of sub-threshold input (positive/negative changes)
High-speed transmission
Bidirectional signalling
Limited flexibility
Advantages of Electrical Synapses (invertebrates and simple vertebrates)
There is a strong correlation between startle responses and e-synapses, indicating their main advantage is extremely fast transmission (compared to chemical synapses) which may be why they evolved in invertebrates and lower (simple) vertebrates
Advantages of Electrical Synapses (mammals)
Synchronisation of network oscillations
Visual information processing in the retina
Circadian rhythms in the suprachiasmic nucleus
(Synaptic delay is negligible in larger mammals, so e-synapses are not advantageous for speeding up processing time)
Advantages of Chemical Synapses
Allows flexibility of response to synaptic input
Can allow amplification and inhibitory responses
Why can chemical synapses have flexible responses?
Flexibility can be related to:
*The presynaptic release process
*Transmission
*Postsynaptic responsiveness.
What are Intrinsic / endogenous properties?
Certain neurons have intrinsic properties allowing them to fire in a certain way. E.g.:
Intrinsic properties: Endogenous bursting
Neuron displays ‘pacemaker’ activity (APs with no stimulation)
Intrinsic properties: Plateau
Persistent depolarised state. Once triggered, cell can fire APs without excitation
Intrinsic properties: Post-inhibitory rebound
Increased neuronal excitability (activity) after period of inhibition
Intrinsic properties: Spike adaptation
spike adaptation – frequency decreases during maintained depolarization
Simple ‘building blocks’ of larger circuits:
A Recurrent excitation -> neurons activate each other
B Mutual (reciprocal) inhibition -> neurons inhibit each other
C Recurrent inhibition -> neuron A excites neuron B, which inhibits neuron A
D Cyclic inhibition -> neuron A excites neuron B, which excites neuron C, which excites neuron A.
E Coupling by way of directly opposed electrotonic junctions
F Electrical coupling via presynaptic fibres
Monogamous voles and dopamine receptors
Dopamine receptors D1 and D2 exist in the neurons of the nucleus accumbens.
In a bachelor vole, dopamine activates receptor D2. D2 helps consolidate the link between a bachelor vole and his partner
In a bonded vole, dopamine activates receptor D1, not D2. As D2 can no longer be activated, no new bonds can be made, making the vole monogamous. If a new female vole arrives the bonded male vole will chase it off.
