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Neurobiology > Neural circuits > Flashcards

Neural circuits Flashcards

1
Q

What are neural circuits and why are they important?

A

One neural circuit is many neurons interconnected via synapses. Many neural circuits interconnect to form brain networks.

Neural circuits control complex behaviours, such as motor co-ordination and memory.

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2
Q

What are the 2 types of neurons in neural circuits?

A
  • Projection neurons

- Interneurons

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3
Q

What are the properties of projection neurons?

A
  • They receive information in one area of the nervous system and transmit this information to a distinct part.
  • They have long axonal projections which allows them to transmit info across different areas of the nervous system.
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4
Q

What are the properties of interneurons?

A
  • They receive and transmit information within the same anatomical region of the nervous system.
  • They connect sensory and motor neurons.
  • They have many dendrites and one axon.
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5
Q

What type of receptors do projection and interneurons use?

A

Both neurons use ionotropic receptors for fast transmission and GPCRs for slow modulatory transmission.

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6
Q

Describe the knee jerk reflex

A
  1. The knee is struck with a reflex hammer.

2. The leg is jerked and extends up.

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7
Q

Explain the knee jerk reflex by what occurs in the spinal cord

A
  • When the knee is hit, this causes the tendon in the quadricep to stretch. This is sensed by stretch receptors in the quadricep.
  • These receptors activate sensory neurons which take information into the spinal cord.
  • In the spinal cord, the sensory neurons send glutamate to activate motor neurons controlling the quadricep.
  • The motor neuron causes the quadricep to contract via Ach. This causes the leg to extend.
  • The activated sensory neurons in the spinal cord also activate a local inhibitory interneuron 1a via GABA. These interneurons release GABA onto motor neurons which act upon the flexor muscle. This inhibits contraction of the flexor muscle, so only the quadricep contracts.
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8
Q

What do Renshaw cells do and what neurons do they act upon?

A
  • Renshaw cells are interneurons.

- They send glycine to the motor neurons which excite them, so inhibit the motor neurons.

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9
Q

Explain how the Renshaw cell works in the knee jerk reflex

A
  • The motor neuron acting on the flexor muscle sends an activating signal to the Renshaw cell.
  • The Renshaw cell releases glycine onto the same motor neuron, which is inhibitory.
  • The Renshaw cell also sends an inhibitory signal to the 1a inhibitory interneuron which acts upon the flexor muscle. This means that disinhibition occurs and the flexor muscle remains relaxed.

This is a negative feedback loop.

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10
Q

Which centres in the brain have a modulatory influence on the Renshaw cells?

A

The cortex:

  • Has descending pathways which releases glutamate to excite and increase the activity of the extensor muscle (quadricep).
  • Quickly releases GABA to increasingly inhibit the extensor muscle, while decreasingly inhibiting the flexor muscle.
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11
Q

Which neurons initiate voluntary movement in spinal circuits?

A
  • Renshaw cells
  • Cholinergic (Ach) motor neurons
  • Excitatory cortico-spinal projection neurons
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12
Q

What is the function and properties of GPCRs?

A
  • Metabotropic GPCRs work to have a slow modulatory effect.

- They all have 7 transmembrane domains and a ligand binding site.

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13
Q

What are the different types of G protein?

A
  • Gs
  • Gi
  • Gq
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14
Q

What NTs are GPCRs sensitive to?

A
  • Ach: muscarinic receptors
  • Serotonin
  • Norepinephrine
  • Dopamine
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15
Q

What are muscarinic receptors?

A

They are acted upon by Ach, and there are 4 different classes.
- They activate all 3 G proteins.

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16
Q

What are Ach muscarinic receptors important for in the CNS, and how is this achieved?

A

They are important in learning, memory, cognition and wakefulness.
- This is achieved by the receiving of Ach input by the cortex and hippocampus by the basal forebrain.

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17
Q

What are Ach muscarinic receptors important for in the PNS?

A

They function in postganglionic neurons in the PNS.

18
Q

How does Ach regulate the specific K+ channel involved in the regulation of the M current?

A
  • The muscarinic receptors are activated. This activates a transmembrane receptor on the basal forebrain projection.
  • This receptor inhibits opening of the K+ channel.
  • The K+ channel closes, causing a slow depolarisation of the neuron & increasing excitability.
19
Q

Where are serotonin and norepinephrine neurons found?

A

In clusters in the brain stem nuclei. There are small numbers of both.

20
Q

Where are dopamine neurons found?

A

In the midbrain. There are small numbers present.

21
Q

Properties of serotonin neurons? Where they project from, what they activate and what they modulate.

A
  • Project from Raphe nuclei in the medulla.
  • There are 7 families and they activate all 3 G proteins.
  • They modulate circuits involved in locomotion, respiration, sleep, wake, aggression, social dominance and perception.
22
Q

Properties of norepinephrine neurons? Where they project from, what they activate and what they modulate.

A
  • Project from the locus coeruleus in the pons.
  • They activate beta-adrenergic receptors via Gs and Gi.
  • Modulates attention, arousal and enhances learning.
  • > Modulates excitability in the hippocampus, by closing calcium gated K+ channels, to increase K+ and cause slow depolarisation.
  • In the PNS, they are involved in modulating cardiac function for fight/flight.
23
Q

Properties of dopamine neurons? Where they project from, what they act on and what they modulate.

A
  • Project from the midbrain (substantia nigra).
  • They act on adenylyl cyclase. D1 and D1 act on Gs to increase excitability, and D2-4 act on Gi to decrease exctability of neurons.
  • They modulate motor function via the basal ganglia.
24
Q

Why are sea slug neurons used as models?

A
  • They are simple; have 20,000 - 30,000 neurons.
  • The neurons have a very large cell body.
  • Can have identified neurons.
25
Q

Explain the gill withdrawal reflex

A

This is when the siphon is touched, the gill retracts.

  1. When the siphon is touched, sensory neurons are activated.
  2. These synapse onto motor neurons via glutamate.
  3. The motor neurons are activated and cause contraction of muscles connected to the gill. This causes the gill to withdraw.
26
Q

How does serotonin modulate the neurons involved in the gill withdrawal reflex?

A

When the sensory neuron is activated, it also activates serotonigeric neurons which release serotonin onto both the sensory and motor neuron.
This allows for modulation of the neurons.

27
Q

How does sensitisation (non-associated learning) bring about facilitation?

A

Sensitisation releases serotonin, which increases the strength of the synaptic potential, causing facilitation, which is a stronger synaptic potential.

28
Q

What is the basal ganglia and its main function?

A

A collection of nuclei in the midbrain.
It is connected to the cortex, brain stem and the thalamus.
Its main function is to regulate activity in these linked areas of the brain.

29
Q

How does the basal ganglia interact with its linked parts of the brain?

A
  • The basal ganglia integrates sensory and motor information from the cortex.
  • This information is taken to the thalamus which takes it back to the cortex.
  • The cortex then passes the information to the brain stem, and then to the spinal cord.
30
Q

Which neurotransmitter regulates movement in the basal ganglia?

A

Dopamine

31
Q

Which neurons are mainly found in the basal ganglia circuits?

A

Medium spiny neurons.

32
Q

What are properties of medium spiny neurons?

A
  • They are projection neurons.

- They use GABA and are inhibitory.

33
Q

What are the 3 nuclei in the basal ganglia?

A
  • Striatum
  • Globus Pallidus external (GPe)
  • Globus Pallidus internal (GPi)
34
Q

How is the thalamo-cortico output regulated?

A
  • The GPi gives an inhibitory output to the thalamus, reducing thalamo-cortico excitation. The motor cortex can’t activate the striatum so excitation is reduced.
35
Q

What are the 2 pathways via which medium spiny neurons respond to dopamine?

A

Direct and indirect pathway

36
Q

What is the direct pathway?

A

The motor cortex input activates medium spiny neurons in the striatum.

  • The excitation then jumps over GPe and inhibits GPi, thus reducing inhibitory outflow.
  • There is less inhibition in the thalamus.
  • Movement is facilitated.
37
Q

What is the indirect pathway?

A

This involves medium spiny neurons which connect the GPi via GPe.

  • The striatum is excited by the motor cortex.
  • Inhibitory input from the striatum reaches GPe, causing inhibition of GPe.
  • This means there is less inhibition on GPi, so GPi has a stronger inhibitory output on the thalamus.
  • Movement is inhibited.
38
Q

Which dopamine receptors are expressed in the direct pathway and what do they do?

A

D1.

They increase excitability by activating Gs.

39
Q

Which dopamine receptors are expressed in the indirect pathway and what do they do?

A

D2.

They decrease excitability by activating Gi.

40
Q

How do D1 receptors work?

A

They bind dopamine, which increases excitability of medium spiny neurons. This increases inhibition of Gi, thus facilitating movement and reinforcing the direct pathway.

41
Q

How do D2 receptors work?

A

They bind dopamine and decrease excitability of medium spiny neurons. This reduces inhibition of the striatum on the GPe, leading to an increased inhibitory output from GPi and less inhibition in the thalamus. As a result, movement is facilitated, this opposing the indirect pathway.

42
Q

How does Parkinsons arise?

A

Arises due to the degeneration of dopaminergic neurons in the basal ganglia. This causes a decrease in movement.

Neurobiology (15 decks)

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