Chapter 2 Flashcards
How did Charles Sherrington use behavioural observations to infer the major properties of synapses?
Sherrington studied reflexes. Once, he strapped a dog into a harness and pinched one of the dog’s feet. After a short period of time, the dog flexed its leg (reflexed) even when he made a cut that disconnected the spinal cord from the brain - even sometimes being stronger or weaker reflexes. He therefore found:
1. Reflexes are slower than the conduction of action potentials along an axon due to synapses
2. Several weak stimuli presented at nearby places or times combine their effects - temporal summation.
3. When one set of muscles becomes excited, a different set becomes relaxed.
How do EPSPs and IPSPs produce temporal and spatial summation?
- EPSPs - excitatory postsynaptic potential: graded depolarisation due to sodium ions entering the neuron.
- IPSPs - inhibitory postsynaptic potential: graded hyperpolarisation produced by flow of negatively charged chloride ions into the cell.
To produce temporal and spatial summation, EPSPs and IPSPs are in play as summation only occurs with graded potential with either depolarisations or hyperpolarisations.
Why is inhibition important in the nervous system?
Inhibitory synapses are important in inhibiting firing rate.
What are the sequences of events at a synapse, from neurotransmitter synthesis, to the disposition of transmitter molecules?
- The neuron synthesizes neurotransmitters, either in the cell body or at the end of the axon.
- Action potentials travel down the axon. At the presynaptic terminal, the depolarization enables calcium to enter the cell. Calcium releases neurotransmitters from the terminals and into the synaptic cleft, the space between the presynaptic and postsynaptic neurons.
- The released molecules diffuse across the narrow cleft, attach to receptors, and alter the activity of the postsynaptic neuron in any of several ways.
- The neurotransmitter molecules separate from their receptors.
- The neurotransmitter molecules may be taken back into the presynaptic neuron for recycling, or they may diffuse away.
- Some postsynaptic cells send reverse messages to control the further release of neurotransmitter by presynaptic cells
What is the difference between ionotropic and metabotropic receptors?
Metabotropic effects are slower
How do ionotropic receptors work?
Ionotropic receptors twist the receptor to open its central channel that allows the ion to pass through
How do metabotropic receptors work?
When a neurotransmitter attaches to a metabotropic receptor, it bends the receptor protein that goes through the membrane of the cell. The other side of that receptor is attached to a G protein—a protein coupled to guanosine triphosphate (GTP), an energy-storing molecule. Bending the receptor protein detaches that G protein, which is then free to take its energy elsewhere in the cell. The result of that G protein is increased concentration of a second messenger, such as cyclic adenosine mono- phosphate (cyclic AMP), inside the cell. Just as the “first messenger” (the neurotransmitter) carries information to the postsynaptic cell, the second messenger communicates to areas within the cell.
How do certain drugs affect behaviour via the synapse?
- Hallucinogenic - drugs that distort perception chemically resemble serotonin. They attach to serotonin receptors and stimulate them for longer-than-normal durations
- Opiate - Opiates are chemically similar to endorphins. Opiates relieve pain by acting on receptors in the brain as well as in the skin.
- Amphetamine - increases release of dopamine, decreasing reuptake and prolonging the effects of the neurotransmitters. Prolonging the dopamine effects produces increased arousal.
- Methylphenidate - similar to 4 but it is more gradual
- MDMA - increases serotonin release and blocks reuptake
- MAO - prevents breakdown od serotonin
- Nicotine - chemically similar to the neurotransmitter that binds to the nicotonic acetylchlonine receptors
- Cannabis - THC binds to anandamide + 2 AG on presynaptic neuron which prevents the release of some neurotransmitters.
What are some hormones and their effects?
What happens when a neurotransmitter reaches a receptor?
It can open ligand gated ion channels (and more). Different ionotropic receptors react differently these channels:
1. Acetylcholine receptor - has excitatory effects, opening these channels for Na+ to flow in causing depolarisation
2. GABA receptor - has inhibitory effects, opening these channels for Cl- to flow in causing hyperpolarisation
What happens when Cl- enters the cell and K+ exits?
It leads to an Inhibitory Post-Synaptic Potential (IPSP) which inhibits the action potential at the axon hillock
What happens when Na+ enters the cell?
It leads to an Excitatory Post-Synaptic Potential (EPSP) exciting the action potential at the axon hillock
What did Eccles find?
Post-synaptic potentials are graded as various degreees of inhibitory/excitatory potentials. This leads to summation such as temporal where more stimulation leads to more depolarisation and spatial where stimulating 2 presynaptic neurons at the same time can reach the threshold
What forms serotonin and dopamine?
These neurotransmitters are formed by diet with raphe nuclei forming serotonin and ventral tegmental and substantia nigra form dopamine
What can agonist psychoactive drugs do?
- Increase release of neurotransmitters
- Decrease reuptake of neurotransmitters
- Block breakdown of neurotransmitter
- Stimulate postsynaptic receptors
What do antagonist psychoactive drugs do?
- Cause vesicles to leak the neurotransmitter
- Block postsynaptic receptors
