Excitatory and Inhibitory Neurotransmission in the CNS

Question 1

The Neurone: has morphological regions with distinct roles
Dendrites: receive —– from other neurones and convey —– electrical signals —– to the soma.

Cell body (soma): —– and —- centre. Contains the nucleus, ribosomes, mitochondria and endoplasmic reticulum. Integrates incoming electrical signals that are conducted passively to the ——.

Axon hillock and initial segment: site of initiation of the——-action potential

Axon: conducts —– signals as action potentials to the presynaptic terminal. Mediates transport of materials between the soma and presynaptic terminal (——) and vice versa (——) by slow and fast axonal transport
Synapse: point of chemical (usually) communication between neurones (or other cells)

Answer 1

inputs
graded
passively

synthetic and metabolic
axon hillock

‘all or none’

output
anterograde direction
retrograde direction (One function of this process is recycling of materials originally transported from cell body to axon.)

Question 2

Several virus (e.g. herpes, polio, rabies) exploit —– to infect neurones.

Answer 2

retrograde transport

Question 3

Regardless of type, most neurones have four functional regions? what are the regions?

Answer 3

Question 4

Types of neurons?

Answer 4

Question 5

The Action Potential in Neurones?

Answer 5

Question 6

Action Potentials Allow Electrical Signals to be Conducted Over Large Distances Without Decaying.
T/F
why?

Answer 6

T
The nerve cell membrane is ‘leaky’ (i.e. not a perfect insulator): passive signals do not spread far from their site of origin due to current loss across the membrane accompanied by a reduced change in potential (imagine a leaky garden hose as an analogy for the axon)

• The membrane potential change is a passive process that decays exponentially with distance; λ = length constant
• The distance over which the current spreads depends on membrane resistance (rm) and the axial resistance of the axoplasm (ri) (the resistance inside of the cell.)
• • increasing the ratio rm/ri—>increases λ

Question 7

Increasing passive current spread factors?

Answer 7

• Decrease ri (increase axon diameter)
• Increase rm (add myelin)
  
  • Provided by Schwann cells in PNS and oligodendrocytes in CNS
• Conduction in myelinated axons is much faster than in nonmyelianted axons of the same diameter
• Saltatory conduction: in myelianted axons, the action potential ‘jumps’ from one node of Ranvier to the next
  
  • Voltage-activated Na+ channels cluster at nodes i.e. where the depolarisaton occur

Question 8

Overview of chemical neurotransmission at the synaptic cleft

Answer 8

Question 9

Structural elements of the chemical synapse?
synaptic cleft?
a matrix?
vesicles?
Pre and post-synaptic membranes?

Answer 9

• Pre and post-synaptic membranes separated by a narrow synaptic cleft
• A matrix of fibrous extracellular protein within the cleft that holds the pre- and post-synaptic membranes together
• Vesicles wihin the presynaptic terminal that store neurotransmitter
• Membrane differentiations
  
  • Presynaptically, the active zones around which vesicles cluster
  • Postsynaptically, the postsynaptic density containing neurotransmitter receptors

Question 10

Type of synapses?

Answer 10

location of the presynaptic terminal upon the postsynaptic cell

Question 11

Synapses can be classified functionially as being excitatory or inhibitory. what are the main NTs and their receptors? what ion causes each?

Answer 11

Excitatory synapse: in the CNS the transmitter is most frequently glutamate

Inhibitory synapse: in the CNS the transmitter is most frequently GABA or glycine

Question 12

Neurotransmitters may act directly or indirectly on ion channels. what are those channels?

Answer 12

• Direct gating is by ionotropic receptors (the ligand-gated ion channel) - the receptor is an integral component of the molecule that forms the channel it controls, gating of channel is rapid
• Indirect gating is mediated by activation of metabotropic receptors (G-protein-coupled receptors) - receptor and the channel in controls are distinct, gating of channel is slower

Question 13

Glutamate?

Answer 13

Glutamate activates postsynaptic, cation selective, inotropic, glutamate receptors generating a local, graded, excitatory (depolarising) response - the excitatory postsynaptic potential (e.p.s.p.)

Question 14

GABA?

Answer 14

GABA or glycine activates postsynaptic, anion selective, ionotropic, GABA/glycine receptors generating a local, graded, inhibitory (hyperpolarising) response - the inhibitory postsynaptic potential (i.p.s.p.)

Question 15

Synaptic integration; spatial and temporal summation?

Answer 15

• Spatial summation: many inputs converge along a neurone to determine its output
• Temporal summation: a single input may modulate output by variation in action potential frequency of that input
• Physiologically, spatial and temporal summation are not isolated, but complementary, processes

Question 16

NTs:

Acetylcholine, amino acids and amines are released from —–
Peptides are released from —–
Glutamate, GABA, glycine, acetylcholine, and 5-HT can activate ———-. These mediate —– neurotransmission.
All, except glycine, can also activate —— receptors. These mediate relatively —– neurotransmission

Answer 16

synaptic vesicles

secretory vesicles

ionotropic ligand-gated ion channels (LGICs)
fast

metabotropic G-protein-coupled
slow

Question 17

Cholinergic receptors?

Answer 17

• Cholinergic synaptic transmission in autonomic ganglia display both direct and indirect transmitter actions
  
  • Fast EPSP is due to activation of nicotinic (ionotropic) ACh receptors, channels conduct Na+ and K+
  • Slow EPSP follows activation of muscarinic (GPCR) ACh receptors, ACh closes a K+ channel (M-type)

Question 18

Glutamate/glutamate receptors?

Answer 18

• Glutamate is the major excitatory neurotransmitter but may also have inhibitory effects via its response at metabotropic glutamate receptors
• Ionotropic glutamate receptors directly gate ion channels; may be classified via their response to non-endogenous agonists that mimic glutamate
  
  • Non-NMDA receptors bind the agonists kainate or AMPA controlling a channel permeable to Na+ and K+ - fast excitatory synaptic transmission in the CNS
  • NMDA receptor controls a channel permeable to Na+, Ca2+ and K+ - slow component to the excitatory synaptic potential
    
    • Certane anaethetic agents e.g. ketamine and psychomimetric agents e.g. phencyclidine are selective blockers of NMDA-operated channels
• Metabotropic glutamate receptors don’t have an integral ion channel but exert their effect by activation of a second messenger cascade
  
  • Role is modulation of neurotransmission e.g. presynaptic inhibition
• Ionotropic and metabotropic glutamate receptors are important in discriminating between on and off retinal pathways