Session 5: Synaptic signal transmission - electrical and chemical Flashcards
Define a synapse.
Anatomical place where signal transfer at a functional junction occurs.
Classify physiological synapses into two major structural and functional classes and name the most prominent (general) locations of these two classes.
- Electrical synapse (myocardial and visceral smooth muscle cells or neurons): are usually between cells of the same type
- Chemical synapse (neuron-neuron, neuron-muscle and neuron-gland): may be between cells of different types
Define the concept “synaptic signal transmission” and list anatomic sites where chemical and electrical signal transfer occur.
Synaptic signal transmission: process by which neurons communicate with other neurons, glands and muscles
Chemical signal transfer anatomic sites: neuron-neuron, neuron-muscle, and neuron-gland
Electrical signal transfer anatomic sites: myocardial and visceral smooth muscles or neurons
Briefly describe the five basic functions of a chemical synapse.
- Transmit action potentials from neuron to neuron
- Prevents propagation of weak signals
- Amplifies, modifies or redirects signals
- One impulse can be converted into repetitive
impulses - An impulse can be integrated with other impulses
Describe the 5 steps (events) in chemical synaptic transmission.
- An action potential in the presynaptic cell causes depolarisation of the presynaptic terminal
- Ca++ enters presynaptic terminal via Ca++ channels
- 200-300 vesicles bind to attachment sites and release neurotransmitter into the synaptic cleft
- Neurotransmitter combines with postsynaptic cell membrane receptors
- Membrane permeability for ions change
Briefly describe the five basic functions of a chemical synapse.
- Transmit action potentials from neuron to neuron
- Filters:
* Prevents propagation of weak signals
* Amplifies, modifies or redirects signals
* One impulse can be converted into repetitive
impulses
* An impulse can be integrated with other impulses
Identify general characteristics of chemical synapses and identify characteristics of synaptic transmission, including convergence and divergence, synaptic fatigue, synaptic delay and unidirectional conduction.
Convergence: signals from multiple inputs unite to excite a single neuron
Divergence: an input signal spreads to an increasing number neurons as it passes through successive orders of neurons in its path
Synaptic fatigue: when excitatory synapses are repetitively stimulated at a rapid rate, the number of discharges by the postsynaptic neuron is at first very great, but the firing rate becomes progressively less in succeeding milliseconds, this is known as fatigue
Synaptic delay: the minimal period of time required for all the events of synapses to take place. It is about 0.5 milliseconds; these processes are included
Unidirectional conduction: chemical synapses always transmit signals in one direction because because the presynaptic membrane has neurotransmitters but no receptors and the postsynaptic membrane has receptors but no neurotransmitters. This allows signals to be directed to one specific goal.
Distinguish three different anatomical types of chemical synapses.
- Axodendritic: a synapse in which the axon of one neuron comes in contact with the dendrites of another neuron
- Axosomatic: a synapse in which the axon of one neuron comes in contact with the cell body of another neuron
- Axo-axonal: a synapse in which the axon of one neuron comes into contact with the axon of another neuron
Distinguish different types of chemical synapse arrangements
One-to-one
Many-to-one
Briefly explain the phenomenon of synaptic fatigue and state one physiological purpose thereof.
When excitatory synapses are repetitively stimulated at a rapid rate, the number of discharges by the postsynaptic neuron is at first very great, but becomes progressively less in succeeding milliseconds.
Discuss inhibition, excitation, and facilitation at chemical synapses.
They describe the signal transmission characteristics of synapses. Inhibition and excitation are the membrane mechanisms (EPSP/IPSP). The importance of having inhibitory as well as excitatory types of receptors is that this allows the restraint of nervous action as well as excitation.
Define and describe the phenomena “excitatory and inhibitory postsynaptic potentials” at chemical synapses , by referring to the cellular mechanisms.
An excitatory
Define and describe the phenomena “excitatory and inhibitory postsynaptic potentials” at chemical synapses , by referring to the cellular mechanisms.
An excitatory postsynaptic potential (EPSP) develops when a neurotransmitter alters the membrane permeability in such as way that the membrane is HYPOPOLERISED
An inhibitory postsynaptic potential (IPSP) develops when a neurotransmitter alters alters the membrane permeability in such a way that the membrane is HYPERPOLARISED
Explain the term “summation of postsynaptic potentials” and distinguish two types of summation.
Summation is when the effects of the terminals of neurons add to one another until neuronal excitation does occur
Two types:
* Spatial summation: increasing numbers of parallel
nerve fibers which carry signals simultaneously to
another neuron
- Temporal summation: repeated sequential action
potentials in one presynaptic neuron
Identify four important mechanisms by which the effect of already released neurotransmitters is terminated.
- Re-uptake (by active transport), by presynaptic
terminals - Uptake by neuroglial cells
- Diffusion out of synaptic cleft and removal by
circulation and eventual hepatic breakdown - Enzyme inactivation
Describe the effects of the following on chemical synaptic signal transfer:
- acidosis and alkalosis
- hypoxia
- drugs
Alkalosis and acidosis:
- alkalosis greatly increases neuronal excitability
- acidosis greatly depresses neuronal activity
Hypoxia:
* neuronal activity is also greatly dependent on the
adequate supply of oxygen
* cessation of oxygen for only a few seconds can cause
complete inexcitability of some neurons
* this effect is observed when the brain’s blood flow is
temporarily interrupted because within 3 to 7
seconds, the person becomes unconscious
Drugs:
* caffeine, theophylline and theobromine, which are
found in coffee, tea, and cocoa respectively all
increase neuronal excitability
Discuss dendrites with specific reference to:
- their physiological anatomy
- transmission of an electrotonic current down
dendrites
- decrement of electrotonic conduction in dendrites
- summation of excitation and inhibition of dendrites
Transmission of an electrotonic current down dendrites: transmission of electronic current means direct spread of electrical current by ion conduction in fluids of the dendrites without generation of action potentials
Decrement of electrotonic current down dendrites:
the decrease in membrane potential as it spreads electrotonically along dendrites towards the soma is called decremental conduction
Summation of excitation and inhibition: dendrites can summate IPSPs in the same way that soma can
Identify three anatomic locations (steps) in chemical synaptic transmission where abnormalities may occur and identify one cause of an abnormality at each of these steps.
- Presynaptically
* Auto-antibodies in Lambert-Eaton syndrome inhibit
calcium ion entry into pre-synaptic terminal which
leads to muscle weakness
* Botulism food poisoning: Clostridium toxin leads to
protease cleaving proteins necessary for
neurotransmitter release - Synaptically
* Nerve gases (chemical warfare) inhibit
acetylcholinesterase as a result acetylcholine is not
cleared and accumulates which leads to later
repetitive action potentials becoming impossible to
send which then leads to paralysis of muscles, for
example the inter alia respiratory muscles - Postsynaptically
* Myasthenia gravis: auto-antibodies directed against
nicotinic cholinergic receptor which leads progressive
muscle fatigue and weakness
List examples of cell membrane excitants and depressants, with a few clinical applications.
Membrane excitants:
- hypocalcemia: destabilises the resting membrane
action
- hyperkalemia: depolarises, moves the resting
membrane towards the threshold
Membrane depressants include:
- temperature: hypothermia slows membrane
processes down
- hypercalcemia:
List examples of cell membrane excitants and depressants, with a few clinical applications.
Membrane excitants:
- hypocalcemia: destabilises the resting membrane
action
- hyperkalemia: depolarises, moves the resting
membrane towards the threshold
Membrane depressants include:
- temperature: hypothermia slows membrane
processes down
- hypercalcemia: raises the threshold by increasing the
membrane stabilisation
- hypokalemia: hyperpolarises the resting potential,
raises the threshold
- procaine: a local anesthetic, blocks sodium
conductance
- crush injury: larger fibres injured first, with a decrease
in impulse conduction, sparing smaller fibres
- hypoxia: effect is proportional to axon diameter; larger
fibres are depressed first
