Synaptic Transmission in the CNS Flashcards
Whats the basic flow of information (input/output) in the CNS?
Sensory receptor-> sensory neurone (afferent) -> CNS-> Motor neurone (efferent) -> Effector
sensory receptor= ears, eyes, taste
effector= muscles or glands
CNS= spinal chord/ brain
What are interneurones?
Have LOADS of dendrites (so lots of input) but only one output
eg. Purkinje neurons (cerebellum)
What are the basic components of the knee jerk reflex?
- Muscle spindle (sensory receptor) (tendon)
- Skeletal muscle (effector) (Tendon)
- Sensory neurone (afferent)
- Motor neurone (efferent)
Muscle spindle is embedded within skeletal muscle
How does the structure of the muscle spindle allow it to measure its length?
Intrafusal muscle spindle lies on top of extrafusal muscle fibre (the ones that contract)
When muscle fibre contracts/stretchs it activates spindle which sends info down sensory neuron afferents (y-motorneuron) to CNS
Sensory neurone synapses with the motor neurone (a-motorneuron) of the muscle fibres = mono synaptic reflex arc
This is called a mono synaptic reflex arc because there is a single chemical synapse between the sensory neuron and the motor neuron in the central nervous system. This whole system is designed to allow the muscle to constantly monitor its own length independent of any conscious thought.
How can you tell the difference between sensory neurons, interneurons, and efferent neurons?
efferent= motor
Motorneurones: have their cell bodies and receive information in the spinal cord and send long axons to muscles
Sensory neurons: receive information at their dendrites but their shape can look somewhat different and harder to distinguish dendrites from synapses as the cell body is separated.
Interneurons: are found in the brain and send information over much shorter distances to link different neurones together in networks
How is action potential frequency related to how long a muscle fibre is?
Action potentials increase when muscle is stretched
Action potentials decrease when mucle is contracted
What is the GTO and what does it do?
Golgi tendon organ
Its a sensory receptor
It monitors tension in a muscle
Stretch= GTO sends low freq action potentials
Contract= GTO sends high freq action potentials
Describe the neuronal mechanism of keeping tension in the body?
Its a polysnaptic arc
GTO (sensory receptor)-> activates sensory neurone-> activates inhibitory interneurone -> inhibits motor neurone that activates skeletal muscle
When muscle is relaxed, GTO sends no action potentials
As tension gets too high, GTO gets activated and it just relaxes=> to prevent damage
How can skeletal muscles work in antagonistic pairs in the knee jerk reflex?
Quad needs to contract while hamstring needs to relax- can do this because of muscle spindle and GTO
Sensory neurone can synapse with both a- motorneurone that goes to quad and inhibitory interneurone that innervates hamstring
So when tendon is stretched- it activates quad to contract and hamstring to stretch at the same time
Muscle spindle monitors length of muscle
GTO monitors tension of muscle
How is K+, Na+, Ca2+ distrubted across cell membranes?
intracellular conc vs extracellular conc
K= HIGH inside, LOW outside
Na= LOW inside, HIGH outside
Ca= LOW inside, HIGH outside
this is all relative ie ca is HIGHER outside but not acc v high
K= 140 mM inside and 15 mM outside
Na= 15 mM inside and 150 mM outside
How do electrochemical gradients create membrane potentials?
Eg. if membrane becomes permeable to K, then K starts to flow out of cell (because its moving down its chemcial gradient)
This makes the inside of the cell more negative, meaning K is going against its electrical gradient
K is trying to reach its equilibrium potential
cell becomes more neg because K (cation) is leaving so becomes less pos
Electrical gradient= ions flow toward regions of opposite charge
Chemical gradient= ions flow from high to low conc
What does equilibrium potential mean?
aka reversal potential
The membrane potential at which net flux of ions= 0
Where Fc = -(Fe)
Fc= flux down chemical gradient; Fe= flux down electrical gradient
What is the membrane potential at rest?
~70 mV
- If membrane became only permeable to K, then K will leave cell until Ek= -84 mV
- If membrane became only permeable to Na then Na will enter cell until Ena= 58 mV
How do cells communicate electrically by changing selective permeability of membranes to Na and K?
When membranes are only permeable to Na, membrane potential increases to +58 mV (DEPOLARIZATION)
When membranes are only permeable to K, membrane potential decreases to -84 mV (REPOLARIZATION)
What happens during depolarization, repolarization, and hyperpolarization?
Depolarization: influx of +ve ions (Na), cell becomes more +ve
Repolarization: efflux of +ve ions (K), cell becomes less +ve
Hyperpolarization: greater efflux of +ve ions (K) than at rest
How long is a typical action potential? Whats the amplitude of a typical action potential?
- Last 1-2 ms
- Peak -10 to +30 mV
At what point in the action potential do voltage Na channels open and close?
OPEN Right after stimulus reaches threshold (-55mV) the channels open and Na ENTERS cell causing POSITVE charge in the cell
CLOSE at the peak, Na stops entering the cell because channels close
At what point in the action potential do voltage gates K channels open?
OPENS Right after the peak, Na gates close and K channels slowly open and K LEAVES the cell causing loss of positivity
CLOSES slowly which is what causes hyperpolarization
Describe the different gates of Na and K channels?
M gate= (Na) opens quickly with depolarization (voltage gated)
H gate= (Na) blocks open channel
N gate= (K) opens slowly with repolarization, closes slowly (voltage gated)
At rest, m and n are closed, h is open. During depolarization m opens. At peak h closes and n opens, m closes during repolarization and n is slow to close during hyperpolarization
Whats the difference between absolute and relative refractory period?
Absolute: Na channels still blocked (H gate) so CANNOT generate another action potential -> sets max AP frequency
Relative: Cld generate AP but would need a stronger stimulus to get to threshold; bigger the stimulus the quicker an AP can go
What are channelopathies are due to Na or K channel defects?
K: Acquired neuromyotonia, Episodic ataxia/myokymia syndrome, Long QT syndrome, Familial persistent hyperinsulinemia hypoglycemia of infancy
Na: Epilepsy
Acquired neuromyotonia : hyperexcitability of motor nerves
Episodic ataxia/myokymia syndrome : uncoordination, nerve action potential does not efficiently repolarize
Long QT syndrome: dysfunction of cardiac repolarization
Familial persistent hyperinsulinemia hypoglycemia of infancy
Whats a compound action potential?
Change in extracellular voltage as the SUM of all APs from all the axons in a particular population
- size of the peak= the number of axons in each population
- time to the peak following stimulation= the conduction velocity for that populations of axons
- Delay in peak= conduction velocity of different axons
Compound action potential allows us to measure conduction velocity as well as probe the function of different types of neurone in a peripheral nerve
What would increase conduction velocity?
- increase in axon diameter
- increase in axon myelination
What are the conduction properties of the following fibre types:
A-alpha
A-beta
A-gamma
A-delta
B
C
A-alpha: muscle spindle afferent, 12-20im, +++, 70-120ms
A-beta: touch/pressure, 5-12im, +++, 30-70ms
A-gamma: muscle spindle, 3-6im, +++, 15-30ms
A-delta: pain/touch, 2-5im, ++, 12-30ms
B: preganglion automatic, 3im, +, 3-15ms
C: pain/temperature, 0.4-1.2im, +, 0.5-2ms
+ refers to how heavily mylinated they are
im is unit of their diameter
How does nature increase the axon length constant?
By increasing Rm (membrane resistance)
Bigger length constant= higher conduction velocity
How does axon diameter effect Ri (internal resistance) and therefore velocity conduction?
Smaller diameter= high Ri, slow
Larger diameter= low Ri, fast
small diameter means high cytoplasm resistance, means slow velocity
large diameter means low resistance, means high velocity, depolarization maintained for longer
What happens when mylein sheaths are degraded?
- Multiple sclerosis
- Guillan-barre syndrome
membrane resistance isnt as high and leads to increased leak of ions across membrane
