Nervous System Flashcards
What maintains the RMP of the nerve cell?
- the sodium/potassium pump pumps out 3 Na+s and pumps in 2K+
- sodium is mainly extracellular
- potassium is mainly intracellular
What is the Nernst equation?
Can be used to calculate the electrical potential across the cell membrane
Why is the RMP always negative?
It refers to the intracellular charge - it’s dependent on the ratio of extracellular and intracellular ions
How does a rise in extracellular potassium cause a decrease in the RMP?
Using the Nernst equation: (K+ 5 vs 7)
K+ = 61.54 mV log [K+]o
[K+] i
- Eeq K+ = 61.54 log (5/148) = -90 mV
- Eeq K+ = 61.54 log (7/148) = -81 mV
How does synaptic transmission occur at the motor endplate?
- Nerve impulse propagated alone pre-synaptic membrane.
- Calcium influx through voltage-gated cells.
- Migration of ACh vesicles to the pre-synaptic membrane.
- Release of ACh into synaptic cleft (20nm wide) and binds to post-synaptic receptors
- Pre-synaptic receptor activation stimulates more mobilization of ACh
- Post synaptic activation allows Na into the cell along the Na channel, altering the membrane potential
- ACh unbinds from receptors and is broken down by ACh-esterase
What is the chemical structure of ACh?
Describe what happens at the ACh junction
What does the adult nicotinic ACh receptor look like?
Made of 5 subunits arranged around a central pore
All are transmembrane proteins
Is a ligand-gated ion channel
How is ACh broken down by ACh-esterase?
The Acetylcholinesterase has 2 binding sites
- an anionic site which electrostatically attracts the N+
- a serine esteric site
This breaks down the ACh into acetic acid and choline
What is the nerve action potential like?
4 phases of depolarization
- Resting state. Potential maintained by Na/K pump.
- Depolarization. Sodium influx into cell with rise in RMP.
- Repolarization. Potassium efflux out of cell.
- Refractory period - sodium gates can’t open, time required to re-establish Na/K gradients
How many ACh vesicles does an AP release?
125 ACh vesicles
Where is ACh-esterase bound to?
ACh-esterase is bound to the basal lamina of connective tissue within the cleft.
Are all Aα fibres myelinated?
Yes
What is the diameter if Aα fibres?
12 - 20 microns
What is the conduction speed of Aα fibres?
70 - 120 m/s
What classification of Aα are there?
Aα - motor, sensory fibres from muscle tendon Golgi organs
- 12 - 20 microns
- type Ia = proprioception (12 - 20 microns)
- type Ib = proprioception (12- 30 microns)
What are Aβ fibres? What is their diameter, conduction speed and are they myelinated?
- Type II
- touch, pressure, proprioception
- 5- 12 microns diameter
- 30 - 70 m/s conduction speed
- yes, they’re myelinated
What are Aγ fibres? What is their diameter, conduction speed and are they myelinated?
- motor (muscle spindle)
- 3 -6 microns diameter
- 15- 30 m/s conduction speed
- yes they’re myelinated
What are Aδ fibres? What is their diameter, conduction speed and are they myelinated?
- Type III
- pain, cold, temperature, touch
- 2 - 5 microns diameter
- 12 - 30 m/s conduction speed
- yes, they’re myelinated
What are B fibres? What is their diameter, conduction speed and are they myelinated?
- preganglionic autonomic fibres
- <3 microns diameter (small)
- conduction speed 3 - 14 m/s
- some are myelinated
What are C dorsal route fibres? What is their diameter, conduction speed and are they myelinated?
- Type IV
- pain, warm and cold temp, touch
- 0.4 - 1.2 microns diameter (tiny)
- 0.5 - 2 m/s conduction speed
- NOT myelinated
What are C sympathetic fibres? What is their diameter, conduction speed and are they myelinated?
- postganglionic autonomic fibres
- 0.3 - 13 microns diameter
- 0.7 - 2.3 m/s conduction speed
- NOT myelinated
How is a painful stimulus transmitted to the brain?
- peripheral stimulation of the nerve ending
- pre-synaptic cell bodes in the dorsal root ganglia
- impulses transmitted via secondary neurones in the contralateral spinothalamic tracts to the thalamus
- impulses then transmitted to somatosensory cortex
- descending inhibitory pathways originating in the hypothalamus and periaqueductal grey matter
What speed do unmyelinated C fibres conduct at?
1 m/s
What do Aγ fibres innervate?
Muscle spindles.
The sympathetic nervous system contains B and C fibres
Do fibres of type Aβ transmit from vibration sensing organs?
Yes, they also transmit proprioception impulses
How fast can unmyelinated fibres transmit?
Lower velocities (0.5 - 2 m/s)
How are axons classified?
According to conduction velocity
Axons connecting sympathetic ganglia are what colour?
Preganglionic sympathetic fibres are white rami and are myelinated (myelin makes them white)
Can axons conduct in both directions?
Yes but synapses won’t work both ways
What is the absolute refractory period related to in terms of ion channels?
The absolute refractory period refers to the inactivation of sodium channels, not potassium conductance
Why are myelinated fibres faster?
Up to 50 times faster due to saltatory conduction - jumping between Nodes of Ranvier
Is nerve conduction slower or faster if you cool the nerve?
Slower
What speed do pre-ganglionic fibres conduct at?
These are in group B, conducting at 3 - 14 m/s
Therefore, not the fastest conducting ones.
Where in the spinal cord is proprioception carried?
The dorsal columns carry fine touch and proprioception