eLFH - Nervous System Physiology Flashcards
Nernst equation
Nernst equation use
Calculate electrical potential across a cell membrane
Removing constants from equation shows that resting membrane potential depends on ration of intracellular to extracellular ions
Resting membrane potential charge
Always negative
Refers to intracellular charge
How is resting membrane potential maintained
Sodium/Potassium ATPase - 3 Na+ out and 2 K+ in
Membrane freely permeable to K+ so K+ moves out of cell
Synaptic transmission at the motor endplate / neuromuscular junction
1) Nerve impulse propagated along pre-synaptic membrane
2) Calcium influx through voltage gated Ca2+ channels
3) Migration of ACh vesicles to pre-synaptic membrane via activation of SNARE proteins by Ca2+
4) Release of ACh into synaptic cleft + binds receptors on post synaptic membrane
5) ACh also stimulates receptors on pre-synaptic membrane - mobilises more ACh
6) Post-synaptic receptor activation allows Na+ ions into post-synaptic cell via its sodium channel - alters membrane potential
7) ACh unbinds from receptors - broken down by Acetylcholinesterase
SNARE proteins description
Proteins on vesicular membranes and on the pre-synaptic membranes
Control the docking and exocytosis of ACh vesicles
SNARE protein examples on vesicle membranes
Synaptobrevin
Synaptotagmin
SNARE protein examples on presynaptic membrane
Syntaxin
SNAP 25
Substance which inactivates SNARE proteins
Botulinum toxin
Therefore prevents ACh release and causes flaccid paralysis
How does Ca2+ also mobilise vesicles in reserve pool within presynaptic cell
Ca2+ triggers phosphorylation of Synapsin molecules holding vesicles in reserve pool
This releases the vesicles into the available pool
How is action potential at neuromuscular junction propagated to rest of the muscle fibre
Action potential spreads down t-tubular system and results in Excitation-Contraction coupling
See flashcard in Cardiovascular physiology
Distance of synaptic cleft
~ 20 nm
Number of vesicles containing ACh released following action potential
~ 125 vesicles released into synaptic cleft
Number of ACh molecules within each vesicle released by pre-synaptic cell
10,000 - 12,000 molecules of ACh
Acetylcholine chemical structure
Acetylcholine synthesis, release and breakdown cycle
CoA forms Acetyl-CoA (acetyl-coenzyme A)
Acetyl group binds with Choline which is reabsorbed after ACh breakdown - forms ACh and CoA
ACh released
ACh broken down by acetylcholinesterase to form Choline + Acetate
Choline reabsorbed into pre-synaptic cell to re-form ACh with Acetyl-CoA
Adult nicotinic ACh receptor structure and binding sites
2 alpha subunits
Beta subunit
Delta subunit
Epsilon subunit
ACh binding sites on alpha subunits
Foetal nicotinic ACh receptor structure
Same as adult except Epsilon subunit replaced with Gamma subunit
I.e.
2 alpha subunits
Beta subunit
Delta subunit
Gamma subunit
Breakdown of ACh by acetylcholinesterase process
Anionic binding site and Esteric binding site
Where is acetylcholinesterase in synaptic cleft bound
Bound to Basal lamina of connective tissue within cleft
Action potential in nerve fibre propagation
By voltage gated ion flux down concentration gradients
Action potential in nerve fibre phases
Phase 0 - Resting state
Phase 1 - Depolarising
Phase 2 - Repolarising
Phase 3 - Refractory period
Back to phase 0
Phase 0 of neuronal action potential
Resting membrane potential maintained by Na/K ATPase
Phase 1 of neuronal action potential
Depolarisation
Sodium influx into cell once threshold potential reached
Rise in membrane potential
Phase 2 of neuronal action potential
Repolarisation
Potassium moves out of cell
Phase 3 of neuronal action potential
Refractory period
Undershoots membrane potential
Sodium gates cannot open - time required to re-establish Na/K gradients
Resting membrane potential value in nerve cells
- 70 mV
Peak positive membrane potential reached during action potential
+ 35 mV
Nerve fibre classification overview
Usually classify according to fibre type
Also a I - IV classification of sensory fibres but don’t worry about those
Nerve fibre types
A alpha
A beta
A gamma
A delta
B
C dorsal root
C sympathetic
A alpha nerve fibres - Modality served
Proprioception
Motor
A alpha nerve fibres - Diameter
12 - 20 micrometres for Motor and type Ia proprioception A alpha fibres
12 - 30 micrometres for type Ib proprioception A alpha fibres
A alpha nerve fibres - Conduction speed
70 - 120 m/s
A alpha nerve fibres - Myelination
Yes
A beta nerve fibres - Modality served
Touch
Pressure
Proprioception
A beta nerve fibre - Diameter
5 - 12 micrometres
A beta nerve fibre - Conduction speed
30 - 70 m/s
A beta nerve fibre - Myelination
Yes
A gamma nerve fibre - Modality served
Motor (muscle spindle)
A gamma nerve fibre - Diameter
3 - 6 micrometres
A gamma nerve fibre - Conduction speed
15 - 30 m/s
A gamma nerve fibre - Myelination
Yes
A delta nerve fibre - Modality served
Pain
Temperature
Touch
A delta nerve fibre - Diameter
2 - 5 micrometres
A delta nerve fibre - Conduction speed
12 - 30 m/s
A delta nerve fibre - Myelination
Yes
B nerve fibre - Modality served
Preganglionic autonomic fibres
B nerve fibres - Diameter
< 3 micrometres
B nerve fibres - Conduction speed
3 - 14 m/s
B nerve fibres - Myelination
Some
C dorsal root nerve fibres - Modality served
Pain
Temperature
Touch
C dorsal root nerve fibres - Diameter
0.4 - 1.2 micrometres
C dorsal root nerve fibres - Conduction speed
0.5 - 2 m/s
C dorsal root nerve fibres - Myelination
No
C sympathetic nerve fibres - Modality served
Postganglionic autonomic fibres
C sympathetic nerve fibres - Diameter
0.3 - 13 micrometres
C sympathetic nerve fibres - Conduction speed
0.7 - 2.3 m/s
C sympathetic nerve fibres - Myelination
No
Pain pathway - sensation transmission
Peripheral stimulation of nerve ending by painful stimulus
Pre-synaptic cell bodies located in ipsilateral dorsal root ganglion in spinal cord
Impulses transmitted via secondary neurones in contralateral spinothalamic tract to thalamus
Impulses transmitted from thalamus to somatosensory cortex
Some descending inhibitory pathways originating in hypothalamus and periaqueductal grey matter
Spinal cord tracts and which are ascending sensory vs descending motor
Ipsilateral spinal cord tracts
Dorsal columns - cuneate and gracile fasciculus
Contralateral spinal cord tracts
Lateral + Anterior spinothalamic tracts
Spinocerebellar tracts
Modalities of dorsal columns
Touch
Vibration
Proprioception
Modalities of spinocerebellar tracts
Proprioception
Modalities of lateral spinothalamic tract
Pain
Temperature
Modalities of anterior spinothalamic tract
Light touch
Pressure
Role of lateral corticospinal tract
Contralateral motor innervation
Role of anterior corticospinal tract
Ipsilateral motor innervation
Predominantly to trunk
Role of extrapyramidal tracts
From brainstem nuclei to lower neurones
Primarily for posture + muscle tone
Spinal shock
Flaccid paralysis with loss of limb reflexes
Occurs immediately after cord transection
When does hyper-reflexia occur post spinal cord injury
2 - 6 weeks after injury
Brown-Sequard syndrome effects
Ipsilateral:
- Paralysis
- Loss of proprioception
- Loss of vibration
Contralateral:
- Loss of pain
- Loss of temperature
Monosynaptic reflex arc phases
Stimulation of receptor (muscle spindle) when stretched
Signal carried by type Ia afferent fibres
Neuron enters dorsal root of spinal cord
Ia neuron synapses directly with A alpha motor neurons
Signal to neuromuscular junction and muscle activation
Is knee reflex monosynaptic or polysynaptic reflex
Technically polysynaptic as Ia fibres also synapse with inhibitory interneurons to cause relaxation of hamstring muscles as well as contraction of quadriceps
Types of muscle fibre
Extrafusal - normal muscle
Intrafusal - contain muscle spindle
Innervation of extrafusal muscle fibres
A alpha motor fibres
Innervation of muscle spindles
A gamma fibres
Function of Golgi tendon organs
Protect the muscle
Produce inhibitory impulse following overstretch of the muscle
Location of Golgi tendon organs and their innervation
In tendons
Type Ib fibres
Sympathetic nervous system outflow
T1 to L2
Synapses in sympathetic chain
Adrenal medulla is supplied by pre-ganglionic fibres
Parasympathetic nervous system outflow
Cranial nerves 2, 7, 9, 10
Sacral nerves S2 to S4
75% outflow is via Vagus nerve
Location of parasympathetic ganglia
In the effector organs themselves
Therefore all parasympathetic nerves are pre-ganglionic
Role of vagus nerve and parasympathetic supply to respiratory system
Bronchoconstriction
Role of vagus nerve and parasympathetic supply to cardiovascular system
Bradycardia
Vasodilation
Role of vagus nerve and parasympathetic supply to GI system
Stimulates stomach and intestine motility
Stimulates gastric and pancreatic secretion
Autonomic nerve fibres that use Acetylcholine as neurotransmitter
All pre-ganglionic fibres - therefore:
- All parasympathetic fibres use ACh as neurotransmitter
- Sympathetic innervation to adrenal glands use ACh
Also post-ganglionic sympathetic innervation of sweat glands and piloerector muscles use ACh (exception to the rest of post-ganglionic sympathetic fibres)
Autonomic nerve fibres that use Noradrenaline as neurotransmitter
Post-ganglionic (therefore sympathetic) fibres
EXCEPT postganglionic sympathetic innervation of sweat glands, piloerector muscles and renal vessels
Autonomic nerve fibres that use Dopamine as neurotransmitter
Post-ganglionic sympathetic innervation of renal vessels
Glands which receive only Parasympathetic fibre innervation (not under dual control)
Lacrimal glands
Glands which receive only Sympathetic fibre innervation (not under dual control)
Piloerector muscles
Adipose tissue
Juxtaglomerular apparatus
