eLFH - Nervous System Physiology Flashcards by Thomas Eraifej

Nernst equation

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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

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Resting membrane potential charge

Always negative
Refers to intracellular charge

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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

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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

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SNARE proteins description

Proteins on vesicular membranes and on the pre-synaptic membranes

Control the docking and exocytosis of ACh vesicles

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SNARE protein examples on vesicle membranes

Synaptobrevin

Synaptotagmin

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SNARE protein examples on presynaptic membrane

Syntaxin

SNAP 25

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Substance which inactivates SNARE proteins

Botulinum toxin

Therefore prevents ACh release and causes flaccid paralysis

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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

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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

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Distance of synaptic cleft

~ 20 nm

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Number of vesicles containing ACh released following action potential

~ 125 vesicles released into synaptic cleft

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Number of ACh molecules within each vesicle released by pre-synaptic cell

10,000 - 12,000 molecules of ACh

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Acetylcholine chemical structure

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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

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Adult nicotinic ACh receptor structure and binding sites

2 alpha subunits
Beta subunit
Delta subunit
Epsilon subunit

ACh binding sites on alpha subunits

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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

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Breakdown of ACh by acetylcholinesterase process

Anionic binding site and Esteric binding site

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Where is acetylcholinesterase in synaptic cleft bound

Bound to Basal lamina of connective tissue within cleft

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Action potential in nerve fibre propagation

By voltage gated ion flux down concentration gradients

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Action potential in nerve fibre phases

Phase 0 - Resting state

Phase 1 - Depolarising

Phase 2 - Repolarising

Phase 3 - Refractory period

Back to phase 0

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Phase 0 of neuronal action potential

Resting membrane potential maintained by Na/K ATPase

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Phase 1 of neuronal action potential

Depolarisation
Sodium influx into cell once threshold potential reached

Rise in membrane potential

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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

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

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