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
Where are temperature and pain carried in the spinal cord?
The contralateral spinothalamic tracts
What do fibres carrying fine touch sensation form?
The gracile and cuneate nuclei
After complete transection of the spinal cord, what would be found below the lesion?
- spinal shock ensues immediately after cord transection
- loss of reflexes (hyperreflexia occurs 2-6 weeks later)
- urinary incontinence
- flaccid paralysis with loss of limb reflexes
- sensation is lost
- joint position sense also lost
Draw and label a diagram of the cross section of the spinal cord.
What ipsilateral ascending pathways does the spinal cord have?
- dorsal columns from the fasciculi cuneatus and gracilis
What contralateral ascending pathways does the spinal cord have?
- lateral spinothalamic tract
- anterior spinothalamic tract
- spinocerebellar tracts
What do the dorsal colums carry?
Touch, vibration and proprioception
What does the ascending spinocerebellar tract carry?
Proprioception
What does the lateral spinothalamic tract carry?
Pain and temperature
What carries light touch and pressure?
The anterior spinothalamic tract
What ipsilateral descending pathways does the spinal cord have?
- anterior spinothalamic tract (extrapyramidal)
- rubro/tecto/vestibulospinal tracts
What contralateral descending pathways does the spinal cord have?
- lateral spinothalamic tract (pyramidal)
What descending pathway is responsible for contralateral motor innervation?
Lateral corticospinal tract
What descending tract is responsible for extrapyramidal tracts from brainstem nuclei to lower neurones?
The reticulospinal tract
What descending tract is responsible for ipsilateral motor innervation?
Anterior corticospinal tract
What descending tract is responsible for extrapyramidal tracts from brainstem nuclei to lower neurones?
Vestibulospinal tract
Where is fine touch transmitted?
The posterior white column in the medial and lateral fasciculi, connecting to their respective cuneate and gracile nuclei
What is the pyramidal tract?
A descending motor tract.
The posterior and anterior spinocerebellar tracts ascend in the lateral column and transmit proprioception to the cerebellum without crossing.
Where are the cell bodies of Aδ and C fibres?
They reside within the dorsal root ganglion of the spinal cord and are both first order neurones
Where do Aδ and C fibres synapse?
Aδ fibres synapse with cells in laminae I and V of the dorsal horn whereas C fibres synapse with cells in the substantia gelatinosa (laminae II and III) in the dorsal horn.
Where are most ascending neurones?
The anterolateral columns.
Most second order neurones cross within a few segments and ascend in the anterolateral columns (spinothalamic tract).
Does the substantia gelatinose project directly to higher levels?
No
What does Brown-Sequard syndrome (spinal cord hemisection) cause?
- ipsilateral paralysis
- ipsilateral loss of proprioception
- ipsilateral loss of vibration sense
- contralateral loss of pain sensation
- contralateral loss of temperature sensation
What is the knee-jerk reflex?
- monosynaptic reflex
- muscle spindle is stimulated
- afferent pathway via Aγ fibres
- neurone enters the dorsal root of spinal cord, synapses in grey matter
- synapse is either direct with Aδ motor neuron or via interneurones
- muscle activation
What is a muscle spindle?
- either extrafusal (normal muscle, innervated by Aδ motor fibres)
- or intrafusal (containing the muscle spindle)
- spindles are innervated by Aγ nerves which are either type Ia (responding to stretch and ‘rate of stretch’) or type II (responding to stretch only)
- Golgi tendon organs produce an inhibitory impulse if the muscle is over-stretched, these are found in the tendon and innervated by type Ib fibres
Where can reflex actions occur?
- glands
- skeletal muscle
- smooth muscle
- cardiac muscle
How are reflex pathways influenced by higher centres?
Via the upper motor neurone descending tracts
eg rubro/cortico/reticulo/vestibulo/tecto-spinal tracts
Where are the receptors involved in the knee jerk reflex?
Spindles in the muscle
What is the knee jerk sensory fibre and which spinal segment does it propagate through?
A Type Ia fibre which synapses with an alpha motor neurone at the level of L2
What nerve does the knee jerk reflex involve?
The femoral nerve
Where is sympathetic outflow from?
T1-L2
Where are the sympathetic nerve system synapses?
In the sympathetic chain.
Some nerves synapse in ganglia above or below the level they arise from and then continue as post-ganglionic fibres.
What is the adrenal medulla supplied by?
Pre-ganglionic sympathetic fibres
Where is parasympathetic outflow from?
Cranial nerves 2, 7, 9, 10 and and sacral nerces S2-4
Where is the largest proportion of parasympathetic outflow from?
75% of outflow is from the vagus nerve
Why are the parasympathetic nerves pre-ganglionic?
Because parasympathetic ganglia are in the effector organs themselves
What effect does the vagus nerve have on the respiratory system?
Bronchoconstriction
What effect does the vagus nerve have on the CVS?
Bradycardia
Dilates arterioles
What effects does the parasympathetic nerve have on the GI system?
Stimulates stomach and intestine motility, gastric and pancreatic secretion
What neurotransmitters does the autonomic NS use?
Achetylcholine
Noradrenaline
What are all pre-ganglionic fibres?
Cholinergic
What are all post-ganglionic sympathetic nerves? What are the exceptions?
They’re all noradrenergic except sweat-glands and piloerector muscles
What neurotransmitters does the vagus nerve use?
Acetlycholine for both pre and post-ganglionic
What neurotransmitters do the majority of sympathetic nerves use?
Acetylcholine at preganglionic synapses
Noradrenaline at postganglionic sympathetic adrenergic synapses to heart and vessels (alpha and beta receptors).
Dopamine at postganglionic sympathetic dopaminergic synapses to renal vessels (D1 receptors).
What is the only sympathetic pathway using acetylcholine?
Sympathetic innervation of sweat glands/piloerector muscles.
Pre ganglia Acetycholine
Post ganglionic sympathetic holinergic acetlycholine (M2 receptors)
What neurotransmitters are used for sympathetic innervation to the adrenal glands?
Preganglionic acetylcholine.
No post synaptic nerve.
Adrenals produce adrenaline and noradrenaline which go into blood to act on the beta and alpha receptors at the heart and vessels.
Does the vagus nerve contain both afferent and efferent fibres?
Yes
Where do the parasympathetic nerves go?
The preganglionic fibres run almost to the organ innervated and then synapse in ganglia within the organ ie they do not have post ganglionic fibres
Does the vagus nerve play a role in bladder emptying?
No, because parasympathetic innervation of the bladder originates from S2-4 spinal cord segments and preganglionic fibres passing through the pelvic nerves.
Parasympathetic stimulation causes detrusor contraction and trigone + sphincter relaxation.
What effect does vagal stimulation have on dead space?
It decreases anatomical dead space, therefore decreasing physiological dead space - because it causes bronchoconstriction.
What effects does vagal stimulation have on insulin, gastrin, gastric acid and bile secretion?
All are increased.
Which organs are not under control of both sympathetic and parasympathetic nervous systems?
- lacrimal glands - parasympathetic only
- piloerector muscles - sympathetic only
- adipose tissue - sympathetic only
- juxtaglomerular apparatus - sympathetic only
