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Summary NHB (Neurobiology and Human Behaviour) Neurophysiology Practical Notes
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University of Cambridge (Medicine 1st class): Summary of key points of NHB (Neurobiology and Human Behaviour) Neurophysiology practicals for 2nd year.

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  • June 29, 2023
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  • 2022/2023
  • Summary

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NHB (Neurobiology and Human Behaviour)
Neurophysiology Practical Notes
Structure and function of eye
Mapping visual field and blind spot
Using perimeter
Meridians (i.e. angles from the vertical)
Nasal and temporal parts of eye field swapped on retina
Hunting animals have forward-facing eyes so larger binocular than monocular
field, prey have inverse
Fovea is central 5˚ (1-1.5mm, macula is ~6mm) but 30-50% of V1
Peripheral vision is needed for posture and walking
Range of accommodation is difference in power at near and far
Refractive error = (1/F - 2)
● 2 is for a lens
● F is furthest distance (m) that is in focus

Mapping blind spot
Paper is 28.5cm away from eye so 1 cm along the paper is ~2˚
● TOA: O is 1, A is 28.5, so angle is tan^-1(1/28.5) ~= 2˚
● Since 1 radian = 57˚, 28.5cm along the paper would be ~57˚, 1/28.5 of
that would be ~2˚
Blind spot is centred at 16-18˚ temporally (on visual field) from centre
Blind spot is 5-10˚ in diameter so 1.5 to 3mm on retina

Measuring visual acuity
An object 1.0 cm in size, viewed from a distance of 57.0 cm subtends an angle
of 1.0 degree.
300 µm on the retina is about 1 degree.
The slenderest foveal cone has a diameter of about 2.3 µm (30 seconds)
Retina is 17mm from the optics of the eye
Eye is ~20mm in diameter
Opticians charts are based on ‘normal’ acuity being 1 minute but its actually
40-30 seconds
you are nearly ten times better at detecting a vernier offset than the gap
between the prongs of a letter E
Thickness of retina
● Parafovea (where GC layer is thickest): ~418µm
○ Near optic disk is also thick because of GC axons not cell bodies
● Peripherally: ~200µm
Thickness of innermost layer of axons (GC axons)
● Parafovea: ~30µm
● Near optic disk: ~200µm

Histology of retina
The slender, lightly-stained outer segments of the receptors contain the
photopigments. The more substantial inner segments contain the densely-

, staining (pink) ellipsoids (bigger in cones than in rods).
The layer of GC axons is separated from the vitreous humour by the inner
limiting membrane, which is made of the dilated ends of Müller's cells
Viewing the retina
● (Scanning laser) ophthalmoscope for retina
○ Light does not go through lens
● Slit lamp (with Volk lens) for cornea
● Optical Coherence Tomography

Cutaneous sensation and ischaemic nerve block
Detection, localisation (is second stimulus in same place as former) and
resolution (are two close stimuli perceived as one) threshold
Detection
● Use Von Frey hair on non-hairy skin (otherwise indentation will also be
caused by disturbing hairs)
● compression is related to the length and thickness of the bristle

Ischaemia
Can cut off blood supply by inflating cuff to 160mmHg
All function is usually lost aft ~20mins, not safe to keep going past 30mins
Motor function
● Pincer grip plastic, record force to pull from grasp

Order of loss (theoretically)
● Large fibres (touch, vibration (Pacinian corpuscles have some of the
largest myelinated fibres), body position sense (proprioception),
muscle strength (Aa))
○ But grip strength can survive after light touch which goes against

muscles being all Aa (bigger) and touch being Ab
● small myelinated fibres (pin-prick pain, cool/cold)
● unmyelinated C fibres (warmth (pleasant sensation), itch, unpleasant
burning pain).

Observations
● 2-7mins: mild pins and needles that can be relieved by rubbing
● 10-15mins: paralysis of small hand muscles
○ Touch, deep pressure and position sense are lost before warmth,

cold and pain
○ But all are lost distal to proximal
○ Slow pain is lost later and becomes more unpleasant
● 12-15mins: numbness in fingertips
● 20-25mins: arm below cuff is mostly paralysed and anaesthetised
(except for slow pain)
● Last stages, only C fibres left
○ stimuli feel painful which had previously felt innocuous or (at least)

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