Chapter 7

Nerves, sensation, and accessible design

Perception, cognition, action, feedback — four capabilities that fail independently

Human Biology for the Medical Maker

Why this chapter exists

"Can the user operate this control?" is at least four questions
Ample strength, insufficient pinch precision
Operates it perfectly, cannot see whether it latched
Sees and understands it, cannot reach it
Does it all, cannot feel the strap cutting in

The nervous system, organised

Central: brain and spinal cord; peripheral: the nerves
Afferent carries information toward the centre
Efferent carries commands away
Autonomic: heart rate, glands, the physiology of anxiety

How a neuron works

Dendrites receive; the cell body integrates
The axon conducts, sometimes over a metre
Myelin insulates in segments: the signal jumps
Conduction from about 0.5 to 120 metres per second

The resting potential and the action potential

All or nothing — intensity is coded by firing frequency and by recruiting more fibres

Synapses and the neuromuscular junction

Calcium entry releases neurotransmitter across a narrow cleft
The receiving cell sums excitatory and inhibitory inputs
Nerve to muscle: acetylcholine at the neuromuscular junction
Many drugs and toxins act exactly here

The somatosensory receptors

Receptor	Senses	Adaptation
Merkel complex	Sustained touch, edges, texture	Slow
Meissner corpuscle	Light touch, flutter, slip	Rapid
Pacinian corpuscle	Vibration, deep pressure	Very rapid
Ruffini ending	Skin stretch, sustained pressure	Slow
Free nerve endings	Warm, cold, potentially damaging stimuli	Varies
Muscle spindle / tendon organ	Muscle length / tension	Slow

Rapidly adapting receptors stop reporting a constant stimulus — a design hazard.

Two-point discrimination: receptor density varies

Site	Approximate threshold
Fingertip	2 to 4 mm
Palm	about 10 mm
Forearm	30 to 40 mm
Back	40 to 50 mm or more

Two raised dots 3 mm apart on a handle gripped by the palm are one dot.

Proprioception — the sense people forget

Body position and movement: spindles, tendon organs, joint receptors
Without it, you must watch to find your hand or grade a grip
Every task then consumes visual attention

Controls operable by feel — distinct shapes, clear detents — help everyone whose eyes are busy.

Pain is not a sensor reading

Absence of a pain report is not evidence of absence of harm. A design must not use the user's discomfort as its safety mechanism.

Reflexes — faster than awareness

The withdrawal has already happened by the time it hurts — and sustained pressure triggers no reflex

Engineering bridge: biosignals

Why this table is introductory biomedical instrumentation

The signals and their sizes

Signal	Typical amplitude	Bandwidth
ECG — heart	0.5 to 4 mV	0.05 to 150 Hz
Surface EMG — muscle	0.05 to 5 mV	10 to 500 Hz
EEG — brain	10 to 100 microvolts	0.5 to 100 Hz
EOG — eye movement	50 to 3500 microvolts	0 to 30 Hz
Nerve action potential	1 to 20 microvolts	100 to 2000 Hz
PPG — pulse	Optical	0.5 to 20 Hz

An absolute boundary

Never connect anything you have built to a person.

Patient-connected equipment is governed by IEC 60601-1. Bench and USB-powered equipment do not meet its isolation and leakage limits.

Psychophysics: the numbers behind "noticeable"

Just-noticeable difference grows with the baseline — Weber's law
A cue sized for a quiet workshop may vanish in a busy ward
Simple visual reaction about 200 ms; choices take longer
Your interface timeout is a design parameter with human limits behind it

Decompose any task: perception, cognition, action, feedback

Each stage can fail independently, and each is a separate intervention

Worked through: "can use the control"

Stage	Demand	Could fail because
Perception	See a small symbol	Acuity, contrast, lighting, glare
Cognition	Know what it means and the sequence	Unfamiliar convention, memory load
Action	Reach and pinch the latch	Range of motion; pinch force versus precision
Action	Hold it open; slide the part	Endurance; tremor; two hands needed
Feedback	Know it is fully engaged	Is there any indication at all?

The same decomposition is why WCAG is perceivable, operable, understandable, robust.

Designing for variation, not for a diagnosis

A diagnostic label is not a specification. The person is the expert on their own experience, priorities, and context.

Ask: what are you trying to do? How do you do it now? What becomes difficult, and when?

Accessible feedback: redundancy, not substitution

Single-channel cue	Fails for	Add
Colour only	Colour vision deficiency (about 8% of men of northern European descent), poor light	Shape, position, text, pattern
Sound only	Hearing loss, noisy ward, facing away	Visible state change, mechanical detent
Small text only	Low vision, wet or gloved hands	Larger type, contrast, tactile feature
Position only	Symmetrical parts get assembled wrongly	Asymmetric geometry

The strongest control is one where the wrong thing cannot be done.

Devices, software, and training technology

Clinical devices

EEG, EMG, nerve conduction, evoked potentials
TENS, functional electrical stimulation, deep brain stimulation
Cochlear implants and hearing aids
AAC devices; switch and alternative access hardware

Software and training

Screen readers — NVDA is free; accessibility test tools
Eye gaze, switch scanning, AAC apps, MNE-Python
Lumbar puncture and nerve block trainers
Monofilaments, tuning forks, two-point discriminators

The highest-leverage contribution in the book

Software: free tools, public standards, no patient contact
Run axe over something; navigate with a screen reader
Hardware: switch interfaces, mounting arms, wipeable enclosures, straps
Each individual, each made with an OT and the user

Education-grade biosignal kits are for learning only — never for a clinical judgement.

Worked example: two identical cord ends

First attempt: red and green — fails for colour deficiency
Text: "PULL TO BEND" and "SUPPORT — DO NOT PULL"
Different tab shapes, distinguishable by touch
A curved arrow, a fixed position, a line from pull point to segment
Keep colour — as a redundant cue

Then test it on someone who has never seen it, without hints.

Workshop exercise and exit criteria

Decompose a touchscreen button press into at least eight demands
Barrier review: five barriers, each tied to a stage, redesigned with costs
Apply redundant coding to your model and test it
Measure two-point discrimination on fingertip and forearm
State ECG, EMG, and EEG amplitudes — and why nothing you build touches a person

What this chapter does not establish

You cannot assess anyone's sensation, strength, or cognition
You cannot build patient-connected electronics
A one-handed or gloved trial generates hypotheses; it does not validate
Accessibility for a particular person is established with that person
You have not learned neurology

Next — The head and neck — eye, ear, nose, throat, and mouth.