Sensory systems 1-4

What is the main job of sensory systems?

To convert environmental energy (touch, temperature, light, sound, chemicals) into electrical signals the CNS can interpret.

What must all sensory information become before entering the CNS?

Graded receptor potentials, which then trigger action potentials in sensory neurons.

What determines whether a sensation becomes conscious?

Signals must reach the appropriate cortical area (somatosensory, visual, auditory cortex); without cortex, no conscious perception occurs.

What is the difference between retinal and cortical blindness?

Retinal blindness means no signal reaches the brain; cortical blindness means the eyes work but the cortex cannot interpret signals, causing "blindsight."

What is topographic organization in sensory systems?

The rule that neighboring receptors remain neighbors in the CNS, preserving body maps like somatotopy, retinotopy, and tonotopy.

What is sensory transduction?

The process where a receptor converts a stimulus into a graded receptor potential by opening ion channels.

How does the brain encode stimulus intensity using activity code?

A single neuron increases its firing rate when the stimulus becomes stronger.

How does the brain encode intensity using population code?

More neurons become activated when the stimulus is stronger, increasing total activity.

What is the difference between phasic and tonic receptors?

Phasic receptors fire only when the stimulus changes (rapidly adapting), while tonic receptors fire continuously as long as the stimulus is present (slowly adapting).

What is the labelled-line principle?

Each sensory pathway is dedicated to one modality, so the brain interprets any activity in that pathway as that modality regardless of the true stimulus.

What is an example of the labelled-line principle in action?

Paradoxical cold, where stimulating a warm receptor accidentally triggers the cold pathway, and the brain incorrectly "perceives" cold.

What is pattern coding?

A method where sensations are represented by activation patterns across multiple receptors, allowing complex perception from limited receptor types.

Where do all somatic sensory neurons enter the CNS?

Through the dorsal root ganglion, which contains the sensory neuron cell bodies.

What do nociceptors detect?

Actual or potential tissue damage, including harmful mechanical, thermal, or chemical stimuli.

Why does injured tissue become more sensitive?

Inflammatory chemicals lower nociceptor thresholds, making them fire more easily and increasing perceived pain.

What do extreme mechanoreceptors respond to?

Intense mechanical pressure such as pinching, cutting, or crushing, producing sharp, localized pain.

What do extreme temperature receptors respond to?

Very hot or very cold temperatures outside normal ranges, producing burning or freezing pain.

What do polymodal nociceptors respond to?

Multiple noxious stimuli including mechanical stress, chemical irritation, and inflammatory mediators like prostaglandins and bradykinin.

Why don't nociceptors adapt?

Because continued firing is necessary to warn the brain of ongoing harm or potential tissue damage.

What do free nerve endings detect?

Pain, temperature, and light touch; they adapt slowly and are widely distributed across the skin.

Why are Merkel discs important for fine touch?

They adapt slowly and continue firing during sustained pressure, allowing detection of shapes, edges, and details.

Where are Meissner corpuscles found and what do they detect?

They are found in glabrous skin and detect fine touch and low-frequency vibration; they adapt rapidly and signal changes in contact.

What do Pacinian corpuscles detect?

Deep pressure and high-frequency vibration, but they adapt so rapidly that they're not useful for palpation.

What do Ruffini endings detect?

Skin stretch, joint deformation, finger position, and deep warmth; important for proprioception of hand shape.

What do muscle spindles detect?

Muscle stretch; they use intrafusal fibers and trigger reflex contraction when stretched.

What do Golgi tendon organs detect?

Muscle tension, especially during contraction, helping regulate force output.

How does cutaneous proprioception work?

Skin stretch during movement activates mechanoreceptors, supplementing joint and muscle proprioception.

What happens to proprioception after joint or muscle injury?

Joint, tendon, and muscle receptor damage reduces proprioceptive feedback, increasing reinjury risk.

How does kinesiology tape improve proprioception?

By increasing skin stretch input to cutaneous receptors and enhancing awareness when deeper proprioceptors are damaged.

What do rapidly adapting receptors tell the brain?

They detect changes in a stimulus, firing mainly at the beginning or end, helping signal movement or transitions.

What do slowly adapting receptors tell the brain?

They detect sustained stimuli, continuing to fire as long as the stimulus is present, helping with continuous touch or pressure perception.

Why does clothing become unnoticeable after a few minutes?

Phasic mechanoreceptors adapt quickly and stop firing once the stimulus is constant.

Why are tonic receptors important for reading braille?

They continue firing under sustained pressure, allowing continuous edge and texture detection.

How does the CNS determine the duration of a stimulus?

By analyzing whether receptors continue firing (tonic) or only fire at onset/offset (phasic).

What determines the quality or type of sensory stimulus?

The specific sensory pathway (labelled line) that carries the signal to the brain.

Why can electrical stimulation of the optic nerve make someone "see" light?

Because the optic pathway always produces visual perception when activated, regardless of the actual stimulus.

Why do different chocolates (e.g., Hershey vs Cadbury) taste different?

They activate different combinations of taste receptors, creating distinct pattern codes.

What is somatotopy?

The preserved mapping of body surface regions onto corresponding regions of the somatosensory cortex.

What is retinotopy?

The preserved spatial mapping of the retina onto the visual cortex, maintaining visual field organization.

What is tonotopy?

The mapping of sound frequency along the cochlea and onto the auditory cortex.

What triggers a receptor potential?

The opening of stimulus-specific ion channels in response to mechanical, thermal, chemical, or light energy.

What determines whether a receptor potential produces an action potential?

The receptor potential must reach threshold at the sensory neuron's trigger zone.

Why can stronger stimuli produce more action potentials even though AP size is always the same?

Stronger stimuli produce higher firing frequency, not larger action potentials.

What does a higher action potential firing rate indicate?

A stronger stimulus intensity at the receptor.

Why does pressing harder on the skin increase sensation?

The stronger stimulus activates more receptors (population code) and increases firing rate (activity code).

Why do cold and warm receptors use separate pathways?

To ensure precise temperature discrimination and to preserve the labelled-line organization.

Why can extreme temperatures feel painful?

Once temperature crosses safe physiologic ranges, nociceptors are activated and generate pain signals.

Why do burn injuries hurt more hours after the injury?

Inflammatory chemicals sensitize polymodal nociceptors, lowering their threshold and increasing firing.

What is the functional significance of Merkel discs in palpation?

They provide sustained, detailed pressure information necessary for detecting texture, edges, and shape.

Why are Meissner corpuscles important during object manipulation?

They detect slipping or changes in contact, allowing fine grip adjustments.

What makes Pacinian corpuscles unsuitable for deep palpation?

They adapt extremely quickly and stop firing almost instantly under sustained pressure.

Why are Ruffini endings important during grasping movements?

They detect skin stretch and finger position, contributing to hand-shape proprioception.

Why is proprioception impaired after an ankle sprain?

Joint capsule receptors and mechanoreceptors are damaged, reducing feedback needed for balance and movement control.

What neural structure contains the cell bodies of primary sensory neurons?

The dorsal root ganglion (DRG).

Why does stretching a muscle cause a reflex contraction?

Muscle spindles detect the stretch and activate a spinal reflex to resist it.

Why do GTOs fire more during active contraction than passive stretch?

Contraction increases tendon tension strongly, activating Ib afferents that monitor force output.

How can taping help an athlete recovering from a rotator cuff injury?

It enhances cutaneous feedback, improves proprioception, and assists drainage of inflammatory fluid.

Why does loss of proprioception increase reinjury risk?

Without accurate position sense, joints move less predictably, increasing microtrauma and instability.

What is the role of mechanoreceptors in joint stability?

They provide continuous feedback about joint position and movement, allowing protective reflexes and coordinated control.

Why does the brain require receptor potentials before action potentials?

Receptor potentials encode stimulus intensity and duration, which APs alone cannot represent.

Why is the retina considered part of the CNS?

Because it develops from the neural tube and contains central nervous system neurons

What is the primary function of the retina?

To convert light energy into electrical signals

Why does damage to different retinal regions cause different visual deficits?

Because photoreceptor and ganglion cell density varies across the retina

What retinal structure provides the highest visual acuity?

The fovea

Why does the fovea provide high acuity vision?

It has extremely high cone density and minimal convergence

What type of photoreceptors dominate the fovea?

Cones

What is the macula?

A retinal region surrounding the fovea responsible for highest central visual acuity

Why is the vertebrate retina described as "backwards"?

Light passes through neural layers before reaching photoreceptors

Which structure focuses light but does not perform transduction?

The lens

What path does light take before reaching photoreceptors?

Cornea → lens → vitreous humor → retina

What anatomical structure causes the blind spot?

The optic disc

Why are there no photoreceptors at the optic disc?

It is occupied by ganglion cell axons forming the optic nerve

Why is the blind spot usually not perceived?

The other eye compensates and the brain fills in missing information

Which photoreceptors mediate low-light vision?

Rods

Which photoreceptors mediate color and fine detail vision?

Cones

Approximately how many rods are in the retina?

About 120 million

Approximately how many cones are in the retina?

About 5 million

What is the resting membrane potential of photoreceptors in the dark?

Approximately −40 mV

Why are photoreceptors depolarized in the dark?

cGMP-gated sodium channels are open

What second messenger keeps sodium channels open in darkness?

cGMP

What neurotransmitter is continuously released in the dark?

Glutamate

What happens to rhodopsin when light strikes it?

It splits into retinal and opsin

Which G-protein is activated in phototransduction?

Transducin

What enzyme does transducin activate?

cGMP phosphodiesterase (PDE)

What is the effect of PDE on cGMP levels?

It decreases cGMP

What happens to sodium channels when cGMP decreases?

They close

What happens to photoreceptor membrane potential in light?

It hyperpolarizes to about −70 mV

What happens to glutamate release in light?

It decreases or stops

What is the key phototransduction rule for exams?

Light causes hyperpolarization and decreased glutamate release

What type of glutamate receptors do OFF bipolar cells have?

Ionotropic AMPA receptors

How does glutamate affect OFF bipolar cells?

It excites them

When are OFF bipolar cells most active?

In darkness

What visual information do OFF bipolar cells encode?

Decreases in light (darkness)

What type of glutamate receptors do ON bipolar cells have?

Metabotropic glutamate receptors (mGluR)

How does glutamate affect ON bipolar cells?

It inhibits them

Why does light depolarize ON bipolar cells?

Reduced glutamate removes inhibition, allowing sodium channels to open

What visual information do ON bipolar cells encode?

Increases in light

Which retinal neurons fire action potentials?

Ganglion cells

Which cells use graded potentials instead of action potentials?

Photoreceptors and bipolar cells

When do ON-center ganglion cells fire action potentials?

When light increases in the center of the receptive field