NOT DONE Lecture 6: Somatosensory, Taste, and Smell

What are sensory receptors?

Specialized cells or groups of cells that detect changes in the internal or external environment.

What are the two structural types of sensory receptors?

Primary and secondary receptors

What is a primary sensory receptor?

A receptor that is the ending of a sensory neuron itself.

What is a secondary sensory receptor?

A specialized epithelial cell that synapses onto a sensory neuron.

Give an example of a primary sensory receptor

Free nerve endings (pain, temperature).

Give an example of a secondary sensory receptor

Taste receptor cells and photoreceptors.

What does adaptation of sensory receptors mean?

A decrease in receptor response during a constant stimulus.

Why is adaptation important?

It maintains sensitivity to changes rather than constant stimuli.

What is a modality of sensation?

A principal type of sensation such as touch, pain, sight, or sound.

What is the adequate stimulus of a receptor?

The stimulus to which a receptor responds most easily.

What determines how a sensation is perceived?

The receptor’s characteristics and the central connections of its axon

What is the labeled-line principle?

Each receptor type has a direct line to the brain for one specific modality

Name the five types of sensory receptors

 • Mechanoreceptors
• Thermoreceptors
• Chemoreceptors
• Photoreceptors
• Nociceptors

Sensory receptors are __________ cells that detect changes in the environment.

specialized

Primary sensory receptors are part of the __________ neuron.

sensory

Secondary sensory receptors release __________ onto sensory neurons.

neurotransmitter

Taste receptor cells are __________ receptors.

secondary

Pain receptors are __________ receptors.

primary

The stimulus a receptor responds to most easily is its __________ stimulus.

adequate

The specificity of nerve fibers for one modality is called the __________ principle.

labeled-line

Mechanoreceptors detect __________.

deformation

Thermoreceptors detect __________.

temperature

Nociceptors detect ________

tissue damage

True or false: Sensory receptors are membrane proteins.

False; Sensory receptors are specialized cells or groups of cells. Some receptors use membrane proteins, but the receptor itself is a cell (e.g., taste receptor cells, photoreceptors).

True or false: Sensory receptors adapt to constant stimuli.

True

True or false: Primary receptors are separate cells from sensory neurons.

False: Primary receptors are the endings of sensory neurons themselves, not separate cells.

True or false: Secondary receptors generate action potentials directly.

False: Secondary receptors generate receptor potentials and release neurotransmitter; the sensory neuron generates the action potential.

True or false: Each receptor transmits only one type of sensation.

True

True or false: Chemoreceptors detect light.

False: Chemoreceptors detect chemicals (taste, smell, CO₂). Light is detected by photoreceptors.

What is a receptor potential?

A graded electrotonic potential generated in a sensory receptor.

Are receptor potentials graded or all-or-none?

Graded

Are receptor potentials local or propagated?

Local (non-propagated)

What is a generator potential?

A receptor potential that occurs in a primary sensory neuron.

What is a receptor potential (in secondary receptors)?

A graded potential that causes neurotransmitter release onto a sensory neuron

What triggers action potentials in primary receptors?

The generator potential reaching threshold at the trigger zone.

What triggers action potentials in secondary receptor systems?

Neurotransmitter release onto the afferent neuron.

Do receptor potentials have a refractory period?

No

Can receptor potentials summate?

Yes!

What ion conductance increases in a Pacinian corpuscle?

Sodium (Na⁺)

What causes receptor excitation?

A change in membrane potential (ΔVm)

When is an action potential generated?

When receptor potential current exceeds threshold.

Receptor potentials are __________ potentials.

Graded

Receptor potentials decay with __________ and __________.

time; distance

Mechanical distortion increases __________ conductance in Pacinian corpuscles.

Na+

Selective decreased Na⁺ permeability occurs in __________ receptors.

photoreceptors

True or false: Receptor potentials propagate without decrement.

False; Receptor potentials do not depend on voltage-gated Na⁺ channels like action potentials do, so drugs like lidocaine that block APs do not block receptor potentials.

True or false: Receptor potentials can be depolarizing or hyperpolarizing.

True

What is the relationship between stimulus strength and receptor potential?

They are proportional

What determines action potential frequency?

The size of the receptor potential.

What determines neurotransmitter release?

Action potential frequency.

What does intensity coding depend on?

The size and duration of the receptor potential

Stimulus strength is proportional to __________ potential.

receptor

Action potential frequency is proportional to receptor potential __________.

size

Low-intensity stimuli have __________ resolution.

greater

True or false: Neurotransmitter release depends on AP frequency

true

True or false: High-intensity stimuli have better resolution than low-intensity stimuli.

False; Low-intensity stimuli activate fewer receptors, giving better spatial resolution. High-intensity stimuli spread over more receptors → poorer localization.

What is receptor adaptation?

 A decrease in receptor potential or spike frequency during sustained stimulation.

What are slowly adapting receptors also called?

Tonic receptors

What are rapidly adapting receptors also called?

Phasic receptors

What do tonic receptors detect best?

Constant stimuli and body position.

What do phasic receptors detect best?

Movement and changes in stimulus

Rapidly adapting receptors respond mainly to __________.

change

Slowly adapting receptors keep the brain informed of __________.

body position

Pacinian corpuscles are __________ adapting receptors.

rapidly

True or false: Tonic receptors stop responding quickly to stimuli.

False; Tonic (slowly adapting) receptors continue firing as long as the stimulus is present. They signal constant conditions like posture or stretch.

True or false: Phasic receptors respond mainly at stimulus onset and offset.

true

What is sensory coding?

Conversion of stimulus energy into neural signals.

What is spatial summation?

Increasing the number of stimulated fibers.

What is temporal summation?

Increasing firing rate in the same fibers

What improves stimulus localization?

Lateral inhibition

Signal intensity can be increased by __________ summation or __________ summation.

spacial; temporal

Lateral inhibition increases __________.

acuity

True or false: Lateral inhibition suppresses neurons at the stimulus center.

False; Lateral inhibition suppresses neurons at the edges of the stimulus, not the center. This enhances contrast and helps the brain localize the stimulus more precisely.

True or false: Temporal summation increases firing frequency.

True

Generator potentials occur in __________ receptors.

primary

Secondary receptors convert stimuli into __________ release.

neurotransmitter

Action potentials are produced by the __________ neuron, not the receptor cell.

sensory

What is a receptive field?

The area of the body monitored by a single sensory neuron

What happens when receptive fields are small?

Localization is more precise

What happens when receptive fields are large?

Poor localization

Where are receptive fields smallest?

Fingertips and lips

What is the effect of convergence on sensitivity?

Increases sensitivity but reduces localization.

What is the effect of divergence?

Spreads sensory information to multiple brain areas.

Divergence allows sensory information to reach __________ areas.

multiple

True or false: Divergence allows parallel processing of sensory information.

True!

What is sensory transduction?

Conversion of stimulus energy into a change in membrane potential.

What types of channels are involved in mechanoreceptors?

Stretch-gated ion channels

What type of channels are involved in photoreceptors?

Ligand-gated channels controlled by second messengers

What happens to ion permeability during transduction?

It changes to alter membrane potential

Sensory transduction converts stimulus energy into a __________ potential

receptor

Mechanoreceptors use __________-gated ion channels.

mechanically

Photoreceptors use __________ messenger systems.

second

True or false: All sensory receptors use the same ion channels.

False: Different receptor types use different transduction mechanisms (mechanical, chemical, or second-messenger systems).

True or false: Sensory transduction always causes depolarization.

False: Some receptors hyperpolarize (e.g., photoreceptors).

How many neurons are typically in an ascending sensory pathway?

3 or more

What does the first-order (primary) sensory neuron do?

Carries information from the receptor to the spinal cord or medulla

What does the second-order (secondary) neuron do?

Decussates and projects to the thalamus.

What does the third-order (tertiary) neuron do?

Projects from the thalamus to the somatosensory cortex

Where are primary sensory neuron cell bodies located?

Dorsal root ganglia (spinal) or sensory cranial nerve ganglia

Where do secondary neuron cell bodies lie?

Spinal cord or medulla