Chap 5- Sensory system

What are sensory receptors?

Specialized neuron cells that detect stimuli and produce receptor potentials

Where are sensory receptors located?

In the PNS; they convey information to specific CNS targets

What is transduction?

Conversion of one form of energy into another (physical stimulus → electrical signal)

What is sensory transduction?

Conversion of pressure, touch, heat, etc. into neural impulses

What is a receptor potential?

Electrical signal produced by sensory receptors when stimulated; encodes intensity and duration of stimulus

How are receptor potentials produced?

By opening and closing of ion channels

Do all receptor potentials elicit action potentials?

No

What do receptor potentials encode?

Stimulus intensity (size of potential) and duration (duration of potential)

What is a receptive field?

Particular area in periphery where application of adequate stimulus causes the receptor to respond

Each receptive field corresponds to what?

One afferent fiber/receptor

What is a cutaneous receptive field?

An area of skin where a receptor's endings reside

What is an auditory receptive field?

A range of sound frequency

What is a color receptive field?

A range of wavelength

What does receptive field size encode?

The location of the stimulus

Which body areas have the smallest receptive fields?

Fingertips, lips, and tongue (finest touch discrimination)

How is receptive field size tested?

Two-point discrimination using calipers

What does perception of two points in two-point discrimination indicate?

Each point is activating different fibers

What does perception of one point in two-point discrimination indicate?

Both points are within a single receptive field

What is sensory receptor adaptation?

Receptors become less sensitive during a maintained stimulus

Which receptors are the EXCEPTION to adaptation?

Nociceptors (pain receptors) - they do NOT adapt

What are slowly adapting receptors good for?

Detecting static position, size, shape, and duration

What are rapidly adapting receptors good for?

Detecting change and movement of stimuli

What are the two types of afferent fiber endings?

Encapsulated and non-encapsulated (free nerve endings)

What are encapsulated endings?

Afferent endings surrounded by a capsule that modifies mechanical stimuli

How does encapsulation affect adaptation?

Capsules act as mechanical filters → rapidly adapting receptors

What are non-encapsulated endings?

Free nerve endings

How do afferent fibers differ by diameter?

Largest = proprioception; Intermediate = touch; Smallest = pain and temperature

List the 5 types of sensory receptors by stimulus type.

Chemoreceptors, Photoreceptors, Thermoreceptors, Mechanoreceptors, Nociceptors

What do chemoreceptors detect?

Smell, taste, pH, metabolite concentration

What do photoreceptors detect?

Photons (light) - located in retina

What do thermoreceptors detect?

Temperature

What do mechanoreceptors detect?

Physical deformation (touch, muscle tension, auditory, vestibular)

What do nociceptors detect?

Pain

Name 3 special animal receptors.

Magnetoreceptors (birds), infrared receptors (snakes), electroreceptors (fish)

What is somatic sensation?

Sensory information from skin and muscle including touch, proprioception, pain, and temperature

Where are cell bodies of sensory neurons located?

Dorsal root ganglia and cranial nerve ganglia

What are the 4 types of touch receptors in skin?

Merkel cells, Meissner corpuscles, Pacinian corpuscles, Ruffini endings

What type of touch do Merkel cells detect?

Fine touch (slowly adapting)

What type of touch do Meissner corpuscles detect?

Light touch (rapidly adapting)

What type of touch do Pacinian corpuscles detect?

Deep pressure and vibration (rapidly adapting)

What type of touch do Ruffini endings detect?

Skin stretch (slowly adapting)

What is proprioception?

Internal information about limb position, muscle force, tension, and joint stress

Where are muscle spindles found?

In most skeletal muscles

What activates muscle spindles?

Stretching of muscle fibers

What information do muscle spindles provide?

Changes in muscle length and velocity of stretch

Which muscles have dense populations of muscle spindles?

Jaw muscles and tongue

What are the fiber types for proprioception?

Ia and II fibers

What is the diameter of Ia/II fibers?

13-20 μm

What is the conduction velocity of Ia/II fibers?

80-120 m/s

What are the fiber types for touch?

Aβ fibers

What is the diameter of Aβ fibers?

6-12 μm

What is the conduction velocity of Aβ fibers?

35-75 m/s

What are the fiber types for pain and temperature?

Aδ and C fibers

What is the diameter of Aδ fibers?

1-5 μm

What is the conduction velocity of Aδ fibers?

5-30 m/s

What is the diameter of C fibers?

0.2-1.5 μm

What is the conduction velocity of C fibers?

0.5-2 m/s

What does "nociception" mean?

From Latin "nocere" = to hurt; pain sensation

When do nociceptors begin to respond?

When stimulus reaches intensive levels

How do nociceptors respond as stimulus intensity increases?

Their response continues to increase

How do thermoreceptors differ from nociceptors during painful stimuli?

Thermoreceptors do NOT increase firing rate in pain range

What did the pain experiment demonstrate?

Pain involves specialized neurons (nociceptors), not just overstimulation of thermoreceptors

What are Aδ nociceptors?

Rapidly conducting, thinly myelinated fibers responding to intense mechanical and/or thermal stimuli

What are C fibers?

Unmyelinated, polymodal nociceptors responding to thermal, mechanical, and chemical stimuli

What is "first pain"?

Sharp, fast pain mediated by Aδ fibers

What is "second pain"?

Delayed, diffuse, aching pain mediated by C fibers

What are Aδ fibers selective for?

More selective to type of stimuli (mechanical or thermal)

What are C fibers selective for?

Polymodal - respond to all three stimulus types (thermal, mechanical, chemical)

What is the tactile and proprioceptive pathway?

Dorsal Column-Medial Lemniscus pathway

Describe the 3-neuron pathway for touch.

1st: primary afferents to ipsilateral medulla; 2nd: decussates in medulla to thalamus; 3rd: thalamus to postcentral gyrus

Where do touch/proprioception fibers synapse?

In the ipsilateral medulla

Where do touch/proprioception fibers decussate?

In the medulla

Describe the 3-neuron pathway for pain.

1st: cell bodies in dorsal root ganglia; 2nd: decussates at spinal level to thalamus; 3rd: thalamus to cortex

Where do pain fibers decussate?

At the spinal level (spinothalamic tract)

Where are cell bodies for pain fibers?

Dorsal root ganglia and cranial nerve ganglia

What happens after pain fibers enter the spinal cord?

Processes run up and down several levels for multiple synaptic contacts

What happens with damage to one side of spinal cord at level A?

Ipsilateral loss of touch; Contralateral loss of pain (below lesion)

What happens with damage above medulla at level B?

Contralateral loss of BOTH touch and pain

Where is the primary somatosensory cortex located?

Postcentral gyrus

What are the four subdivisions of primary somatosensory cortex?

Area 3a, 3b, 1, and 2

What is the function of Area 3a?

Proprioception

What is the function of Area 3b?

Cutaneous stimuli

What is the function of Area 1?

Cutaneous stimuli

What is the function of Area 2?

Integrates proprioception and cutaneous stimuli

Where is somatosensory association cortex (SII) located?

Upper bank of lateral sulcus

Where does SII receive input from?

Converging projections from SI

Where does SII project to?

Amygdala and hippocampus

Why are SII projections to amygdala and hippocampus important?

For tactile memories and learning

Where else does SI project?

To parietal association areas

Where do parietal association areas project?

To motor areas (forms cortical sensorimotor circuit)

Are there direct projections from SI to MI?

Yes, rapid projections

What is the stretch reflex?

A monosynaptic reflex involving muscle spindles (mentioned in slide 10)

What is the 3-neuron pathway concept?

Sensory info travels via 1st (periphery to CNS), 2nd (crosses to thalamus), and 3rd (thalamus to cortex) neurons

How do encapsulated endings affect adaptation?

They are rapidly adapting due to mechanical filtering by the capsule

What is the adequate stimulus?

The type of stimulus to which a receptor is most sensitive

Can outputs from multiple receptors be combined?

Yes, to produce richer and more complex sensations

What are the two subsystems of somatic sensation?

1) Mechanical stimuli (touch, vibration, pressure, proprioception); 2) Painful stimuli and temperature

What is the function of dorsal root ganglia?

House cell bodies of sensory neurons

What is the function of cranial nerve ganglia?

House cell bodies of sensory neurons for cranial nerves

What is the relationship between stimulus intensity and receptor potential?

Greater intensity = larger receptor potential