Chapter 10a

Sensory Physiology

About this Chapter

• General properties of sensory systems• Somatic senses• Chemoreception: smell and taste• The ear: hearing• The ear: equilibrium• The eye and vision

Table 10-1 (1 of 2)

General Properties: Sensory Division

Table 10-1 (2 of 2)

General Properties: Sensory Division

Sensory Pathways

• Stimulus as physical energy sensory receptor• Receptor acts as a transducer

• Intracellular signal usually change in membrane potential

• Stimulus threshold action potential to CNS

• Integration in CNS cerebral cortex or acted on subconsciously

Figure 10-1a

Stimulus

Cell body

Unmyelinatedaxon

Free nerve endings

(a)

Somatosensory Receptors

Figure 10-1b

Stimulus

Enclosed nerveendingLayers of connectivetissue

Myelinated axon

Cell body

(b)

Somatosensory Receptors

Figure 10-1c

Stimulus

Myelinated axon

Cell body ofsensory neuron

Synaptic vesicles

Specialized receptorcell (hair cell)

Synapse

(c)

Somatosensory Receptors

Table 10-2

Sensory Receptors

Sensory Transduction

• Stimulus energy converted into information processed by CNS• Ion channels or second messengers initiate

membrane potential change

• Adequate stimulus: Preferred form of stimulus

• Threshold: Minimum stimulus• Receptor potential: Change in sensory

receptor membrane potential

Figure 10-2

The receptive fields of three primary sensory neuronsoverlap to form one large secondary receptive field.

Primary sensoryneurons

Secondarysensoryneuron

SECTION THROUGH SPINAL CORD

Information from thesecondary receptive

field goes to the brain.

The primary sensory neuronsconverge on one secondarysensory neuron.

Receptive Fields of Sensory Neurons

Figure 10-3a

One signal goes to the brain.

Compass with pointsseparated by 20 mm

Primarysensoryneurons

Skin surface

Secondarysensoryneurons

(a)

Sensory Neurons: Two-Point Discrimination

• Two-point discrimination varies with the size of the secondary receptive field

Two signals go to the brain.

Compass with pointsseparated by 20 mm

Primarysensoryneurons

Skin surface

Secondarysensoryneurons

(b)

Figure 10-3b

Sensory Neurons: Two-Point Discrimination

• Two-point discrimination varies with the size of the secondary receptive field

Integration by CNS

• Sensory information• Spinal cord to brain by ascending pathways• Directly to brain stem via cranial nerves

• Visceral reflexes integrated in brain stem or spinal cord usually do not reach conscious perception

• Perceptual threshold: level of stimulus necessary to be aware of particular sensation

Sensory Pathways

• Each major division of the brain processes one or more types of sensory information

Sensory Pathways

Figure 10-4

3

2

1

1

2

3

Olfactory pathways fromthe nose project throughthe olfactory bulb to theolfactory cortex.

Equilibrium pathways projectprimarily to the cerebellum.

Most sensory pathways projectto the thalamus. The thalamusmodifies and relays informationto cortical centers.

Eye

Nose

Tongue

Equilibrium

Sound

Brainstem

Visualcortex

Auditorycortex

Gustatory cortex Primary somaticsensory cortex

Olfactory cortex

Olfactory bulb

Cerebellum

Thalamus

Somaticsenses

Properties of Stimulus: Modality

• Indicated by where• Sensory neurons are activated • Neurons terminate in brain

• Specific to receptor type• Labeled line coding • 1:1 association of receptor with sensation

Properties of Stimulus: Location

• According to which receptive fields are activated

• Auditory information is an exception• Sensitive to different frequencies

• Lateral inhibition• Increases contrast between activated receptive

fields and inactive neighbors

• Population coding• Multiple receptors functioning together

Properties of Stimulus: Location

• The brain uses timing differences rather than neurons to localize sound

Figure 10-5

Properties of Stimulus: Location

• Lateral inhibition enhances contrast and makes a stimulus easier to perceive

Figure 10-6

Stimulus Stimulus

Primary neuronresponse is proportional

to stimulus strength.

Pathway closest tothe stimulus inhibits

neighbors.

Inhibition of lateralneurons enhances

perception of stimulus.

Tonic level

A B C

A B C

Tonic level

Skin

Pin

Primarysensoryneurons

Secondaryneurons

Tertiaryneurons

A B C

Fre

qu

ency

of

acti

on

po

ten

tial

sF

req

uen

cy o

f ac

tio

n p

ote

nti

als

Properties of Stimulus

• Intensity• Coded by number of receptors activated and

frequency of action potentials

• Duration• Coded by duration of action potentials• Some receptors can adapt or cease to respond

• Tonic receptors versus phasic receptors

Properties of Stimulus

• Sensory neurons use action potential frequency and duration to code stimulus intensity and duration

Figure 10-7

(a) Moderatestimulus

(b) Longer andstrongerstimulus

Duration

Amplitude

Axon terminalCell bodyMyelinated axonTrigger zoneTransduction site

Stimulus

Receptor potentialstrength andduration vary withthe stimulus.

Receptor potentialis integrated at thetrigger zone.

Neurotransmitterrelease varies withthe pattern of actionpotentials arrivingat the axon terminal.

Frequency of actionpotentials is proportionalto stimulus intensity.Duration of a series ofaction potentials isproportional to stimulusduration.

20

0

-20

-40

-60

-800 5 10 0 5 10 0 5 10

20

0

-20

-40

-60

-80

Time (sec)

0 5 10 0 5 10 0 5 10

Threshold

Mem

bra

ne

po

ten

tia

l (m

V)

Mem

bra

ne

po

ten

tia

l (m

V)

1 2 3 4

Tonic and Phasic Receptors

Figure 10-8a

Tonic and Phasic Receptors

Figure 10-8b

Somatic Senses: Modalities

• Touch• Proprioception• Temperature• Nociception• Pain• Itch

Somatic Senses Pathways

Fine touch,proprioception,

vibration

Nociception,temperature,coarse touch

SPINAL CORD

MEDULLA

THALAMUS

Pain, temperature, andcoarse touch cross themidline in the spinal cord.

Fine touch, vibration,and proprioceptionpathways cross themidline in the medulla.

Sensations are perceivedin the primary somaticsensory cortex.

Sensory pathwayssynapse in the thalamus.

Primary sensory neuron

Secondary sensory neuron

Tertiary neuron

KEY1 1

2 2

3 3

4 4

Figure 10-9

The Somatosensory Cortex

Figure 10-10

Touch Receptors in the Skin

Figure 10-11

Hair

Hair root

Free nerve ending ofnociceptor respondsto noxious stimuli.

Merkel receptorssense steady pressureand texture.

Meissner’s corpuscleresponds to flutter andstroking movements.

Sensory nervescarry signals tospinal cord.

Ruffini corpuscleresponds to skinstretch.

Pacinian corpusclesenses vibration.

Free nerve endingof hair root senseshair movement.

Free nerveending

Temperature Receptors

• Free nerve endings• Terminate in subcutaneous layers• Cold receptors• Lower than body temperature

• Warm receptors• Above body temperature to about 45°C• Pain receptors activated above 45°C

Nociceptors

• Free nerve ending• Respond to strong noxious stimulus that may

damage tissue• Modulated by local chemicals• Substance P is secreted by primary sensory

neurons• Mediate inflammatory response• Inflammatory pain

Nociceptors Pathways

• Reflexive protective response• Integrated in spinal cord• Withdrawal reflex

• Ascending pathway to cerebral cortex• Becomes conscious sensation (pain or itch)

Somatosensory Nerve Fibers

Table 10-5

Nociceptors: Pain and Itch

• Itch• Histamine activates C fibers causing itch

• Pain • Subjective perception• Fast pain • Sharp and localized—by A fibers

• Slow pain • More diffuse—by C fibers

The Gate-Control Theory of Pain

Figure 10-12a

The Gate Control Theory of Pain Modulation

Figure 10-12b

The Gate Control Theory of Pain Modulation

Figure 10-12c

Referred Pain

Figure 10-13b

Skin(usual stimulus)

Kidney(uncommon stimulus)

Primary sensoryneurons

Secondarysensoryneuron

Ascending sensorypath to somatosensorycortex of brain(b)

Pain

• Ischemia• Lack of adequate blood flow

• Chronic pain is a pathological pain• Analgesic drugs• Aspirin • Inhibits prostaglandins and slows transmission of

pain to site of injury