Sensory Physiology
Sections 3.3-3.6
Regulatory Mechanism
Controller
Effector
Sensor
(Feedback)
Simple Nerve Pathway
CNS (Interneurons)
Sensor
Effector
E.g. Reflex - simple, stereotyped response; very fast
Complex Nerve Pathway
Other sensors
Sensor
Other sensors
Other motor
CNS
Other motor
Effector
More complex, modulated response; slower
Simple Nerve Pathway:
Spinal Reflexes
• Stretch spindle fiber in
muscle → sensory
neuron
• sensory neuron
synapses directly with
motor neuron in CNS
• Motor neuron →
muscle
Inhibitory Stretch Reflex
• Golgi tendon organ
• sensory neuron synapses
w/interneuron, which
synapses w/motor neuron
• Inhibits MN, thus
prevents contraction
Reciprocal Innervation
• Activation of both
excitatory and inhibitory
motor responses in
antagonistic muscles
• Prevents both muscles
from contracting
simultaneously
Experiment: Spinal Reflexes
• Whack your partner!
– Patellar reflex
– Achilles reflex
– Biceps reflex
Punctate Distribution of
Cutaneous Sensors
• Different sensations
perceived at different
points on surface of the
skin
• Different sensor types
distributed throughout skin.
• Perception localized to
specific points
Acuity
• Acuity
– ability to discriminate size, shape of an object in the
environment
• Determined by size of receptive field
– area that, if stimulated, will cause a response from a
single sensory neuron
•  receptor density,  receptive field size, acuity
Touch Acuity
Large Fields, Low Density
Small Fields, High Density
Experiment:
Two Point Touch Discrimination
• Subject should have eyes closed
• Start with tines ~2.5 cm apart
• Touch to subject’s skin w/ both tines
simultaneously
• If they can feel two points, close
slightly and repeat.
• Repeat until subject can feel only
one pt.
• Distance btw tines at pt where
subject loses ability to feel two pts
= diameter of receptive field
• Test
–
–
–
–
Index finger
Palm
Lower Arm
Nape of Neck
Sensory Adaptation
• Response of sensors to constant
stimulation
• Phasic receptors
– exhibit sensory adaptation
– firing rate of receptor (# AP’s)
decreases with constant stimulus
• Tonic receptors
– exhibit little adaptation
– maintain constant firing rate as
long as stimulus is applied
Experiment:
Thermoreceptor Adaptation
• Place one hand in cold water, the other in
hot water for 60 seconds
• Place both hands simultaneously in warm
water.
• What do you feel in each hand?
Sensory Pathways and Perception
• sensors are transducers
– convert environmental change into an electrical signal
• CNS interprets electrical signals
– not the environmental changes directly
– e.g. blow to head perceived as flash of light
• CNS interprets origin of stimuli based upon
neurons that deliver sensory info. into CNS
– Can deceive CNS with respect to origin
Sensory Pathways and Perception
• Referred pain
– perception of pain originating from location other than
actual site of tissue damage
– E.g. phantom limb pain
• Irritation of severed nerve endings induces AP’s
• CNS perceives stimulus being applied to limb not present
– E.g. angina pectoris
• Damage to heart perceived in left chest, shoulder & arm
• pain sensors of visceral organs often use shared pathways of
interneurons leading to the brain
Sensory Pathways and Perception