Sensory Physiology

Sections 3.3-3.6

Regulatory Mechanism

Sensor

Controller

Effector

(Feedback)

Simple Nerve Pathway

Sensor

CNS (Interneurons)

Effector

Senso

ry Neu

ron Motor Neuron

E.g. Reflex - simple, stereotyped response; very fast

Complex Nerve Pathway

Sensor

CNS

Effector

Senso

ry Neu

ron Motor Neuron

More complex, modulated response; slower

Other sensors Other motor

Other sensors Other motor

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