Sensory & Motor MechanismsChapter 50

Sensory receptors transduce stimulus energy and transmit signals to the CNS

Stimuli = forms of energy Sensation involves converting energy

into a change in the membrane potential of sensory receptors

Sensations are action potentials that reach the brain via sensory neurons

The brain interprets sensations, giving the perception of stimuli

Sensory pathway

Fig. 50-2

Slight bend:weakstimulus Stretch

receptor

Mem

bra

ne

pote

nti

al

(mV

)

Axon

Dendrites

Strong receptorpotential

Weakreceptorpotential

Muscle

–50

–70

Mem

bra

ne

pote

nti

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(mV

)

–50

–70

Action potentials

Action potentials

Mem

bra

ne

pote

nti

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(mV

)Large bend:strongstimulus

Reception

Transduction

0

–70

0

–70

1 2 3 4 5 6 7

Mem

bra

ne

pote

nti

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(mV

)

Time (sec)

1 2 3 4 5 6 7Time (sec)

Transmission Perception

Brain

Brain perceiveslarge bend.

Brain perceivesslight bend.

1 2

34

1

2 3 4

0

0

Sensory Reception Detection of stimulus Sensory receptors

Detect heat, light, pressure, chemicals Blood pressure, body position

Sensory transduction Conversion of stimulus to change in

membrane potential Charge difference in membrane due to ions

Transmission Passage of nerve impulse along axons

and across synapses Sensory cells without axons release

neurotransmitters at synapses with sensory neurons

Larger receptor potentials generate more rapid action potentials

Integration of sensory information begins when information is received

Perception Interpretation of sensory system input

by brain Ex: colors, smells, sounds, tastes Is there a sound if a tree falls and no

one is around to hear it? Action potentials = all or none!!

Modification of stimuli Amplification

Strengthening of stimulus energy During transduction Produces many product molecules by one

enzyme Adaptation

Decrease in responsiveness Allows you to filter stimulus

Types of Sensory Receptors Mechanoreceptors

Sense physical deformation Pressure, touch, stretch, motion, sound

Chemoreceptors Both general and specific General = total solute concentration Specific = chemicals that attach to specific receptor

proteins Electromagnetic receptors

Detect electromagnetic radiation Light, electricity, magnetism

Types of Sensory Receptors Thermoreceptors

Detect heat and cold Pain receptors

Extreme pressure or temperature Nocireceptors

Detect noxious conditions

Ex: Hearing & Equilibrium Mechanoreceptors produce receptor potentials

settling particles or moving fluid cause deflection of cell surface structures

Hairs Different stiffness and lengths Cause vibrations of different frequencies

Statocysts Sense gravity & maintain equilibrium Grains of sands Gravity settles sand to bottom stimulates receptor

Hearing in mammals Ear converts energy of pressure waves

to nerve impulses Mechanoreceptor = hair cells Signal is amplified before it reaches the

hair cell

Hearing in mammals (cont) 1. Moving air causes tympanic membrane to

vibrate 2. 3 bones transmit vibrations to oval window

– membrane on cochlea’s surface 3. when bone vibrates on oval window,

pressure waves created in fluid 4. in vestibular canal, pressure causes hairs

to vibrate up and down 5. mechanoreceptors open or close ion

channels in membrane

Fig. 50-8

Hair cell bundle froma bullfrog; the longestcilia shown areabout 8 µm (SEM).

Auditorycanal

EustachiantubePinna

Tympanicmembrane

Ovalwindow

Roundwindow

Stapes

Cochlea

Tectorialmembrane

Incus

Malleus

Semicircularcanals

Auditory nerveto brain

Skullbone

Outer earMiddle

ear Inner ear

Cochlearduct

Vestibularcanal

Bone

Tympaniccanal

Auditorynerve

Organ of Corti

To auditorynerve

Axons ofsensory neurons

Basilarmembrane

Hair cells

Sound variables Volume

Determined by amplitude of sound wave Larger volume = greater bending of hairs

Pitch Determined by sound wave’s frequency High frequency = high pitch

Equilibrium in mammals Inner ear detects movement, position and

balance Utricle & Saccule

Chambers located behind oval window Sheet of hair cells that go into a gelatinous

material Contains otoliths

Semicircular canals Connected to utricle Detect turning of the head

Muscle Contraction Skeletal muscle

Striated Connected to bones

Thick filaments Staggered arrays of myosin

Thin filaments 2 strands of actin and 2 strands of a

regulatory protein coiled

Skeletal muscle Sarcomere

Repeating unit Z lines M lines

Fig. 50-25b

TEM

Thickfilaments(myosin)

M line

Z line Z line

Thinfilaments(actin)

Sarcomere

0.5 µm

Sliding Filament Model Thin and thick filaments slide past each

other increasing the overlap of the fibers

Head of myosin Binds ATP to provide energy for muscle

contraction Tail of myosin

Adheres to other tails of myosin to form the thick filament

Muscle fiber contraction Myosin head is bound to ATP (low

energy)

Muscle Fiber Contraction Myosin hydrolyzes ATP to ADP now in

high E

Muscle Fiber Contraction

Myosin head bindsto actin = cross-bridge

Muscle Fiber Contraction

ADP is released, myosin returns to low E, thin filamentslides