1. Sensory and Motor Mechanisms
Chapter 50
Campbell Biology – 9th Edition

2. You must know
The location and function of several types of sensory receptors
How skeletal muscles contract
Cellular events that lead to muscle contraction

3. Sensory Receptors
Mechanoreceptors: physical stimuli – pressure, touch, stretch, motion, sound
Thermoreceptors: detect heat/cold
Chemoreceptors: transmit solute conc. info – taste (gustatory), smell (olfactory)
Electromagnetic receptors: detect EM energy – light (photoreceptors), electricity, magnetism
Pain receptors: respond to excess heat, pressure, chemicals

4. This rattlesnake and other pit vipers have a pair of infrared receptors, one between each eye and nostril. The organs are sensitive enough to detect the infrared radiation emitted by a warm mouse a meter away.
Eye
Infrared
receptor
Some migrating animals, such as these beluga whales, apparently sense Earth’s magnetic field and use the information, along with other cues, for orientation.
Chemoreceptors: antennae of male silkworm moth have hairs sensitive to sex phermones released by the female

5. Reception: receptor detects a stimulus
Sensation = action potentials reach brain via sensory neurons
Perception: information processed in brain

6. Muscles always contract
Muscles work in antagonistic pairs to move parts of body
Biceps
contracts
Human
Triceps
relaxes
Forearm
flexes
extends
Extensor
muscle
Flexor
Grasshopper
Tibia

7. Skeletal Muscle Structure
Bundle of
muscle fibers
Single muscle fiber
(cell)
Plasma membrane
Nuclei
Muscle
Myofibril
Dark band
Sarcomere
Z line
Light
band
I band
TEM
A band
0.5 µm
M line
Thick filaments
(myosin)
H zone
Thin filaments
(actin)
Attached to bones by tendons
Types of muscle:
smooth (internal organs)
cardiac (heart)
Skeletal (striated)
1 long fiber = single muscle cell
Each muscle fiber = bundle of myofibrils, composed of:
Actin: thin filaments
Myosin: thick filaments

8. Sarcomere: basic contractile unit of the muscleZ H A
Relaxed muscle fiber I
Contracting muscle fiber
Fully contracted muscle fiber
Z lines – border
I band – thin actin filaments
A band – thick myosin filaments

9. (Note: Filaments do NOT shorten!)
Muscle Contraction:
Sarcomere
0.5 µm Z H A
Relaxed muscle fiber I
Contracting muscle fiber
Fully contracted muscle fiber
Sarcomere relaxed: actin & myosin overlap
Contracting:
Muscle fiber stimulated by motor neuron
Length of sarcomere is reduced
Actin slides over myosin
Fully contracted: actin & myosin completely overlap
Sliding-filament model: thick & thin filaments slide past each other to increase overlap
(Note: Filaments do NOT shorten!)

10. Muscle fibers only contract when stimulated by a motor neuron
Ca2+ released
from sarcoplasmic
reticulum
Mitochondrion
Motor
neuron axon
Synaptic
terminal
T tubule
Sarcoplasmic
Myofibril
Plasma membrane
of muscle fiber
Sarcomere

11. Muscle fiber depolarizes Ca2+ released Initiate sliding of filaments
Synaptic terminal of motor neuron releases acetylcholine
Muscle fiber depolarizes
Ca2+ released
Initiate sliding of filaments
Ca2+
CYTOSOL SR
PLASMA
MEMBRANE
T TUBULE
Synaptic cleft
Synaptic terminal
of motor neuron
ACh

12. Depolarization of muscle cell releases Ca2+ ions  bind to troponin  expose myosin sites on actin
Myosin-binding sites blocked.
Myosin-binding sites exposed.
Tropomyosin
Ca2+-binding sites
Actin
Troponin complex
Myosin-
binding site
Ca2+

13. Hydrolysis of ATP by myosin  cross-bridge formed  thin filament pulled toward center of sarcomere
Thin filaments
Thick filament
Thin filament
Thick
filament
Myosin head (low-energy
configuration)
Cross-bridge
binding site
Myosin head (high-
energy configuration)
Actin
Myosin head (low-
Thin filament moves
toward center of sacomere.

14. Speed of muscle contraction:
Fast fibers – brief, rapid, powerful contractions
Slow fibers – sustain long contractions (posture)