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

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

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

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

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

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

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

Sarcomere: basic contractile unit of the muscle Z 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

(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!)

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

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

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+

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.

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