Chapter 49

A Brief Overview of Sensory & Motor Mechanisms – Focus on

Muscle Contraction

Sensory Reception

Sensory receptors are modified neurons or cells that detect stimuli & typically trigger action potentials in neurons.

The action potentials are propagated until they reach an integration center – located in the CNS.

Then, responses can be initiated.

Sensory Reception

So, if you put your hand on a hot oven, pain receptors in your skin trigger action potentials and your brain triggers a response – your muscles contract and you remove your hand.

You have many types of receptors: those that respond to pressure, touch, sounds, pain, temperature, chemicals, light…

Muscle Structure & Contraction

1.) Our skeletal muscle (the muscle we can control the movement of) is composed of a bundle of muscle cells called fibers.

a.) Each fiber is a bundle of myofibrils consisting of myofilaments (protein strands):

i.) Thin filaments consisting of the protein actin.

ii.) Thick filaments consisting of the protein myosin.

Muscle Structure & Contraction

2.) The regular arrangement of the filaments produces a pattern of bands and repeating units.

a.) The repeating units are called sarcomeres – these are the functional contracting units of muscles.

Muscle Structure & Contraction

3.) Structure of sarcomere:

a.) The end of each sarcomere is called the Z line.

b.) Thin filaments (actin) originate at Z lines and project toward the center.

c.) Thick filaments (myosin) lie in the center forming what is called the A band.

Muscle Structure & Contractiond.) There is an area of overlap of thick & thin filaments. The area where thin filaments don’t reach is called the H zone.

e.) The area where only thin filaments are is the I band.

Muscle Structure & Contraction

4.) Sliding-filament model of muscle contraction:

a.) Thick & thin filaments don’t change in length – they just slide past each other to shorten the sarcomere causing the muscle to shorten = contract.

b.) Myosin hydrolyzes ATP, changes shape & binds to actin filament.

c.) Myosin then relaxes & pulls the actin filament toward the center of the sarcomere.

d.) The cycle repeats for furthering shortening of the filaments.

Muscle Structure & Contraction

5.) Calcium regulates contraction:

a.) At rest, the myosin binding sites on actin are blocked by a protein (tropomyosin).

b.) When an action potential in a neuron reaches the synapse b/w a neuron & a muscle cell (called the neuromuscular junction), the NT acetylcholine is released.

Muscle Structure & Contraction

c.) This causes Ca to be released from the sarcoplasmic reticulum (an organelle in the muscle cell).

d.) Ca binds to tropomyosin, changes its shape & exposes the actin so that myosin can bind and shorten the sarcomere = contraction.

Check out the following animations:http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter42/animations.html#

http://www.wisc-online.com/objects/index_tj.asp?objID=AP2904