Single motor neuron (nerve cell) and all of the muscle fibers it supplies; Nerve=bundle of neurons/nerve cells

Smallest recordable contraction; response to single threshold stimulus (just large enough to generate an AP across the sarcolemma); 3 Phases:

1. Latency

2. Contraction

3. Relaxation

Relative Twitch Duration: varies with muscle fiber type; postural muscles have slower twitch (not expected to move quickly); muscles necessary for rapid movement (like the eye) have faster twitch

Period between stimulus and initiation of the muscle twitch response; excitation is occuring; AP release of calcium and initial binding of myosin head to actin (no tension yet because no power stroke)

Power Stroke; onset of shortening to peak of tension development; cross-bridge formation

Calcium taken back up into terminal cisternae & cross bridge detaches; no more AP from ATP binding to myosin

Muscle has a number of ways to increase tension:

1. Summation: Temporal- increase rate of stimulus delivery; Tetany- no relaxation (charlie horse)

2. Multiple Motor Unit Summation: more motor units are recruited as stimulus intensity increases; smallest motor units recruited first, followed by larger ones; end result is smooth, steady increase in force generated

3. Treppe: warm up effect; with repeated stimuli, muscle "warms up," enzymes become more efficient and stronger contraction ensues from increased temperature

**Treppe vs. Temporal Summation: both have multiple stimuli, but relaxation differs; Treppe- complete relaxation between contractions; Temporal Summation- does not have complete relaxation

Contraction=force generated by cross bridges; force on object=tension; weight of object=load

1. Isotonic: same tension but length of muscle changes; 2 types: Concentric- contractions that shorten muscle (only contraction explained by Huxley's theory) & Eccentric: contraction that lengthens muscle (putting heavy object back down

2. Isometric: muscle stays same length but force changes; ex: trying to push wall

3. Isokinetic: mechanically induced; not typical contraction performed daily; performed at same speed with controlled angular velocity of joint; ex: exercise equipment

1. Number of Fibers and Stimuli: temporal summation and recruitment of fibers

2. Size of Muscles: Larger muscles generate larger forces

3. Elastic Elements: non-contractile proteins & structures

4. Tension: internal- some tension generated must be directed to overcome slack of elasticity of non-contractile elements; external tension- overcomes natural load

5. Muscle Length: length-tension relationship- when skeletal muscle begins contraction at perfect length, you will get greatest contraction; too much overlap- not much room to pull; too little overlap- myosin heads cannot reach actin (muscle overstretched)

Non-Contractile Elements of Muscle Cells:

• Connective Tissue
• Tendons
• Organelles
• Sarcolemma

Supporting Proteins:

• Titin: elastic protein (z-line to thick filament)
• Nebulin: core of thin filament

1. Load and Duration: the greater the load, the less a muscle shortens and shorter the contraction; the greater the load, the slower the contraction

2. Fiber Typing: mixed cell types within people; 3 types:

Slow Oxidative (Red), Fast Glycolytic (White), and Fast Oxidative-Glycolytic (Red)

-Similar to skeletal muscle: excitable, can be stretched, contracts, and moves something along

-Different from skeletal muscle: shape and size of cell, lack of striations, and location

Location: walls of different organs; ex: respiratory tract

Organization: consists of sheets of muscle cells; 2 layers- longitudinal & circular layer; Functional outcome of layers- squeeze & push material through tube

Innervation: different than skeletal in that it does not have a 1:1 relationship with neuron; Supplied by autonomic nervous system; nerve endings- called bulbous varicosities; poorly organized neuromuscular junction- called diffuse junctions

Cells: do not run entire length of organ; small & spindle shaped; may join together to form sheets; joined by gap junctions (allow communication) unlike skeletal muscle

Connective Tissue: has  thin layer of endomyosium surrrounding each smooth muscle cell

Sarcoplasmic Reticulum: less well-developed; touches plasma membrane; stores calcium; PM has caveoli (calcium storage cavities)

T-Tubules: none; no need since SR touches PM

Filaments (Contractile): ratio of thick to thin is 1:16 (opposed to Skel muscle 1:2); thick (myosin), thin (just actin & tropomyosin); thick & thin filaments spiral down long axis of muscle cell; not arranged in perfect bands

AP Stimulus: neurotransmitter released from bulbous variscosity; SR receives stimulus directly; Ca enters cytoplasm from SR & caveoli, causing contraction

• Initiated by calcium entry into cytoplasm
• Because there is no troponin-tropomyosin complex, Ca binds to the protein calmodulin
• Once activated, calmodulin activates kinase 
• Kinase transfers phosphate to myosin head
• Cross-bridge formation, similar to skeletal muscle of actin (thin films.) and myosin heads (thick films.)

1. Single Unit: cells contract as a unit; rhythmical; gap junctions allow current flow directly; found in organs

2. Multi Unit: muscle fibers that are independent from one another; gap junctions and spontaneous APs are rare; respond to hormones and have autonomic NS innervation; have to be more closely regulated (receive specific innervation); ex: regulate pupil size, arrector pili, large air ways to lungs; NO pace maker cells

by:

1. Neurotransmitters: acetylcholine & norepinephrine depending on organ system (only ach in skeletal m.)

2. Receptors: that receive neurotransmitters can be inhibitory or excitatory-can determine whether an AP occurs (skeletal m. always excitable)

3. Stretching: More vigorous contraction if stretched

4. Hormones & Local Factors: can be stimulating or inhibitory depending on receptor

• Length Tension: stretch stimulates contraction
• Hyperplasia: Increase # of cells (ex pregnancy)
• Secretion: of elastin and collagen of CT

• Myosin ATPase slow to cleave ATP into ADP + P
• Lots of mitochondria
• Lots of myoglobin (stores oxygen)
• Dependent on O2 for ATP production
• Blood Supply
• Lots of capillaries
• Low glycogen stores
• Good for endurance exercising

• Contains fast ATPase
• Less myoglobin
• Not O2 dependent
• High glycogen storage
• Not used for endurance- used for sprinting

• Fast ATPase
• Some dependence on O2
• Moderate glycogen stores
• Little endurance