These animations have been prepared with the objective of serving as teaching tutorials to assist undergraduate students in conceptualizing the complex dynamics of physiological processes -- especially as they relate to insects. I am sharing these animations with my colleagues that wish to link to them, or refer them to their students for the purpose of illustrating course lecture topics.

As a courtesy, to help me evaluate the usefulness of the animations, if you use them in your teaching, please send me an e-mail at: llkeeley@tamu.edu, and let me know if they are helpful. I would also point out that these are works in progress, and will be updated occasionally with more scenes, voice-over, etc. to improve their usefulness for instruction

Instructions for playing the animation

The movie starts on opening and the title page takes several seconds to play.

The menu control allows you to review any scene without viewing the preceding scenes.

play: (4),

stop: (<),

restart the scene: ( 9)

back up one frame: ( 7 )

advance one frame (8)

The MENU (78) button allows you to load and view any scene without viewing the preceding scenes.

Scene 1 -- Vertebrate Innervation

Each muscle fiber (= muscle cell) is innervated by a motor neuron that controls the contraction of the fiber. Each fiber in the muscles of vertebrate animals receives innervation by a single motor neuron. Conversely, a single motor neuron may innervate more than one fiber. A motor neuron plus all of the muscle fibers that it innervates comprise a motor unit. In this scene there are four neurons (R,B,Y,P) controlling four motor units. The motor units are: R-1,3 (R = neuron R; 1,3 refers to muscle fibers 1 and 3); B-2; Y-4,6 and P-5.

For vertebrate muscles, each motor neuron contacts the fiber at a neuromuscular junction and may consist of one or several endplates. Endplates are analogous to the synapse between neurons. Endplates are neuronal termini closely apposed to the sarcolemma of the fiber. They stimulate the fiber by releasing neurotransmitter chemicals that interact with receptors on the fiber surface. The resulting stimulus to the fiber results in a local depolarization (endplate potential). If the endplate potential is sufficiently great, it results in a propagated depolarization, comparable to a neuronal action potential, that spreads over the surface of the fiber. The propagated depolarization results in a maximal contraction of the fiber.

Both excitatory and inhibitory neurons may innervate muscles. Acetylcholine is the excitatory neurotransmitter chemical at the endplate of vertebrates, and g-aminobutyric acid (GABA) is the inhibitory neurotransmitter.

When a motor neuron conducts action potentials to the fibers in its motor unit, all the fibers in the motor unit undergo contraction. The number of motor units in a muscle determine the magnitude of muscle contraction. A muscle contracts proportionally to the number of motor units present and how many motor units are activated during a given contraction. The more motor units, the finer the control of the overall muscle contraction.

Scene 2 -- Insect Innervation

Insect muscles consist of bundles of fibers enveloped within a tracheolated membrane that divides it from its neighbors (= muscle units). Each muscle unit is innervated by one, two or three neurons (polyneuronal innervation) that innervate all the fibers within the muscle unit. There is no overlap of innervation between muscle units.

There are three types of neurons: fast, slow and inhibitory. All insect muscle fibers receive fast innervation. Fast neurons produce a maximum contraction by the fibers. Some fibers receive both fast and slow innervation. Slow neurons produce a graded response. Sometimes there is an inhibitory neuron that blocks contractions.

Glutamate is the stimulatory neurotransmitter for the fast and slow neurons, instead of acetylcholine; whereas GABA is the inhibitory neurotransmitter.

Insect muscle fibers do not propagate a depolarization by motor neurons. Therefore, insect neurons have multiterminal innervation with endplates at 30-80 mm intervals along the surface of the fibers. Each endplate depolarizes a localized area of the fiber surface that results in regional contractility.

Fast neurons release a maximum of neurotransmitter from each endplate and cause a maximum contraction of all fibers of the unit.

By contrast, slow neurons release small amounts of the neurotransmitter and result in small contractions. Repeated impulses of the slow neuron cause accumulation of the neurotransmitter at the junction with increasingly greater contraction of the muscle.

A fast neuron produces a maximum, "twitch" response by the muscle unit.

A slow neuron produces a graded response by the muscle unit as the neurotransmitter chemical accumulates.

Graded responses of insect muscles are a function of both the number of muscles units that respond, and the magnitude of contraction within each unit -- if they receive slow innervation. If the muscle receives only fast innervation, then the magnitude of contraction is proportional to the number of muscle units undergoing contraction since each unit undergoes a maximal contraction.

The animation shows two muscle units of an insect muscle: an upper unit controlled by neuron A and a lower unit controlled by neuron B. Since both muscle units A and B are innervated by a single neuron and give a maximum contraction in response to nerve impulses, the innervation must be by fast neurons.