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, or remodeled, scenes 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 control panel embedded in the movie allows you to control any scene.
The MENU button allows you to view any scene without viewing the preceding scenes
Scene 1 - Neuron Structure
The purpose of the nervous system is to collect and integrate information from both external and internal sources and to use that information as a basis for behavior that is significant to the survival of the individual and the species.
The neuron is the unit cell of the nervous system for the conduction and integration of information. The neuron consists of the dendrite, the soma, the axon and the terminal, axon arborization.
The dendrite is a region of information input. If the neuron is a sensory neuron, the dendrite may be modified to form a specialized sensory receptor that can detect chemical, mechanical or visual stimuli. If the neuron is in the central nervous system, the dendrite will receive input from other neurons via the terminal, axon arborization.
The soma (= perikaryon) is the cell body of the neuron and contains the nucleus and typical cellular organelles such as Golgi, mitochondria, and endoplasmic reticulum.
The axon is a long, thin cytoplasmic extension that conducts the nerve impulse. The axon cone is a morphologically distinct region at the start of the axon that serves as the origin of the nerve impulse.
The terminal, axon arborization is a branching of the axon terminus that forms interneuronal contacts to convey information between neurons. In the case of motor neurons that control muscle contraction, the axon termini contact muscle fibers (see the animation: Comparative Vertebrate – Invertebrate Muscle Innervation and Contraction)
Scene 2 - Neuron Information Coding
A sensory neuron innervates a sense receptor, which is a structure modified to detect a specific form of sensory stimulus. A sensory stimulus can be light (photons of a specific wavelength), chemicals in solution (taste, = gustation), chemicals in gas phase (odor, = olfaction) or sound waves and other types of mechanical pressures. The sense receptor is modified to detect the sensory stimulus and transfer information about the intensity of the stimulus to the dendrite of the sensory neuron. The intensity of the stimulus can be brightness of light for vision, concentration of the chemicals for taste and odor or loudness of the sound or amount of pressure for mechanoreception.
The dendrite transduces the intensity of the stimulus into an electrical depolarization (the dendrite membrane develops a wave of negativity compared to its normal positive charge). This depolarization is referred to as the generator potential. The generator potential spreads decrementally over the surface of the dendrite.
If the stimulus intensity is sufficiently great to cause the generator potential to reach the axon cone, it will elicit all-or-none action potentials in the axon. The frequency (number per unit time) of the action potentials is proportional to the intensity of the original stimulus and the magnitude of the generator potential. The greater the stimulus intensity, the larger the generator potential and the more frequent the action potentials.
The animation shows three intensities of stimuli: weak, intermediate and strong, and their resulting generator potentials and increasing frequencies of action potentials.
When the action potentials reach the axon arborization, they pass along the branches and cause the release of neurotransmitter chemicals from the axonal termini. The quantities of the neurotransmitter chemicals released are, again, proportional to the number of action potentials.
If the axonal termini of branches contact other neurons to form a synapse, the amounts of the neurotransmitter chemicals released cause localized depolarizations in the synaptic membrane of the post-synaptic neuron. The amount of post-synaptic depolarization is proportional to the amount of neurotransmitter chemicals released. Subsequently, the neurotransmitter chemicals stimulate the post-synaptic neurons to generate a new frequency of action potentials in the post-synaptic neurons.
The events of the synapse are not illustrated in detail in Scene 5.
Scene 3 - Insect Nervous System - A CNS Model
A ganglion (singl.) is an identifiable region of the nervous system that contains a concentration of neuron cell bodies (soma). The typical insect nervous system consists of a series of ganglia (pl.). The "brain" or supraesophageal ganglion is located in the insect head and is a fusion of three ganglia: the protocerebrum, the deutocerebrum, and the tritocerebrum. The protocerebrum is the center for visual processes. The deutocerebrum is the center for olfactory processes, and the tritocerebrum monitors feeding activities.
The brain is connected to the insect ventral nerve cord by the circumesophageal connectives that encircle the esophagus. The ventral nerve cord consists of a series of segmental ganglia that are joined by paired interganglionic connectives.
The animation depicts how information about a stimulus is detected by a sense receptor (mechanoreceptor), converted to electrical information in the sensory neuron, and transferred to the central nervous system.
The sense receptor illustrated is a trichoid (hair) sensillum located on the femur of a mesothoracic leg of the insect. The seta of the sensillum is connected to a neuron that runs to the central nervous system.
The animation demonstrates how the hair transmits information about the intensity of a mechanical stimulus. The mechanical stimulation causes the hair to bend. When the hair bends, it causes pressure on the dendrite of the sensory neuron at its attachment point to the hair. The pressure causes mechanical bending of the hair.
The pressure on the dendrite causes depolarization of the dendrite. The depolarization of the dendrite in a sense receptor is called the generator potential. The generator potential is an electrotonic (not propagated) depolarization that spreads away from its source decrementally, over the dendritic membrane.
However, if the generator potential is sufficiently strong, it spreads across the dendrite and the soma until it reaches the junction between the soma and the axon (= axon cone). If the depolarization is sufficiently great to reach the axon cone, it causes propagated, all-or-none depolarizations (= action potentials) to be generated in the axon.
Action potentials are propagated along the axon without diminishing until they reach the terminal arborization and a junction point (= synapse) with another neuron located at an axon terminal.
The animation demonstrates that the increasing magnitude (intensity) of the stimulus (depicted by the increasing thickness of the red arrows) causes an increasing generator potential in the dendrite, and results in a higher frequency (number per unit time) of action potentials to be generated in the axon.
Scene 4 - Information Coding by a Receptor and Transfer Between Neurons in the CNS
This animation depicts the transfer of information from an external hair mechanoreceptor on an insect leg – through a synapse to a neuron in a ganglion of the central nervous system. This is a cut-away view of the hair receptor, its sensory neuron and the path of the sensory axon from the leg through a lateral nerve, into a ganglion of the ventral nerve cord. The sensory axon inter-communicates with the axons of ganglionic neurons in the neuropile region of the ganglion.
Brushing against an object, such as a leaf, bends the hair and provides a mechanical stimulus. The dendrite of the sensory neuron is depolarized proportional to the amount of bending. The depolarization of the dendrite is re-interpreted in the axon as a frequency of action potentials. The action potentials in the axon of the sensory neuron are transmitted to a secondary, ganglionic neuron in the neuropile region of the ganglion.
The region of transfer of the primary information from the sensory neuron to the secondary, ganglionic neuron is a point of contact called the synapse. The information conveyed by the presynaptic sensory neuron as a certain frequency of action potentials is interpreted into a different frequency of action potentials by the postsynaptic ganglionic neuron.
Scene 5 - Synapse Coding and Transfer of Information
The synapse is a junction between two neurons. It is functionally the point where information can be modulated and interpreted.
Information comes into the synapse from a presynaptic neuron in the form of action potentials. Vesicles of neurotransmitter chemicals are stored in the synaptic terminus of the presynaptic neuron. The arrival of action potentials at the terminal causes the presynaptic vesicles to fuse with the axonal cell membrane and release the neurotransmitter chemicals into the synaptic cleft, a gap between the pre- and postsynaptic neurons.
The neurotransmitter chemicals diffuse across the synaptic cleft and bind to specific neurotransmitter receptors on the synaptic membrane of the postsynaptic neuron. This interaction between the neurotransmitter chemical and its receptor causes opening of ion channels in the postsynaptic membrane resulting in diffusion of Na+ into the postsynaptic neuron and depolarization of the synaptic membrane. This depolarization is termed the excitatory postsynaptic potential (= EPSP).
The EPSP is a non-propagated, electrotonic depolarization that spreads decrementally from its source along the postsynaptic membrane. If EPSPs accumulate sufficiently rapidly, they become additive (spatial and temporal summation) and form a grand EPSP at the axon cone that generates action potentials in the axon of the postsynaptic neuron.
After interaction with its receptor on the postsynaptic membrane, enzymes located at the synapse may degrade the neurotransmitter chemical. This is the case for acetylcholine, a common neurotransmitter, and cholinesterase, its degradative enzyme. Subsequently, the acetate and choline are taken up by the pre-synaptic neuron and re-synthesized into acetylcholine. Other common neurotransmitter chemicals of insects are biogenic amines such as: dopamine, octopamine, g-aminobutyric acid and serotonin, and in vertebrate animals epinephrine and norepinephrine. Peptides can also serve as neurotransmitter chemicals.
Summary of Information Transfer at the Synapse: In the synapse, the information arrives encoded as the frequency of the action potentials. Each action potential causes a number of synaptic vesicles to rupture and release their enclosed neurotransmitter chemicals. The information conveyed in the axon as frequency of action potentials is converted in the synapse to the concentration of neurotransmitter chemicals based on the number of vesicles that release their contents. The accumulation of the neurotransmitter chemical opens cation channels and results in an increasing depolarization (EPSP) within the postsynaptic membrane. When the EPSP is sufficiently intense, a new frequency of action potentials are generated in the postsynaptic axon.
Scene 6 - Synaptic Information Interpretation
The synapse modulates and interprets the primary information that comes to it from the presynaptic neuron. The frequency of outgoing action potentials is not equivalent to that of incoming action potentials.
The postsynaptic neuron may receive input from hundreds or thousands of presynaptic neurons through their synaptic contacts. Each incoming action potential on a presynaptic neuron causes a quanta of neurotransmitter chemicals to be released from the pre-synaptic vesicles and results in a region of localized depolarization (EPSP) on the postsynaptic membrane.
As the numbers of active presynaptic neurons simultaneously increase, the localized synaptic depolarizations (EPSPs) spread over the surface of the postsynaptic neuron and accumulate at the axon cone, or hillock. The additive effects of each individual EPSP is to produce a grand EPSP at the axon cone that results in generating action potentials at a new frequency in the axon of the postsynaptic neuron.
Temporal Summation
Temporal summation involves action potentials arriving at the terminus of one presynaptic axon at close intervals so that each succeeding postsynaptic EPSP builds on the previous ones. In this way, the postsynaptic, electrotonic depolarizations accumulate to form a grand EPSP that results in a new frequency of action potentials in the axon of the postsynaptic neuron.
Spatial Summation
Counterpoint to temporal summation, in spatial summation, the information input (action potential frequency) in a number of presynaptic neurons arrives sufficiently close together to release enough neurotransmitter chemical to accumulate and depolarize a sufficient area of the postsynaptic membrane to attain a grand EPSP and generate a new frequency of action potentials in the postsynaptic axon.
Summary of Information Transfer by Postsynaptic Spatial Summation:
In the living state, spatial and temporal summations occur concurrently. The action potentials from numerous presynaptic neurons converge on a postsynaptic neuron at sufficiently close intervals to accumulate their EPSPs and generate an axonal response in the postsynaptic neuron.