The photo shows the stomatogastric ganglion (STG) of a Jonah crab (Cancer borealis), taken by Marie Suver (California Institute of Technology; with permission) during the Neural Systems & Behavior course at the Marine Biological Laboratory in Woods Hole, MA. The STG contains the somata of 26 neurons that belong to two central pattern generating neural circuits (gastric mill and pyloric circuit)This photo shows the pyloric dilator neuron, a pacemaker neuron in the STG that was stained via intracellular injection of a fluorescent dye. The inset shows a recording of the rhythmic oscillations of the PD membrane potential. The recordings were taken by Christopher Goldsmith in the lab of Wolfgang Stein at Illinois State University.
Because of the many similarities between vertebrate and invertebrate systems, especially with regards to basic principles of neuronal function, invertebrate model systems such as the crustaceanstomatogastric nervous system continue to provide key insight into how neural circuits operate in the numerically larger and less accessible vertebrate CNS.
Understanding how neuronal networks enable animals and humans to make coordinated movements is a continuing goal of neuroscience research. The stomatogastric nervous system of decapod crustaceans, which controls aspects of feeding, has contributed significantly to the general principles guiding our present understanding of how rhythmic motor circuits operate at the cellular level.
Rhythmic behaviors include all motor acts that at their core involve a rhythmic repeating set of movements. The circuits underlying such rhythmic behaviors, central pattern generators (CPGs), all operate on the same general principles. These networks remain rhythmic in the completely isolated nervous system, even in the absence of all rhythmic neuronal input, including feedback from sensory systems. Although the details differ in each circuit, all CPGs use the same set of cellular-level mechanisms for circuit construction. More importantly, CPG circuits are usually not dedicated to producing a single neuronal activity pattern. This flexibility results largely from the ability of different neuromodulators to change the cellular and synaptic properties of individual circuit neurons. When the properties of circuit components are changed, the output of the circuit itself is modified. These aspects of CPG operation are often shared by other circuits, enabling a general understanding of neuronal circuit operation.
The STNS contains a set of distinct but interacting motor circuits. The understanding of this multifunctional network contributed importantly to the general understanding of neural circuit operation. The value of this system has resulted from its accessibility, the use of several innovative techniques, and the combined research effort of around 15 laboratories over the past ~30 years.
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a vertebrate's body. In a human, the cerebral cortex contains approximately 14–16 billion neurons, and the estimated number of neurons in the cerebellum is 55–70 billion. Each neuron is connected by synapses to several thousand other neurons. These neurons typically communicate with one another by means of long fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells.
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves or efferent nerves, while those nerves that transmit information from the body to the CNS are called sensory nerves or afferent. Spinal nerves are mixed nerves that serve both functions. The PNS is divided into three separate subsystems, the somatic, autonomic, and enteric nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the sympathetic and the parasympathetic nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit from the cranium are called cranial nerves while those exiting from the spinal cord are called spinal nerves.
The development of the nervous system, or neural development (neurodevelopment), refers to the processes that generate, shape, and reshape the nervous system of animals, from the earliest stages of embryonic development to adulthood. The field of neural development draws on both neuroscience and developmental biology to describe and provide insight into the cellular and molecular mechanisms by which complex nervous systems develop, from nematodes and fruit flies to mammals.
A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a condensation of neurons or, in more advanced forms, a central nervous system, organisms with radial symmetry are associated with nerve nets, and are found in members of the Ctenophora, Cnidaria, and Echinodermata phyla, all of which are found in marine environments. In the Xenacoelomorpha, a phylum of bilaterally symmetrical animals, members of the subphylum Xenoturbellida also possess a nerve net. Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.
In the vertebrate embryo, a rhombomere is a transiently divided segment of the developing neural tube, within the hindbrain region in the area that will eventually become the rhombencephalon. The rhombomeres appear as a series of slightly constricted swellings in the neural tube, caudal to the cephalic flexure. In human embryonic development, the rhombomeres are present by day 29.
A neural circuit is a population of neurons interconnected by synapses to carry out a specific function when activated. Neural circuits interconnect to one another to form large scale brain networks.
Central pattern generators (CPGs) are self-organizing biological neural circuits that produce rhythmic outputs in the absence of rhythmic input. They are the source of the tightly-coupled patterns of neural activity that drive rhythmic and stereotyped motor behaviors like walking, swimming, breathing, or chewing. The ability to function without input from higher brain areas still requires modulatory inputs, and their outputs are not fixed. Flexibility in response to sensory input is a fundamental quality of CPG-driven behavior. To be classified as a rhythmic generator, a CPG requires:
"two or more processes that interact such that each process sequentially increases and decreases, and
that, as a result of this interaction, the system repeatedly returns to its starting condition."
Neural oscillations, or brainwaves, are rhythmic or repetitive patterns of neural activity in the central nervous system. Neural tissue can generate oscillatory activity in many ways, driven either by mechanisms within individual neurons or by interactions between neurons. In individual neurons, oscillations can appear either as oscillations in membrane potential or as rhythmic patterns of action potentials, which then produce oscillatory activation of post-synaptic neurons. At the level of neural ensembles, synchronized activity of large numbers of neurons can give rise to macroscopic oscillations, which can be observed in an electroencephalogram. Oscillatory activity in groups of neurons generally arises from feedback connections between the neurons that result in the synchronization of their firing patterns. The interaction between neurons can give rise to oscillations at a different frequency than the firing frequency of individual neurons. A well-known example of macroscopic neural oscillations is alpha activity.
The scratch reflex is a response to activation of sensory neurons whose peripheral terminals are located on the surface of the body. Some sensory neurons can be activated by stimulation with an external object such as a parasite on the body surface. Alternatively, some sensory neurons can respond to a chemical stimulus that produces an itch sensation. During a scratch reflex, a nearby limb reaches toward and rubs against the site on the body surface that has been stimulated. The scratch reflex has been extensively studied to understand the functioning of neural networks in vertebrates. Despite decades of research, key aspects of the scratch reflex are still unknown, such as the neural mechanisms by which the reflex is terminated.
The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output. Because arthropods have an open circulatory system,decapitated insects can still walk, groom, and mate - illustrating that the circuitry of the ventral nerve cord is sufficient to perform complex motor programs without brain input.
The neural basis of prey detection, recognition, and orientation was studied in depth by Jörg-Peter Ewert in a series of experiments that made the toad visual system a model system in neuroethology. He began by observing the natural prey catching behavior of the common European toad.
Eve Marder is a University Professor and the Victor and Gwendolyn Beinfield Professor of Neuroscience at Brandeis University. At Brandeis, Marder is also a member of the Volen National Center for Complex Systems. Dr. Marder is known for her pioneering work on small neuronal networks which her team has interrogated via a combination of complementary experimental and theoretical techniques.
Models of neural computation are attempts to elucidate, in an abstract and mathematical fashion, the core principles that underlie information processing in biological nervous systems, or functional components thereof. This article aims to provide an overview of the most definitive models of neuro-biological computation as well as the tools commonly used to construct and analyze them.
Spinal locomotion results from intricate dynamic interactions between a central program in lower thoracolumbar spine and proprioceptive feedback from body in the absence of central control by brain as in complete spinal cord injury (SCI). Following SCI, the spinal circuitry below the lesion site does not become silent rather it continues to maintain active and functional neuronal properties although in a modified manner.
The evolution of nervous systems dates back to the first development of nervous systems in animals. Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes. Primitive systems, like those found in protists, use chemical signalling for movement and sensitivity; data suggests these were precursors to modern neural cell types and their synapses. When some animals started living a mobile lifestyle and eating larger food particles externally, they developed ciliated epithelia, contractile muscles and coordinating & sensitive neurons for it in their outer layer.
The stomatogastric ganglion (STG) is a much studied ganglion found in arthropods and studied extensively in decapod crustaceans. It is part of the stomatogastric nervous system.
Central pattern generators are biological neural networks organized to produce any rhythmic output without requiring a rhythmic input. In mammals, locomotor CPGs are organized in the lumbar and cervical segments of the spinal cord, and are used to control rhythmic muscle output in the arms and legs. Certain areas of the brain initiate the descending neural pathways that ultimately control and modulate the CPG signals. In addition to this direct control, there exist different feedback loops that coordinate the limbs for efficient locomotion and allow for the switching of gaits under appropriate circumstances.
The neuroscience of rhythm refers to the various forms of rhythm generated by the central nervous system (CNS). Nerve cells, also known as neurons in the human brain are capable of firing in specific patterns which cause oscillations. The brain possesses many different types of oscillators with different periods. Oscillators are simultaneously outputting frequencies from .02 Hz to 600 Hz. It is now well known that a computer is capable of running thousands of processes with just one high frequency clock. Humans have many different clocks as a result of evolution. Prior organisms had no need for a fast responding oscillator. This multi-clock system permits quick response to constantly changing sensory input while still maintaining the autonomic processes that sustain life. This method modulates and controls a great deal of bodily functions.
The following Diagram is provided as an overview of and topical guide to the human nervous system:
Segmentation is the physical characteristic by which the human body is divided into repeating subunits called segments arranged along a longitudinal axis. In humans, the segmentation characteristic observed in the nervous system is of biological and evolutionary significance. Segmentation is a crucial developmental process involved in the patterning and segregation of groups of cells with different features, generating regional properties for such cell groups and organizing them both within the tissues as well as along the embryonic axis.
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