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 0.02 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.
Autonomic rhythms The autonomic nervous system is responsible for many of the regulatory processes that sustain human life. Autonomic regulation is involuntary, meaning we do not have to think about it for it to take place. A great deal of these are dependent upon a certain rhythm, such as sleep, heart rate, and breathing.
Circadian rhythms Circadian literally translates to "about a day" in Latin. This refers to the human 24-hour cycle of sleep and wakefulness. This cycle is driven by light. The human body must photoentrain or synchronize itself with light in order to make this happen. The rod cells are the photoreceptor cells in the retina capable of sensing light. However, they are not what sets the biological clock. The photosensitive retinal ganglion cells contain a pigment called melanopsin. This photopigment is depolarized in the presence of light, unlike the rods which are hyperpolarized. Melanopsin encodes the day-night cycle to the suprachiasmatic nucleus (SCN) via the retinohypothalamic tract. The SCN evokes a response from the spinal cord. Preganglionic neurons in the spinal cord modulate the superior cervical ganglia, which synapses on the pineal gland. The pineal gland synthesizes the neurohormone melatonin from tryptophan. Melatonin is secreted into the bloodstream where it affects neural activity by interacting with melatonin receptors on the SCN. The SCN is then able to influence the sleep wake cycle, acting as the "apex of a hierarchy" that governs physiological timing functions. "Rest and sleep are the best example of self-organized operations within neuronal circuits".
Sleep and memory have been closely correlated for over a century. It seemed logical that the rehearsal of learned information during the day, such as in dreams, could be responsible for this consolidation. REM sleep was first studied in 1953. It was thought to be the sole contributor to memory due to its association with dreams. It has recently been suggested that if sleep and waking experience are found to be using the same neuronal content, it is reasonable to say that all sleep has a role in memory consolidation. This is supported by the rhythmic behavior of the brain. Harmonic oscillators have the capability to reproduce a perturbation that happened in previous cycles. It follows that when the brain is unperturbed, such as during sleep, it is in essence rehearsing the perturbations of the day. Recent studies have confirmed that off-wave states, such as slow-wave sleep, play a part in consolidation as well as REM sleep. There have even been studies done implying that sleep can lead to insight or creativity. Jan Born, from the University of Lubeck, showed subjects a number series with a hidden rule. He allowed one group to sleep for three hours, while the other group stayed awake. The awake group showed no progress, while most of the group that was allowed to sleep was able to solve the rule. This is just one example of how rhythm could contribute to humans unique cognitive abilities.
Central pattern generation A central pattern generator (CPG) is defined as a neural network that does not require sensory input to generate a rhythm. This rhythm can be used to regulate essential physiological processes. These networks are often found in the spinal cord. It has been hypothesized that certain CPG's are hardwired from birth. For example, an infant does not have to learn how to breathe and yet it is a complicated action that involves a coordinated rhythm from the medulla. The first CPG was discovered by removing neurons from a locust. It was observed that the group of neurons was still firing as if the locust was in flight. In 1994, evidence of CPG's in humans was found. A former quadrapalegic began to have some very limited movement in his lower legs. Upon lying down, he noticed that if he moved his hips just right his legs began making walking motions. The rhythmic motor patterns were enough to give the man painful muscle fatigue. A key part of CPG's is half-center oscillators. In its simplest form, this refers to two neurons capable of rhythmogenesis when firing together. The generation of a biological rhythm, or rhythmogenesis, is done by a series of inhibition and activation. For example, a first neuron inhibits a second one while it fires, however, it also induces slow depolarization in the second neuron. This is followed by the release of an action potential from the second neuron as a result of depolarization, which acts on the first in a similar fashion. This allows for self-sustaining patterns of oscillation. Furthermore, new motor patterns, such as athletic skills or the ability to play an instrument, also use half-center oscillators and are simply learned perturbations to CPG's already in place.
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