Biological Rhythms Flashcards
What are the types of synchronized rhythmic Oscillations?
- Collective entrainment
2. Pacemaker
What cyclical activities are defined by period?
Ultradian —> less than 1 day to complete a cycle
Respiration, heart rate
Circadian. —> approx. 1 day to complete a cycle
Sleep- wake cycle
Infradian —> above 1 day to complete a cycle
Menstrual
What are the pacemaker systems?
Regulatory input —> pacemaker activated secondary oscillators which then regulate rhythmic output
How is the respiratory rhythm generated in the medulla?
Respiratory rhythms are already established in utero and a child can breathe immediately after birth. Breathing is not only modulated by the needs of gas exchange (intake of oxygen and disposal of carbon dioxide), it needs quite sophisticated modulation, for example during speech. Respiration is ultimately linked to life. One cannot stop breathing for very long, as the endogenous respiratory drive will quickly override the desire not to breathe.
Respiration happens rhythmically, with phases of inspiration followed by phases of expiration. Inspiration is primarily driven by neuronal output of motor neurons located in spinal cord segments C3 to C5, via their ventral roots and the axons forming the phrenic nerves, which innervate the diaphragm. Activation of these motor neurons constricts the striated muscle fibers of the diaphragm and causes inspiration.
While the precise mechanism of central respiratory rhythm generation is not yet fully understood, there is consensus that the central respiratory rhythm generation takes place in small bilateral and symmetrical regions of the ventral medulla. The clusters of neurons contributing to the central rhythm generation are part of a longer columns of respiratory neurons, named ventral respiratory columns, extending from the rostral to the caudal medulla.
Over many years, research on the respiratory neuronal network has become more and more sophisticated. Historically, different parts of the brain and brainstem were surgically removed and the effects of these lesions on respiration were studied. Following these experiments, isolated slices of the medulla have been demonstrated to produce rhythmic output related to respiration.
More recently, studies on isolated brainstem-spinal cord preparations from rodents have allowed simultaneous optical recordings from large numbers of medullary neurons, together with electrophysiological recordings from spinal nerve roots of the upper cervical segments (C3 to C5). These experiments allowed direct comparison of the activity of neurons expressing respiratory rhythms in the medulla, with the motor output of nerve roots forming part of the phrenic nerve.
An example of these recordings is shown on the next slide. Timing of optical recording frames is indicated by lines linking them to the corresponding phases of the electrophysiological recording below. The optical recording shows medullary neurons during highest activity (in red) corresponding with highest phrenic inspiratory motor activity.
Describe respiratory rhythm generation
Optical recordings from isolated rodent brain stem preparations
Electrophysiological recordings from the spinal nerve roots of the upper cervical segments (C3-C5)
Rhythmic bursting from the ventral medulla stimulates inspiratory motor activity in the phrenic nerve roots
What are the steps of respiratory rhythm generation
1. Pons Pneumotaxic Center (Pontine Respiratory Group): Essential for maintaining normal breathing pattern; inhibits apneustic center Apneustic Center: lower pons; tonic facilitation of inspiration by excitatory input to pre-Bötzinger complex.
- Medulla
Dorsal Respiratory Group (DRG): Inputs from the pulmonary stretch receptors, CN IX, CN X, and outputs to the ventral respiratory group.
Ventral Respiratory Column: A group of nuclei in the ventro-lateral medulla.
- Pre-Bötzinger complex is the respiratory rhythm generator; primary source of rhythmic inspiratory drive
- Bötzinger complex feedback loop with Pre-Bötz; expiratory -Rostral VRG contains inspiratory neurons.
- Caudal VRG contains expiratory neurons.
- Spinal Cord
(Phrenic Motor Nucleus) PMN is located in the ventral horn of the cervical spinal cord (C3-C5 level
Describe modulation of the respiratory rhythm through neuronal input
The respiratory rhythm generation in the ventral respiratory column (VMC) is also influenced via neuronal input through the dorsal respiratory group (DRG) of the medulla, which constitutes the caudal half of the nucleus of the solitary tract (NTS).
The NTS is the primary target of visceral afferents carrying sensory information related to respiratory function, via the glossopharyngeal (CN IX) and vagus (CN X) nerves.
In addition, neuronal input occurs via a respiratory center in the pons called pontine respiratory group (PRG).
Describe chemical modulation of respiratory rhythm
Respiratory rhythms maintain adequate oxygen and carbon dioxide levels in arterial blood and are modulated by chemical input:
Increase in respiration and respiratory rate is induced by:
Decrease in oxygen
Decrease in pH
Increase in carbon dioxide
Decrease in respiration and respiratory rate is induced by:
Increase in oxygen
Increase in pH
Decrease in carbon dioxid
Describe chemical modulation of respiratory activity
Peripheral Chemoreceptors
• Carotid bodies – At fork of common carotid artery – Send info mainly through CN IX (glossopharyngeal nerve) – Sensitive to: pO2, pCO2, and pH
• Aortic bodies
– On aorta
– Send sensory info to medulla through CN X (vagus nerve)
– Sensitive to: pO2, pCO2 but NOT pH
Describe the components of respiratory rhythm neural modulation
Dorsal Respiratory Group (DRG):
• Signals ventral respiratory group
• Integrates peripheral signals
• Receives visceral afferents
Pontine Respiratory Group (PRG):
• Signals ventral respiratory group
• Fine tunes breathing rhythm in sleep, speech, and exercise
How does hypothalamic control and cortical input aid in neural modulation?
Hypothalamic controls
• Signals ventral respiratory group
• Pain, strong emotion influence breathing rate and depth
• Increased temperature increases breathing rate
Cortical controls
• Voluntary control of breath
What CNS disorders produce Abnormal Breathing Problems?
Normal breathing is characterized by rhythmic activity with limited variations regarding lung volume changes during inspiration and expiration, as well as duration of each period.
Lesions of corticospinal and corticobulbar fibers may lead to Cheyne-Stokes respiration characterized by a periods no breathing followed by a period of waxing and waning of respiratory depth, and so on. Patients suffering from these lesions may completely lose voluntary control of respiration.
Pontine lesions may lead to apneustic breathing, which is characterized by prolonged inspiratory plateaus of high lung volume followed by prolonged expiratory pauses with low lung volume.
Medullary lesions may lead to ataxic breathing, with almost randomly occurring large and small breaths alternating with prolonged periods of apnea.
What is suprachiasmatic nucleus?
SCN is in the anterior hypothalamus bordering the 3rd ventricle
vol. < 0.3mm3
SCN neurons among smallest in CNS
What is the function of Suprachiasmatic Nucleus?
Retino-hypothalamic tract fibers synapse in the suprachiasmatic nucleus of the hypothalamus and reset the circadian clock
Light activates photo-sensitive cells in the retina
Circadian clock signals reach the pineal gland, to induce the release of high melatonin levels during darkness
Describe the structure function of suprachiasmatic nucleus
Paraventricular Nucleus Hypothalamus - pineal gland pathway: melatonin release
Dorsomedial Nucleus Hypothalamus - ventrolateral preoptic region (GABA/galanin) and lateral hypothalamus (hypocretin/orexin): sleep/wake cycle
Subparaventricular Zone Hypothalamus- medial preoptic region (hypothalamus): body temperature
