10. Control of Breathing - Asleep Flashcards
Summarise the control of breathing throughout sleep
Control of breathing during sleep is purely autonomic, i.e. there are no voluntary or emotional influences from the motor cortex or limbic system
Outline sleep
Sleep can be seen on a brain EEG across the general cortical areas, whereby brainwaves are of fast frequency and low voltage
Consists of 4 stages and REM:
o With each stage, there is an increase in the amplification of the electrical activity coming from the brain
Stage 1 - transitional, slow rolling eye movements, postural movements, auditory response present
Stage 4 - no auditory response present
REM - rapid eye movement, dreaming sleep, all muscles functionally paralysed except eyes and diaphragm
o REM in-between each stage; circadian rhythm
o Purpose: consolidate memory
Hypogram of healthy adult; 90 minute schedule; sleep deprivation results in missing of the 1st REM cycle
Outline EEGs
Electroencephalography (EEG) is an electrophysiological monitoring method to record electrical activity of the brain
It is typically noninvasive, with the electrodes placed along the scalp, although invasive electrodes are sometimes used such as in electrocorticography
EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain
In clinical contexts, EEG refers to the recording of the brain’s spontaneous electrical activity over a period of time, as recorded from multiple electrodes placed on the scalp
Briefly contrast the control of breathing when awake with that whilst asleep
Control of breathing when awake:
o Reflex/automatic control (by the brainstem)
o Voluntary/behavioural control (by the motor cortex)
o Emotional control (by the limbic system)
Control of breathing during sleep:
o Only involves reflex/automatic control
o Does not involve voluntary/behavioural or emotional control
Outline the 5 stages of sleep
There are five stages of sleep; these include stages 1-4 (non-REM sleep) followed by REM sleep:
o Stage 1 is light sleep where you drift in and out of sleep and can be awakened easily; in this stage, the eyes move slowly and muscle activity slows; during this stage, many people experience sudden muscle contractions preceded by a sensation of falling
o In stage 2, eye movement stops and brain waves become slower with only an occasional burst of rapid brain waves; the body begins to prepare for deep sleep, as the body temperature begins to drop and the heart rates slows
o When a person enters stage 3, extremely slow brain waves called delta waves are interspersed with smaller, faster waves; this is deep sleep; it is during this stage that a person may experience sleepwalking, night terrors, talking during one’s sleep, and bedwetting; these behaviors are known as parasomnias, and tend to occur during the transitions between non-REM and REM sleep
o In stage 4, deep sleep continues as the brain produces delta waves almost exclusively; people roused from this state feel disoriented for a few minutes
o During REM (rapid eye movement) sleep, brain waves mimic activity during the waking state; the eyes remain closed but move rapidly from side-to-side, perhaps related to the intense dream and brain activity that occurs during this stage
Outline the reflex/automatic control of breathing
Reflex/automatic control of breathing: brainstem respiratory neurones
Pre-Bötzinger Complex: the area in the brainstem which controls respiratory rhythm generation
It contains 2 types of rhythm-generating neurones:
o Inspiratory neurones - (dorsal root ganglion)
o Expiratory neurones - (ventral root ganglion)
The inspiratory and expiratory neurones exhibit reciprocal inhibition
Outline the pre-Bötzinger Complex
The pre-Bötzinger complex is a cluster of interneurons in the ventral respiratory centre of the medulla of the brainstem
This complex has been proven to be essential for the generation of respiratory rhythm in mammals
The exact mechanism of the rhythm generation and transmission to motor nuclei remains controversial and the topic of much research
Rhythmic respiratory breathing continuously adapts to the organism’s posture, activity level, speech, and can reveal whether someone is calm, agitated, or scared
Plasticity of the mechanisms involved in respiratory behavior is modulated in part by the pre-Bötzinger complex
Outline the factors of respiratory control which are affected by sleep
Respiratory muscles in the upper airway and pump muscles
Respiratory control centres
Blood gases and chemosensitivity (i.e. pCO2 and pO2)
Outline breathing during sleep (vs. breathing when awake)
Minute ventilation and tidal volume (TV) fall by ~10%
Alveolar ventilation falls by ~15%
Frequency of breathing and oxygen saturation (SaO2) do not change
Outline changes in SaO2 and CO2 during sleep
Changes in SaO2 during sleep:
o Tidal Volume decreases, leading to a decrease in pO2; therefore SaO2 remains virtually constant in healthy individuals
o COPD patients live on the steep part of the oxygen-dissociation curve; therefore SaO2 decreases significantly when pO2 decreases
Changes in CO2 during sleep:
o Tidal Volume decreases, leading to an increase in pCO2 of ~3-4 mmHg
o The increase in pCO2 stimulates respiratory centres to continue breathing
o N.B. if pCO2 does not increase during sleep, this results in death
Outline the ventilatory sensitivity to CO2 during sleep
Sensitivity to CO2 decreases during sleep; this causes ventilation to decrease during sleep
Decrease in ventilation leads to an increase in pCO2 (hypercapnia)
Hypercapnia is mandatory for breathing during sleep
Define apnoea
Temporary cessation of breathing, especially during sleep
Outline apnoea and the hypercapnia apnoeic threshold
Hypercapnic apnoeic threshold: the amount of pCO2 which is critical for breathing during sleep
pCO2 must rise above the hypercapnic apnoeic threshold for regular breathing to continue during sleep
Respiratory muscles of the upper airway assist breathing; if their activity decreases, this results in obstructive sleep apnoea
Muscles of the upper airway which reduce their activity during sleep:
o Tongue (genioglossus)
o Levator palatini
o Tensor palatine
These muscles stiffen the soft palate (in the pharyngeal region at the back of the throat):
o When they are active, they prevent airway constriction and collapse
o When they are not active, the airway is prone to collapse
The airway at the back of the throat (i.e. the pharynx)
is distensible since it does not contain cartilage
Influences of sleep on the airway include:
o The muscles of the upper airway relax and the airway constricts
o Pharyngeal resistance increases; therefore ventilation becomes more difficult
o Hence more effort is required to achieve the same amount of ventilation
o N.B. in some people, turbulent airflow is setup over the vocal chords, leading to snoring
Outline obstructive sleep apnoea
Obstructive sleep apnoea results from a mechanical obstruction of the airway during sleep
Airflow stops; respiratory effort increases
There is no impairment of respiratory control
Positive pressure leads to airway collapse
Mechanism:
- Sleep
- Upper airway muscle function decreases
- Apnoea –> hypoxia and hypercapnia –> increased respiratory effort
- Arousal:
- Consequence: paradoxical breathing:
o Pressure moves between the thorax and the abdomen during breathing
o Air does not move, as the patient tries to breathe, they expose the thorax to large negative pressures at a time when the O2 saturation has fallen; this is dangerous for the heart
Symptoms:
o Loud snoring
o Partner witnesses lack of breathing
o Profound sleepiness during the day
Obstructive sleep apnoea mainly affects middle-aged men
Outline central sleep apnoea
Central sleep apnoea; pCO2 decreases and gets closer to the apnoeic threshold
Airflow stops; there is no respiratory effort due to lack of brain control
Rare unless congenital; congenital hyperventilation syndrome
Most patients with this condition have heart failure
