control of respiration 1/ neural control of respiration (R6) Flashcards
respiratory rhythm
-inspiration (diaphragm contracts and moves down,rib cage expands as rib muscles contract and air is inhaled) followed by expiration (diaphragm relaxes and moves up, rib cage gets smaller as rib muscles relax and air is exhaled)
what generates the respiratory rhythm
- until recently it was thought that the dorsal respiratory group of neurons generate the basic rhythm of breathing
- it is now generally believed that the breathing rhythm is generated by a network of neurons called the Pre-Botzinger complex:
- these neurons display pacemaker acitivity.
- they are located near the upper end of the medullary respiratory centre
the rhythm =
inspiration followed by expiration
respiratory rhythm generator
- medulla (medulla oblongata) is major rhythm generator:
- fairly normal ventilation is retained if breathing rhythm is generated in sagittal section of brainstem above medulla (pons)
- ventilation ceases if breathing rhythm is generated in sagittal section of brainstem below medulla (spinal cord)
- > the rhythm is generated in the medulla and can be modified by inputs from the pons (pneumotaxic center and apneustic center)
respiratory control centers in the brain stem
- pons respiratory centers (pneumotaxic center and apneustic center)
- medullary respiratory center (made up of doral respiratory group and ventral respiratory group)
- > pre-botzinger complex are located near the upper end of the medullary respiratory centre
normal neural control of respiration
- dorsal respiratory group neurons (inspiratory) located in the medulla are stimulated by the pre-botzinger complex,
- the firing of these neurons leads to contraction of the inspiratory muscles causing inspiration
- when firing of these neurons stops, passive expiration occurs
- in normal quiet breathing, ventral neurons do not activate expiratory muscles
inspiration
before inspiration:
-external intercostal muscles are relaxed
-diaphragm is relaxed
inspiration:
-diaphragm contracts and lowers on contraction, increasing vertical dimensions of thoracic cavity
-contraction of external intercostal muscles causes elevation of ribs which increases side to side dimension of thoracic activity
neural control of ‘active’ expiration during hyperventilation
- firing of dorsal neurons (stimulated by the pre-botzinger complex, causing inspiration and expiration normally occurs when the firing stops)
- However increased firing of dorsal neurons excites a second group (ventral respiratory group neurons) during hyperventilation
- ventral respiratory group neurons excite internal intercostals, abdominals etc causing forceful/active expiration
- in normal quiet breathing, ventral neurons do not activate expiratory muscles
major muscles of inspiration
-contract every inspiration, relaxation of these muscles causes passive expiration
=diaphragm and external intercostal muscles
accessory muscles of inspiration
-contract only during forceful inspiration
= sternocleidomastoid and scalenus muscles
muscles of active inspiration
-contract only during active/forced expiration
-normally expiration occurs by the relaxation of inspiratory muscles
= internal intercostal muscles and the abdominal muscles
how is the rhythm generated in the medulla modified by neurons in the pons (pneumotaxic center)
- stimulation of the pneumotaxic center (type of pons respiratory center) terminates inspiration
- the pneumotaxic center (PC) is stimulated when dorsal respiratory neurons fire which inhibits inspiration
- without the pneumotaxic center (PC), breathing is prolonged inspiratory gasps with brief expiration - APNEUSIS
apneusis
- prolonged inspiratory gasps with brief expiration
- occurs when the pneumotaxic center is NOT stimulated (as stimulation of the pneumotaxic center inhibits inspiration)
how is the rhythm generated in the medulla modified by neurons in the pons (apneustic center)
-impulses from the neurons of the apneustic center excite the inspiratory area of the medulla causing prolonged inspiration
what can modify the rhythm of breathing that is generated in the medulla
inputs from pons:
- pnemotaxic centre (terminates inspiration)
- apneustic centre (prolongs inspiration)
factors that influence the respiratory centers
stimulation received from different types of receptors including:
- higher brain centers (eg. cerebral cortex, limbic system, hypothalamus)
- stretch receptors in the walls of bronchi and bronchioles -juxtapulmonary/J receptors
- joint receptors
- baroreceptors
- central chemoreceptors
- peripheral chemoreceptors
hypothalamus
stimulated by changes in body temperature, thirst, hunger, and other homeostatic systems
receptor
an organ or cell able to respond to light, heat, or other external stimulus and transmit a signal to a sensory nerve
Juxtapulmonary/J receptors
stimulated by:
- pulmonary capillary congestion
- pulmonary oedema (caused by left heart failure)
- pulmonary emboli (rapid shallow breathing)
joint receptors
stimulated by joint movement
effect of stimulation of baroreceptors on respiratory centers
increased ventilatory rate in response to decreased blood pressure
examples of involuntary/reflex modifications of breathing
- pulmonary stretch receptors hering-breur reflex
- joint receptors reflex in exercise
- stimulation of respiratory center by temperature, adrenaline or impulses from cerebral cortex
- cough reflex
- sneezing reflex
pulmonary stretch receptors (reflex modification of breathing)
- activated during inspiration, afferent discharge inhibits inspiration (hering-breur reflex - guard against hyperinflation)
- only activated at large tidal volumes (»1 litre)
- maybe important in new born babies
- may prevent over inflation of the lungs during hard exercise
joint receptors reflex in exercise (reflex modification of breathing)
-impulses from moving limbs reflexly increase breathing and probably contribute to the increased ventilation during exercise
factors that may increase ventilation during exercise
- reflexes originating from body movement
- adrenaline release
- impulses from the cerebral cortex
- increase in body temperature
- later : accumulation of CO2 and H+ generated by active muscles
ventilatory response to exercise (graph)
- ventilation increase is a jump from rest to exercise (stage I)
- then during exercise ventilation slowly increases (stage II) before leveling off (stage III)
- then jumps down from exercise to recovery before slowly decreasing back to resting ventilation levels
cough reflex (reflex modification of breathing)
- vital part of body defence mechanisms
- helps clear airways of dust, dirt or excessive secretions
- activated by irritation of airways or tight airways (eg.asthma)
- control center is in the medulla
- afferent discharge stimulates: short intake of breath, followed by closure of the larynx, then contraction of the abdominal muscles (increases intra-alveolar pressure) and finally opening of the larynx and expulsion of air at a high speed
sneezing reflex (reflex modification of breathing)
- activated by irritation of mucosa of upper respiratory passages especially the nasal mucosa eg. by dust, pollen, irritant substances, viruses, mechanical obstruction, or excess build up of fluid in the nasal passages
- control center is in the medulla
- complex afferent discharge stimulates eyes, face and neck muscles, soft palate, uvula, abdominal muscles, diaphragm and chest muscles
- results in sudden forceful expulsion of air through the nose and mouth
