Respiratory Physiology Lecture 6: Control of Respiration and Muscles

Control of Respiration Muscles

Muscles of Inhalation and Exhalation

MUSCLES OF INHALATION

MUSCLES OF EXHALATION

Sternocleidomastoid

Scalenes

Sternum:

Exhalation

Internal

intercostals

- Inhalation

External

intercostals

Diaphragm

Diaphragm:

Exhalation

Inhalation

External

oblique

- Internal

obliqu

Transversus

abdominis

Rectus

abdominis

• Quiet inhalation; The most important muscle of inhalation is the diaphragm, a dome shaped muscle, that forms the floor of the thoracic cavity. Contraction of the diaphragm causes it to flatten and increase the vertical height of the thoracic cavity. During normal quiet inhalation the diaphragm descends about 1 cm, producing a pressure difference of 1- 3 mm Hg and the inhalation of about 500 ml of air. The contraction of the diaphragm is responsible for 75% of inhaled air. The other muscles that are involved are the external intercostal muscles. When these muscles contract they elevate the ribs, and increase the width and depth of the thoracic cavity. The external intercostal muscles are responsible for about 25% of the air entering the lungs.
• Inhalation during strenuous exercise; In strenuous breathing the diaphragm may descend 10 cm, which produces a pressure difference of 100 mm Hg and the inhalation of 2-3 litres of air. During deep, forceful , inhalations, the accessory muscles of inspiration participate in increasing the volume of the thoracic cavity. The accessory muscles are the sternocleidomastoideoles that elevate the sternum, the scalene muscles that elevate the first two ribs, and the pectoralis minor muscles that elevate the 3-5th ribs. Both normal quiet inhalation and inhalation during exercise involve muscle contraction and are therefore active processes.
• Advanced pregnancy, excessive obesity or confining abdominal clothing can prevent complete descent of the diaphragm.

Role of the Respiratory Center

The size of the thorax is altered by respiratory muscles that contract in response to signals transmitted from nerve centres in the brain stem, and relax in the absence of impulses. The muscles are under control of a dispersed group of neurons, collectively called the respiratory centre, which can be divided into 3 areas on the basis of their function;

1. the medullary rhythmicity centre in the medulla oblongata
2. the pneumotaxic area in the pons
3. the apneustic area also in the pons

Respiratory Center Sagittal Plane

Sagittal

plane

RESPIRATORY

CENTER:

Midbrain

Pneumotaxic area

Apneustic area

Pons

Medullary rhythmicity

area:

Inspiratory area

Medulla

oblongata

Expiratory area

Spinal

cord

Sagittal section of brain stem

O Ventilation is controlled by neurons in pons & medulla oblongata in the brain

3 groups of neurons

1. medullary rhythmicity area in the medulla oblongata
2. pneumotaxic area in the pons
3. apneustic area in the pons

Medullary Rhythmicity Area

Activates

INSPIRATORY AREA

ACTIVE

INACTIVE

INSPIRATORY AREA

ACTIVE

EXPIRATORY AREA

2 seconds

3 seconds

Diaphragm and external

intercostals actively

contract

Diaphragm and external

intercostals relax, followed

by elastic recoil of chest

wall and lungs

Diaphragm,

sternocleidomastoid

and scalene muscles

contract

Internal intercostal

and abdominal

muscles contract

active

active

passive

active

Normal quiet inhalation

Normal quiet exhalation

Forceful inhalation

Forceful exhalation

(a) During normal quiet breathing

(b) During forceful breathing

• Controls basic rhythm of respiration
• Normal quiet breathing the expiratory area is inactive . During forceful breathing the inspiratory area activates the expiratory area, which is active during high ventilation rates

The function of the medullary rhythmicity area is to control the basic rhythm of respiration. There are inspiratory and expiratory areas within the medullary rhythmicity area. The figure shows the relationship between these two areas during normal quiet breathing and forceful breathing. During quiet breathing the inspiratory area generates nerve impulses for about 2 seconds which are transmitted via nerves to the external intercostal muscles and the diaphragm, causing them to contract and inhalation to occur. This is therefore an active process. After 2 seconds the inspiratory centre becomes inactive and the external intercostal muscles and diaphragm relax for about 3 seconds in the passive process of exhalation. Exhalation is also due to the elastic recoil of alveolar walls.

• The neurons of the expiratory centre remain inactive during quiet breathing. However, during forceful breathing impulses from the inspiratory area activate the expiratory area. Impulses from the expiratory area thencause contraction of the internal intercostal muscles and the abdominal muscles to cause forceful exhalation, for example during exercise or playing a wind instrument. The inspiratory centre is regulated by other sites in the brain stem, the pneumotaxic area and the apneustic area, which alter the rate of transition from inhalation to exhalation and therefore the rate of breathing.

Role of the Respiratory Center: Pneumotaxic Area

The pneumotaxic

area sends inhibitory

impulses to the

inspiratory area

Sagittal

plane

RESPIRATORY

CENTER:

Midbrain

Pneumotaxic area

Apneustic area

Pons

- Medullary rhythmicity

area:

Inspiratory area

Medulla

oblongata

Expiratory area

Spinal

cord

These signals

shorten the duration

of inhalation.

Sagittal section of brain stem

Role of the Respiratory Center: Apneustic Area

The apneustic area

sends stimulatory

impulses to the

inspiratory area

Sagittal

plane

RESPIRATORY

CENTER:

Midbrain

Pneumotaxic area

Apneustic area

Pons

+ Medullary rhythmicity

area:

Inspiratory area

Medulla

oblongata

Expiratory area

These signals

prolong the duration

of inhalation.

Spinal

cord

Sagittal section of brain stem

Chemical Regulation of Respiration

• Central chemoreceptors in medulla oblongata detect changes in CSF, PCO2 and H+
• Peripheral chemoreceptors - in carotid bodies detect changes in PO2, PCO2 and H+ - in aortic bodies detect changes in PO2, PCO2 and H+

Chemoreceptors and Nerve Impulses

Medulla oblongata

1

1

Sensory axons in

glossopharyngeal nerve

(cranial nerve IX)

Internal carotid

artery

Carotid

body

External carotid

artery

Carotid sinus

Common carotid

artery

Sensory axons

in vagus nerve

(cranial nerve X)

1

Arch of aorta

Aortic bodies

Heart

Notes on Chemoreceptors

NOTES

• Central chemoreceptors are found in the medulla oblongata of the brain stem, and respond to changes in PCO2 and H+ in the cerebrospinal fluid.Peripheral chemoreceptors are located in the aortic bodies and in the carotid bodies. They detect changes in PO2, PCO2 and H+ .
• Aortic bodies are found in the wall of the arch of the aorta. Carotid bodies are found in the left and right carotid arteries. In both cases they are located close to the baroreceptors that detect blood pressure. Sensory nerves from the peripheral chemoreceptors send impulses to the respiratory centre.
• CO2 is lipid soluble and readily enters cells where it combines with water and in the presence of carbonic anhydrase generates carbonic acid, which subsequently dissociates to H+ and HCO3 -. The resulting increase in H+ is detected following even a very small change in PCO2 from the normal value in arterial blood of 40 mm Hg. An increase in PCO2 in arterial blood is called hypercapnia. Hypercapnia is detected by both central and peripheral chemoreceptors. . However, in addition, the peripheral chemoreceptors can also detect and respond to a deficiency in O2. When PO2 in arterial blood falls below a normal value of 100 mm Hg, but is still above 50 mm Hg, the peripheral chemoreceptors are stimulated. . However, if PO2 falls below 50 mm Hg, the severe deficiency in 02 suppresses the activity of central chemoreceptors and the inspiratory area, which then do not respond well and send fewer impulses for inspiration. As the respiratory rate falls and PO2 falls lower and lower, a positive feedback cycle is established with fatal results.

Chemoreceptor Regulation of Respiration

• A slight increase in Pco2 (and thus H+), a condition called hypercapnia, stimulates central and peripheral chemoreceptors
• As a response to increased Pco2, increased H+ and decreased PO2, the inspiratory area is activated and hyperventilation, rapid and deep breathing, occurs is lower than 40 mm Hg, a condition . If arterial Pco2 called hypocapnia, the chemoreceptors are not stimulated and the inspiratory area sets its own pace until CO2 accumulates and Pco2 rises to 40 mm Hg.
• Severe deficiency of O2 depresses activity of the central chemoreceptors and respiratory center

Negative Feedback Regulation of Breathing

Some stimulus disrupts

homeostasis by

Increasing

Arterial blood Pco2

(or decreasing pH or Po2)

Receptors

Central

chemo-

Peripheral

chemo-

receptors receptors

in

medulla

in aortic

and

carotid

bodies

Input

Nerve

impulses

Control center

Inspiratory area in

medulla oblongata

Output

Nerve

impulses

Effectors

Muscles of

inhalation and

exhalation

contract more

forcefully and

more frequently

(hyperventilation)

Decrease in arterial

blood Pco2, increase in

pH, and increase in Po2

Return to homeostasis

when response brings

arterial blood Pco2, PH,

and Po2 back to normal

Negative feedback

regulation of breathing

• Negative feedback control of breathing
• Increase in arterial pCO2
• Stimulates chemoreceptors
• Sensory nerves send signals to the inspiratory center
• Muscles of respiration contract more frequently & forcefully
• pCO2 Decreases