Control of Breathing 2

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Nervous control of breathing


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Physiological control systems involving the

nervous system usually have three components.

These are:

a central controlling area

an afferent pathway (takes sensory nerve

impulses to the central control area)

an efferent pathway (takes motor nerve impulsesaway the central control area to the respiratory

muscles)

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Nervous control of respiration


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1. The central controlling area for breathing, called the

respiratory centre, is in the brain stem

2. It consists of a group of neurons which regulate minute

ventilation (involuntarily) in response changes in oxygen,

carbon dioxide and pH:

a. Inspiratory neurons are active during inspiration and

inactive during expiration

b. Expiratory neurons are active during expiration but not

inspiration3

Central Controlling Area


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Central controlling area

3. These two groups of neurones automatically maintain a

rhythmic cycling pattern of inspiration and expiration.

This automatic rhythm can be modified by the afferent

information.

4. The central controlling area receives input from higher

brain centers and peripheral receptors, and their output

fine tunes the breathing rhythm during activities such as

speaking, sleeping, or exercising

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Efferent pathways

The efferent nerves from the respiratory centre pass

down the spinal cord to the diaphragm, intercostal

muscles and accessory muscles of inspiration in the

neck. The diaphragm is supplied by the phrenic nerve that is

formed in the neck from the spinal nerves, C3, 4 and 5.

The intercostal muscles are supplied by the segmental

intercostal nerves that leave the spinal cord between TI

and TI2.

The accessory muscles in the neck are supplied from the

cervical plexus.

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Efferent pathways

During normal breathing, inspiration is an active

muscular process.

Expiration is passive and relies on the natural elasticity

of the tissues to deflate the lung. The most importantmuscle for inspiration is the diaphragm.

Any disease that affects the efferent pathways from the

respiratory centre to C3, 4 and 5 and then the phrenic

nerve to the diaphragm, may cause severe difficulty in

breathing. Trauma to the cervical cord, above C3, is

normally fatal for this reason

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Afferent pathways to respiratory

centre

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Cortical influences on central

controlling areaPermits voluntary control of ventilation by interacting with

and over-riding the autonomic centers in the medullary

rhythmicity area

Examples include the control of ventilation during speech

and singing, as well as deliberate forceful inspirations,

expirations, or attempts at breath holding; pain and certain

emotional states may also influence the rate and depth of

ventilation in this fashion

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Central chemoreceptorssensory neuron located in the brain stem or hypothalamus

responds to changes oxygen, carbon dioxide and pH in

the blood or cerebrospinal fluid

Peripheral chemoreceptors

sensory neuron located in the wall of a blood vessel (e.g.,

the aortic body and the carotid body)

responds to oxygen, carbon dioxide and pH (as above)

Chemoreceptors


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carotid bodyA group of chemoreceptors located near the bifurcations

of the carotid arteries

monitor changes in the oxygen and CO2 content and pH of

the blood

Aortic body

group of peripheral chemoreceptors located in the arch of

the aorta

monitor changes in the oxygen and CO2 content and pH ofthe blood

Chemoreceptors


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Copyright 2009, John Wiley &Sons, Inc.


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Chemoreceptors and

Homeostatic control of breathing If oxygen level falls or carbon dioxide levels risetoo greatly from the set point, a negative

feedback mechanism increases pulmonary

ventilation. This brings in more oxygen andexpels more carbon dioxide.

Humans are most sensitive to carbon dioxide

levels because the amount of carbon dioxide

varies most in respiration in response to different

metabolic and environmental conditions.

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Stretch receptors in the respiratory

system and the Herring-Breuer Reflex The respiratory tract from bonchi to terminal

bronchioles consists of smooth muscle, and this

muscle contains stretch receptors.

If the smooth muscle is stretched due to a largerthan normal breath (e.g. during vigorous

exercise), the stretch receptors send afferent

signals to the central nervous system to STOP

inspiration.

This is a protective reflex to prevent over-

expansion and tearing of the lung tissue.

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Proprioceptors and control of

breathing At the start of exercise, stimulation of peripheral stretchreceptors (or proprioceptors) in skeletal muscle and

joints (tendon organs) results in afferent signals to the

brainstem and this stimulates an increase in minute

ventilation.

This effect of breathing occurs within seconds of starting

exercise

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Drugs and the control of breathing

Some drugs (Narcotics, barbiturates and

opiates) have a direct effect on the

respiratory centre usually to slow it down

and can cause respiratory failure.

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Copyright 2009, John Wiley &Sons, Inc.

End of Chapter 23

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Control of Breathing 2