Control Mechanisms (HL IB Biology)

• A hormone is a chemical messenger produced by an endocrine gland and carried by the blood
  • They are chemicals which transmit information from one part of the organism to another and bring about a change
  • They alter the activity of one or more specific target organs
• Hormones are used to control functions that do not need instant responses
• The endocrine glands that produce hormones in animals are known collectively as the endocrine system
  • A gland is a group of cells that produces and releases one or more substances (a process known as secretion)
• Control of the endocrine system is primarily by the hypothalamus and the pituitary gland

The hypothalamus 

• • The hypothalamus monitors the blood as it flows through the brain and, in response, releases hormones or stimulates the neighbouring pituitary gland to release hormones
    • The hypothalamus plays an important role in some homeostatic mechanisms
  • Hypothalamus functions include
    • Regulating body temperature
      • The hypothalamus monitors blood temperature and initiates a homeostatic response if this temperature gets too high or too low
    • Osmoregulation
      • Cells in the hypothalamus monitor the water balance of the blood and releases the hormone ADH if the blood becomes too concentrated
        • ADH increases absorption of water in the kidneys
    • Regulating digestive activity
      • The hypothalamus regulates the hormones that control appetite as well as the secretion of digestive enzymes
    • Controlling endocrine functions
      • The hypothalamus causes the pituitary gland to release hormones that control a variety of processes e.g. metabolism, growth and development, puberty, sexual functions, sleep, and mood

The pituitary gland

• The pituitary gland is located below the hypothalamus
• Its role is to produce a range of hormones
  • Some of these directly influence and regulate processes in the body while some stimulate the release of further hormones from other endocrine glands
• The pituitary gland is divided into two sections; the anterior pituitary and posterior pituitary
  • The anterior pituitary produces and releases hormones
  • The posterior pituitary stores and releases hormones produced by the hypothalamus e.g. ADH and oxytocin

• There are several circumstances that can cause an individual's heart rate to increase, such as during exercise
• The brain is involved in this response of the heart however it does not require any thinking and is under unconscious control
• There is a specific region of the brain that plays a vital role in controlling the heart rate
  • This cardioregulatory centre in the brain is called the medulla
• The medulla is found at the base of the brain near the top of the spinal cord
• The medulla is made up of two distinct parts:
  • The acceleratory centre, which causes the heart to speed up
  • The inhibitory centre, which causes the heart to slow down
• Both centres are connected to the sinoatrial node (SAN) by nerves
• These specific nerves are different from the nerves that control conscious activities. They make up the autonomic nervous system
• The autonomic nervous system is self-controlling

Control of heart rate diagram

The location of the medulla helps to keep it protected from harm. It has an essential function as a cardioregulatory centre.

Increasing heart rate

• Once the acceleratory centre has been activated impulses are sent along the sympathetic neurones to the SAN
• Norepinephrine is secreted at the synapse with the SAN
• Noradrenaline causes the SAN to increase the frequency of the electrical waves that it produces
• This results in an increased heart rate

Decreasing heart rate

• Once the inhibitory centre has been activated impulses are sent along the parasympathetic neurones to the SAN
• Acetylcholine is secreted at the synapse with the SAN
• This neurotransmitter causes the SAN to reduce the frequency of the electrical waves that it produces
• This reduces the elevated heart rate towards the resting rate

Chemoreceptors and baroreceptors

• Exercise causes several internal conditions to change, creating internal stimuli:
  • Carbon dioxide concentration in the blood increases
  • There is an initial fall in blood pressure caused by the dilation of muscle arterioles
• These internal stimuli can be detected by chemoreceptors and baroreceptors (pressure) receptors located in the aorta (close to the heart) and in the carotid arteries (they supply the head with oxygenated blood)
  • Chemoreceptors detect changes in blood pH and oxygen and carbon dioxide levels
  • Baroreceptors monitor changes in blood pressure

Location of the baroreceptors and chemoreceptors

Baroreceptors are located on the arch of the aorta and on the enlargement of the carotid arteries called the sinus. Chemoreceptors are located near the baroreceptors but on the outside of the aorta and carotid arteries

• These receptors release nerve impulses that are sent to the acceleratory and inhibitory centres (coordinators)
• The frequency of the nerve impulses increases or decreases depending on how stimulated the receptors are:
  • Lower frequency impulses activate the inhibitory centre to slow down the heart rate and stroke volume
  • Higher frequency impulses activate the acceleratory centre to speed up the heart rate and stroke volume

The processes involved in the control of the heart rate. The internal stimuli are detected by chemoreceptors and baroreceptors that send impulses to coordinators (accelerator centre or inhibitory centre). The coordinators send signals to the effector (SAN) which produces a specific response.

• Ventilation rates in the body are controlled by cells called respiratory centres which are located in the medulla of the brain
• At rest, action potentials, produced at random, travel to the diaphragm and intercostal muscles to initiate contraction and therefore ventilation; this occurs at a stable and slow pace
• During exercise, higher levels of carbon dioxide are produced due to an increase in respiration
• Waste carbon dioxide produced during respiration diffuses from the tissues into the blood 
• This waste carbon dioxide is transported around the body in different ways
  • In the blood plasma in the form of hydrogen carbonate ions (HCO^3-); around 85 % of carbon dioxide is transported in this way
  • Around 5 % of carbon dioxide dissolves directly in the blood plasma
  • Bound to haemoglobin as carbaminohaemoglobin; this accounts for around 10 % of carbon dioxide transport in the blood

pH changes in the blood

• Peristalsis is series of muscle contractions in the walls of the oesophagus or small intestine that pass like a wave along the alimentary canal
  • This wave forces the bolusof food along the alimentary canal
  • These contractions are controlled unconsciously by the autonomic nervous system, specifically the enteric nervous system (ENS)
    • The ENS consists a web of sensory neurones, relay neurones and motor neurones embedded in the tissues of the alimentary canal
• Peristalsis is controlled by circular and longitudinal muscles which initiate a mechanism called the peristaltic reflex
  • These muscles are smooth muscle (not striated)
• The bolus of food is detected by stretch receptors (sensory neurones of the ENS) as the alimentary canal becomes distended
• An action potential is passed to relay neurones of the ENS which synapse with two different motor neurones
  • One motor neurone releases an excitatory neurotransmitter which stimulates
    • Longitudinal muscles to contract behind the bolus to reduce the length of that section the oesophagus or the small intestine
      • This forces the food forwards through the alimentary canal
    • Circular muscles contract to reduce the diameter of the lumen of the oesophagus or small intestine
      • This prevents the food moving backwards towards the mouth
  • A second motor neurone releases an inhibitory neurotransmitter which stimulates smooth muscle ahead of the bolus to relax and open the lumen of the alimentary canal

Control of peristalsis diagram

The control of peristalsis is initiated by the bolus and is carried out by two different motor neurones