BioNinja Notes - Equilibrium

BODY SYSTEMS

In multicellular organisms, each individual cell needs to communicate with other cells in order to maintain organismal survival. Cells group together to form tissues; different tissues interact to form organs and organs are integrated into body systems. The collective actions of these different structures combine to create new synergistic effects – known as emergent properties. For example, individual cells are incapable of conscious behaviours but millions of interacting nerve cells enable the capacity for critical thought and decision making. Animals contain a myriad of body systems that function to promote growth and survival:

COMMUNICATION NETWORKS

In order for multicellular organisms to function, there must be a mechanism for communication between the different body systems. Animals contain two means by which individual cells can become integrated:

·       Nervous system: A rapid communication system involving the conduction of electrochemical impulses

·       Endocrine system: A widely distributed network that uses chemical messengers for prolonged effects Additionally, specific materials and energy can be transferred between cells via the blood (vascular) system.

NERVES VS HORMONES

The nervous and endocrine systems differ in a number of key aspects which influence and inform the roles that they perform in multicellular communication. Some of the differences between the systems include:

NERVOUS SYSTEM

The nervous system coordinates the actions of complex organisms via the transmission of electrochemical signals. These signals are transmitted by a specialised network of cells called neurons. The nervous system can be divided into two key parts – the central nervous system (CNS) and peripheral nervous system (PNS).

The central nervous system is made up of the brain and spinal cord. It functions as a central information integration organ by processing information from a variety of inputs and coordinating necessary responses. The brain is an integrating centre for conscious processes and is responsible for a range of higher order cognitive functions – including learning, memory, emotion and critical thought. The spinal cord functions as a connection point between the brain and the peripheral nerves. It also acts as an integrating centre for certain unconscious processes that do not require the involvement of the brain (these are reflex actions).

The peripheral nervous system is made up of sensory and motor neurons that transmit information to or from the central nervous system. These neurons are organised into bundles of nerve fibres that collectively form a nerve. Each nerve enables communication with a specific region of the body. Afferent nerves send signals to the central nervous system (via sensory neurons), while efferent nerves function to send signals from the central nervous system (via motor neurons). Individual neurons may be wrapped within a layer of fatty tissue called the myelin sheath, which insulates the nerve cell and improves transmission speeds (i.e. faster signalling). However, myelinated fibres take up more space and have greater energy requirements, meaning not all nerve fibres are myelinated.

STIMULUS RESPONSE MODEL

The basic pathway for a nerve impulse is described by the stimulus-response model. A stimulus represents any change to the environment (external or internal) that is detected by a receptor. Receptors convert the stimulus into an electrical impulse that is transmitted via sensory neurons to the central nervous system, where processing of the signal occurs. When a response is determined (either conscious or unconscious), an impulse is transmitted via motor neurons to an effector organ – which is either a muscle or a gland. The effector then generates a response. Muscles are stimulated to contract and enable movement, while glands release chemicals (endocrine glands release into the bloodstream, exocrine glands release onto surfaces).

REFLEX ACTIONS

A reflex is a rapid and involuntary response to a stimulus, resulting from a simple signalling pathway called a reflex arc. Reflex actions do not involve the brain – instead sensory information is relayed directly to the motor neurons via the spinal cord. This results in a fast response that does not involve conscious thought or deliberation. Reflex actions are especially beneficial in survival situations, when quick reactions are needed to avoid permanent damage. An example of a reflex arc is a pain response pathway. The free nerve ending of a sensory neuron acts as a pain receptor and transmit a signal to a single interneuron within the grey matter of the spinal cord. The impulse is transmitted via a motor neuron to the local muscles, which then contract and move the impacted appendage away from the source of discomfort.

STIMULUS

SPINAL CORD

RESPONSE

THE BRAIN

The brain functions as the main integration centre of the central nervous system. While the spinal cord can regulate certain unconscious processes (reflex actions), only the brain is capable of the coordination and integration of more complex actions – including learning, memory, emotions and consciousness. The brain is organised into three main structures – the cerebral hemispheres, the cerebellum and the brain stem.

Cerebral Hemispheres

The cerebral hemispheres (collectively called the cerebrum) are responsible for the processing of most higher order functions. They are heavily folded (gyrification) to increase cognitive capacity and are divided into four topographical lobes that specialise in the integration and coordination of distinctive functions:

·       Frontal lobe – Controls voluntary motor activities, speech production and tasks involving dopamine

·       Parietal lobe – Responsible for sensory perceptions, such as touch sensation (tactility), smell and taste

·       Occipital lobe – The visual processing centre of the brain (used for sight perception and interpretation)

·       Temporal lobe – Involved in auditory processing (i.e. hearing) and the comprehension of language

Not all complex tasks are equally represented by both cerebral hemispheres – some activities are localised to a single side. For example, speech production is coordinated by Broca’s area, which is situated in

the left frontal lobe of the brain. Additionally, each hemisphere processes sensory and motor information for the opposite side of the body. Information can be passed between the two hemispheres by a bundle of myelinated nerve fibres embedded within the brain. These fibres form the corpus callosum to facilitate interhemispheric communication.

Cerebellum

The cerebellum is a separate structure located at the base of the brain. It is responsible for the coordination of complex motor actions – which includes balance and proprioception. The cerebellum coordinates gait, controls your posture and is involved in maintaining muscle tone. While the cerebellum helps to control voluntary muscle activity, it does not initiate contractions – that is controlled by the primary motor cortex in the frontal lobe. The term cerebellum is derived from Latin and essentially translates to ‘little brain’.

Brainstem

The brainstem is the structure that acts to connect the cerebrum to the spinal cord and the cerebellum. It consists of the midbrain, pons and medulla oblongata. The brainstem controls involuntary and unconscious functions – like breathing and heart rate. These actions do not require conscious intervention, which is why people in vegetative states may still be alive – as the brainstem maintains these critical survival functions.

ENDOCRINE SYSTEM

The endocrine system consists of a network of ductless glands that release chemical messengers that are called hormones directly into the bloodstream. This chemical release allows for a more widespread and longer lasting response. The hormones bind to specific receptors and only activate cells with the particular receptor. Hence, endocrine activity can be controlled by modifying either the amount of hormone released from the endocrine gland or the quantity of receptor expressed on the appropriate target cell. Hormones can either be hydrophilic proteins that bind to external receptors or hydrophobic lipids (i.e. steroids) that can freely cross the phospholipid bilayer and bind to receptors within the cytoplasm of the target cells.

HYPOTHALAMUS

The endocrine system is controlled by a region of the brain called the hypothalamus. The hypothalamus functions as a homeostatic control centre and regulates hormonal secretion via a ‘master gland’ called the pituitary gland. This gland lies adjacent to the hypothalamus and consists of anterior and posterior lobes. The hypothalamus produces releasing factors which trigger the release of hormones synthesised by the anterior lobe. Additionally, the hypothalamus also produces certain hormones itself, which are released from the posterior lobe. The hormones released from the pituitary gland may act directly on body tissues, but may also act to stimulate the activity of other endocrine glands (e.g. TSH stimulates the thyroid gland).

ENDOCRINE GLANDS

Endocrine glands secrete their product (hormones) directly into the bloodstream, rather than through a duct (exocrine glands).

Endocrine glands involved in homeostatic regulation include:

·       Pancreas – makes insulin and glucagon (control blood sugar)

·       Thyroid gland – produces thyroxin (regulates metabolism)

·       Adrenal gland – secretes adrenaline (‘fight or flight’ actions)

·       Pineal gland – releases melatonin (sets circadian rhythms)

·       Ovaries – synthesises female sex hormones (e.g. estrogen)

·       Testes – makes the male sex hormone (e.g. testosterone)

The hypothalamus and the pituitary gland are neuroendocrine glandsandfunctiontolinkthenervousandendocrinesystems, asbothsystemsarenecessarytomaintaininternalequilibrium.

HOMEOSTASIS

Organisms must maintain certain physiological conditions in order to ensure the continuation of essential metabolic reactions needed for survival. Examples of conditions that must maintain an internal equilibrium include body temperature, blood glucose concentrations, blood pH levels and the osmotic concentration of the blood. Homeostasis is the tendency for a cell or organism to maintain a constant internal environment within physiological tolerance limits. A failure to maintain internal conditions within preset limits will result in the development of disease. Homeostatic regulation involves the use of communication systems (nerves and hormones) to detect and then respond to changing internal conditions via feedback loops.

NEGATIVE FEEDBACK

Physiological variations are detected by a variety of receptors within the body. Different receptor types detect different specific stimuli – for example, chemoreceptors detect chemicals while baroreceptors detect pressure changes. These receptors convert the stimulus into biological signals that can be processed. In order to maintain internal equilibrium, an organism must enact physiological responses that will function to negate the change detected – so as to restore conditions to within the required preset limits. This process, whereby a response is the reverse of the detected change, is known as negative feedback.

All homeostatic mechanisms involve negative feedback. Positive feedback will not promote homeostasis as it involves a response that acts to reinforce the stimulus (it will amplify the variation). Any unregulated condition that disrupts normal physiology will cause a disease. Conditions that must be regulated include blood sugar levels (normally between 75 – 95 mg/dL) and body temperature (between 36 – 38ºC). Certain physiological activities produce predictable changes to internal conditions and need to be regulated. These include circadian rhythms, vigorous physical exercise and digestive movement in response to eating a meal.

BLOOD GLUCOSE REGULATION

Blood glucose concentrations are controlled by a set of antagonistic hormones secreted by the pancreas. Insulin (secreted by b cells) lowers blood sugar levels by increasing the uptake of glucose by the liver and adipose tissue (stored as glycogen). Glucagon (secreted by a cells) raises blood sugar levels by increasing the release of glucose by the liver and adipose tissue. Blood sugar levels may be increased following a meal and will decrease as a response to vigorous exercise (glucose is used in aerobic and anaerobic respiration).

🡇 blood sugar levels  

Glucagon released

DIABETES

Diabetes mellitus is a metabolic disorder that occurs when the body is unable to regulate its blood glucose concentrations. The body is unable to either produce or respond to insulin, resulting in hyperglycaemia.

There are two types of diabetes that differ in their physiological consequences, risk factors and treatment:

THERMOREGULATION

Homeotherms are animals that maintain a stable body temperature. Changes to core body temperature are detected by thermoreceptors in the skin and hypothalamus. These trigger a variety of mechanisms (either cooling or heating) to restore equilibrium. Responses include:

Cooling Mechanisms:

·       Vasodilation – The arterioles widen to allow more heat to be lost

·       Sweating – The production of sweat enables evaporative cooling

·       Behavioural changes – Burrowing or decreasing physical activity

Heating Mechanisms:

·       Vasoconstriction – The arterioles will narrow to retain more heat

·       Shivering – Repetitive muscular contractions will generate heat

·       Piloerection – Bristling hairs act to trap warm air against the skin

Body temperature is also regulated by the hormone thyroxin. The hypothalamus triggers the production of thyroxin by the thyroid gland by stimulating the pituitary to secrete TSH (thyroid stimulating hormone). This hormone increases metabolic activity, producing heat as a by-product. Brown adipose tissue is particularly proficient at generating heat, as these cells have an uncoupling protein that prevents the energy released from aerobic respiration from being transferred to ATP. Instead, the released energy is converted into heat.

CIRCADIAN RHYTHMS

Circadian rhythms are the body’s physiological responses to a 24-hour cycle of day and night. Circadian rhythms are regulated by the hormone melatonin.

Light exposure (i.e. day time) is detected by the suprachiasmatic nucleus in the hypothalamus, which suppresses melatonin secretion. This means that levels are lower during the day and higher at night. Melatonin functions to promote activity in nocturnal animals and conversely promotes sleep in diurnal animals. Over a prolonged period, melatonin secretion becomes entrained to anticipate

theonsetofdarknessandtheapproachofday.Melatoninlevelswillnaturally decrease with age, leading to changes in the sleeping patterns of the elderly.

EXERCISE INTENSITY

Physiological changes occur when an organism performs vigorous physical activity. The active cells must undertake aerobic respiration in order to produce large quantities of ATP. This requires both oxygen and glucose, while carbon dioxide is produced as a by-product. Gases are exchanged at the lungs (respiratory system), while all materials are transported in the bloodstream (vascular system). To prepare for vigorous activity, the amygdala (an area of the brain involved in emotional processing) sends stress signals to the hypothalamus to initiate a ‘fight or flight’ response. The hypothalamus then stimulates the release of adrenaline (also called epinephrine) from the adrenal glands. Adrenaline will trigger a variety of responses:

·       Cardiac output (heart rate and stroke volume) will increase to allow for greater systemic blood flow

·       Ventilation rate will increase and bronchioles will widen to improve the exchange of respiratory gases

·       Sugars will be released from storage organs and blood flow will be redirected to active tissues (muscles)

Cardiac Output

Cardiac output is autonomically controlled by the medulla oblongata (brainstem). Baroreceptors in the aortic arch and carotid sinuses detect any changes in blood pressure and send signals to the medulla to trigger adjustments to either heart rate or vessel diameter. Additionally, chemoreceptors in the blood vessels, along with central chemoreceptors in the medulla, may regulate heart rate according to changes in blood pH levels (a build-up of carbon dioxide lowers the pH of blood).

Ventilation Rate

Ventilation rate is regulated according to the changes in blood pH (which reflects the concentration of carbon dioxide in the blood). The chemoreceptors will send signals to the medulla to make adjustments that reflect the body’s requirements (vigorous activity will increase the levels of carbon dioxide in the blood and will trigger an increase in the ventilation rate to enable the removal of this gas). The brainstem controls ventilation by signalling to the diaphragm and the intercostal muscles within the chest cavity to increase the depth and frequency of breaths.

DIGESTIVE CONTROL

Consuming a meal prompts certain physiological adjustments to allow for the transit of food through the digestive tract and its subsequent assimilation into the body. The initiation of swallowing and the egestion of faeces are under voluntary control by the CNS, however the passage of food between these points is involuntary. The enteric nervous system (ENS) coordinates the sequential contraction of longitudinal smooth muscles to move the food distally through the alimentary canal (via a process called peristalsis).