Coordination & RegulationIn order to survive a wide range of environmental conditions, the human body must be able to detect and respond to changesThe maintenance of a constant internal environment within tolerable limits is called homeostasis and is regulated by negative feedbackTwo main body systems coordinate homeostatic regulation - the nervous system and the endocrine system

Comparison of the Nervous System and Endocrine System

Signalling Pathways Mediated via Communication between Nerves and Hormones

Homeostasis:

Homeostasis is the tendency of an organism or cell to maintain a constant internal environment within tolerance limits Internal equilibrium is maintained by adjusting physiological processes, including:

Body temperature  (normally 36 - 38°C) Blood pH  (normally 7.35 - 7.45) Carbon dioxide concentration  (normally 35 - 45 mmHg) Blood glucose concentration  (normally 75 - 95 mg / dL) Water balance  (varies with individual body size)

Negative Feedback Most homeostatic control mechanisms operate through a negative feedback loop When specialised receptors detect a change in an internal condition, the response generated will be the opposite of the change that

occurred When levels have returned to equilibrium, the effector ceases to generate a response  If levels go too far in the opposite direction, antagonistic pathways will be activated to restore the internal balance

Negative Feedback Loop

Homeostatic Control Systems Homeostasis is maintained by the concerted effort of body systems communicating via both electrical (nervous) and chemical (hormonal)

systems

Both nerves and hormones are specific in their actions - nerves terminate in specific parts of the organism, while hormones only produce activity in specific target cells

The actions of both nerves and hormones involve chemical substances - hormones are chemicals themselves, while nerves use chemicals called neurotransmitters to facilitate electrical signalling

Nerves tend to bring about a response very rapidly, while hormonal responses are much slower but tend to be longer lasting The initiation of homeostatic responses results from an external or internal stimulus, which is detected by a specific type of receptor

Types of Receptors

Homeostatic Control via the Nervous SystemThermoregulationAnimals capable of temperature regulation within a given range are called homeotherms and maintain a constant body temperature through a negative feedback loop

The hypothalalmus acts as a control centre in thermoregulation by detecting fluctuations in body temperature The skin also possesses thermoreceptors and relays this information to the hypothalamus, which coordinates corrective measures

When body temperature rises, the following cooling mechanisms may occur: Vasodilation:  The skin arterioles dilate, bringing blood into closer proximity to the body surface and allowing for heat transfer (convective

cooling)

Sweating:  Sweat glands release sweat, which which is evaporated at the cost of latent heat in the air, thus cooling the body (evaporative cooling)

When body temperature falls, the following heating mechanisms may occur: Vasoconstriction:  The skin arterioles constrict, moving blood away from the surface of the body, thus retaining the heat carried within the

blood  Shivering:  Muscles begin to shake in small movements, expending energy through cell respiration (which produces heat as a by-product)

Other mechanisms through which homeotherms may regulate their body temperature include: Piloerection:  Animals with furry coats can make their hair stand on end (piloerection), trapping pockets of warm air close to the body

surface Behavioural responses:  Animals may physically respond to environmental conditions in a bid to regulate temperature (e.g. bathing,

burrowing, etc.)

Thermoregulation by the Nervous System

Homeostatic Control by the Endocrine SystemBlood Glucose Regulation

The body requires volumes of glucose in order to make ATP, however the amount of ATP demand will fluctuate according to need and thus the body regulates its release of glucose into the bloodstream as high levels of glucose in the bloodstream can damage cells (creates hypertonicity)

Two hormones, insulin and glucagon, are responsible for controlling blood glucose concentration (they have antagonistic functions) These hormones are released from different groups of cells with pancreatic pits (called the islets of Langerhans) and act principally on the

liver

When blood glucose levels are high (e.g. after feeding): Insulin is released from beta cells in the pancreas and causes a decrease in blood glucose concentration This may involve stimulating glycogen synthesis in the liver (glycogenesis), promoting glucose uptake into the liver and adipose tissue or

increasing the rate of glucose breakdown (increase cell respiration)

When blood glucose levels are low (e.g. after strenuous exercise): Glucagon is released from alpha cells in the pancreas and cause an increase in blood glucose concentration This may involve stimulating glycogen breakdown in the liver (glycogenolysis), promoting glucose release from the liver and adipose tissue

or decreasing the rate of glucose breakdown (decrease cell respiration) 

Blood Glucose Regulation by the Endocrine System

Homeostatic Control by Nervous and Endocrine SystemsOsmoregulation

All terrestrial animals regulate their body fluid levels by controlling the amount of water released from the body as urine The medullary region of the kidneys is hypertonic and will draw water out of the collecting ducts and back into the circulating blood Osmoreceptors in the hypothalamus detect water levels in the blood and coordinate the release of the neurohormone, anti-diuretic

hormone (ADH)  Neurohormones are hormones released from nerve cells that target distant cells (as opposed to neurotransmitters which target nearby

neurons)

When blood water levels are low (e.g. dehydration): More ADH is released from the posterior pituitary ADH stimulates the production of aquaporins in the collecting ducts of the kidneys, making them more permeable to water More water is reabsorbed into the bloodstream and less water is lost in urine

When blood water levels are high: Less ADH is released from the posterior pituitary Less aquaporins are produced in the collecting ducts of the kidneys, making them less permeable to water Less water is reabsorbed into the bloodstream and more water is lost in urine

Osmoregulation by the Nervous and Endocrine Systems

The Nervous System:

The nervous system contains a specialised network of cells called neurons and coordinates the actions of complex organisms via the transmission of electrochemical signalsThe nervous system can be divided into two main parts:

Central Nervous System (CNS):  Made up of the brain and the spinal cord Peripheral Nervous System (PNS):  Made of peripheral nerves which link the CNS with the body's receptors and effectors

Divisions of the Nervous System

NervesThere are three main types of neurons in the nervous system:

Sensory Neurons:  Conduct nerve impulses from receptors to the CNS (afferent pathway) Relay Neurons:  Conduct nerve impulses within the CNS (also called interneurons or connector neurons) Motor Neurons:  Conduct nerve impulses from the CNS to effectors (efferent pathway)

Structure of a Motor Neuron

The Stimulus-Response ModelThe basic pathway for a nerve impulse is described by the stimulus-response model

A receptor converts a stimulus into a nerve impulse, which is transmitted by a sensory neuron to the CNS (spinal cord) Relay neurons within the CNS will transmit this signal to a control centre (usually the brain), where the information is processed Motor neurons will transmit a resultant nerve impulse from the CNS to an effector organ (a muscle or gland), eliciting an appropriate

response

The Stimulus-Response Pathway

A reflex is a rapid and involuntary response to a stimulus and results from an even simpler pathway called a 'reflex arc' Reflex actions do not involve the brain in the decision making process - instead sensory information is relayed directly to motor pathways

within the spine This results in a reaction without conscious thought, which may be important in survival situations when quick reactions are necessary to

avoid permanent damage or pain

Transmission of a Nerve Impulse WITHIN a Neuron Transmission of a nerve impulse within a neuron occurs via the movement of an electrical potential along the length of the neuron When a neuron is not firing, the charge difference across the membrane is negative (-70 mV) - this is known as the resting potential When a neuron is firing, the charge difference changes to become slightly positive (~ 30 mV) - this is known as the action potential The change in membrane polarity when the neuron is firing (from resting potential to action potential) is called depolarisation  Restoration of the resting potential is known as repolarisation

Generation of a Resting Potential The sodium-potassium pump (Na+/K+ pump) maintains the electrochemical gradient of the resting potential (-70 mV) It is a transmembrane protein that uses active transport to exchange Na+ and K+ ions across the membrane (antiport mechanism) It expels 3 Na+ ions for every 2 K+ ions admitted (in addition, some of the K+ ions will leak back out of the cell) This makes the inside of the membrane relatively negative when compared to the outside (-70 mV = resting potential)

Transmission of an Action Potential

Sodium and potassium channels in nerve cells are voltage-gated, meaning they can open and close depending on the voltage across the membrane

In response to a signal at a sensory receptor or dendrite, sodium channels open and sodium enters the neuron passively The influx of sodium (Na+ in) causes the membrane potential to become positive (depolarisation) If a sufficient change in membrane potential is achieved (threshold potential), adjacent voltage-gated sodium channels open, generating a

wave of depolarisation (action potential) that spreads down the axon The change in membrane potential also activates voltage-gated potassium channels, causing potassium to exit the neuron passively The efflux of potassium (K+ out) causes the membrane potential to become negative again (repolarisation) Before the neuron can fire again, the original distribution of ions (Na+ out, K+ in) must be re-established by the Na+/K+ pump The inability to propagate another action potential during this time (refractory period) ensures nerve impulses only travel in one direction

Generation of an Action Potential

Transmission of a Nerve Impulse BETWEEN Neurons The junction between two neurons is called a synapse, it forms a physical gap between the pre-synaptic and post-synaptic neurons

An action potential (electrical signal) cannot cross the synaptic gap, so it triggers the release of chemicals (neurotransmitters) to continue the signal

Chemical Transfer Across Synapses When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels Calcium ions (Ca2+) diffuse into the cell and promote the fusion of vesicles (containing neurotransmitters) with the plasma membrane The neurotransmitters are released from the axon terminal by exocytosis and cross the synaptic cleft Neurotransmitters bind to appropriate neuroreceptors on the post-synaptic membrane, opening ligand-gated channels Excitatory neurotransmitters (e.g. noradrenaline) open ligand-gated sodium channels (depolarisation) Inhibitory neurotransmitters (e.g. GABA) open ligand-gated potassium or chlorine channels (hyperpolarisation) The combination of chemical messengers received by dendrites determines whether the threshold is reached for an action potential in the

post-synaptic neuron Neurotransmitter molecules released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity)

Overview of Synaptic Transfer

Drugs and ToxinsNeurotransmitters can have a variety of responses depending on the cell type activated:

Drugs and toxins may affect animal behaviour and physiological responses by effecting the transmission of nerve impulses between neurons

Drugs that stimulate the nervous system are called agonists, while drugs that inhibit the nervous system are called antagonists Drugs are commonly used to treat medical conditions or recreationally (to alter mood and perception) Toxins are toxic substances (poisons) produced by organisms which can cause pain, paralysis and even death

The Endocrine System:The endocrine system is a system of ductless glands that secrete hormones into the bloodstream to regulate body functionsIt is different to the exocrine system, which releases products from ducted glands into cavities (e.g. digestive tract) or into the external environment While the endocrine system describes chemical messengers that act on distant sites, hormones may also act locally via autocrine or paracrine signalling

Types of Cell Signalling

Types of HormonesThere are three main classes of hormones:

Steroid Hormones Steroid hormones are lipophilic - meaning they can freely diffuse through the plasma membrane They bind to receptors either in the cytoplasm or in the nucleus of the target cell, to from an active receptor-hormone complex This activated complex will move to the nucleus and bind the DNA directly, acting as a transcription factor for the regulation of specific

genes

Mechanism of Action for a Steroid Hormone

Protein Hormones Protein hormones (peptides and amines) are lipophobic - meaning they cannot freely pass through the plasma membrane They bind to receptors on the surface of the plasma membrane, which generates a chemical signal within the cell The receptor is typically coupled to a G-protein (or adenylate cyclase), which activates an intracellular molecule called a second

messenger This process is called signal transduction, because the initial chemical signal (the hormone) is transduced via a G protein to an

intermediary molecule (second messenger) within the cell Examples of second messengers include:  cyclic AMP (cAMP), protein kinases, phosphatases, calcium ions (Ca2+), nitric oxide (NO)

Mechanism of Action for a Protein Hormone

Hormone Action Hormones may induce cell activity in a number of ways, including: Changing plasma membrane polarity or permeability by opening or closing protein channels - e.g. regulating glucose uptake into adipose

tissue Regulating expression of functional proteins (via gene activation or suppression) - e.g. producing cytoskeletal proteins to alter cell

morphology Moderating enzyme activity (via activation or deactivation) - e.g. altered cell metabolism Inducing or supressing release of secretory products - e.g. secretion of ovarian hormones in response to stimulation from pituitary

hormones Stimulating mitosis and cell division A small stimulus can cause a large response if it involves many intermediaries, as each step can activate multiple molecules at the next

step (amplification)

The Endocrine System The endocrine system releases hormones into the bloodstream to act on distant target cells within the body The hormone will only be able to activate cells that possess the appropriate receptor (the receptor and hormone share specificity) The endocrine system is slow to initiate, but may have a much more prolonged and sustained response when compared to the nervous

systemGeneral Overview of the Endocrine System

Plant Hormones (Phytohormones):

Plant hormones (or phytohormones) are chemical substances that are used to control growth and development in plantsThey are produced in relatively small amounts and can be transported in the xylem or phloem if the target cell is differentPlant hormones may have different effects in different areas of the plant, making it difficult to definitively assign a function for a given phytohormone

Types of Plant HormonesAuxins (e.g. IAA)

Auxins are a class of plant hormones principally involved in plant growth and elongation They may increase the rate of cell elongation in response to directional stimuli, and are thus important in tropic responses (e.g.

phototropism) They promote apical dominance - where the apex / tip of a plant grows while the lateral buds don't develop They may increase the rate of cell division

Cytokinins Cytokinins are important in growth as they promote cell division (cytokinesis) They play a role in ensuring the roots and shoots grow at equal rates (important for continued survival) and also stimulate growth of fruit Whereas auxins are responsible for primary (vertical) growth, cytokinins are responsible for secondary (lateral) growth - they control

branching

Gibberellins Gibberellin (or gibberellic acid) is necessary for seed germination

In the presence of water, it stimulates the production of amylase (converts starch into maltose), allowing for the formation of ATP (via glucose)

The energy produced in the embryo - as a result of the action of gibberellin - is used to facilitate germination The glucose produced may also be used to synthesis cellulose - for cell wall formation Gibberellin also causes stem elongation by promoting both cell elongation and cell division

Ethylene A gas which acts as a hormone and stimulates maturation and ageing (senescence)  It is responsible for the ripening of certain fruit (auxins and gibberellins promote fruit growth but inhibit ripening) It is also responsible for the ageing and loss of leaves (abscission) and the death of flowers

Abscisic Acid (ABA) Abscisic acid principally inhibits plant growth and development It promotes the death of leaves (abscission) and is responsible for seed dormancy It generally initiates stress responses in plants (like winter dormancy in deciduous trees) ABA prevents excess water loss in dehydrated plants by causing the closure of stomata

General Overview of Role of Plant Hormones in Growth and Development

External Factors The growth and development of a plant is often dependent on external environmental factors For instance, germination is dependent on the presence of water and some plants may only flower after exposure to cold temperatures

(vernalisation) Tropisms are growth responses prompted by directional stimuli (e.g. phototropism = response to light ; geotropism = response to gravity)

Phototropism Phototropism is the growing or turning of an organism in response to a unidirectional light source Auxins (e.g. IAA) are plant hormones that are produced by the tip of a shoot and mediate phototropism Auxin makes cells enlarge or grow and, in the shoot, are eradicated by light The accumulation of auxin on the shaded side of a plant causes this side only to lengthen, resulting in the shoot bending towards the light Auxin causes cell elongation by activating proton pumps that expel H+ ions from the cytoplasm to the cell wall The resultant decrease in pH within the cell wall causes cellulose fibres to loosen (by breaking the bonds that hold them together) This makes the cell wall flexible and capable of stretching when water influx promotes cell turgor Auxin can also alter gene expression to promote cell growth (via the upregulation of expansins)

The Role of Auxin in Phototropism

Photoperiodism Flowering is controlled by phytochrome, which is affected by light (photoperiodicity) Phytochrome exists in two forms:  A red (Pr) form absorbs red light (~660 nm) and is converted into a far red form (Pfr) A far red (Pfr) form absorbs far red light (~730 nm) and is converted into a red form (Pr) The Pfr form is the active form of phytochrome, while the Pr form is the inactive form of phytochrome Sunlight contains more red light, so the Pfr form is predominant during the day, with the gradual reversion to the Pr form occurring at night In long day plants, the active Pfr form is a promoter of flowering and so flowering is induced when the night period is less than a critical

length and Pfr levels are high In short day plants, the active Pfr form is an inhibitor of flowering and so flowering is induced when the night period is greater than a critical

length and Pfr levels are low

The Control of Flowering in Plants

Pheromones:Pheromones are chemicals released by an organism into its environment, enabling it to communicate with other members of its own species

Insect PheromonesAlarm Pheromone

When an ant is disturbed, it can releases a pheromone that attracts other ants and causes an alarm response  High concentrations cause the ants to run about as they work to remedy the disturbance  Unless additional amounts of the alarm pheromone are released, it soon dissipates  This ensures that once the emergency is over, the ants return quietly to their former occupations

Trail Pheromone Certain ants lay down a trail pheromone, which will attract and guide other ants to a source, such as food  The trail pheromone evaporates quickly, so the more pheromone that exists on the path, the greater the probability other ants will follow it This ensures that ants always take the shortest route around an obstruction - as more ants can traverse a shorter path in a given time

period, the pheromone will accumulate quicker on this path

Ant Paths as Determined by Trail Pheromones

Sex Attractants Hundreds of pheromones are known with which one sex (usually the female) of an insect species attracts its mates  Other species of animal and plant may mimic these sex pheromones for their own survival purposes: One species of spider, Mastophora cornigera, releases a scent that mimic the sex pheromone of the moth species it preys upon A number of orchid species emit a scent that mimics the sex pheromone of female wasps and bees - attracting males for the purpose of

pollination  Many types of sex attractants are available commercially for use in insect baiting (used in conjunction with conventional insecticides) 

Mammalian PheromonesReleaser Pheromones

Many mammals (e.g. dogs and cats) deposit chemicals in and around their established "territory" (often released in the urine) As these vaporize, they signal to other members of the species of the presence of the occupant of the territory Domestic rabbit mothers release a mammary pheromone that triggers immediate nursing behavior by their babies (pups) 

Primer Pheromones

Rats and mice give off pheromones that elicit mating behavior - however the response is not immediate as it is for the sex attractants of insects 

Instead, detection of the pheromone primes the endocrine system of the recipient to make the changes (e.g. ovulation) needed for successful mating

Human Pheromones In human females, two uncharacterised pheromones are secreted from armpits, synchronizes menstrual cycles and ovulation in closely-

living groups One pheromone is released prior to ovulation and speeds up the onset of ovulation of those close by  The other pheromone is released after ovulation and delays the onset of ovulation in others, effectively 'synchronizing’ the group