Regulatory Mechanisms in Animals

Regulatory Pathways

In animals the two regulatory pathways are the endocrine system and the nervous system. The overall aim of these two pathways is to maintain a stable internal environment in the face of changing conditions: to maintain homeostasis.

Both the nervous system and the endocrine system exert their effects by highly specific interactions with a receptor on, or within, the responding or target cell. This means that both systems use chemical communication.

Negative Feedback System

The negative feedback system involves the nervous or hormonal systems, or both acting together to promote stability of the internal environment. They are stimulus-response mechanisms in which the response produced reduced the effect of the original stimulus. The response provides feedback that has a negative effect on the stimulus.

Response which restores original state

Stimulus

Receptor

Control Centre

Effector

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Hormones:

‘Hormone’ does not refer to a particular type of chemical compound but to a diverse group of compounds that act as intercellular messengers and regulate cell function.

• In animals hormones are produced by cells in one o[part of the body and is transported throughout the organism via internal transport systems

• They transmit their signal to target cells which have receptors that are specific for certain hormones.

• Hormones exert their effects either directly by passing straight through the plasma membrane into the cell or by interactging with the receptor on the outside of the cell.

Hormone Specificity and Speed:Hormonal communication is specific for two reasons:

Firstly:• A particular stimulus will only affect a specific group of

hormone secreting cells.

Secondly:• Although hormones pass throughout the organism only

those cells which posses specific receptors are capable of responding to the hormone.

Hormonal effects are generally slower than nervous responses, longer in duration and affect cells which are widely distributed around the body.

Types of HormonesThe are two main groups of hormone: hormones produced from fatty acids and hormones produced from amino acids.

Hormones produced from fatty acids:• Are small and lipid soluble• Can easily pass through plasma membranes• Usually interact eith genes in the nuleus to cause a change

or response• Include hormones such as oestrogen and testosterone

Hormones produced from amino acids:• Are water soluble• Cannot pass through the plasma membrane• Binds to receptors on the outside of the cell which cause a

response inside the cell• Include hormones such as adrenaline and growth hormone

Endocrine Glands:

In animals cells that produce hormones are usually clustered together in discrete organs known as endocrine glands.

Examples of hormones in animals:Gland Hormone Tangent FunctionAdrenal cortex

Glucocorticoids Many cell types

Promotes synthesis of glucose

Mineralocorticoids Kidney tubes Regulates reabsorption of salts

Adrenal medulla

Adrenaline Heart, blood vessels, liver, fat

Increases cardiac output, mobilises glucose, prepares body for action

Anterior pituitary

Adrenocorticotrophic Hormone (ACTH)

Adrenal cortex Promotes release of adrenal cortex hormone

Growth Stimulating Hormone (GSH)

Bone, muscle Promotes protein synthesis and growth

Follicle Stimulating Hormone (FSH)

Ovary Promotes development of follicle and secretion of oestrogen

Luteinising Hormone (LH) Ovary Promotes ovulation and the secretion of progesterone

Prolactin Mammary glands

Stimulates milk secretion

Thyroid Stimulating Hormone (TSH)

Thyroid Promotes production and release of thyroxine

Gland Hormone Tangent Function

Hypothalamus

Several releasing hormones

Anterior pituitary

Controls release of anterior pituitary hormones

Ovary Oestrogen Reproductive tract, body generally

Promotes menstruation cycle, development of female features and behaviours

Progesterone Uterus Prepares uterus for and maintains pregnancy

Pancreas Insulin Muscle, liver, fat

Lowers blood glucose

Glucagon Liver, fat Raises blood glucoseParathyroid Parathyroid

HormoneBone, kidney Raises blood calcium

Posterior Pituitary

Oxytocin Mammary gland

Causes release of milkAntidiuretic hormone Kidney Promotes reabsorption

of eater from collecting tubules

Pineal Melatonin Brain Daily and seasonal cycles

Gland Hormone Tangent FunctionTestis Testosterone Reproductive tract,

the body generallyDevelopment of masculine features and behaviour

Thyroid Thyroxine Most cells Regulates cellular metabolic rate

The Pituitary Gland:

The pituitary gland plays a central role in overall endocrine regulation. More than half of the hormones in mammals are either released from the pituitary gland or are used to release hormones from the pituitary gland.

The pituitary gland secretes hormones involved in:• The regulation of growth • Lactation• Reproductive state• Skin pigmentation• Fat tissue• Kidney function• The activity of the thyroid gland• The activity of the adrenal glands

The pituitary gland lies immediately below the hypothalamus. The hypothalamus receives information from the body regarding its overall state of well being, such as the level of hunger or thirst, smell, pain and emotions. This information is used to regulate hormone release from the pituitary gland.

The Nervous System:

• The nervous system is usually a more direct mode of communication between parts of the body than hormones.

• Control by nerves is generally rapid, short in duration and precisely located.

• Nervous responses require more energy than hormonal responses

Reflex Responses:

The ability to detect and quickly respond to changes in the internal and external environments in fundamental to the survival of all animals. Many reflex response protect the body from further injury.

A common example of a reflex response occurs when you prick your finger. You will find that your finger has already been removed form the stimulus before you feel the pain. This occurs because the scnsory receptor on your finger has sent a message via the sensory neuron to the CNS where interneurons connect this neuron to motor neruron to send a message back to an effector mucle to contract and escape the stimulus causing the pain.

The Human Nervous System:

Mammals have a central nervous system and a peripheral nervous system.

Central Nervous System:

• Is made up of the brain and the spinal cord.• Different regions of the brain are associated with different functions:

- The cerebral cortex has areas associated with motor activity , sensory input, speech, sight and hearing

- The hypothalamus recieves information relating to the well-being of the body and functions in maintaining homeostasis

- The cerebellum is involved in the coordination of muscular activity, including posture, balance and movement

- The brainstem has centres associated with the control of the heart, blood vessels and lung ventilation

The Peripheral Nervous System:

• Includes sensory nerves which carry information towards the CNS and motor nerves which carry information away from the CNS to effector organs such as muscles and glands

• The peripheral nervous system can also being split into two different divisions:

The somatic nervous system:• which is voluntary movement such as movement by skeletal muscles

The autonomic nervous system:• it conveys signals to smooth muscle, heart muscle and gladnular tissues, and

regulates the activities of the digestive, cardiovascular, respiritory andf endocrine systems

System/Role FunctionCardiovascular system

Controls the rate and strength od the heartbeat and the distribution of blood to different organs by change in the diameter of the arteries

Digestive system Controls mixing and movement of food through gut and secretion of various enzymes

Respiratory system Controls the diameter of major airways of lungs and the secretion of mucus over surface

Excretory system Promotes emptying of the bladder and the rate urine production by kidneys

Reproductive system Controls contraction of reproductive tract and thus passage of eggs, sperm and embryo

Metabolic regulation Control the formation and release of hormones affecting overall metabolism

Temperature regulation

Controls cutaneous blood flow and sweating

Eye function Controls the diameter of the pupil to regulate incoming light and focusing of lens

The Senses:Sense Receptor/

ResponseStructure Picture

Vision: Photoreceptor cells which contain light sensitive pigments interact with the light to produce an electrical signal in a sensory nerve.

There are two types of eyes:

Simple eyes:• Are single

chambered eyes which rely on a lens, cornea or both to form an image on the

photoreceptors of the retina. • This type of

creates a higher resolution than compound eyes.

• Humans have a simple eye.

Compound eyes: • Are found in

insects.• They have

several lens systems which are all used to form the one image.

Hearing: Sound travels as vibrations through the air, water and solids. Animals can detect sound using mechanoreceptors, which are sensory neurons that can detect minute vibrations

Taste and Smell Chemoreceptors are cells sensitive to different chemicals. Many animals have receptors for specific chemicals which can signal sexual readiness, or the proximity of predators or prey.

Mechanoreception:

Cutaneous mechanoreceptors detect external stimuli such as pressure and touch. Whilst a range of receptors detect internal mechanical stimuli such as joint position, muscle tensions and tension in the walls of the lungs and stomach

Pain: All animals with a nervous system avoid encounters with harmful external stimuli.

Neurons

Neurons, the functional unit of the nervous system, consist of:• A cell body which performs normal cell functions (such as making

proteins)• Dendrites which carry impulses towards the cell body• Axons which carry impulses away from the cell body to the next neuron

There is only one axon leaving the cell body but many dendrites bringing impulses into the cell body.

Synapses are found at the axon terminals. Synapses are specialised regions where information is communicated from a nerve cell to another nerve cell of effect muscle.

Myelin sheath is a fatty tissue which insulates the axon.

Neurons are grouped together into bundles called nerves. A single nerve may have several hundred axons running side by side

Neurons are excitable cells:

In excitable cells movement of ions across the membrane causes changes in the level of polarisation causes a physiological response by the cell.

There are 3 basic steps involved in the function of nerve cells and the way they conduct signals in the nervous system:1. Generation of a nerve impulse (action potential) by sensory

neurons2. Conduction (propagation) of an impulse along axons3. Chemical transmission of a signal to another cell across a synapse

Sensory nerves respond to stimuli by depolarising the nerve cell. It does this by making the inside of the cell less negative. If depolarisation is large enough an action potenial will be generated.

The action potential will then be conducted along the axon to the axon terminal

At the terminals neurotransmitters are released and they diffuse across the synapse and bind to specific receptors on the postsynaptic plasma membrane

The Action Potential:

• A nerve impulse or action potential is a wave of electrical change which passes rapidly along the axon membrane.

• The action potential does not vary in size. It is either sufficient to generate the action potential (pass the threshold potential) or it is not. This is often referred to an the ‘all or none’ law.

• The intensity of a stimulus is conveyed by the number of action potentials generated.

• A region that has just generated an action potential cannot produce another one for a brief period known as a refractory period.

Glossary:

HomeostasisNegative feedback

Target cellsEndocrine glands

Pituitary glandNeuron

Effector cellsReflex

InterneuronsFlexion

ExtensionAction potential

ConductionTransmission

SynapseDepolarisation

Threshold potentialRefractory period

Complete the following:

Key Questions:1, 2, 3, 4, 5, 6, 7, 9, 11, 12, 13, 14

Chapter Review Questions:1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12