Maintenance of a relatively stable internal environment (keeping everything in balance or at equilibrium)

Minor fluctuations around an optimum level

Includes: Water balance Blood sugar levels Body temperature And regulation of many other substances

Homeostasis happens via a series of steps known as the stimulus response model.

Organisms must first detect a stimulus before they can respond.

This is the job of receptors. Receptors are specialised

areas that respond to certain stimuli like light, heat sound or pain.

The message is then sent to an organ, gland or muscle that acts as an effector.

The effector then causes a response

Stimuli in the external and internal environments are detected by receptors. Electrical impulses travel to the CNS where coordination takes place. Impulses are sent to effectors that carry out a response.

Hormones are released directly into the bloodstream and transported throughout the body. Target tissues involved in regulating cell activities respond.

Oxygen from the air diffuses from the lungs into the capillaries and is carried to all cells.

Carbon dioxide from the cells is carried tothe lungs in the blood and diffuses into thealveoli from where it is exhaled.

Removal of CO2 helps regulate pH.

Blood distributes warmth, hormones, O2, nutrients including glucose, fatty acids and amino acids to cells and removes wastes including CO2.

The products of digestion – simple molecules – are absorbed into the blood and lymph vessels in the wall of the intestine from where they are supplied to other parts of the body.

Undigested material is eliminated.

Nitrogenous wastes (urea), excess water and salts are excreted from the body in urine.

Water balance (osmoregulation) is regulated in this way.

Evaporation of sweat from the surface helps regulate temperature.

The skin is a barrier between the internal and external environments and reduces the chance of entry of micro-organisms.

Feedback is a process whereby some proportion of the output signal of a system is passed (fed back) to the input.

This is often used to control the dynamic behaviour of the system.

Positive feedback is a feedback system in which the system responds to an alteration of a body system/s in the same direction as the alteration.

In contrast, a system that responds to an alteration of a body system/s in the opposite direction is called a negative feedback system.

The term "positive" means responding in the same direction as the original alteration whereas "negative" means responding to the opposite direction.

Characterised by a stimulus (condition)

Stimulus is detected by a receptor

This message is sent to a coordinating centre in the body

A message is sent from there to an effector (muscle or gland), which carries out a response

Internal body temperature rises

Detected by thermoreceptors in skin

Hypothalamus compares against optimum

Increased blood flow to skin, sweat glands,

Adrenal gland activity slows

Nervous System Rapid response Electrochemical messages

Endocrine System Slow response Messages transmitted via the blood stream Messengers may be peptide hormones or

steroid hormones

Fast Control

Nelson p 250

Nerve impulse travel along defined pathways

Sensory neurones (PNS)

receptors

Interneurones (CNS)

brain and

spinal cord

Motor neurones (PNS)

effectors

Nelson p 237

Motor Neuron: Transmit impulses from the

central nervous system to: ▪ Muscles▪ Glands

Most motor neurons are stimulated by interneurons, although some are stimulated directly by sensory neurons.

Sensory Neuron: Respond to different stimulus

receptors:▪ Touch Smell

▪ Taste Sight▪ Sound

Interneuron: Found exclusively within the

spinal cord and brain Stimulated by signals from:▪ Sensory neurons▪ Other interneurons

Martin & Kinnear p 302

Receptors are nerve cells capable of detecting changes in the environment

Chemoreceptors – detect smell , taste, oxygen levels, glucose levels, etc

Mechanoreceptors – detect pressure, touch , sound vibrations, position and posture of body

Photoreceptors – detect light eg. retinal cells or body surface cells in invertebrates

Thermoreceptors – detect heat and cold, located in skin and hypothalamus of brain

Pain receptors – are free nerve ending in skin

Grivas & Carter p 246

•Resting nerve cell is negatively charged

•Excitation due to a stimulus results in an electrical impulse

• Impulse travels along the nerve in a ‘Mexican Wave’

•Myelinated nerve cells transmit nerve impulses more rapidly

Nelson p 244

Nerve impulse travels along axon to the synaptic knob of the neurone

This causes the movement of synaptic vesicles to the end of the knob

There neurotransmitters are released from the vesicle into the synaptic gap

Neurotransmitters pass across the synaptic gap and bind briefly to the neurotransmitter receptors on the next neuron

This then stimulates an action potential in the next neuron

Automatic reflex response

Quick Response - as the impulse does not go all the way to brain, just to the spinal cord

Important survival mechanism –protection

Synaptic gaps occur between sensory neurones, between interneurones and between motor neurones

Synapses allow a variety of interconnections between nerve cells to be made

Synapses ensure that nerve impulses are transmitted only in one direction

Slow but sustained control of the body

Martin & Kinnear p 308

Chemical substances that are released directly into blood stream

Target & ‘excite’ particular cells and organs to respond in some way

Target tissue may be a long way from the gland that secretes the hormone

Only minute quantities of hormones are produced

Pituitary gland – master gland - produces many hormones that affect hormone production by other glands

Nelson p 252