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THE BRAIN The brain contains 100,000,000,000 neurons. The grey matter (site of interneurons) is located on the outside of the organ, with white matter (largely composed of myelinated axons) located within. The relative location of the white and grey matter in the spinal cord is reversed. Brain components include: Cerebrum: Controls all conscious actions. Cerebellum: Coordination of complex muscular movements (eg: posture). Hypothalamus: Temperature control, water balance, hunger etc. Brain stem (midbrain, pons, medulla oblongata): Control of heartbeat, blood vessels, breathing, salivation and swallowing. Meninges: Three membranes which enclose the brain and spinal cord. Cerebrospinal fluid between the meninges and CNS protects the brain and spinal cord from knocks. It also circulates through the ventricles of the brain. dmacc.eA The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 15 Fever The hypothalamus raises body temperature in response to pyrogens (inc. bacterial toxins), creating fevers during periods of infection. QUESTION 1 Which organelles would be required to produce neurotransmitters? _________________________________________________________________________ QUESTION 2 By what process would the neurotransmitter pass out of the axon terminal into the synapse? Explain. _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ REFLEX ARCS A reflex arc is a pathway from a receptor to an effector that does not involve the brain. Instead, the involuntary response in a reflex arc is simply determined by direct connections within the spinal cord. With less distance for impulses to travel, and few synapses involved, the response to the stimulus is very rapid. Stimulus Receptor CNS Effector Response As a consequence, reflex arcs are involved in situations that could cause damage to the individual, eg. stepping on a drawing pin. The more rapid the response, the less tissue damage experienced. A B C D G F E Other examples of reflexes include the spontaneous withdrawal of a hand from a hot object and the knee jerk reflex. The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 16 QUESTION 3 Which of the following organelles produces a neurotransmitter such as acetylcholine? A B C D Nucleus Mitochondria Chloroplast Ribosome QUESTION 4 The part of the neuron that receives impulses is the: A B C D Axon Cell body Dendrite Myelin sheath QUESTION 5 The key feature about the nervous system is that it is: A B C D Long lasting Slow Only occurs in mammals Fast acting QUESTION 6 Neurotransmitter moves across the synaptic cleft by: A B C D Diffusion Facilitated diffusion Osmosis Active transport QUESTION 7 The role of the myelin sheath is to: A B C D Protect the cell body Connect sensory and motor neurons Release neurotransmitter at synapses Increase the speed of impulses QUESTION 8 A person has unfortunately stepped on a nail in the family garage. The response to the stimulus would be determined by the: A B C D Brain PNS Spinal cord Hypothalamus The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 17 THE EYE Label the vertebrate eye: N M A Suspensory ligaments L B K C J D E F I G H Complete the following table of eye components: Structure Function Cornea Iris Pupil Ciliary Muscles Lens Vitreous Humour Retina Choroid Sclera The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 18 fine focussing The shape of the lens is controlled by the ciliary muscles. The flexibility of the lens allows the eyes to focus on both near and distant objects. Near Vision Distant Vision (Campbell N A et al, 1999) THE RETINA The human retina contains 125 million rods and 6 million cones (70% of all sensory receptors in the body!) Rods: More sensitive to light than cones, but do not distinguish colours. Provide night vision, but only in black and white. In humans, concentrated at peripheral regions of the retina, and totally absent from fovea. Cones: 3 types (‘red’, ‘green’ & ‘blue’) provide sharp colour vision in daylight. Require more light than rods to be stimulated, so do not function at night. Concentrated at the fovea (150,000/mm2) of the retina – some birds have over a million cones/mm2 (eg. hawks can spot the movement of a mouse from a kilometre in the air!). The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 19 The Trichromatic theory states that the different colours and shades we perceive are due to the varying degree of stimulation of each cone type, i.e. the equal stimulation of all 3 types of cones results in perception of white light. Colour vision is found in all vertebrate classes, but not all of their species History of Cones In vertebrates, four different types of cones exist: SWS1: short wavelength sensitive, i.e. sensitive to the violet region (and in some cases UV region) of the spectrum. SWS2: blue sensitive. MWS: medium wavelength sensitive, in the green region. LWS: long ...in the green/yellow to red, depending on species. The 4 types were present in ancestral vertebrates (eg. dinosaurs). Today, colour vision is best developed in teleost fish, birds and reptiles, each with tetrachromatic vision. Birds and reptiles also have UV vision. During their evolution, when they took refuge from predators and competitors by adopting nocturnal behaviour, placental mammals lost two types of cones (SWS2 and MWS). Today their colour vision is reduced to dichromacy; they can discriminate yellow from blue, but have lost red-green discrimination. Primates, including humans, redeveloped a third cone type from the duplication and modification of the LWS gene. They now have a trichromatic colour system, including SWS1, M/LWS green and M/LWS red. Trichromacy is characterised by the discrimination of red-green colours on top of blue-yellow colours. Australian marsupials did NOT follow the same route as other mammals. They have three cone types (trichromacy), but they retained an ancestral cone type (MWS) and did not reevolve a third one from gene duplication as did primates. Fat-tailed Dunnart Many marsupials (eg. fat-tailed dunnarts, honey possums, bandicoots) also possess UV vision. Some mammal species have lost three cone types. These mammals have monochromatic vision, only possessing LWS cones. They have no colour vision (eg. marine mammals and a couple of nocturnal rodents) (Cathy Arrese, UWA) The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 20 HORMONES AND OTHER CHEMICAL MESSENGERS Hormones are organic molecules which are produced and secreted from one type of cell (eg: of an endocrine gland), and travel via extracellular fluid (often via the bloodstream) to act on specific target cells, causing profound effects in tiny quantities. Only target cells contain the specific receptors which will bind to a particular hormone. Although most mammalian hormones travel in the bloodstream to exert their effects in a different area of the organism, some “chemical messengers” act on the same cell that produced them, and others may be secreted into tissue fluid and act on cells that are close by. Hormones are slow acting, and yet they can exert long lasting effects. Three main groups of hormones exist: Proteins (peptides), eg: insulin, glucagon, adrenalin. Steroids (lipids), eg: testosterone and oestrogen. Amino acid derivatives, eg: thyroxin In the case of protein/peptide or amino acid derivative hormones, the hormonesecreting cell typically releases its contents via exocytosis. Steroid hormones, and some amino acid derivative precursor molecules, however, pass out of their hormonesecreting cells by simple diffusion. Chemical messengers, often all loosely called hormones, are found in both plants and animals. Hormones attach to receptors on or in target cells via lock and key mechanisms. The hormone attaches to a receptor site (usually a protein) that is only found in target cell tissues. If a cell does not have the required receptor site, then the hormone will not bind with the cell and will not affect the cell’s activity. Receptor proteins are found in two locations. Receptors on the outside of the cell membrane attach to hormones that are not able to pass through the cell membrane such as polypeptide (protein) based hormones. Receptors within the cytoplasm of the cell combine with steroid based hormones and simple amino acid hormones such as thyroxine. These hormones have no difficulty in passing across cell membranes to reach their receptors. Some hormones can bind with receptors found on many different kinds of cells in the organism, whilst others are specific, only binding with one type of cell or tissue. The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 21 CHEMICAL NATURE OF HORMONES Type of Hormone Steroid Examples Features Availability & Lifespan Testosterone, oestrogen, cortisone, progesterone. Protein/Peptide Water soluble, travel in bloodstream; Unable to pass across cell membranes. In active form, most are water soluble & travel freely in bloodstream *thyroid hormones require a carrier protein; Amino Acid Derivative The School For Excellence 2011 thyroid hormones are able to pass across cell membranes nuclear receptors; others cannot cross and require membrane-bound receptors. Made in advance by cell and stored in secretory vesicles, leaving by exocytosis; Short. Made in advance by cell and stored in secretory vesicles. Leave cell by exocytosis (or by diffusion if precursor); *Short. The Essentials – Year 11 Biology – Book 1 Page 22 ACTION OF WATER-SOLUBLE AND LIPID-SOLUBLE HORMONES Based on their mode of operation, hormones can be divided into two broad groups, i.e. water-soluble and lipid-soluble hormones. Water-soluble hormones include: protein hormones (>200 amino acids in size) peptide hormones (<200 amino acids in size) and some amino acid derivative hormones. These hormones dissolve readily in water, but are unable to cross membranes. In contrast, lipid-soluble hormones, i.e. steroid hormones and some amino acid derivative hormones(eg: thyroxin) are not soluble in water, and can freely pass through membranes. In either case, a target cell must contain a protein receptor molecule for that particular hormone. In the case of water-soluble hormones, the receptors are located on the surface of the cell membrane of the target cells. The hormone-receptor complex activates other proteins within the cell (eg: G proteins) which are responsible for the cascade of events (signal transduction) within the cell leading to the cellular response. In the case of lipid-soluble hormones, the receptors are found within the target cell, either within the cytosol or the nucleus. Again, the hormone binds to the waiting receptor. The hormone-receptor complex attaches to a particular site on a chromosome within the nucleus, leading to the cellular response. The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 23 CELL SIGNALLING Cell signalling occurs in the following manner: Signal Reception (At the cell membrane in the case of protein hormones) Signal Transduction (Usually a pathway of several steps, with each molecule in the pathway bringing about a change in the next) Cell’s Response (Brought about by the last molecule in the pathway) SIGNAL TRANSDUCTION Signal transduction at the cellular level refers to the cascade of events originating outside the cell leading to a specific cellular response. With each step in a signal transduction pathway, the number of molecules involved is typically amplified, i.e. many more molecules are typically involved at each subsequent step in the pathway. The outcome is an alteration in cellular activity, which can be the result of changes in gene expression within the cell. (Campbell NA et al, 1999) Signal transduction induced by a peptide hormone The movement of signals can be simple, eg. acetylcholine acts in a simple manner on receptor molecules in the post-synaptic membrane. Acetylcholine allows signals (in the form of small ions, such as sodium) to be passed mainly by diffusion, either into or out of the cell via membrane proteins. Enough ionic movement at the one time can generate electrical impulses that travel along the nerve cell. More complex signal transduction pathways involve many intracellular events. Signal transduction takes place in as little time as a millisecond or as long as a few seconds. The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 24 FUNCTIONING OF A G PROTEIN-LINKED RECEPTOR: If a protein or peptide hormone activates a membrane receptor. This typically in turn activates a G protein. The activated G protein causes a cascade of amplified changes (signal transduction). finally leading to the cell’s response. G protein receptor systems are very widespread and diverse in function, eg. they are involved during * Embryonic development * Vision and smell reception Note: up to 60% of all medicines used today exert their effects by influencing G protein pathways. The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 25 WATER SOLUBLE v. LIPID SOLUBLE HORMONE SUMMARY (a) (b) Water Soluble Hormone: Binds to receptor in cell membrane. Triggering a signal transduction pathway. Leading to a change in cytoplasmic function or gene transcription. Lipid Soluble Hormone: Passes through cell membrane and binds to intracellular receptor (either in cytoplasm or nucleus). Hormone-receptor complex activates gene expression. Response G protein Water soluble hormone Hormone binds to receptor on cell membrane or within cell Membrane receptor Signal transduction: Signal detected within cell Lipid soluble hormone cascade of molecular events linking external signal to a specific cellular response Cell activates response to signal Response Cytosol/nucleus Cytosol/nucleus receptor (Campbell NA et al, 2002) The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 26 A SELECTION OF IMPORTANT ENDOCRINE HORMONES Endocrine Gland Hormone(s) Produced Target Cells Function/Role Heart Adrenal Liver ADH Regulates water reabsorption FSH Follicle growth Pituitary In females: Ovulation, corpus luteum development LH Most Cells Decrease blood sugar levels Liver and Muscle Increase blood sugar levels Pancreas Thyroid Thyroxin Most Cells Oestrogen Ovary Uterus Progesterone Testes Most Cells Regulate menstrual cycle and secondary sexual characteristics Regulates secondary sex characteristics and growth processes QUESTION 9 Which organelle would make the hormone: (i) Insulin? _____________________________________________________________________ (ii) Testosterone? _____________________________________________________________________ The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 27 COMPARISON OF COORDINATION SYSTEMS Nervous System Type of Message Endocrine System electrochemical Speed of Message slow Duration of Response seconds - years Transmission via nerves Specificity high (direct nerve pathway) Note: The endocrine and nervous systems work together in co-ordinating the activities of multicellular organisms. The School For Excellence 2011 The Essentials – Year 11 Biology – Book 1 Page 28