System Regulation PDF

SYSTEM REGULATION HOMEOSTASIS Maintenance of a constant internal environment within physiological tolerance limits Responsible for preserving equilibrium within cells Uses both communication systems: - The nervous system allows for rapid responses to change - Endocrine glands enable wides...

SYSTEM REGULATION HOMEOSTASIS Maintenance of a constant internal environment within physiological tolerance limits Responsible for preserving equilibrium within cells Uses both communication systems: - The nervous system allows for rapid responses to change - Endocrine glands enable widespread/sustained effects If homeostasis is not maintained, disease will occur à diseases disrupt normal physiology - Has an impact on health variables used for homeostasis NEGATIVE FEEDBACK - regulates A feedback loop involves the use of an outcome as an input for action (effects will feed mini back into a system) Feedback loops can be either positive or negative: - Positive: Response reinforces the input ( change) - Negative: Response opposes the input ( change) As homeostasis requires variables to oscillate around a set point, variables are regulated by negative feedback VITAL FUNCTIONS Internal equilibrium is maintained by adjusting various conditions: - Blood glucose levels (normally ranges between 75 – 95 mg/dL) - Body temperature (normally ranges between 36 – 38 oC) - Blood pH (normally ranges between 7.35 – 7.45) Blood osmotic concentrations (varies according to body size) Certain physiological activities produce changes to these levels that need to be regulated: - Circadian rhythms describe the physiological changes that occur over a day/night cycle - Intense bouts of physical activity will alter the metabolic requirements of respiring cells - Eating and fasting will impact the digestive processes by which nutrients are absorbed BLOOD SUGAR REGULATION High levels of blood glucose can damage cells by creating a hypertonic solution that draws water Blood sugar levels are detected by chemoreceptors and controlled by two sets of pancreatic ~ hormones - Insulin: Lowers blood glucose concentrations - Glucagon: Raises blood glucose concentrations ↓ pancreatic hormones DIABETES Diabetes mellitus is a metabolic disorder resulting from abnormally high blood glucose levels THERMOREGULATION control hypothalasum - center When body temperature changes, peripheral thermoreceptors in the skin transmit signals to the hypothalamus (the homeostasis ‘control center’) - The pituitary gland releases TSH (thyroid stimulating hormone) Thyroxin is produced by the thyroid gland to increase cell respiration Brown adipose tissue possesses an uncoupling protein that prevents ATP from being produced from the breakdown of organic compounds (lipids) 6 Instead, the released energy is converted into heat & (thermogenesis) Brown adipose cells have more mitochondria and many fat droplets of Atp stops production CIRCADIAN RHYTHMS Body’s physiological responses to a day-night cycle (promotes a cycle of sleeping and waking) - It is controlled by melatonin from the pineal gland (brain) Melatonin secretion is inhibited by light exposure Melatonin promotes sleep in diurnal (day-time) animals and promotes activity in nocturnal (night-time) animals Melatonin supplements are often used to manage jet-lag PHYSICAL EXERTION Epinephrine (adrenaline) is the body’s natural stress hormone and activates the sympathetic nervous system (‘fight or flight’ response) It is released from the adrenal glands located above the kidneys Epinephrine is released to prepare the body for vigorous activity: - Heart rate is elevated to pump more blood around the body - Arterioles to the skeletal muscles dilate (more blood to muscles) - Ventilation rate increases to improve gas exchange (O2 uptake) - Glucose and fats are released from storage organs (more ATP) HEART RATE Vigorous activity increases heart rate to improve blood flow to tissues - Respiring tissues consume oxygen and produce carbon dioxide (which lowers blood pH) & Heart rate is under the autonomic control of the medulla (brainstem) - Chemoreceptors in the aorta and carotid arteries detect changes in the blood pH levels - - - Baroreceptors in the aortic arch and carotid sinuses detect changes in blood pressure - - VENTILATION RATE Medulla > - diaphragm Vigorous activity also increases ventilation rate, as contracting muscles require more energy (necessitating uptake of O2 and removal of CO2) Carbon dioxide reacts with water in the blood to form C carbonic acid - Blood pH drops and is detected by chemoreceptors in the brainstem The brainstem will signal to the diaphragm and thoracic muscles to increase ventilation rate and ventilation volume (more gas exchange) - This is an autonomic response and is not under voluntary control - Expelling excess CO2 will restore blood pH to homeostatic set points  nerve pathways A DIGESTIVE CONTROL The movement of food is regulated by both voluntary and involuntary nerve pathways - The CNS controls the initiation of swallowing and the egestion of feces (both voluntary) - The movement of food between these points is involuntary Peristalsis involves the contraction of longitudinal smooth muscles in the alimentary canal  SYSTEM INTEGRATION BODY SYSTEMS Interactions between these components create new functionalities – emergent properties ORGAN SYSTEMS INTEGRATION Coordination between the different organ systems is needed to allow multicellular organisms to collectively perform the functions of life and meet the requirements for ongoing survival - Support metabolism and help to maintain homeostasis COMMUNICATION Animals possess two distinct communication systems which allow for coordination between the different body organs - Nervous System: Consists of a network of nerve cells (neurons) that transmit electrochemical impulses - Endocrine System: Consists of ductless endocrine glands that release chemical messengers (called hormones) directly into the bloodstream NERVOUS SYSTEM The nervous system can be divided into two main parts: - CNS (Central) – Composed of the brain and spinal cord - PNS (Peripheral) – Links CNS to receptors and effectors The CNS acts as an information processing center, while the PNS functions to transfer information to and from this system Nerve cells - neurons STIMULUS-RESPONSE PATHWAYS The peripheral nervous system communicates information through a stimulus-response pathway Stimuli (either internal or external) are converted into electrical impulses by receptors Effectors (muscles or glands) convert the electrical impulses into a response Information is transferred via the central nervous system by sensory and motor neurons - Sensory neurons relay signals to the CNS, while motor neurons relay signals from the CNS RECEPTORS Sensory organs contain specialized receptor cells that can detect a specific stimulus and generate a nervous impulse - Impulses are relayed to the CNS by the sensory neurons Some of the main types of sensory receptors include: - Photoreceptors – detect light energy (found in retinas) - Thermoreceptors – detect changes in body temperature - Chemoreceptors – detect chemicals (smell, taste, blood) - Mechanoreceptors – detect touch, pressure, and stretch EFFECTORS An effector is any organ or a cell that acts in response to a stimulus by triggering a response - Signals are transmitted to effectors from the central nervous system by motor neurons The two main types of effectors are muscles (create movement via contraction) and glands - Endocrine glands release chemicals called hormones into the bloodstream (are ductless) - Exocrine glands secrete chemicals onto surfaces or cavities via ducts NERVES Signals are sent to and from body regions along tracts called nerves - Each nerve consists of bundles of sensory and motor neurons that target a specific anatomical location C Nerve fibers may be insulated by fatty layers called the myelin sheath which improves nerve conduction speeds (dark rings in the bottom image) SPINAL CORD Peripheral nerves connect to the body’s central processing unit (brain) by a column of nerve fibers that collectively form a body’s spinal cord The peripheral nerves for a specific region of the body feed into the spinal cord at a particular site - Injury to a spinal section affects all body parts ventral to that region Acts as an integration centre for certain unconscious processes (reflex actions) - do not require communication with the brain to occur nerve fibres unconcious processes REFLEX ACTIONS A reflex is a rapid and involuntary response to stimuli, with skeletal muscles as the effector - A reflex arc does not involve the brain, only the spinal cord Reflex arcs allow for faster responses that do not involve conscious thought or deliberation - An example of a reflex arc is the response to a pain S stimulus (detected by nociceptors) BRAIN The brain is an integration and coordination organ that processes information from various systems It is responsible for various higher-order functions, including memory, emotions, and consciousness The human brain is composed of three main parts: - Cerebrum – The predominant processing center - Cerebellum – Used for balance/proprioception - Brainstem – Connects the brain to the spinal cord CEREBRUM The largest part of the brain, responsible for most of its complex cognitive tasks Two hemispheres divided into four lobes with distinct functionalities - Frontal Lobe – Controls voluntary motor activity - Parietal Lobe – Controls touch sensation (tactility) - Occipital Lobe – Used in visual processing (sight) - Temporal Lobe – Auditory/language processing CEREBRAL HEMISPHERES some specialisation - Left: Involved in analysis and calculations (logical) - Right: Responsible for spatial abilities (creativity) Each hemisphere processes sensory and motor information for the opposite side of the body - The hemispheres are joined by the corpus callosum Damage to this area causes ‘split brain’ syndrome M CEREBELLUM At the base of the brain corpus callosum coordinates skeletal muscle contraction It is responsible for balance and proprioception Collectively, the cerebellum is involved in controlling posture and gait, as well as maintaining muscle tone - Does not initiate muscle contractions Those signals originate within the motor cortex - valanceeception BRAIN STEM Connects the cerebrum to the spinal cord and the cerebellum - Consists of the midbrain, pons, and medulla oblongata The brainstem regulates involuntary and unconscious functions – like breathing and control of heart rate ductless glands - release hormones into ENDOCRINE SYSTEM - bloodstream 2 Network of ductless glands that release hormones directly into the bloodstream Hormones bind to specific receptors and only activate cells with the appropriate receptor - Activity can be controlled by moderating the quantity of either the hormone or receptor - Hormones can be proteins (bind to external receptors) or lipids (bind to internal receptors) Sind to specific receptors and endo. sends signals to pituarity gland > - new. HYPOTHALAMUS Connects the nervous and endocrine systems T - It functions as a homeostasis control center within the brain - Receives information from nerves and relays the processed signals to the adjacent pituitary gland The pituitary gland is composed of two lobes and controls the release of hormones from other endocrine glands in the body - Anterior lobe secretes hormones to activate other glands anterior & - Posterior lobe secretes hormones from the hypothalamus ↓ Posterior ENDOCRINE GLANDS Pancreas – Insulin and glucagon (blood glucose) Adrenal gland – Adrenaline (‘fight or flight) Thyroid gland – Thyroxin (raises body heat) Pineal gland – Melatonin (circadian rhythms) Gonads – sex hormones (testosterone/estrogen)  IMMUNE SYSTEM PATHOGENS A pathogen is a disease-causing agent that disrupts the normal physiology of the host body - Pathogens can be either cellular (living organisms) or non-cellular (infectious molecules) DISEASE Disease is a particular kind of illness, with characteristic symptoms Main causes: - Genetic - Environmental - Pathogen infection Pathogens can be opportunistic – usually don’t cause a disease but can if the immune system is compromised IMMUNE SYSTEM The function of an immune system is to provide protection against pathogenic infections SURFACE BARRIERS The first line of defense against infection is the surface barriers that prevent pathogen entry - Physical barriers (skin and mucous membranes) and chemical barriers Skin The thick and tough layer of predominantly dead cells containing large amounts of keratin Hair follicles contain sebaceous glands – secrete sebum – maintains moisture and lower pH →acidity inhibits the growth of bacteria and fungi Mucous Membranes Thinner and softer – vagina, penis, airways Secrete mucus – glycoproteins Protects internal structures – pathogens are trapped in it Antiseptic properties – lysozyme – an antibacterial enzyme CLOTTING If the skin is broken, it will no longer act as an impenetrable barrier – this is repaired by clotting Formation of a blood clot - Platelets adhere to one another to form a compact, sticky plug - They release clotting factors that trigger a cascade of reactions that result in the production of thrombin → Converts fibrinogen to insoluble fibrin which forms a mesh The clot is in the form of a gel until it comes into contact with air→hard scab COAGULATION CASCADE phagocytichocytes INNATE IMMUNITY > The second line of defense against infection ① · is the innate immune - Activated when the surface barriers have been breached => system - non-specific (there is no differentiation between specific types of pathogens) - non-adaptive (the response is the same upon every infection – there is no memory) o The primary component of the innate immune response is the phagocytic leukocytes They recognize generic pathogen-associated molecular patterns (PAMPs) – such as cell walls and protein coats PHAGOCYTOSIS - destruction of phatogen Type of endocytosis - Phagocytes use pseudopodia to travel to the site of infection (amoeboid movement) - The pseudopodia surround the pathogen and fuse– form an intracellular vesicle→transported to a lysosome→ pathogen destroyed by digestive enzymes ADAPTIVE IMMUNITY The third line of defense Initiation is slower but the response is more effective than the innate responses - specific - differentiates between specific pathogens and responds accordingly - adaptive - produces a faster response upon re-exposure (memory) à lymphocytes (B and T cells) H Produce large amounts of specific proteins (antibodies) that can recognize specific pathogens and mediate their destruction ANTIGENS AND ANTIBODIES Antibodies recognize markers on the surface of pathogens - Antigens (proteins or glycoproteins) – any molecule that stimulates an immune system response - The lymphocytes produce antibodies that specifically target the antigen – irreversible binding - One lymphocyte produces only antibodies with one type of hypervariable region drain excessfluidfroa body LYMPHATIC SYSTEM D Jessers A system of vessels that drain excess fluid from body tissues + lymph nodes - swollen structures that contain large numbers of lymphocytes that produce antibodies Antibodies can: - make a pathogen more recognizable to phagocytes - easier engulfment →Prevent viruses from docking to host cells - can become antigen-presenting LEUKOCYTES AND LYMPHOCYTES cells When phagocytic leukocytes (macrophages) engulf a pathogen, they can become antigen-presenting cells They drain into the lymphatic system and end up in lymph nodes where they activate only the specific T lymphocytes that recognize the antigenic fragment H expressed on the leukocytes’ surface Lymphocytes (B and T cells) produce specific antibodies H HELPER T LYMPHOCYTES G When the T cell binds to the macrophage it activates →releases cytokines (signaling H molecules) and binds to the B lymphocyte that is specific for the same antigen →cytokines and binding activate the B lymphocyte Pathogens may have multiple antigenic fragments on their surface so multiple B cells may be activated (polyclonal activation) B LYMPHOCYTES - After activation, a B cell is going to divide (mitosis) to form many clones - Most of the clones differentiate into short-lived plasma B cells with large rER and Golgi →mass production of antibodies - A small number differentiates into long-lived memory cells for ongoing immunity - binds antigen to the ANTIBODY ACTION The primary way an antibody functions is by binding to the antigen and making it easier to be detected by the non-specific s e - phagocytic leukocytes – -opsonization Other mechanisms of antibody action include precipitation, agglutination, and neutralization SECONDARY IMMUNE RESPONSE Memory B-cells are inactive – if the same pathogen infects the body again they are activated and respond very rapidly Immunity to an infectious disease is due to having either antibodies against the pathogen, or memory cells that allow rapid production of the antibody. The secondary immune response (upon re-exposure) is faster and more potent SUMMARY IMMUNODEFOCOENCY Immunodeficiency occurs when the immune system becomes compromised or is absent - The body cannot fight off pathogens and becomes susceptible to opportunistic infections Acquired Immuno-Deficiency Syndrome (AIDS) is caused by HIV →infects T cells H - Reduces the ~ lymphocyte numbers – limits the body’s capacity to make antibodies against infections DAIDS - Prevents an adaptive immune response from being initiated HIV After infection, the virus undergoes a period of inactivity→infected T cells H reproduce When the virus becomes active, the entire population of T cells is effectively destroyed H - HIV is destroyed by exposure to light or air and is transmitted by the exchange of body fluids ZOONOSES A zoonosis is an infectious disease that can be transmitted from other species to humans ANTIBIOTICS Antibiotics are biological compounds that specifically target prokaryotic metabolism - Do not harm the infected eukaryotic organism Antibiotics can be bactericidal (kills bacteria) or bacteriostatic (inhibits bacterial growth) - Antibiotics are not effective against viral infections – viruses don’t have a metabolism ANTIBIOTIC RESISTENCE Some bacteria have developed resistance to antibiotics - Bacteria can exchange genes via plasmid conjugation to become multi-resistant organisms - The prevalence of these strains is increasing due to the use of broad spectrum antibiotics VACCINES AND IMMUNIZATION Vaccines are weakened / attenuated forms of pathogens that cannot cause the disease A vaccination triggers a primary immune response→memory cell production - When exposed to the real pathogen, a more potent secondary immune response is triggered →the disease symptoms do not develop (immunity) HERD IMMUNITY Non-immunized individuals may be protected from exposure to a pathogen by the large number of vaccinated individuals – heard immunity/protection - important for the immunocompromised, extremely young, and elderly OUTBREAKS An outbreak is a sudden rise in the occurrence of an infectious disease within a population An epidemic is an outbreak that occurs within a particular community or geographic area A pandemic is an outbreak that occurs across a wider geographical area (e.g. globally) Outbreaks occur when a pathogen is introduced into a community that has no immunity to it - New strains of pathogens emerge when pre-existing pathogens develop new antigenicity The spread of an outbreak may be driven by several different factors including: - The globalization of trade (enables transmission between isolated communities) - Increased urbanization and climate change (increases the spread of zoonotic diseases) COVID-19 The COVID-19 pandemic was a global outbreak caused by a novel coronavirus (SARS- CoV2) - To effectively manage and contain its spread, epidemiologists needed to evaluate data relating to its distribution, infectivity, and overall rate of spread (hospital admissions) - Two calculations used to assess the data were percentage difference and percentage change  RESPIRATORY SYSTEM GAS EXCHANGE All living organisms must exchange gases with the environment Gases can be exchanged with atmosphere (if terrestrial) or water (if aquatic) Happens through diffusion – relatively slow – large surface area needed Small organisms (e.g. unicellular) – through their outer surface Larger organisms – require specialized tissues with increased surface area – alveoli in lungs, spongy mesophyll in leaves CONCENTRATION GRADIENT Necessary for gas exchange – must be maintained through cellular respiration picture inresentation ↑ ↳alvely singtiple a # RESPIRATORY SURFACE has to be : permeable—oxygen and carbon dioxide can diffuse across freely large—the total surface area is large about the volume of the organism moist—the surface is covered by a film of moisture in terrestrial organisms so gases can dissolve thin—the gases must diffuse only a short distance, in most cases through a single layer of cells. LUNGS Specialized respiratory structures - Air enters through the nose or the mouth C - The air travels down the trachea (windpipe) 2 - Then in two bronchi ↓ranch - Each bronchus branches to bronchioles ↑ - The bronchioles end in alveoli (groups of 5-6 at each alveolar duct) Lung Lung - Alveoli exchange respiratory gases with the bloodstream (via capillaries)  Pulmonary makes surface tension unexistable surfactant Surfactant laires y alveolus has a between & one from alveaulus - ALVEOLI one capilary Spherical air sacs surrounded by a dense network of capillaries from Composed of two types of cells - pneumocytes - Type I: Flattened cells that are responsible for gas exchange - Type II: Granular cells that secrete a pulmonary surfactant A high surface tension would prevent alveoli from inflating →a surfactant is needed Pulmonary surfactant – secreted by cells in the walls of alveoli, a structure similar to phospholipids BREATHING MECHANISM The trachea and bronchi have6 cartilage in their walls to ensure they remain open The bronchioles have smooth muscle fibers in their walls, allowing their width to vary Spushi kos During ventilation the pressure inside the thorax drops below atmospheric pressure→air is drawn to the lungs→pressure equalizes→muscle contractions rise it above atmospheric→air is forced out - ↓ ↑ - - - - - - - - - - - - - - - LUNG CAPACITY Lung volumes can be measured in several different ways: Total lung capacity: The maximal volume of air in the lungs differentwaae Tidal volume: Amount of air exchanged in a normal breath ↑ to Residual volume: Amount of air always present in the lungs Vital capacity: Maximum air volume that can be exchanged Spirometer – measures lung volumes  VASCULAR SYSTEM CIRCULATORY Moves materials around a body - It carries essential materials to cells while removing wastes - It helps maintain body temperature and is involved in signaling Three main components: - Blood – the fluid medium in which materials are transported - Vessels – the network of ‘pipes’ by which the blood is moved - Heart – a muscular ‘pump’ that drives the movement of blood pulmonary and systemic BLOOD Blood is the fluid medium in which materials are transported around the body Three types of cells – red blood cells, white blood cells and platelets The liquid plasma is involved in transporting dissolved solutes, electrolytes and proteins Materials that are transported in the blood plasma include: VESSELS Arteries carry blood at high pressure away from the heart Veins carry blood at low pressure back towards the heart Capillaries facilitate the exchange of materials with tissues The vascular system is a closed network – all vessels connect Major arteries branch into smaller connecting arterioles Veins branch into venules - Branching causes pressure to drop between arteries and veins  ARTERIES The wall of the artery is composed of several layers: - tunica externa—a tough outer layer of connective tissue with collagen fibres - tunica media—a thick layer containing smooth muscle and elastic fibres made of the protein elastin - tunica intima—a smooth endothelium forming the lining of the artery; in some, includes a layer of elastic fibers PULSE FLOW Blood flows through arteries in repeated surges (pulses) that correspond with the contraction of the heart muscle Elastic fibers in the arterial wall can stretch and retract to accommodate the passage of blood through the lumen ↳ - This elastic recoil maintains pressure between pulses stratch or narrow - The muscle fibers can also contract to narrow the lumen - This results in vasoconstriction (or vasodilation), which is used to regulate blood pressure and temperatures VEINS Veins carry blood to the heart from the tissues of the body and lungs under low pressure Veins have a relatively wide lumen when compared to the thickness of the outer wall Veins have a specialized structure to support their role in carrying blood back to the heart: - The outer wall is quite thin (with less muscle and elastin) because the pressure is lower 2 T - They possess valves to prevent backflow (stops the blood from pooling at the jet extremities) · I ⑮ VALVES Valves ensure the unidirectional flow of blood and prevent the pooling of blood in the lower extremities Flaps of tissue that prevent backflow Exist only in veins Veins can be compressed by contractions of skeletal muscles to promote blood flow against gravity Often run parallel to arteries so they can be compressed by arterial bulges created by a pulse CAPILLARIES Responsible for all material exchange between the blood and the body tissues They have a very small lumen diameter (only allows the passage of a single cell at a time) The capillary wall is made of a single-cell layer to minimize diffusion distance – and has a gel coating (basement membrane) that acts as a filter Some capillaries may be fenestrated (contain pores) to maximize the rate of exchange All active cells in the body are close to a capillary. Two exceptions: the lens and the cornea of the eye - must be transparent so there are no blood vessels Tissue fluid–part of the blood plasma that leaks through the pores of capillaries (contains O2, glucose, etc.) - Flows between cells allowing them to take in useful molecules and excrete waste products→reenters the capillary network HEART A muscular pump that moves the blood around the body through vessels Four chambers – two atria and two ventricles - The atria act as reservoirs (collection of blood from veins) - The ventricles act as pumps (sending blood into arteries) - The heart is functionally divided into a left and right side - The right side transports blood to the lungs (pulmonary) - The left side transports blood to body tissues (systemic) HEART RATE Heart rate is measured by counting the number of arterial pulses Each pulse corresponds to one contraction of the heart muscle - This pressure wave can be felt as a pulse where an artery is close to the body surface bpm – beats per minute The pulse can be felt at the wrist and the neck→Two or three fingertips are pressed lightly against the skin where the artery is located (the thumb should not be used because it has a pulse) Digital meters – pulse oximeter CORONARY ARTERIES AND CORONARY HEART DISEASE Coronary arteries are the blood vessels that surround and nourish the heart tissue Can be narrow or blocked by fatty deposits (lipids, including cholesterol) - atheroma (plaque) - The blockage - an occlusion An occlusion can become impregnated with calcium salts→damage to the anterior wall→triggers formation of a blood clot (thrombosis)→can block the flow of blood to part of the heart wall, depriving it of oxygen and preventing normal contractions→a heart attack (myocardial infarction) RISK FACTORS hypertension—raised blood pressure increases the chance of blood clot formation smoking—raises blood pressure because nicotine causes vasoconstriction eating too much-saturated fat and cholesterol—promotes plaque formation obesity—associated with raised blood pressure and high blood cholesterol concentrations high salt intake—a large quantity of sodium chloride in the diet raises blood pressure drinking excessive amounts of alcohol—associated with raised blood pressure and obesity sedentary lifestyles—a lack of exercise is correlated with obesity and prevents the return of venous blood from the extremities leading to a greater risk of clot formation genetic predisposition—some genes increase the risk of hypertension and thrombosis old age—blood vessels become less flexible.  NERVOUS SYSTEM MEMBRANE POTENTIAL Neurons create electrical signals by creating and maintaining a potential difference in charge across the plasma membrane of the neuron–membrane potential A resting potential – when the neuron is not firing (-70mV) - Maintained by an uneven distribution of ions creating an electrochemical gradient An action potential when the neuron is firing (+30mV) - Resting potential à action potential = depolarization - Restoration of a resting potential = repolarization A nerve impulse – a wave of action potentials spreading along the length of the fan axon RESTING POTENTIAL Maintained by: - The sodium-potassium pump – creates an electrochemical gradient – the inside is negative relative to the outside o Three sodium ions are expelled from the cell for every two potassium ions that are let in - Membrane is 50x more permeable to K+ - Negatively charged proteins inside the cell ↑ ACTION POTENTIAL Action potentials – the rapid changes in membrane polarity that cause a nerve impulse Depolarization – triggered by the opening of sodium channels (inside becomes positive) Repolarization – triggered by the opening of potassium channels (inside returns negative) A few milliseconds are needed to fully restore resting potential – refractory period NERVE IMPULSES The ion channels along the length of an axon are voltage-gated - changing the membrane potential at one position on the axon opens the ion channels at the next position on the axon A nerve impulse is an action potential that starts at one end of a neuron and is propagated along the length of the axon as a wave of depolarization NERVE IMPULSE SPEED about 1 m/s with a 1μm diameter Increasing speed: - Wider diameters - faster conduction speeds (less resistance) - Myelination - nerve impulse can jump from one node of Ranvier to the next (up to 100 m/s) SYNAPSES Synapses are junctions between two neurons, between a neuron and an effector cell (muscle fibers or gland cells), or between sensory receptor cells and neurons The electrical signals within a neuron cannot be transferred across a synaptic cleft (20 - 40nm) + - Depolarization of the presynaptic membrane causes an influx of Ca + - Ca causes the presynaptic neuron to release vesicles with neurotransmitters (exocytosis) - Those bind to receptors on the postsynaptic cell and trigger a new action potential GRADED POTENTIALS The ion channels on the dendrites are ligand-gated When a neurotransmitter binds to a post-synaptic receptor, it causes a change in membrane potential Excitatory post-synaptic potentials cause membranes to depolarise, but inhibitory post-synaptic potentials cause hyperpolarisation to occur across a membrane If the total depolarization is sufficient to reach a set threshold (–55 mV), an action potential happens NEUROTRANSMITTERS Acetylcholine - Released at neuromuscular junctions in order to trigger voluntary muscle contractions - Released within heart tissue to cause the heart rate to slow Acetylcholine must be continuously removed from within the synapse - overstimulation can lead to fatal paralysis An enzyme acetylcholinesterase is present in the synaptic cleft and acetylcholine into acetate & choline The choline is absorbed by the presynaptic cell where it is joined with another acetyl group to reform acetylcholine