Control & Coordination: Neurotransmission Overview PDF

**Control and Coordination** **Types of Information Transfer** **Nervous System** **Endocrine System** -------------------------- ----------------------------------- -------------------------------------- **Form of transmission** Electrical impul...

**Control and Coordination** **Types of Information Transfer** **Nervous System** **Endocrine System** -------------------------- ----------------------------------- -------------------------------------- **Form of transmission** Electrical impulses Chemical messengers (Hormones) **Formed at** Sensory neurone generates impulse Secretory gland **Travel in** Neurones Blood (endocrine) **Speed** Instantaneous Slow **Duration** Short-term Long lasting **effects** Localised Widespread **receptor location** On cell surface membrane Cell surface membrane OR within cell **Energy** Large amount Less required - Both involve cell signalling - Both involve signal molecule binding to receptor - Both involve chemicals **Neurones** 3 different types of neurones: - **Sensory neurone:** Transmits impulses from receptor to CNS - Swelling of spinal cord containing cell body known as ganglion A diagram of a cell Description automatically generated - **Intermediate Neurone (Relay/Connector):** transmit impulse from sensory to motor neurone - Found entirely in CNS - **Motor Neurone:** Transmit impulses from CNS to effector - Cell body lies within CNS and contains the nucleus - Dark specks in cytoplasm are rough ER regions  **Reflex Arc** A **reflex arc** is the pathway along which impulses are transmitted from receptor to an effector without involving the 'conscious' regions of brain - Impulses travel from sensory to relay (not always) and finally to motor neurone - The effector acts before the brain processes the impulse and produces any voluntary movement. - Hence, this is a reflex reaction, which is fast, automatic and is useful in response to danger A diagram of a nerve cell Description automatically generated **Myelin** - Myelin is made when specialised cells called **Schwann Cells which** wrap themselves around the axon, enclosing it within many layers - The uncovered regions between Schwann cells are called **Nodes of Ranvier** - About a third of axons on motor and sensory neurons are surrounded by myelin sheaths  **Speed of Conduction** - Myelination stops depolarisation from occurring, greatly increasing the speed of conduction - It also prevents the leakage of ions and increases insulation, increasing speed of conduction. - Myelin also causes **saltatory conduction** which is when action potentials jump from one node to the next, which is about 50 times faster than unmyelinated axon - **Diameter** also affects speed of transmission; with thinner axons, there is greater resistance, hence, transmission is slower **Transmission of Nerve Impulses** Nerve impulses are signals transmitted along the axon, consisting of waves of depolarisation, causing changes in the potential difference across the membrane (action potential) **Resting potential** Resting axons have a slightly negative electrical potential inside, producing a potential difference of about **-70m**V**V** inside compared to the outside Diagram of a cell membrane Description automatically generated - Achieved by: - Plasma membrane being impermeable to Na^+^ /K^+^ - Sodium-potassium pumps that **actively** pump 3 Na^+^ out and 2K^+^ in, increasing the concentration of K^+^ inside, and Na^+^ outside - There are more K^+^ channels than there are Na^+^, therefore K^+^ diffuses down its concentration gradient (outside the cell) **faster** than Na^+^ diffuses in. - Many large negatively charged molecules inside cell  **Action Potential** It is the change in potential difference across the membrane due to changes in permeability of the membrane to Na^+^/K^+^ ions - An initial stimulus causes the opening of some **voltage**-**gated** channels causing Na^+^ to rush in, down its **electrochemical gradient** - This causes the potential difference across the membrane to become **less negative** and is called **depolarisation**. A diagram of cell membrane Description automatically generated - If this potential difference reaches -50mV*V*, then many more channels open, causing inside to become +30mV*V* - This wave of depolarisation is an example of positive feedback - Hence for an action potential to be produced, the potential must be raised to a minimum **threshold potential** of -50mV*V* - If lower than this, an action potential will not be generated - This is known as the all-or-nothing law as the neurones either transmit impulse or do not - After 1ms, all Na^+^ voltage-gated channels close & K^+^ gated channels open, causing K^+^ to diffuse out, thus repolarising the membrane. - The sodium potassium pump continues pumping these ions and maintaining their concentration across the membrane, allowing more action potentials to occur. - **Local circuits** are set up, where the permeability of the neighbouring region of the axon is increased  - Axons have a **refractory period** after the action potential, where it is unresponsive to new stimulations. Its consequences are: - Action potentials do not merge and so are discrete - There is a minimum time between action potentials occurring at one place on neurone - Length of refractory period determines max frequency at which impulses are transmitted - **Hyper polarisation** occurs when the cell potential becomes more negative than resting potential as there is an excess outflow of K^+^ A diagram of a normal distribution Description automatically generated **How action potentials carry information** - Action potentials do not change in size whether large or small stimulus & has constant peak value of +30mV*V* - The brain receives action potential from **specific position** of neurones and interprets the **nature** of the stimulus eg position: retina, nature: light **Strength** of stimulus - The brain interprets this from the **frequency** of the action potential-stronger stimuli have larger frequency - Also strong stimuli cause more neurones to be stimulated hence the **number** of neurones carrying action potential can tell us about the strength **Receptors** - A receptor cell responds to stimulus by converting energy from one form to electrical impulse, initiating an action potential (acts as a transducer) - Receptor cells are often found in sense organs and are specialised cells which detect specific type of stimulus - Some receptors are the ends of sensory neurones, thus there is no synapse between the receptor cells and sensory neurones. **Tongue** - The tongue is covered in many papillae, each papilla has many taste buds over its surface and within each taste bud lies around 50-100 **chemoreceptors** that detect different chemicals, giving different sensations - **Eg:** sodium chloride **(salt)** as stimulus - Na^+^ ions diffuse through highly selective channels of microvilli and cause depolarisation of the membrane: **receptor potential**. - If sufficient stimulation is produced, voltage-gated Ca^2+^ channels open; Ca^2+^ then enters, causing exocytosis of neurotransmitter vesicles - Neurotransmitters cause action potential in the sensory neurone and eventually reaches the cortex of the brain - **Note:** if receptor potential is below the threshold, it causes a local depolarisation of the receptor cell and doesn't stimulate the sensory neurone to send impulses. **Synapses** - Region where two synapses meet, there is a small gap called the **synaptic cleft** **Cholinergic synapse** - Synapses that have acetylcholine (ACh) - Action potential stimulates the opening of voltage gated Ca^2+^ channels at the presynaptic knob, causing an influx of Ca^2+^ into the cytoplasm - This causes exocytosis of ACh vesicles, which fuse with the pre synaptic membrane, then diffuse across the synaptic cleft - ACh has a complementary shape to the chemically gated receptor protein on the post synaptic membrane, and binds to it - This changes the shape of the protein and opens the channel for the entry of Na^+^ - Na^+^ depolarises that part of the membrane; if pd is above threshold, an action potential is generated - **Acetylcholinesterase** recycles Ach by breaking it into acetate and choline, preventing the permanent depolarisation of the membrane. - Choline returns to presynaptic neurone and combines with Acetyl coA to form Ach again.  **Role of synapses:** 1. Ensures one-way transmission as the receptors are only in post synaptic neurone and vesicles are only in presynaptic neurone 2. Decreases the overload of information in the brain as impulses with low frequencies do not reach the brain 3. Involved in **memory and learning** due to the formation of new synapses that links neurones involved 4. **Interconnection of nerve pathways:** sensory and relay have many dendrite increasing surface area for many synapses. This connects neurones from different parts of the body and spreads information throughout. **Structure of striated muscle** A diagram of muscles and muscles Description automatically generated - Striated muscle is multinucleate (**syncytium**) and consists of several tissues eg connective, nerve, striated muscle, blood. - It is made up of bundles of muscle fibres/cells (fascicles) - Each muscle fibre is made up of regular arrangement of myofibrils, which produce the striated appearance of muscle fibres **Structure of muscle fibre**  - The **sarcolemma** (cell membrane) splits into many infoldings called T-tubules - **Sacroplasm** (cytoplasm) contains many mitochondria that generate ATP for muscle contraction - The **sarcoplasmic reticulum (SR)** (endoplasmic reticulum) have many protein pumps that transport Ca^2+^ into the cisternae of SR **Structure of the Myofibrils** - Myofibrils are made of contractile units called **sacromeres** (between two Z discs) which are made of thin and thick protein filaments - **Myosin:** fibrous protein with globular head that makes up **thick** filament - **Actin:** globular protein; two chains of actin overlap to make up **thin** filament - **Tropomyosin:** fibrous protein twisted around actin chain - **Troponin:** protein that is attached to the actin chain at regular intervals A diagram of a cell Description automatically generated - **Z line:** where actin filaments are attached to - **M line:** where myosin filaments are attached to  A diagram of a band Description automatically generated - **A band**: includes the darker parts in the centre where actin and myosin overlap - **H band**: the grey area within the A band where only myosin is present - **I band**: the white area next to the Z line where only actin is present **How muscles contract** - Muscles movement is caused by contraction, causing Z discs to pull closer together by a process of sliding - The energy comes from the ATP in myosin heads (an ATPase) **Process from stimulation** - Sarcolemma is depolarised by an incoming action potential which spreads along membrane and down the T-tubule - Calcium ions are released from sarcoplasmic reticulum (using ATP) and bind to troponin, causing it to change shape - This in turn causes tropomyosin to move, exposing myosin binding sites on actin filament - Myosin heads bind with this site forming cross-bridges - Myosin heads tilt, pulling actin filaments towards centre of sarcomere (M line) - The heads hydrolyse ATP, providing energy for heads to let go of actin and return to original position so that it can bind again to exposed site - This process continues as long as binding sites are open and ATP is in excess - It can be reversed by relaxation of muscle (no cross bridge) and contraction of antagonist muscle that pulls filaments further away, lengthening sarcomere  - During contraction, the A band is unaffected however both H and I bands decrease in length **Providing for muscle contraction** - ATP can be provided from little ATP found in muscle by respiration and lactic fermentation - Creatine phosphate stores is an immediate source of energy that regenerates ATP in the absence of respiration creatine phosphate + ADP creatine + ATP - When demand for energy has reduced, creatine is recharged to form creatine phosphate in the presence of ATP from respiration - When there is an energy demand and not enough ATP to regenerate creatine phosphate, creatine is converted to creatinine and excreted **Electrical Communication in Plants** **Venus fly trap:** - A carnivorous plant that obtains nitrogen compounds by digesting small animals A close-up of a venus flytrap Description automatically generated - Nectar secreting glands attract insects - Each lobe has three sensory/ trigger hairs that respond when deflected - Outer edges have stiff hairs that interlock to trap insect - Surface of lobes has glands that secrete digestive enzymes **Action of shutting the trap** - The deflection of a sensory hair opens Ca^2+^ channels at cells at the base of hair, causing inflow of Ca^2+^ and generating receptor potential (depolarisation) - If two hairs (or one hair touched twice) are stimulated within 20-35s, action potentials travel across the lobe to close it. - H^+^ ions are pumped into the cell walls, breaking cross links (acid growth hypothesis) - Calcium pectate 'glue' in middle lamella dissolves - Ca^2+^ enters the hinge cells causing water to enter by osmosis hence expanding the hinge cells - Lobes of the leaves flip from convex to concave rapidly (change in elastic tension) - Further deflection of hairs stimulate entry of Ca^2+^ into gland cells, causing exocytosis of vesicles containing digestive enzymes. - Mechanical energy converted to electrical energy **Adaptations to avoid energy waste** - Stimulation of single hair doesn't trigger closure eg rain or debris - Gaps between stiff hairs allow tiny insects to crawl out, so plant doesn't waste energy to consume small meals **Chemical Communication in Plants** - Two types of plant growth regulators - **Auxins**: influence elongation of roots and shoots - **Gibberellins**: seed germination and stem internode elongation - They travel directly from cell to cell (diffusion/active transport) or carried in xylem/phloem sap **Auxins** - Plants make several chemicals known as auxins, of which the main one is IAA - Auxins are synthesised in growing tips/meristems - Auxins bind to a protein receptor which stimulates ATPase to pump in H^+^ into cell walls, reducing its pH. - Proteins called expansins are activated at low pH, loosening cellulose microfibril linkages - Water is absorbed by osmosis and pressure potential causes the wall to stretch, elongating the cell - Auxins also inhibit lateral growth so that plant grows taller **Gibberellins and stem elongation** - The dominant allele ­­­LeLe causes the synthesis of the last enzyme that produces active form of gibberellin, GA~1~ - Active gibberellin stimulates cell division and cell elongation whilst interacting with auxin, causing the plant to grow tall - Plants that are homozygous and have the recessive allele lele produce a non-functional enzyme (due to substitution mutation in its primary structure) - Thus, active gibberellin is not produced and so the plant is genetically dwarf - Applying active gibberellin to plants that would remain short can stimulate them to grow tall **Gibberellins and seed germination** - Seeds are in a state of dormancy; this allows it to survive through cold winters and is only activated when enough water is present  Homeostasis