BIOLOGY UNIT 2:BIOENERGETICS, BIOSYSTEMS AND MAINTENANCE MODULE 2:BIOSYSTEMS MAINTENANCE SPECIAL OBJECTIVES 6.1-6.5 NERVOUS COORDINATION By Ms. Sharina Gerald Nervous Coordination Special Objectives of CXC CAPE Biology Syllabus-Unit 2-Module 2: 6.1 Describe the structure of motor and sensory neurones with the use of annotated diagrams. 6.2 Explain the role of nerve cell membranes in establishing and maintaining the resting potential; 6.3 Describe the conduction of an action potential along the nerve cell membrane; Emphasise the value of myelinated neurons in increasing the speed of transmission. 6.4 Explain synaptic transmission; Structure of cholinergic synapse. Annotated diagrams required. 6.5 Outline the role of synapses. Background Knowledge A good grasp of the excretory system from O’Level Biology which would include: Set Induction https://www.youtube.com/watch?v=XdCrZm_JAp0 The Components of the Human/ Mammalian Nervous System The human nervous system is composed of two parts: Human Nervous System Central Nervous System(CNS) Brain Spinal Chord Peripheral Nervous System(PNS) All the other neurons extending from CNS Types of neurones There are three types of neurones: Sensory(receptors)-responsible for detecting a change in the environment(stimuli). Relay or interneurone- connects other neurones to each other. Motor neurone(effectors)- bring about a change in response to stimuli. Types of neurons The Spinal Arc Resting potential Na+/K+ pumps in the membrane actively pump sodium ions out of the cell and potassium into it. Three sodium ions are removed for every two potassium ions brought in. Some of these ions leak back to where they came from by diffusing through other parts of the membrane. More potassium ions leak out than sodium enter. Overally, these two processes causes the outside of the neuron to be negative with relation to the outside. This difference in charge is called a resting potential and is about -70 mV. Action potential This is how nerve impulses are propagated. Axons have voltage-gated Na+ and K+ channels that A receptor detects stimuli. Sodium channels open. Sodium ions flood back into the cell due to an already existing electrochemical gradient. The inside is now no longer negative with relation to the outside. Rather, it is the reverse and the neuron is said to be depolarised. The sodium channels close. Action Potential cont’d In response to voltage changes, the potassium channels open. Potassium diffuses out of the axon therefore positive charge us removed from the axon. The charge now returns to normal. A process called repolarisation. So many K leave that the inside of the axon becomes more negative than the normal resting potential. This is called hyperpolarisation. Na and K channels close and pumps restore the electrochemical gradient. Very light/weak stimuli does not result in and action potential. This is because there is a threshold potential of -50mV to -60V which must be exceeded. Evaluation 1 Explain how stimuli results in the formation of an action potential. Conduction or transmission of an action potential What we have just studied is an action potential at one point. For nerve impulses to be carried, the action potential needs to move along the length of the axon. For this to happen, depolarisation at one point causes local circuits to be set up in the regions next to the action potential on either side. The area before is in a refractory state since its resting potential has not been restored. Therefore the action potential moves forward. https://www.youtube.com/watch?v=9euDb4TN3b0 How information is carried by action potentials The action potentials are always the same size. A strong or weak stimulation would result in the same size action potential i.e. depolarization from -70 mV to about 30 mV. This is known as the ‘all-ornothing’ law. How is it then we know the difference between a weak stimulation and a strong stimulation? This is achieved by the frequency of the action potentials. Stronger stimulations have more frequent action potentials. Additionally, more neurones are active for a strong stimulus than for a weak stimulus. The nature of the stimulus is determined from the position of the sensory neurone. E.g. all stimulation at the eyes is detected as light. How information is carried by a neurone Saltatory Conduction The myelin sheath is impermeable to the Na+ and K+ ions. Hence, no action potentials are set up in this region. Therefore the action potential has to “jump” from one Node of Ranvier to the other. This is called saltatory conduction. However, assuming there was no myelin sheath, the speed of conduction along the neuron would be too slow for survival. Evaluation 2 Outline the process of the transmission of nerve impulses in a myelinated neuron. Synaptic Transmission When two neurones meet, they do not touch each other. Instead, there is a very small gap between the two neurones. This is called a synaptic cleft. The synaptic cleft together with the presynaptic membrane and postsynaptic membrane make up the synapse. Action potentials do not move across synapses. Instead, a chemical known as a neurotransmitter is secreted from one synapse and detected at the other. There are over 40 known neurotransmitters. Synapses are named after their neurotransmitters. For example,? Noradrenaline-adrenergic synapses. Acetylcholine(Ach)-cholinergic synapses. For our purposes, we will be looking at the cholinergic synapse. Cholinergic synaptic transmission Action potential arrives at the pre-synaptic membrane. Results in the opening of calcium ion channels causing calcium ions to flood into the membrane. This results in the exocytosis of vesźicles containing acetylcholine. Acetylcholine then diffuses across synaptic cleft to post synaptic membrane. Molecules of acetylcholine bind with its specific, complementary receptor protein on the post-synaptic membrane resulting in a conformational change in the protein. This results in Na+ channels opening in the post-synaptic membrane, depolarizing the membrane and causing propagation of the action potential. Excitatory Cholinergic Synapse Cholinergic synaptic transmission cont’d As long as acetylcholine remains in the receptor, there will be an influx of Na+ and depolarization will continue. To restore the potential, acetylcholinesterase(an enzyme found in the cleft) is used to split acetylcholine into choline and acetate. Choline is taken back to presynaptic cleft where it reacts with acetyl CoA to make acetylcholine. The acetylcholine is transported into vesicles, awaiting the next action potential. Functions of synapses Ensuring one-way transmission. Interconnecting nerve pathways Spatial summation Temporal summation EPSP IPSP Memory and learning. Effects of chemicals at synapses Nicotine Mimics the action of acetylcholine. Is not easily broken down. Botulinum toxin(Botox) Acts at the presynaptic membrane. Prevents release of acetylcholine. Organophosphorous insecticides Inhibits the action of acetylcholinesterase. Results in continuous action potentials.
