1 / 67

nervous system

nervous system. The nervous system allows the body to respond to changes in the internal and external environment Receptors detect changes/ stimuli which are rapidly transmitted along neurones to effectors that bring about a corrective response. neurones.

jackkjohnson
Download Presentation

nervous system

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. nervous system

  2. The nervous system allows the body to respond to changes in the internal and external environment • Receptors detect changes/stimuli which are rapidly transmitted along neurones to effectors that bring about a corrective response

  3. neurones • Adapted to transmit information throughout the nervous system. • 3 types:

  4. CNS association neurone motor neurone sensory neurone effector receptor response stimulus

  5. structure of a myelinated motor neurone • Large cell body: containing a nucleus, nucleolus, mitochondria and ribosomes • Dendrons: long thin strands of cytoplasm that carry impulses towards the cell body and connect to many neurones in the CNS • Axon: a long extension that carries impulses away from the cell body and terminates in motor end plates that connect to muscles or glands

  6. Schwann cells: wrap around the axon of myelinated neurones forming the fatty myelin sheath that insulates the impulse. • Nodes of Ranvier: small gaps between adjacent Schwann cells that aid transmission of impulses Schwann cell axon

  7. the generation and transmission of nerve impulses

  8. resting potential • When a nerve cell is at rest positive ions are moved in and out of the axon across the membrane by facilitated diffusion using gated channel proteins and by active transport, using carrier proteins. • More positive ions are moved out of the axon than in which causes the inside of the axon to become negative in relation to the outside; the membrane is polarised • The charge across the membrane, the (potential difference) is called the RESTING POTENTIAL RP is approximately -70mV

  9. Once the RP is achieved the gated channel proteins that allow facilitated diffusion to occur close, making the membrane impermeable to ions. • This prevents ions moving back down their concentration gradient.

  10. polarised membrane + + + + axon membrane - - - - inside - - - - outside + + + +

  11. http://youtu.be/p0lmlWoEEKY

  12. THIS CHARGE CAN BE MEASURED

  13. the resting potential

  14. sodium-potassium pump

  15. all-or-nothing law • A stimulus must reach a specific level in order for an impulse to be generated • A more intense impulse will not produce a bigger impulse; all impulses are the same size • Strong stimuli produce a greater frequency of action potentials (impulses) NB: below threshold, no AP above threshold AP all the same size

  16. Below threshold intensity: no action potentials Action potentials generated Increasing intensity of stimulus Threshold intensity Successive stimuli

  17. action potential • Stimulation of the axon causes the membrane to become depolarised • This causes some of the gated channel proteins to open and the membrane becomes permeable to ions. • The positive ions diffuse down their concentration gradient through channel proteins. • The pd across the membrane changes, causing more gated channel proteins to open in a positive feedback mechanism.

  18. This reverses the potential difference across the membrane, to +40mV • Making the inside positive in relation to the outside • This change in charge evokes (starts) an ACTION POTENTIAL

  19. depolarised membrane depolarisation + + + - + - - - stimulus + - - - + + + -

  20. When the AP reaches 40mV the recovery phase starts. • The positive ions are moved by facilitated diffusion through channel proteins and by active transport by carrier proteins to restore the RP. • When the membrane is REPOLARISEDthe membrane becomes impermeable to ions again. • The length of time it takes for the resting potential to be re-established is called the REFRACTORY PERIOD. • This is necessary in order for further action potentials to pass along the axon.

  21. The refractory period: • Ensures the APs are propagated in one direction only, because the area behind the action potential is in a state of recovery.. Without it APs could be generated in both directions along the axon. • Limits the number of APs that can be fired, ensuring each remains separate from subsequent Aps.

  22. DIAGRAM & POINTS PAGE 53/54

  23. propagation of a nerve impulse • An action potential is generated at a specific part of the axon membrane and moves rapidly as a wave of depolarisation along the neurone. • The area behind the depolarised part of the axon (AP) quickly repolarises. • The membrane adjacent to the depolarised section will have opposite charges as they are polarised.

  24. This results in the creation of local currents between the area where there is an action potential and the resting area next to it. • Positive ions from the depolarised section pass along the inside of the membrane towards the polarised zone immediately in front of it. • On the outsideof the membrane positive ions move from the polarised zone to the depolarised zone. • The flow of current in a series of these localised currents creates a wave of depolarisation that moves rapidly along the neurone. • Similar circuits enable the RP to be restored directly behind the AP

  25. Area behind the AP undergoes repolarisation during the refractory period so is unable to be depolarised, preventing the impulse moving backwards IMPULSE RP AP RP + + + + - + + + + - - - - - - - - + Stimulus generated this end + - - - - - - - - + + + + - + + + + local electrical circuit

  26. factors which influence the speed of transmission 1. Diameter of the axon Thicker axons transmit impulses faster as there is proportionally less leakage of ions. (SA:VOL) Giant axons found in earthworms and squid are associated with the need for rapid escape responses.

  27. 2. Myelination of the axon and saltatory conduction • The myelin sheath acts as an insulator in myelinated neurones. • So areas of the axon that are myelinated cannot be polarised or depolarised. • This is because myelin is a fatty substance that does not allow movement of ions across it. • Myelin is absentat the nodes of Ranvier where the Schwann cells meet, allowing depolarisation of the axon membrane, and AP are generated.

  28. Local circuits form between the nodes only, allowing sections of the neurone to be by-passed. • APs ‘jump’ from one node to the next, speeding up transmission by about 100 times. • This is called SALTATORY CONDUCTION • and is found only in the myelinated axons of vertebrates • Saltatory conduction also saves energy as less is required for the active transport of ions across the axon membrane

  29. There are relatively few ion channels under the myelin sheath, being concentrated at the nodes of Ranvier. • This allows myelinated neurones to have relatively diameters. 3. Temperature also affects speed of the nerve impulse, as it affects the rate of diffusion of ions involved.

  30. NB: impulse ALWAYS moves from cell body away from the axon draw diagram page 55 CCEA Text

  31. structure and function of a synapse

  32. the synapse • Synapses are junctions between the axon of one neurone and the dendrite of the next. • They also occur between neurones and muscle and are called neuromuscular junctions. • The gap between 2 neurones is called the synaptic cleft. • Chemicals called neurotransmitters diffuse across this very small gap. • The neurone that releases the neurotransmitter is called the pre-synaptic neurone, whilst the neurone that it diffuses to is called the post-synaptic neurone.

  33. ADAPTATIONS OF THE PRE & POST SYNAPTIC NEURONES. The end of the pre-synaptic neurone is thickened to form a synaptic bulb. Pre-synaptic bulbs contain many mitochondria (to release energy for neurotransmitter production) and synaptic vesicles, which store the neurotransmitter

  34. The postsynaptic membrane contains receptors that are complementary to the specific neurotransmitter used in a particular synapse. • These are linked to many ion channels will cause depolarisation of the post-synaptic neurone, allowing the impulse to continue to travel along the next neurone.

  35. structure of the synapse • Synaptic cleft: gap between the pre and post synaptic membranes • Presynaptic membrane: neuronemembrane before the synapse • Postsynaptic membrane: neurone membrane after the synapse • Neuromuscular junction: synapse between a motor neurone and a muscle • Neurotransmitter: chemical that carries the impulse across the synaptic cleft, found in synaptic vesicles

  36. structure of the synapse You will be expected to identify and label the following structures from LEM and TEM photographs and diagrams • Synapse: gap between 2 neurones • Synaptic bulb: swelling at the end of an axon • Synaptic vesicles: vesicles containing neurotransmitter found in the synaptic bulb

  37. axon myelin sheath action potential end of axon mitochondrion synaptic vesicle containing neurotransmitter substance synaptic bulb synaptic cleft presynaptic membrane dendrite protein receptor postsynaptic membrane complementary receptor & ion channel postsynaptic cell

  38. synapse at a neuromuscular junction

  39. transmission of an impulse across a synapse • An impulse arrives at the synaptic bulb • Depolarisation of the membrane causes (voltage activated) calcium channels to open • Ca2+ ions enter the synaptic bulb by diffusion • The Ca2+ ions cause the vesicles containing neurotransmitter substance (usually acetylcholine) to move tothepre-synaptic membrane

  40. Vesicles fuse with the pre-synaptic membrane releasing acetylcholine into the synaptic cleft (exocytosis) • Acetylcholine diffuses acrossthesynaptic cleft • and binds to receptors on the post-synaptic membrane • This causes the opening of ion channels and • Na+ ions diffuse into the post-synaptic neurone.

  41. The membrane gradually depolarises and an excitatory post-synaptic potential (EPSP) is generated. • If sufficient depolarisation occurs (depending on the number of neurotransmitter molecules filling the receptors) The EPSP will reach the threshold intensity required to produce an AP in the post-synaptic membrane

  42. The acetylcholine must be removed from the post-synaptic membrane, to prevent it continuously generating a new AP in the post-synaptic neurone. The enzyme acetylcholinesterase, which is attached to the post-synaptic membrane, breaks down acetylcholine into choline and ethanoic acid (acetyl) which are released into the synaptic cleft. They diffuse across the cleft and are reabsorbed by the pre-synaptic bulb, where they are resynthesised into acetylcholine and stored in the synaptic vesicles to be used again. The process requires ATP.

  43. function of the synapse 1. Unidirectionality • As the neurotransmitter is only made in the pre-synaptic neurone and the neurotransmitter receptors are present only on the post-synaptic neurone membrane, impulses can only cross from the pre-synaptic neurone to the post-synaptic neurone.

More Related