1. Unit 3 - Unit 3 - Nerve Cells and Neural Pathways CfE Higher Human Biology 19. Memory

2. Learning Intentions I can identify the structures of a neurone including dendrites, cell body and axon I can identify sensory, motor and inter (relay) neurons I can state that sensory neurons take impulses from sensory receptors to the central nervous system I can state that motor neurons take impulses from central nervous system to an effector (e.g. muscle) I can state that inter neurons transmit impulses between sensory and motor neurons within the central nervous system I can state that myelin sheath is composed of fatty material and surrounds the axon I can explain why myelination increases the speed of impulse transmission I can state that myelination increases from birth to adolescence I can state that glial cells support the function of neurons and produce myelin sheath I can state that glial cells also maintain a homeostatic environment around the neurons and remove debris by phagocytosis

3. The nervous system is made up of a system of nerve cells, known as neurons, which receive and transmit electrical signals called nerve impulses. Glial cells support and maintain these neurons. Neurons

4. Function and Types of Neurone Neurones provide the body with rapid communication and coordinated control. They conduct nerve impulses from on one part of the body to another 3 types of neurones – Sensory – Inter(relay) – motor Inter

5. Structure of neurons All neurons have the same basic structure, they are composed of three key structures: – Dendrites – nerve fibres that receives nerve impulses towards a cell body – a cell body – contains the nucleus and most of the cytoplasm – Axons – nerve fibres that carries nerve impulses away from a cell body. Nerve impulses always travel in the same direction: dendrites cell body axon. Neurons dendrites cell body axon

6. The Function of Parts of a Neuron Control metabolism, contains ribosomes for the production of neurotransmitters

7. Structure of neurons Cell body - The cell body contains a nucleus and cytoplasm. The cytoplasm contains organelles such as mitochondria to provide energy for impulses and ribosomes which synthesise proteins (e.g. enzymes) for the synthesis of neurotransmitters. Dendrites – these fibres receive nerve impulses and carry them towards the cell body Axon – this fibre carries nerve impulses away from the cell body. Neurons

8. Structure of neurons The axons of neurons are surrounded in a layer of fatty material known as the myelin sheath. This insulates the axon. The myelin sheath greatly increases the speed of transmission of a nerve impulse from node to node. The small gaps are called nodes. Neurons axon myelin sheath

9. Structure of neurons Myelination (the extent to which an axon is covered in myelin) is not complete at birth. As a child ages, myelination increases and so does nervous control. The responses of a two year old child are therefore slower than those of an adult. Some diseases, such as Polio, Tay-Sachs and Multiple Sclerosis (MS) can damage the myelin sheath and result in loss of muscular co-ordination. Myelination

10. Neurones Types of neurons There are three main types of neuron: SENSORY NEURONES –From sense organs to CNS MOTOR NEURONES –From CNS to effectors INTER NEURONES –From sensory neurones to motor neurones. –Are in CNS

11. Sensory Neurone DIRECTION OF IMPULSE Has dendrites in contact with sense organs. These dendrites merge to form a myelinated fibre which carries impulses to the cell body. Has a short axon Forms connections with neurons in the CNS

12. Inter Neurone Connects sensory neurons to motor neurons. Has many dendrites which form many complex, connections.

13. Motor Neurone DIRECTION OF IMPULSE Has short dendrites which connect to neurons in the CNS Has a long myelinated axon Axon carries nerve impulses to muscle connections.

15. Diseases Diseases such as multiple sclerosis and poliomyelitis Cause the myelin sheath to become damaged or destroyed Resulting in loss of muscular coordination

16. Glial cells have a number of key functions: –physically support neurons –produce the myelin sheath –control the chemical composition of the fluid surrounding the neuron and so maintain a homeostatic environment. –remove debris by phagocytosis Glial cells Glial cell

17. Glial Cells They provide physical support to neurons –(Oligodendrocytes) form the myelin sheath around axons. –(Astrocytes) provide nutrients to neurons, maintain their extracellular environment, and provide structural support (homeostatic environment). –(Microglia) scavenge pathogens and dead cells by phagocytosis

18. Blood Brain Barrier The blood-brain barrier (BBB) is the close association between projections from certain glial cells and the cells forming the walls of capillary blood vessels. This is the layer that lines the blood capillaries and is made up of cells very closely packed together. This separates the blood in the capillaries separate from the extracellular fluid in the brain. This prevents larger molecules and microorganisms moving into the brain fluid from the bloodstream. Some glial cells are phagocytic so they will remove foreign material by phagocytosis.

19. Questions 1.Describe the structure and function of a neuron. 2.Describe the pathway of a nerve impulse through a neuron. 3.Describe the features and functions of sensory, motor and inter neurons. 4.Describe the structure and function of the myelin sheath. 5.Explain the relationship between myelination, co- ordination and development from birth. 6.Describe the function of the glial cells

20. Answers 1.Describe the structure and function of a neuron. All neurons have dendrites, cell body and axons. Neurons transmit electrical signals called nerve impulses. 2. Describe the pathway of a nerve impulse through a neuron. Impulse travels along a dendrite reaches the cell body and then passes along an axon.

21. Answers (continued) 3. Describe the features and functions of sensory, motor and inter neurons. Sensory neuron Has dendrites in contact with sense organs which merge to form a fibre which carries impulses to the cell body. Has a short axon Motor neuron Has short dendrites which connect to neurons in the CNS. Has a long axon carries nerve impulses to muscle connections Inter neuron Connects sensory neurons to motor neurons.Has many dendrites which form many complex, connections.

22. Answers (continued) 4. Describe the structure and function of the myelin sheath. The myelin sheath is a layer of fatty material which greatly increases the speed of transmission of a nerve impulse. 5. Explain the relationship between myelination, co- ordination and development from birth. Myelination is not complete at birth. Therefore co- ordination and dveopement will improve as this is completed

23. Answers (continued) 6. Describe the function of the glial cells Physically support neurons Produce the myelin sheath Control the chemical composition of the fluid surrounding the neuron and so maintain a homeostatic environment. Remove debris by phagocytosis

24. Neurotransmitters at Synapses

25. Learning Intentions I can describe a synapse as an area of communication between axon of one neuron and dendrite of another I can explain that a neurotransmitter is a chemical that relays the message from the pre synaptic neuron to the post-synaptic neuron I can describe the chemical transmission at a synapse from vesicles containing neurotransmitter crossing the synaptic cleft to receptors I can state why it is important to remove neurotransmitters from the synapse I can state that the type of receptor determines whether a signal is inhibitory or excitatory I can state that insufficient neurotransmitter at the synapse results in failure of transmission of the impulse I can state that a summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse. I can define a converging neural pathway as several neurons sending an impulse to one receiving neuron in order to increase the intensity of the impulse e.g. used to see in the dark I can define a diverging neural pathway as a motor neurone sending an impulse to multiple neurons in order to provide fine motor control and hypothalamus co-ordinated control of body temperature I can define reverberating neural pathways as those which can repeat the original impulse

26. The tiny area between the ending of an axon of one neuron and the dendrite of another is known as a synapse. The plasma membranes of each neuron are in very close contact and are separated by a narrow space called a synaptic cleft. Messages are passed across synaptic clefts by chemicals called neurotransmitters. Two examples are acetylcholine and norepinephrine (also known as noradrenaline). Synaptic Cleft and Neurotransmitters

27. The neuron before the synaptic cleft is known as the presynaptic neuron. The neuron after the synaptic cleft is known as the postsynaptic neuron. Synaptic Cleft presynaptic neuron (axon) postsynaptic neuron (dendrite) synaptic cleft neurotransmitters

28. When a nerve impulse passes through a neuron and reaches the end of the axon (known as the axon terminal), many vesicles containing neurotransmitters are stimulated. These vesicles move to and fuse with the membrane at surface of the axon terminal. The neurotransmitters within the vesicles are then released (by exocytosis) into the synaptic cleft. The neurotransmitter then diffuses across the cleft and binds to receptor molecules on the dendrites of the next neuron; this transmits the impulse to the next neuron. Action of neurotransmitters

29. direction of nerve impulse

30. http://www.youtube.com/watch?v=H_81gwAnjDU http://science.education.nih.gov/supplements/nih2/Addi ction/activities/lesson2_neurotransmission.htmhttp://science.education.nih.gov/supplements/nih2/Addi ction/activities/lesson2_neurotransmission.htm Action of neurotransmitters

31. The type of receptor cells found on the postsynaptic neuron will determine whether the signal is: –excitatory (causes an increase in action e.g. cause muscles to contract) or –inhibitory (cause a decrease in action e.g. slow heart rate) Excitatory & inhibitory signals

32. Neurotransmitters must be rapidly removed as soon as the impulse has been transmitted for the following reasons: –to prevent continuous stimulation of the postsynaptic neuron –so that the membrane is sensitive to the next stimulus –otherwise, the neurotransmitter would continue to have an effect –this allows a neurone to send many separate impulses allowing a variety in the rate of impulse transmission. Action of neurotransmitters

33. Neurotransmitters can be removed from the synaptic cleft by: –enzyme degradation - this occurs with acetylcholine, the products of which are absorbed and used to synthesise new neurotransmitters or –re-uptake - this occurs with norepinephrine, which is reabsorbed by presynaptic membrane. Action of neurotransmitters

34. The continual synthesis and removal of neurotransmitters requires a very large amount of energy. Neurones contain a large number of mitochondria to provide ATP. This is why the brain is so easily damaged by oxygen deprivation. Action of neurotransmitters

35. A nerve impulse will only be transmitted across a synaptic cleft if it causes the release of a sufficient number of neurotransmitter molecules; this is known as the threshold. Weak stimuli are known as sub-threshold stimuli and are too weak to cause the transmission of a nerve impulse. When the stimulus is weak, the synapse acts as a gap which the impulse cannot cross and the stimulus is ‘filtered out’ due to insufficient secretion of neurotransmitters. Weak stimuli

36. A single weak stimulus will not trigger the release of enough neurotransmitters to cause transmission of a nerve impulse. However, a series of weak stimuli from many neurons can bring about an impulse. The cumulative effect of a series of weak stimuli which triggers an impulse is known as summation. Summation

37. If a weak stimulus passed along one axon this would not trigger enough neurotransmitters to be released to reach the threshold. Summation When many axons release their neurotransmitter at the same time or in rapid succession, this releases enough chemical to fire a response.

38. Questions 1.Describe what is meant by a ‘neurotransmitter’ 2.Describe how a nerve impulse is transmitted at the synapse (to include vesicles, synaptic cleft and receptors) 3.Describe how neurotransmitters are removed and explain why this is necessary. 4.What will receptors determine about the signal? 5.Describe how synapses can ‘filter out’ weak stimuli. 6.Describe the what is meant by ‘summation’

39. Answers 1.Describe what is meant by a ‘neurotransmitter’ Neurotransmitters are chemicals which cause messages to pass across synaptic clefts 2. Describe how a nerve impulse is transmitted at the synapse (to include vesicles, synaptic cleft and receptors) vesicles, at the axon terminal, are stimulated, vesicles fuse with the membrane at surface of the axon terminal. The neurotransmitters within the vesicles are then released into the synaptic cleft. The neurotransmitter then diffuses across the cleft and binds to receptor molecules on the dendrites of the next neuron this transmits the impulse to the next neuron.

40. Answers (continued) 3. Describe how neurotransmitters are removed and explain why this is necessary. Enzyme degradation and re-uptake It is necessary to prevent continued stimulation of th post synaptic neuron 4.What will receptors determine about the signal? Whether the signal is excitatory or inhibitory

41. Answers (continued) 5. Describe how synapses can ‘filter out’ weak stimuli. When the stimulus is weak, the synapse acts as a gap which the impulse cannot cross and the stimulus is ‘filtered out’ due to insufficient secretion of neurotransmitters. 6. Describe the what is meant by ‘summation’ The cumulative effect of a series of weak stimuli which triggers an impulse is known as summation.

42. Neural pathways

43. Learning Intentions I can define a converging neural pathway as several neurons sending an impulse to one receiving neuron in order to increase the intensity of the impulse e.g. used to see in the dark I can define a diverging neural pathway as a motor neurone sending an impulse to multiple neurons in order to provide fine motor controle.g. hypothalamus co-ordinated control of body temperature I can define reverberating neural pathways as those which can repeat the original impulse I can describe plasticity of response as the ability to suppress reflex actions and the ability of the brain to bypass areas of brain damage and relearn skills

44. Neurons are connected to others in many different ways in the CNS. This allows many complex interactions to occur between neurons and so allows the nervous system to carry out many complex functions. There are three main neural pathways: –converging –diverging –reverberating Complex neural pathways

45. Converging neural pathways have many neurons coming together and feeding impulses to one neuron. This allows for signals to be brought together for a combined or concentrated effect (e.g. summation) at one neuron. An example of this can be found with the convergence of the neurons from rod cells in the retina of the eye. Converging neural pathways The direction of the impulse on the diagram is very important.

46. Convergence of Neurons in Rods and Cones in Dim Light Rods and cones are visual receptors found in the eye. They contain pigments that break down in the presence of light. This breakdown releases a chemical trigger that sends impulses along a pathway of neurones. Cones – the pigment is not as sensitive so daylight is needed Rods – this pigment is sensitive to very dim light so is rendered temporarily inactive in very bright light. As light intensity decreases, cones stop responding and rods take over. The nerve impulse created from one rod is very weak so cant raise the postsynaptic membrane to threshold. The convergent arrangement of several rods allows impulses to be transmitted simultaneously and so release enough neurotransmitter. To optic nerve

47. Diverging neural pathways have one neuron branching out and feeding impulses to many neurons. This allows for signals from a single source to be sent to several destinations and allows us to co-ordinate control (e.g. when threading a needle. This is fine motor control from the motor area of the cerebrum). Similarly temperature in the hypothalamus is found to diverge into branches that lead to sweat glands, skin arterioles and skeletal muscles. Diverging neural pathways The direction of the impulse on the diagram is very important.

48. Reverberating neural pathways possess neurons later in the pathway which form connections with neurons earlier in the pathway. This allows for nerve impulses to be recycled and repeatedly stimulate the circuit, these impulses will only stop when they are no longer required. Complex reverberating pathways in the brain are involved in the control of rhythmic activities such as breathing. Reverberating neural pathways The direction of the impulse on the diagram is very important.

49. The brain can undergo change depending upon sensory input in the synaptic network during our lifetime. The ability of brain cells to become altered and form new neural pathways as a result of new environmental experiences is known as plasticity of response. This allows new neural pathways to be formed during early development when learning many new skills. Major plasticity of response can occur after brain damage (e.g. stroke) and allows undamaged cells to form new neural pathways to take on the functions of the damaged area. Minor plasticity is used to suppress reflexes (e.g. blinking or prevent the body dropping a hot object) or suppress sensory responses (such as visual distractions). Plasticity of response (P263)

50. Questions 1.Name the three main neural pathways. 2.Describe the features and functions of each of the neural pathways. 3.Describe what is meant by ‘plasticity of response’ 4.Give examples of when plasticity of response occurs

51. Answers 1.Name the three main neural pathways. Converging, diverging and reverberating 2.Describe the features and functions of each of the neural pathways. Converging -many neurons coming together and feeding impulses to one neuron. This allows for signals to be brought together for a combined effect at one neuron. Diverging -one neuron branching out and feeding impulses to many neurons. This allows for signals from a single source to be sent to several destinations and allows us to co-ordinate control Reverberating -neurons later in the pathway form connections with neurons earlier in the pathway. This allows for nerve impulses to be recycled and repeatedly stimulated

52. Answers(continued) 3.Describe what is meant by ‘plasticity of response’ The ability of brain cells to become altered and form new neural pathways as a result of new environmental experiences 4. Give examples of when plasticity of response occurs after brain damage (e.g. stroke) to suppress reflexes to suppress sensory responses

53. Neurotransmitters, mood and behaviour

54. Learning Intentions Continued I can state that endorphins are neurotransmitters that stimulate neurons involved in reducing the intensity of pain I can state that endorphins are also connected to feelings of euphoria, appetite control and release of sex hormones I can state that endorphin production increases in response to severe injury, prolonged exercise, stress and certain foods e.g. chocolate I can describe dopamine as a neurotransmitter that induces feelings of pleasure and reinforces particular behaviours in the reward pathway I can state that some disorders are caused by changes to neurotransmitter release e.g. Alzheimer’s, Parkinson’s and some anxiety disorders I can state that some drugs used to treat neurotransmitters are similar to neurotransmitters e.g. agonists and antagonists I can state that an agonist binds to and stimulates receptors mimicking the neurotransmitter I can state that antagonists bind to specific receptors blocking the action of the neurotransmitter I can state that other drugs inhibit the enzymes which breakdown neurotransmitters or inhibit re-uptake at the synapse

55. Endorphins are chemicals that function like neurotransmitters. They act like natural painkillers by combining with receptors at synapses and blocking the transmission of pain signals. They are produced in the hypothalamus. Endorphin production increases in response to: –severe injury –prolonged and continuous exercise –physical & emotional stress –certain foods (e.g. chocolate and chilli peppers) Endorphins

56. Action of Endorphins

57. Increased levels of endorphins can also bring about other responses within the body, such as: –euphoric feelings (intense happiness) –regulation (modulation) of appetite –release of sex hormones Endorphins

58. Other Ways to Increase Endorphin Production sunlight exercise Listen to music meditation laugh

59. Dopamine is a neurotransmitter produced in several areas of the brain which induces the feeling of pleasure. They can also reduce anxiety and stress. Dopamine is also involve in reinforcing beneficial survival-related behaviour (such as satisfying hunger by eating, thirst or sexual need) by activating the reward pathway. The reward pathway involves neurons which secrete or respond to dopamine. Dopamine

60. Agonists are chemicals that bind to and stimulate specific receptors on the membrane of postsynaptic neurons in a neural pathway. Agonists mimic the action of natural neurotransmitters and so normal cell responses occur (i.e. nerve impulse is transmitted) sometimes at an enhanced level. Agonists

61. Antagonists are chemicals that bind to and block specific receptors on postsynaptic neurons. Antagonists, by blocking the receptor sites, prevent the normal neurotransmitter from acting. Antagonists can greatly reduce or even stop the normal transmission of nerve impulses. Other drugs, known as inhibitors, inhibit the enzymes which degrade neurotransmitters or inhibit re-uptake. Antagonists

62. Agonists & Antagonists

63. Below are some examples of neurotransmitter related disorders: Many drugs which treat neurotransmitter related disorders are similar to neurotransmitters. Neurotransmitter related disorders DisorderCauseTreatment Alzheimer’s disease Loss of cells synthesising acetylcholine. Cholinesterase inhibitors Parkinson’s disease Loss of dopamine synthesising neurons. Monamine oxidase inhibitors and the potential use of adult stem cells SchizophreniaOveractive dopamine systemThe use of dopamine antagonists General anxiety disorders Imbalance in serotonin and norepinephrin The use of GABA agonists and beta blockers DepressionLow levels of serotonin Norepinephrine re-uptake inhibitors and monoamine oxidase enzyme inhibitors

64. Questions 1.State the function of endorphins. 2.Describe the effect of endorphins on the body (i.e. mood) 3.State the factors which result in an increase in endorphin production. 4.State the function of dopamine and its effect on the body. 5.Give examples of neurotransmitter related diseases. 6.Describe the action of agonists and antagonists. 7.Describe the action of inhibitor drugs.

65. Answers 1.State the function of endorphins. Endorphins are neurotransmitters which act like natural painkillers by stimulating neurons which are involved in reducing the intensity of pain. 2. Describe the effect of endorphins on the body (i.e. mood) Euphoric feelings, regulation of appetite and the release of sex hormones

66. Answers (continued) 3. State the factors which result in an increase in endorphin production. Severe injury,prolonged and continuous exercise, physical & emotional stress, certain foods 4. State the function of dopamine and its effect on the body. It is a neurotransmitter which induces the feeling of pleasure. Dopamine is also involve in reinforcing beneficial behaviour by activating the reward pathway.

67. Answers (continued) 5. Give examples of neurotransmitter related diseases. Alzheimer’s disease, Parkinson‘s disease, Schizophrenia, General anxiety disorders, Depression 6. Describe the action of agonists and antagonists. Agonists are chemicals that bind to and stimulate specific receptors on postsynaptic neurons. Antagonists are chemicals that bind to and block specific receptors on postsynaptic neurons 7. Describe the action of inhibitor drugs. Inhibitors, inhibit the enzymes which degrade neurotransmitters or inhibit re-uptake.

68. Mode of action of recreational drugs

69. Learning Intentions Continued I can state that recreational drugs can also mimic neurotransmitters I can explain that as a consequence of taking recreational drugs changes in neurochemistry alter mood, cognition, perception and behaviour I can state that many recreational drugs affect neurotransmission in the reward circuit of the brain I can state that recreational drugs can be either antagonistic or agonistic I can explain that antagonists lead to an increase in sensitivity and number of receptors and as a consequence results in addiction = sensitisation I can explain that agonists lead to a decrease in sensitivity and number of receptors and as a consequence results in drug tolerance = desensitisation

70. Many recreational drugs can mimic the action of neurotransmitters and will affect the transmission of nerve impulses in the reward circuit of the brain. Recreational drugs can stimulate the release of neurotransmitters, act as agonists or antagonists and inhibit their reuptake or enzyme degradation. Recreational drugs

71. Recreational drugs therefore alter a persons neurochemistry and so can lead to changes in: –mood e.g. happier/more confident/more aggressive –cognition person becomes poorer at mental tasks such as problem solving and decision making –perception misinterpretation of environmental stimuli e.g. colurs, sounds, sense of time –behaviour person is able to stay awake for longer and talk about themselves endlessly Recreational drugs

72. Drug addiction is a chronic disease. The sufferer will compulsively seek out and use a drug regardless of the consequences. The initial use of the drug is often voluntary but the changes which occur after use soon override a persons control. Drug tolerance occurs when a persons reaction to an addictive drug decreases in intensity although the concentration is the same. A large dose is then required to bring about the original effect. Drug addiction/tolerance

73. Sensitisation is an increase in the number and sensitivity of neurotransmitter receptors. This occurs as a result of exposure to drugs which are antagonists, which block receptors; the body then responds by increasing the number of these receptors. Sensitisation leads to addiction. Sensitisation

74. Desensitisation is a decrease in the number and sensitivity of neurotransmitter receptors. This occurs as a result of exposure to drugs which are agonists, which stimulate receptors and cause feelings of euphoria. The body responds to this overstimulation by decreasing the number of these receptors and so a larger dose is required to bring about the original effect. Desensitisation leads to drug tolerance. Desensitisation

75. Questions 1.What do recreational drugs mimic? 2.What do changes in neurochemistry caused by recreational drugs cause? 3.Describe the meanings of the terms ‘drug addiction’ and ‘drug tolerance’. 4.Describe the meaning of the term ‘sensitisation’ and explain how this leads to drug addiction. 5.Describe the meaning of the term ‘desensitisation’ and explain how this leads to drug tolerance.

76. Answers 1.What do recreational drugs mimic? the effect of neurotransmitters and will affect the reward circuit in the brain. 2.What do changes in neurochemistry caused by recreational drugs cause? It causes alterations in mood, cognition, perception and behaviour

77. Answers (continued) 3. Describe the meanings of the terms ‘drug addiction’ and ‘drug tolerance’. Drug addiction will compulsively seek out and use a drug regardless of the consequences. Drug tolerance occurs when a persons reaction to an addictive drug decreases in intensity although the concentration is the same.

78. Answers (continued) 4. Describe the meaning of the term ‘sensitisation’ and explain how this leads to drug addiction. Sensitisation is an increase in the number and sensitivity of neurotransmitter receptors. This occurs as a result of exposure to drugs which are antagonists, which block receptors; the body then responds by increasing the number of these receptors which leads to drug addiction

79. Answers (continued) 5. Describe the meaning of the term ‘desensitisation’ and explain how this leads to drug tolerance. This is a decrease in the number and sensitivity of neurotransmitter receptors. This occurs as a result of exposure to drugs which are agonists, which stimulate receptors and cause feelings of euphoria. The body responds to this over stimulation by decreasing the number of these receptors and so a larger dose is required to bring about the original effect. This leads to drug tolerance.