Learning Intention. Name and describe the components of a neuron

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1 c) Neural Pathways

2 Learning Intention Name and describe the components of a neuron

3 Cells of the Nervous System The nervous system consists of a complex network of nerve cells called neurons which receive and transmit electrical signals (nerve impulses) and glial cells which support and maintain the neurons Video

4 Structure of a Neuron node

5 Dendrites Dendrites are nerve fibres which receive nerve impulses and pass them towards a cell body.

6 Cell Body The cell body contains the nucleus and most of the cytoplasm. It is the control centre of cell metabolism Cell body

7 Axon An axon is a single nerve fibre that carries nerve impulses away from a cell body Axon

8 interneurone AXON CELL BODY The direction in which a nerve impulse travels is always: dendrites cell body axon DENDRITE

9 Learning Intention Compare the myelin content of adults and infants

10 Myelin Myelin is the fatty tissue that insulates an axon The presence of myelin greatly increases the speed at which impulses can be transmitted

11 Myelination Myelination is the process by which myelin develops round axon fibres Myelination is not complete at birth and so over the first two years of life many more neurons are myelinated As a result responses to stimuli in the first two years of life are not as rapid as there are slower impulses

12 Myelination Summary Axons are surrounded by a myelin sheath which insulates the axon and increases the speed of impulse conduction from node to node. Myelination continues from birth to adolescence. As a result responses to stimuli in the first two years of life are not as rapid or coordinated as those of an older child or adult. Certain diseases destroy the myelin sheath causing a loss of coordination.

13 Learning Intention State that there are 3 different types of neuron

14 Types of Neuron Sensory neurons, carry impulses into the Central Nervous System (CNS) from sense organs Interneurons, found in the CNS where they connect with other neurons Motor neurons, carry impulses out from the CNS to effectors such as muscles and glands

15 Types of Neuron

16 Learning Intention Discuss the role of glial cells

17 Glial Cells Glial cells do not transmit nerve impulses but physically support the neurons. One type is found in the central nervous system. If damage occurs to the neurons, they multiply and remove debris by phagocytosis

18 Glial Cells Others constantly sample and homeostatically regulate the chemical environment of the neurons, removing excess ions and recycling neurotransmitters so that the neuron functions in very constant conditions. Other types of glial cells are responsible for myelination

19 Glial Cells Summary Glial cells physically support neurons and produce the myelin sheath. They also maintain a homeostatic environment around the neurones and remove debris by phagocytosis.

20 Lesson starter b) Which direction would the impulse move? Why? c) What are the two functions of myelin? d) What are the three types of neuron? e) What are glial cells and what do they do?

21 Learning Intention Describe the structure of a synapse and movement of neurotransmitter

22 Synapses The tiny gap between an axon ending of one neuron and the dendrite of the next neuron in the pathway is called a synapse The plasma membranes of the two neurons are very close but are separated by a space called the synaptic cleft Video

23 Synapses The nerve cell before the synaptic cleft is called the presynaptic neuron synaptic cleft The nerve cell after the synaptic cleft is called the postsynaptic neuron As well as connecting to other each, neurons can also connect with muscle fibers and endocrine glands

24 Neurotransmitter Messages are relayed across synaptic clefts by chemicals called neurotransmitters There are many neurotransmitters passed on at the synapse. Two examples are: ACETYLCHOLINE NORADRENALINE

25 Neurotransmitter Neurotransmitters are stored in vesicles at each presynaptic terminal. When a nerve impulse arrives, the vesicles fuse with the membrane and release neurotransmitter into the cleft The neurotransmitters then diffuse across the cleft and bind to receptors on the postsynaptic membrane

26 Neurotransmitter The vesicles containing neurotransmitters occur on one side of the synapse only. This means nerve impulses travel in one direction only. Video 1 Video 2

27 Learning Intention State the difference between excitatory and inhibitory signals

28 Excitatory and Inhibitory Signals The type of alteration to a postsynaptic membrane that occurs following the binding of a neurotransmitter depends on the type of receptor present The signal generated is determined by the receptor and may be either excitatory or inhibitory

29 Excitatory and Inhibitory Signals Acetylcholine released into the cleft between a motor neuron and a skeletal muscle fibre binds to receptors that have an excitatory effect and make the muscle fibres contract eg peristalsis Acetylcholine released into a cleft between a motor neuron and a heart muscle fibre instead binds with receptors which have an inhibitory effect. This reduces the rate and strength of contraction of heart muscle

30 Neurotransmitter Summary Neurotransmitters relay messages from nerve to nerves within and outwith the brain. Neurons connect with other neurons, muscles fibres and endocrine glands at a synaptic cleft. Neurotransmitters are stored in vesicles and released into the cleft on arrival of an impulse. They diffuse across the cleft and bind to receptors on nerve endings. The receptor determines whether the signal is excitatory or inhibitory.

31 Learning Intention Describe the filtering of weak stimuli and summation

32 Weak Stimuli A nerve impulse is only transmitted across a synapse if there is release of enough neurotransmitter A critical number of neurotransmitter molecules is needed (threshold) to affect a sufficient number of receptors on the postsynaptic membrane This means weak stimuli are filtered out because not enough transmitter molecules reach the next neurone

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34 Summation A series of weak stimuli can combine to trigger enough neurotransmitter to fire an impulse in the post-synaptic neuron, a process known as summation.

35 Curare General name given to several poisons. Acetylcholine is a neurotransmitter that relays messages needed to bring about muscular contraction in vertebrates. When a muscle is wounded by a poison dart or injected with curare, the poison binds with acetylcholine receptors on the postsynaptic membrane of muscle fibres. This results in the neural pathway becoming blocked and no nerve impulses reaching the muscles. These muscles fail to contract and remain relaxed in a state of paralysis. Therefore Curare is described as a muscle relaxant.

36 Learning Intentions Explain the need for and methods of removal of neurotransmitters

37 Removal of Neurotransmitter Between impulses the transmitter molecules are rapidly removed from the synaptic cleft to prevent continuous stimulation of postsynaptic neurones There are 2 types of removal: Re-uptake Enzyme degradation

38 Re-uptake Noradrenaline undergoes reuptake by being reabsorbed directly into the presynaptic membrane that secreted it and is stored in a vesicle ready for use

39 Enzyme Degradation Acetylcholine is broken down by an enzyme into non active products which are then reabsorbed by the presynaptic neuron and resynthesised into active neurotransmitter

40 Removal and Summation Summary Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters. Summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse. Neurotransmitters must be removed from the synaptic cleft to prevent continuous stimulation of post synaptic neurones. Neurotransmitters are removed by enzymes or reuptake.

41 Learning Intention Explain the difference between converging, diverging and reverberating neural pathways

42 Complex Neural Pathways Neurones are found to be connected to one another in many different ways in the CNS Examples of neural pathways are: DIVERGING NEURAL PATHWAY CONVERGING NEURAL PATHWAY REVERBERATING NEURAL PATHWAY

43 Converging Neural Pathways To converge means to come together and meet at a common point. Converging path ways are where two or more neurones feed impulses to one neurone Converging neural pathways increase the sensitivity to excitatory or inhibitory signals.

44 Converging Neural Pathways Rods are visual receptors present in the retina of the eye. They contain pigments which break down in the presence of light. In each case, this breakdown forms a chemical which triggers off nerve impulses along a pathway of neurones

45 Converging Neural Pathways The nerve impulse transmitted by one rod is weak. It would mean not enough neurotransmitter would be released to carry on the impulse. Several rods converge to one rod biploar cell to allow enough neurotransmitter to be released.

46 Diverging Neural Pathways In a diverging neural pathway, the route along which an impulse is travelling divides Diverging neural pathways mean that an impulse in one neurone can have a simultaneous effect in many parts of the body An example of diverging pathways is fine motor control in the fingers

47 Reverberating Pathway Reverberation means a sound that occurs repeatedly, as in an echo In reverberating pathway neurones later in the pathway posses axon branches which synapse with earlier neurones sending the impulse back through the circuit

48 Neural Pathways Summary Converging neural pathways increase the sensitivity to excitatory or inhibitory signals. Diverging neural pathways influence several neurons at the same time. In a reverberating pathway neurones later in the pathway synapse with earlier ones sending the impulse back through the circuit.

49 Learning Intentions Explain the term plasticity Compare minor and major plasticity

50 Development of New Neural Pathways The brain is not hard wired with fixed neural pathways, the neurons undergo change in their synaptic network The brain cells ability to become altered as a result of new environmental experiences is called plasticity of response

51 Plasticity Plasticity of response enables new neural pathways to form, especially during: early development of the brain the learning of new skills response to brain injury

52 Major Plasticity Major plasticity follows brain damage, when undamaged cells form new neural pathways take on the function of the damaged area

53 Major Plasticity in Stroke Victims After a stroke, during the first few months, some sufferers are found to make a significant recovery and regain speech or use of their limb The neurons in the damaged area have not regained their functional state, instead neurons in another region of the brain have formed new neural pathways enabling to take on these jobs

54 Minor Plasticity Minor plasticity is when the brain suppresses reflexes e.g. blinking or suppresses sensory impulses causing distraction from a task

55 Minor Plasticity Plasticity is thought to occur as you have two conflicting messages - one saying to blink and the other not to blink meeting in a convergent pathway If the overall effect at the synapse is excitatory then the nerve impulse is fired and blinking occurs If the overall effect is inhibitory then no impulse is fired and blinking fails to occur This explains why some people can resist blinking whilst others can t help themselves

56 Minor Plasticity If a person is given a task to do that requires a lot of concentration and is subjected to auditory and visual distractions, some people are good at suppressing the sensory impulses from the distractions and perform well each time. Other people find it hard to block out the sensory impulses

57 Plasticity Summary New neural pathways can be developed to create new responses, bypass areas of brain damage or to suppress reflexes or responses to sensory impulses creating a plasticity of response.

58 Blinking Experiment Your partner will have 10 attempts to make you blink using a sterile plastic dropper secured in a clamp stand You will have 10 seconds between each trial to summon maximum willpower Compare your results: who was good at suppressing this reflex action? Who couldn t resist blinking?

59 Learning Intentions Describe the role of endorphins Describe the role of dopamine and the reward pathway

60 Endorphins Endorphins are neurotransmitters that stimulate neurones involved in reducing the intensity of pain Increased levels are also connected with euphoric feelings, appetite modulation and release of sex hormones.

61 Endorphins Endorphin production increases in response to: severe injury prolonged and continuous exercise stress certain foods

62 Dopamine and Reward Pathway The reward pathway involves neurones which secrete or respond to the neurotransmitter dopamine Dopamine induces the feeling of pleasure and reinforces particular behaviours

63 Dopamine and Reward Pathway The neurons of the reward pathway are located in the mid-brain below the cortex, and link to the areas at the base of the cortex and in the frontal areas of the cortex. The reward pathway is activated on engagement of beneficial behaviours, eg eating when hungry

64 Endorphins Summary Endorphins are neurotransmitters that stimulate neurones involved in reducing the intensity of pain. Increased levels are also connected with euphoric feelings, appetite modulation and release of sex hormones. Endorphin production increases in response to severe injury, prolonged and continuous exercise, stress and certain foods.

65 Dopamine Summary The reward pathway involves neurones which secrete or respond to the neurotransmitter dopamine. The reward pathway is activated on engagement of beneficial behaviours, eg eating when hungry. Dopamine induces the feeling of pleasure and reinforces particular behaviours.

66 Learning Intentions Discuss the causes, symptoms and treatment of neurotransmitter disorders Describe the mode of action of agonist, antagonist and inhibitor drugs

67 Neurotransmitter Disorders Many drugs used to treat neurotransmitter related disorders are similar to neurotransmitters. Neurotransmitter disorders include Alzheimer s disease Parkinson s disease Schizophrenia

68 Antagonists bind to specific receptors blocking the action of the neurotransmitter. Agonists and Antagonists Agonists bind to and stimulate receptors mimicking the neurotransmitter and triggering a normal cellular response

69 Inhibitors Other drugs inhibit the enzymes (e.g. cholinesterase) which degrade neurotransmitters (e.g. acetylcholine) Or inhibit re-absorption of the neurotransmitter (e.g. noradrenalin)

70 Neurotransmitter Disorder Summary Many drugs used to treat neurotransmitter related disorders are similar to neurotransmitters. Agonists bind to and stimulate receptors mimicking the neurotransmitter. Antagonists bind to specific receptors blocking the action of the neurotransmitter. Other drugs inhibit the enzymes which degrade neurotransmitters or inhibit reuptake.

71 Learning Intention Describe the mode of action of recreational drugs

72 Recreational Drugs Many recreational drugs affect neurotransmission in the reward circuit of the brain. They cause changes in neurochemistry leading to: changes in mood cognition perception behaviour

73 Recreational Drugs Recreational drugs interact with neurotransmitters in different ways, they can: stimulate the release of neurotransmitters imitate the action (agonists) block their binding to receptors(antagonists) inhibit their re-uptake inhibit their breakdown by enzymes

74 Recreational Drugs Summary Many recreational drugs affect neurotransmission in the reward circuit of the brain. Changes in neurochemistry alter mood, cognition, perception and behaviour can occur. Recreational drugs may stimulate the release of neurotransmitters, imitate their action (agonists), block their binding (antagonists), and/or inhibit their re-uptake/enzymatic degradation.

75 Learning Intention Explain drug desensitisation and sensitisation

76 Drug Addiction Drug addiction can be defined as a chronic disease that causes the sufferer to compulsively seek out and use the drug regardless of the consequences

77 Drug Desensitisation Repeated use of a drug that acts as an agonist results in neurotransmitters (eg those that promote dopamine release) being repeatedly stimulated and increased feelings of wellbeing and euphoria

78 Drug Desensitisation The nervous system compenstates for overstimulation of these receptors by reducing their number and those receptors left become less sensitive to the agonist drug

79 Tolerance A larger dose is needed to stimulate the reduced number of less sensitive receptors in order to gain the original effect this is called tolerance

80 Drug Sensitisation Repeated use of a drug that acts as an antagonist by blocking neuroreceptors prevents normal neurotransmitter from acting on them.

81 Drug Sensitisation The nervous system compensates for the reduced stimulation of the receptors by increasing their number and the receptors become more sensitive to the antagonist drug

82 Addiction Sensitisation results in other psychological changes, which transform ordinary sensations of wanting into addiction

83 Remember Desensitisation is a decrease in the number and sensitivity of receptors as a result of exposure to drugs that are agonists and leads to drug tolerance. Sensitisation is an increase in the number and sensitivity of neurotransmitter receptors as a result of exposure to drugs that are antagonists and leads to addiction

84 Sensitisation and Desensitisation Summary Sensitisation and desensitisation are thought to underlie drug addiction and tolerance. Sensitisation is an increase in the number and sensitivity of neurotransmitter receptors as a result of exposure to drugs that are antagonists and leads to addiction. Desensitisation is a decrease in the number and sensitivity of receptors as a result of exposure to drugs that are agonists and leads to drug tolerance

85 Noradrenaline is the main neurotransmitter of the sympathetic nerves in the cardiovascular system. Noradrenaline produces wide ranging effects on many areas of the body and is often referred to as a 'fight or flight' chemical, as it is responsible for the body's reaction to stressful situations. Noradrenaline normally produces effects such as increased heart rate, increased blood pressure, widening of pupils, widening of air passages in the lungs and narrowing of blood vessels in non-essential organs. This enables the body to perform well in stressful situations. Noradrenaline works by stimulating receptors called adrenoceptors, which are found all over the body. When injected into a vein, noradrenaline acts mostly on a type of adrenoceptor known as an alpha receptor. These are found on muscle inside the walls of peripheral blood vessels. Stimulating these alpha receptors causes the muscle to contract, which makes the blood vessels constrict and narrow. Adrenaline metabolic and homeostasis Noradrenaline involved in the sympathetic nervous and cardiovascular system