The Neuron

1. Biological Level of Analysis - Neurotransmission The Neuron

2. Brain Cells • Your brain is made up of cells. • Brain cells come in two types: • neurons, which talk to one another and to the rest of the body, and • glial cells, which provide essential support to keep the whole show going. • There about 100 billion neurons and many more glial cells.

4. The Neuron Signals within a neuron are carried by electricity. To send electrical signals from one part of the neuron to another, the neuron opens channels that allow the ions to move across the membrane, creating a current that carries an electrical signal down the membrane. Neurons receive inputs through branched, treelike structures called dendrites, which put together information from a bunch of different sources. The neuron then sends an electrical signal down a long, wirelike structure, called an axon, which triggers a chemical signal to another neuron, and so on.

5. The Neuron Axons carry signals over long distances; your longest axons run from your spine to the tips of your toes. Each neuron in the brain receives chemical signals from some neurons and sends chemical signals to others. Communication between neurons relies on chemicals called neurotransmitters, which are released from small areas at the end of the axon when triggered by the arrival of a spike. Spikes are sudden increases in the electrical currents in a neuron.

6. The Neuron Every neuron makes and receives up to several hundred thousand chemical connections, called synapses, with other neurons. Neurotransmitters stick to synaptic receptors on the dendrites or cell bodies of another neuron, triggering further electrical and chemical signals. All of these steps, from release to detection, can take place in a thousandth of a second.

7. The Neuron For the brain to accomplish its many duties, neurons have to take on very specific tasks. Each neuron responds to a small number of events, such as hearing a particular sound, seeing someone’s face, carrying out a certain movement. At any given moment, only a small fraction of your neurons, distributed all over your brain, are active. This fraction is ever shifting; the whole game of thinking depends on which neurons are active and what they are saying to each other and to the world.

8. Nucleus Axon Hillock

9. The Neuron Sensory neurons carry signals from the outer parts of your body (periphery) into the central nervous system. They are specified in the senses of taste, touch, hearing, smell, and sight. They send messages from the sensory receptors to the Central Nervous System. Motor neuronscarry signals from the central nervous system to the outer parts (muscles, skin, glands) of your body. Control muscle contractions. Interneuronsconnect various neurons within the brain and spinal cord. They carry information between motor and sensory neurons.

10. The Neuron Neurons transmit electrical signals called action potentials. Action potentials work a bit like a ‘Mexican wave’ The presence of a myelin sheath on a neuron changes the rate of transmission.

11. Neurons

14. Synapses Synapses are the essential components of communication in your brain. Your thought patterns, basic abilities and functions, and individuality are determined by how strong these synapses are, how many of them you have, and where they are. Neurons use synapses to talk to each other within the brain. Only a small fraction of axons form their synapses outside the brain or spinal cord, sending signals to other organs of the body, including muscles.

16. Neurotransmitters Each neuron has direct synaptic contact with several thousand other neurons. Neurons do not touch each other, instead contact is make through neurotransmitters. Neurotransmitters are the chemical messengers used by neurons to communicate with each other. It has been estimated that there are over 100 neurotransmitters.

17. Transmission at the synapse • Neurotransmitters are stored in the vesicles inside the terminal button of the axon; the vesicles are transported to the edge of the button – then released into the synaptic gap • In the synapse – the neurotransmitter can bind with a receptor if the receptor site is the right type and is vacant. If enough of the neurotransmitter binds to the receptor site, the neuron will ‘fire’ – transmit information across the cell body electrically. • If the neurotransmitter is blocked or replaced, then the messages change. This affects the physiological system – cognition, mood, behaviour

18. Transmission at the synapse • When the information is received at the end of the axon neuron, this chemical process is repeated. • Unused neurotransmitter is eventually absorbed back into the neuron it came from (or enzymes will remove it from the synaptic cleft). This is called reuptake. • Certain drugs can be introduced to the nervous system to encourage or prevent the production or release of neurotransmitters.

19. Transmission at the synapse • Certain drugs can occupy receptor sites that would normally receive a neurotransmitter. This has an effect on the receiving neuron as well as preventing the naturally occurring intended communication between neurons. • Certain drugs prevent the reuptake of neurotransmitters – allowing more time to bind to receiving neurons

20. Common Neurotransmitters • Read Information Box in textbook, pg27. • Monoamines: • Acetylcholine (ACH) – involuntary muscle movement, learning memory and sleep • Norepinephreine (NE) – controls sympathetic nervous system, involved in eating and alertness • Dopamine (DA) – involved with movement, attention, learning and memory. • Serotonin (5-HT) – involved in inducing sleep, sensory perception, temperature regulation, control of mood, appetite and aggressive behaviour • Amino Acids: • GABA – most common and is involved in most aspects of brain functioning – from memory to sleep. • Peptides: there are over 50 peptide neurotransmitters

21. Neurotransmitters - Dopamine • Neurotransmitter involved in motivation (pleasure seeking), control of movement, emotional response and addictive behaviour • Addictive drugs increase the levels of Dopamine • Nicotine – the psychoactive ingredient in tobacco – increases dopamine in the rewards circuit creating a relaxing effect

22. Neurotransmitters - Dopamine • L-Dopa – treatment of Parkinson’s disease – triggers release of dopamine n the motor cortex • Chlorpromazine (and other anti-psychotics) – reduces symptoms of Schizophrenia by reducing dopamine levels • Cocaine – intense feelings of pleasure and faster cognitive activity – prevents reuptake of Dopamine

23. Dopamine in pleasure seeking – Berridge & Kringelbach (2009) • fMRI scans used to study brain areas involved in the subjective experience of pleasure. • Found the orbitofrontal cortex was active when people reported feeling pleasure

24. Dopamine in pleasure seeking – Berridge & Bringelbach (2009) • Researchers concluded that: dopamine & the nucleus accumbens is involved in pleasure seeking. This could explain addictive behaviour. • The orbitofrontal cortex and endorphins perhaps linked to the subjctive experience of pleasure.

25. Neurotransmitters -Acetylcholine • Earliest neurotransmitter discovered in the 1930s – released by motor neurons to activate muscle fibres • Along with other – it forms a part of the cholinergic system associated with higher cognitive function (via interneurons) • Thus acetylcholine is linked to effects with muscle contraction as well as the plasticity of the hippocampus – and therefore development of memory • Martinez & Kesner (1991) – see Crane p41

26. Acetylcholine - Martinez & Kesner (1991) • Aim: to investigate the role of ACh in memory formation. • Used rats trained to run a maze. Divided into three groups: • Group 1 – received scopolamine (blocks ACh receptor sites inhibiting release of ACh) • Group 2 - received physostigmine (blocks production of enzyme cholinesterase which cleans up ACh – leading to more Ach in the synaptic gap • Group 3 – Control group

27. Acetylcholine - Martinez & Kesner (1991) Results: • Group 1 – problems finding way through the maze & made more mistakes • Group 2 – ran quickly through the maze & made few mistakes. Faster than the control (group 3) Evaluation • Shows Ach is imporant in memory – rats showed different memory capacity depending on Ach level. Ach is ONE factor that affects memory – but neurobiology of memory is complex

28. Hormones - Oxytocin • The hormone oxytocin is secreted by the hypothalamus and released (1) into the blood stream via the pituitary gland or • (2) into the brain and spinal cord where it binds to oxytocin receptors. • Oxytocin acts primarily as a neurotransmitter • Oxytocin has been linked to trusting other people. • Experimental manipulation of oxytocin levels has shown increase in trust. • According to evolutionary psychologists, trust is an important social tool in the relationship between humans.

29. Hormones - Oxytocin • Trust is an adaptive mechanism as it helps humans to form meaningful relationships at a personal and professional level. Betrayal disrupts bonds of trust and may result in avoidance of the person who has betrayed you. • Learning who to trust and who to avoid is important for survival and the well-being of an individual. Humans should also be able to move on after experiences of breaching trust if long-term relationships and mental well-being are to be preserved. • Oxytocin could play a role in reducing fear reactions via the amygdala that may arise as a consequence of betrayal.

30. Baumgartner et al. (2008) The role of oxytocin in trust ineconomic behaviour Aim • To investigate the role of oxytocin after breaches of trust in a trust game. Procedure • The participants played a trust game used by economists and neuroscientists to study social interaction. • The "investor" (player 1) receives a sum of money and must decide whether to keep it or share it with a "trustee" (player 2). If the sum is shared the sum is tripled. Then player 2 must decide if this sum should be shared (trust) or kept (violation of trust).

31. Baumgartner et al. (2008) The role of oxytocin in trust ineconomic behaviour • fMRI scans were carried out on 49 participants. They received either oxytocin or placebo via a nasal spray. • Participants played against different trustees in the trust game and against a computer in a risk game. In 50% of the games their trust was broken. They received feedback on this from the experimenters during the games.

32. Baumgartner et al. (2008) - Results • Participants in the placebo group were likely to show less trust after feedback on betrayal. They invested less. • Participants in the oxytocin group continued to invest at similar rates after receiving feedback on a breach of trust. • The fMRI scans showed decreases in responses in the amygdala and the caudate nucleus. The amygdala is involved in emotional processing and has many oxytocin receptors. The caudate nucleus is associated with learning and memory and plays a role in reward-related responses and learning to trust.

33. Baumgartner et al. (2008) - Evaluation • Oxytocin could explain why people are able to restore trust and forgive in long-term relationships. • Scanner research is merely mapping brain activity but nothing definite can be said about what it really means at this point in science. • Giving oxytocin like this in an experiment may not reflect natural physiological processes. The function of oxytocin is very complex

34. Hormones - Cortisol • Cortisol is a hormone produced by the adrenal cortex in response to stress and to restore homeostasis (the body’s normal balance). Chronic stress may result in prolonged cortisol. • It increases blood sugar and suppresses the immune system • Has been associated with both depression and memory problems. • MRI shows that elevated levels of cortisol can damage the hippocampus

35. Newcomer et aL ( 1999) Experiment on cortisol and memory • Aim:To investigate how levels of cortisol interfere with verbal declarative memory. • Procedure: A self-selected sample (recruited through advertisement)of 51 normal and healthy people aged 18-30 was used. • It was a randomized, controlled, double-blind experiment running for four days. • All participants gave informed consent.

36. Newcomer et aL( 1999) Experiment on cortisol and memory • There were three experimental conditions: • 1. A high level of cortisol (tablet of 160 mg per day), equivalent to cortisol levels in the blood as a consequence of a major stressful event. • 2. A low level of cortisol (tablet of 40 mg per day), equivalent to cortisol levels in the blood as a consequence of a minor stressful event. • 3. A placebo (tablet of no active ingredient).

37. Results • The high-level group performed worse on the verbal declarative memory test than the low-level group. • They performed below placebo levels after day 1. • The low-level group (mild stress) showed no memory decrease.

38. Evaluation • This was a controlled randomized experiment so it was possible to establish a cause-effect relationship between levels of cortisol and scores on a verbal declarative memory test. • Ethical issues were observed with informed consent. The negative effect of taking high dosages of cortisol was reversible so no harm was done.

39. Stress – The General Adaptation Syndrome (GAS) • Stress as a Psychobiological Process • Hans Selye (1956) first argued for the General Adaptation Syndrome - a model of stress reponse after experimenting on laboratory animals involving a variety of stressors including toxins, physical restraint, extreme heat and bacterial infections. • The GAS model proposes that there is the same bodily response to ALL stressors. • The Syndrome refers to a typical combination of factors that make up the stress response.

40. Stress – The General Adaptation Syndrome (GAS)

41. GAS – Stage 1 - ALARM • Alarm mobilises the body for swift action - such as the 'fight or flight' response. • Occurs over seconds or minutes in response to a sudden stressor • Heart rate, blood pressure increase, increase in blood sugar from the liver for extra energy • Increased tension in the muscles • Respiration deepens • Blood increases ability to clot • Very severe response can be life threatening - a heart attack can occur when the body goes into shock. • If stressor disappears or body returns to resting level - adaptation has been successful

42. GAS – Stage 2 - Resistance • Occurs when stressor persists • Hormones from pituitary gland and adrenal cortex increase • Alarm systems disappear giving appearance of return to normal physiological functioning - but arousal is still high as resistance to stressor is raised

43. GAS – Stage 3 - Resistance • Occurs when stressor persists but the body’s defencescan no longer cope. • Adrenal glands cease to function normally • Blood sugar levels drop • Symptoms that occur with alarm stage such as raised blood pressure can appear irreversibly • Followed by a massive decline in resistance • Can lead to Psychosomatic illnesses (such as stomach ulcers, heart disease, hypertension)