lecture 10 - cognition and chemicals Flashcards

Question

The four principal parts of a neuron ;

Answer 1

1) The soma, or cell body, is the largest part of the neuron and contains the mechanisms that control the metabolism and maintenance of the cell. The soma also receives messages from other neurons. 2) The dendrites, the tree-like growths attached to the soma, function principally to receive messages from other neurons (dendron means ‘tree’). They transmit the information they receive down their ‘trunks’ to the soma. 3) The nerve fibre, or axon, carries messages away from the soma towards the cells with which the neuron communicates. These messages, called action potentials, consist of brief changes in the electrical charge of the axon. For convenience, an action potential is usually referred to as the firing of an axon. Many axons, especially long ones, are insulated with a substance called myelin which is white and gives some parts of the brain its whiteish appearance. The principal function of myelin is to insulate axons from each other and thus to prevent the scrambling of messages. It also increases the speed of the action potential. The immune systems of people who have multiple sclerosis attack a protein in the myelin sheath of axons, stripping it away. Although most of the axons survive this assault, they can no longer function normally, so, depending on where the damage occurs, people who have multiple sclerosis suffer from various sensory and motor impairments. The terminal buttons are located at the ends of the ‘twigs’ that branch off the ends of axons. Terminal buttons secrete a chemical called a transmitter substance whenever an action potential travels down the axon, that is, whenever the axon fires. These chemicals are called neurotransmitters. The transmitter substance affects the activity of the other cells with which the neuron communicates. Thus, the message is conveyed chemically from one neuron to another. Most drugs that affect the nervous system and hence alter a person’s behaviour do so by affecting the chemical transmission of messages between cells

Answer 2

* In the 1950s, Joseph Altman discovered that new neurons in some parts of the brain – specifically, the hippocampus and the olfactory bulb of rats – could be created. * The term for the creation of new neurons is neurogenesis. The type of neuron generated is a granule cell and these cells are found primarily in a specific part of the brain called the dendate gyrus of the hippocampus (Snyder and Cameron, 2012). * The first demonstration of neurogenesis in humans came in 2009 following the development of a new technique which used a chemical called BrdU, which made it easier to count cells in the brain (Eriksson et al, 1998). * The creation of these cells is seen in many mammals, including humans, and their production can be altered by environmental factors, such as stressors and even physical activity such as running or walking (Leuner and Gould, 2010; Schoenfeld and Gould, 2012; Spalding et al, 2013). * This idea that the creation of new cells can result from an environmental intervention is an example of neuroplasticity – the notion that the structure of the brain can be altered by behavioural and environmental factors (Opendak and Gould, 2015). * Experiments have shown that mild types of stressor can enhance neurogenesis but that unpredictable, chronic and intense periods of stress produced, for example, by sleep deprivation or electric shock, can inhibit it (Snyder et al, 2009; Lucassen et al, 2010). Some studies have found that parenting in non-human mammals can reduce neurogenesis (Leuner et al, 2010), probably because, despite being rewarding, looking after offspring is very stressful.

Answer 3

* One mechanism which may underpin this reduction is the inhibition of progenitor cells. That is, the cells that lead to the creation of new neurons are impaired by the experience of stress (Schoenfeld and Gould, 2012). The ultimate cause of this inhibition is likely to be the production of glucocorticoids, stress-chemicals which are released when an organism experiences stress * Glucocorticoids exert a number of negative effects on behaviour if their release is prolonged; people’s immune system works ineffectively, for example, or their memory and ability to navigate their way around an environment becomes impaired (Conrad, 2008). * Stress also exerts effects on the shape and size of some elements of existing neurons; the complexity of dendrites is reduced, for example, as is the number of spines on dendrites and the number of synapses in the hippocampus (McEwen, 2012). * The opposite response – a behaviour leading to neurogenesis – has been found in animals when they engage in physical activity such as exercise or sex and even when they live in an enriched environment or are administered cocaine (Opendak and Gould, 2015). For example, in non-humans such as rodents, running on a treadmill or a wheel has been associated with increased numbers of neurons in the hippocampus (Vivar and Van Praag, 2013) although there is evidence that if the running is done alone or the physical exercise is very intense then neurogenesis does not happen (Opendak and Gould, 2015). Whether such effects can be produced in humans – where the rewarding nature of the running depends on individual differences and our motivation – is not known. * Whether new cells are function-specific or just add to the pool of neurons that can be recruited to perform a range of behaviours is also unknown. * In their review, Opendak and Gould (2015) noted that when new cells are created as a result of spatial navigation learning, these cells are more responsive when they are exposed to spatial cues than other types of cue, suggesting that they may have a degree of specificity. The type of function that cells have also seems to be determined by where the cells are created. * The other important question is: why are these new cells produced in the first place? According to Opendak and Gould (2015), the hippocampus’s ability to generate new cells which can be altered by environmental factors ‘strongly suggests that new neurons are a substrate for sculpting the brain to produce behaviours that are adaptive for the organism’ (p. 6). * They go on to argue that the reduction of neurogenesis can also be an advantage because the stress which creates this leads to increased anxiety and reduced exploration, which might help survival. It is an intellectual leap, but it is one explanation. There is also some evidence to suggest that once the cells are created they do not simply survive; they have to be used. For example, it is not enough for an animal to be trained for the cells to continue; the animal has to learn. If it learns, the new cells are more likely to survive (Shors, 2014)

Answer 4

There are currently around 50, or so, identifiable neurotransmitters and all are important to behaviour in some way. Some, however, play a greater role than others. The amine group of neurotransmitters, which includes dopamine, noradrenaline and serotonin (5-hydroxytryptamine), appears especially important to psychologists because they are involved in a range of behaviours – emotional expression, decision-making, response to reward, inhibiting inappropriate actions, drug-taking and many others (Azizi, 2020). Their effect on nerve cells is binary: they can inhibit, or they can excite. neurotransmitter table in notes * A part of the brain called the nucleus accumbens appears to be important in this respect and seems to be part of a reward system located in the front and midpart of the brain. It evaluates how salient or important events in the outside world are. It and the system it belongs to are also implicated in impulsive behaviour – the inability to delay reward and be aware of the consequences of actions, and so on (Pothuizen et al, 2005) – and drug addiction (Russo et al, 2010; Everitt and Robbins, 2013). * Norepinephrine and dopamine are thought to increase attention; drugs acting on these are used to manage attention disorders (Cortese, 2020). Drugs which act on serotonin receptors increase impulsivity (Pattij and Vanderschuren, 2008) and this impulsivity, according to some scientists, is linked to the initiation and maintenance of drug-seeking (Krishnan-Sarin et al, 2007). There is also evidence that there is an increase in the density of the spines of dendrites in the nucleus accumbens when cocaine or heroin is administered (Shen et al, 2009; Maze et al, 2010). * The events to which the nucleus accumbens responds include rewarding stimuli (food, water, sex), aversive stimuli (ice, shock) and novel stimuli. Dopamine release increases here, as does the firing of dopamine receptors when an organism is rewarded with food or water (Iversen and Iversen, 2007; Schultz, 2007).Dopamine can increase by as much as 20–100 per cent and last for up to 100 minutes (Schultz, 2007). * If an organism expects a reward, then there is a release of dopamine in these regions, but if an organism is completely rewarded there seems to be little dopamine activation. Dopamine is also released in freely moving organisms, which suggests that it is important for motor movement and the motivation to move. * Of course, the movement disorder Parkinson’s disease, discussed a little later, is treated by a dopamine precursor (called Levodopa), and excessive dopamine is thought to be implicated in some of the symptoms of schizophrenia * Like all the amines, dopamine receptors begin projecting from the brain stem and certain dopamine receptors terminate (end their projections) in the front part of the brain, called the prefrontal cortex (PFC), described below. These receptors, called D1 receptors, appear to be very important to cognitive performance and influence tasks such as our ability to store and manipulate non-verbal information over very short periods of time – a type of memory called working memory Some people have a mutation of an allele which directly affects dopamine (by deactivating it) with the consequence that their cognitive function is impaired (Tunbridge et al, 2006). Another type of dopamine receptor, D2, seems to be reduced in the striatum of drug addicts (Kalivas and Volkow, 2005).

Answer 5

* The message carried by the axon – the action potential – involves an electrical current, but it does not travel down the axon the way electricity travels through a wire. Electricity travels through a wire at hundreds of millions of metres per second. The axon transmits information at a much slower rate – less than 100 metres per second. * The membrane of an axon is electrically charged. When the axon is resting (i.e. when no action potential is occurring), the outside is charged at +70 millivolts (mV, thousandths of a volt) with respect to the inside. * An action potential is an abrupt, short-lived reversal in the electrical charge of an axon. This temporary reversal begins at the end of the axon that attaches to the soma and is transmitted to the end that divides into small branches capped with terminal buttons. * The electrical charge of the axon occurs because of an unequal distribution of positively and negatively charged particles inside the axon and in the fluid that surrounds it. These particles, called ions, are produced when various substances – including ordinary table salt – are dissolved in water. * Normally, ions cannot penetrate the membrane that surrounds axons. However, the axonal membrane contains special submicroscopic proteins that serve as ion channels or ion transporters. Ion channels can open or close; when they are open, a particular ion can enter or leave the axon. Ion transporters work like pumps. They use the energy resources of the cell to transport particular ions into or out of the axon. * The outside of the membrane is positively charged (and the inside is negatively charged) because the axon contains more negatively charged ions and fewer positively charged ions. When an axon is resting, its ion channels are closed, so ions cannot move into or out of the axon. * An action potential is caused by the opening of some ion channels in the membrane at the end of the axon nearest to the soma. The opening of these ion channels permits positively charged sodium ions to enter, which reverses the membrane potential at that location. This reversal causes nearby ion channels to open, which produces another reversal at that point. The process continues all the way to the terminal buttons located at the other end of the axon. Note that an action potential is a brief reversal of the membrane’s electrical charge. As soon as the charge reverses, the ion channels close and another set of ion channels opens for a short time, letting positively charged potassium ions out of the axon. This outflow of positive ions restores the normal electrical charge. Thus, an action potential resembles the ‘Mexican wave’ that football fans often make in a stadium. People in one part of the stadium stand up, raise their arms over their heads, and sit down again. People seated next to them see that a wave is starting, so they do the same – and the wave travels around the stadium. Everyone remains at the same place, but the effect is that of something circling in the stands around the playing field. Similarly, electricity does not really travel down the length of an axon. Instead, the entry of positive ions in one location reverses the charge at that point and causes ion channels in the adjacent region to open, and so on. The ion transporters pump sodium ions out of the axon and pump potassium ions back in, restoring the normal balance

Answer 6

* Neurons communicate with other cells by means of synapses. A synapse is the conjunction of a terminal button of one neuron and the membrane of another cell – neuron, muscle cell or gland cell. * The terminal button belongs to the presynaptic neuron – the neuron that sends the message. When terminal buttons become active, they release a chemical called a transmitter substance. The neuron that receives the message (detects the transmitter substance) is called the postsynaptic neuron. * A neuron receives messages from many terminal buttons, and in turn its terminal buttons form synapses with many other neurons. * thousands of terminal buttons can form synapses with a single neuron * A motor neuron is one that forms synapses with a muscle and controls its contractions. When the axon of a motor neuron fires, all the muscle fibres with which it forms synapses will contract with a brief twitch. * A muscle consists of thousands of individual muscle fibres. It is controlled by many motor neurons, each of which forms synapses with different groups of muscle fibres. The strength of a muscular contraction, then, depends on the rate of firing of the axons that control it. If they fire at a high rate, the muscle contracts forcefully; if they fire at a low rate, the muscle contracts weakly.

Answer 7

* There are broadly two types of synapse: excitatory synapses and inhibitory synapses. Excitatory synapses do just what their name implies. When the axon fires, the terminal buttons release a transmitter substance that excites the postsynaptic neurons with which they form synapses. The effect of this excitation is to make it more likely that the axons of the postsynaptic neurons will fire. Inhibitory synapses do just the opposite. When they are activated, they lower the likelihood that the axons of the postsynaptic neurons will fire. * The activity of the synapses on the dendrites and soma of the cell determines the rate at which a particular axon fires. If the excitatory synapses are the more active, then the axon will fire at a high rate. If the inhibitory synapses are the more active, then the axon will fire at a low rate or perhaps not at all. * How do molecules of transmitter substance exert their excitatory or inhibitory effect on the postsynaptic neuron? When an action potential reaches a terminal button, it causes the terminal button to release a small amount of transmitter substance into the synaptic cleft, a fluid-filled space between the terminal button and the membrane of the postsynaptic neuron. * The transmitter substance causes reactions in the postsynaptic neuron that either excite or inhibit it. These reactions are triggered by special submicroscopic protein molecules embedded in the postsynaptic membrane called receptor molecules. * A molecule of a transmitter substance attaches to a receptor molecule in the way that a key fits in a lock. After their release from a terminal button, molecules of transmitter substance find their way to the receptor molecules, attach to them and activate them. * Once they are activated, the receptor molecules produce excitatory or inhibitory effects on the postsynaptic neuron. They do so by opening ion channels. The ion channels found at excitatory synapses permit sodium ions to enter the neuron; those found at inhibitory synapses permit potassium ions to leave it * The excitation or inhibition produced by a synapse is short-lived; the effects soon pass away, usually in a fraction of a second. At most synapses, the effects are terminated by a process called reuptake. The transmitter substance is released by the terminal button and is quickly taken up again. It has, therefore, only a short time to stimulate the postsynaptic receptor molecules. * The rate at which the terminal button takes back the transmitter substance determines how prolonged the effects of the chemical on the postsynaptic neuron will be. The faster the transmitter substance is taken back, the shorter its effects will be on the postsynaptic neuron. As we will see, some drugs affect the nervous system by slowing down the rate of reuptake, thus prolonging the effects of the transmitter substance

Answer 8

* Terminal buttons excite or inhibit postsynaptic neurons by releasing transmitter substances. These chemicals travel a very short distance and affect receptor molecules located on a small patch of the postsynaptic membrane. But some neurons release chemicals that get into the general circulation of the brain and stimulate receptor molecules on many thousands of neurons, some located at a considerable distance away. * The chemicals these neurons release are called neuromodulators, because they modulate the activity of the neurons they affect. * We can think of neuromodulators as the brain’s own ‘drugs’. Because these chemicals diffuse widely in the brain, they can activate or inhibit many different circuits of neurons, thus exerting several behavioural and physiological effects. These effects act together to help achieve a particular goal. * The best-known neuromodulator is a category of chemicals called endorphins, or opioids (‘opium-like substances’). * Opioids are neuromodulators that stimulate special receptor molecules (opioid receptors) located on neurons in several parts of the brain. Their behavioural effects include decreased sensitivity to pain and a tendency to persist in ongoing behaviour. Opioids are released while an animal is engaging in important species-typical behaviours, such as mating or fighting. The behavioural effects of opioids ensure that a mating animal or an animal fighting to defend itself is less likely to be deterred by pain; thus, conception is more likely to occur, and a defence is more likely to be successful. * Many years ago, people discovered that eating or smoking the sap of the opium poppy decreased their sensitivity to pain, so they began using it for this purpose. * They also discovered that the sap produced pleasurable effects: people who took it enjoyed the experience and wanted to take more. * In recent times, chemists have discovered that the sap of the opium poppy contains a class of chemicals called opiates. They also learned how to extract and concentrate them and to produce synthetic versions with even greater potency. * In the mid-1970s, neurobiologists learned that opiates produce their effect by stimulating special opioid receptor molecules located on neurons in the brain (Pert et al, 1974). Soon after that, they discovered the brain’s opioids (Terenius and Wahlström, 1975). Thus, opiates mimic the effects of a special category of neuromodulators that the brain uses to regulate some types of species-typical behaviours. The brain produces other neuromodulators. Some help organise the body’s response to stress, while others reduce anxiety and promote sleep. Some promote eating, while others help end a meal.