The Central Nervous System and Human Behaviour
The following report provides a basic introduction to the central nervous system and how it affects behaviour. There are two communication systems—the central nervous system (CNS) and the hormonal system. Part 1 describes neuronal communication in the CNS, while Part 2 illustrates how human behaviour—mood, emotion, motivation, aggression and ideomotor control—is affected by neuronal systems in the brain.
Neurons are cells which transmit and process information. There are two main components of the neuron—the ‘cell body’, which contains the nucleus, and a long extension called ‘a process’. Neurons are found throughout the body. Neuronal transmission can be subdivided into two discrete structures—the central nervous system (CNS), which represents the largest part of the nervous system, and the peripheral nervous system, which controls the communication of neurons outside the CNS. The CNS, contained within the dorsal cavity, comprises the brain (cranial cavity) and the spinal cord (spinal cavity).
Toates (2007) describes basic neuronal activity in the context of a simple reflex reaction. He points out that, when someone’s foot comes into contact with a sharp object, the neurons at the skin’s surface act as detectors, and, once stimulated, an electrical-chemical reaction takes place while messages are immediately directed towards the spinal cord and onto the brain.
In this example, for a split second, there is a significant increase in electrical activity in the foot—viz., the amount of voltage in a certain number of neurons in the skin of the foot has increased. The sudden change in electrical excitation, and its return to a base value, is called action potential. Action potentials travel incredibly quickly. These neurons, which convey information to the CNS through the spinal cord and then on to the brain, are sensory neurons: the brain then interprets the messages as pain. These messages, in most cases, will lead to an appropriate motor response—i.e., the person will take his foot away from the offending object. The action potential in this neuron will initiate muscle movement. These neurons are known as motor neurons.
Neurons communicate information through the synapse—a minute gap between cells. This is known as synaptic transmission. In this process, where one neuron passes on information to another neuron, the first neuron (‘sending neuron’) is known as the pre-synaptic neuron, while the second (‘receiving neuron’) is referred to as the post-synaptic neuron. It is important to note that, although the transmission of information in the brain is electrical, neuronal communication is a chemical conduction (Toates, 2007). When the action potential reaches the synapse, it releases neurotransmitters (chemical transmitter substances) which move across the synaptic cleft and interact with the specific receptors in the post-synaptic membrane (Kalat, 2000).
The chemical change at the synapse can be excitatory or inhibitory. In the first instance, excitation, there is an increased possibility of the post-synaptic neuron to exhibit action potential. However, a neurotransmitter can display inhibition; here, the second cell is less likely to show action potentials and there is a suppression of activity. One needs to be cautious when making assumptions about the link between psychological processing and neurochemical activity; nevertheless, biologically-orientated psychologists (for example, Toates, 2001; Stevens, 1996; Lefkowitz, Caron and Stiles, 1984) believe that, to a certain extent, our mood, emotion, action, motivation and body regulation are controlled by, and inextricably interconnected with, the neuronal pathways in the CNS. And, further, some reductionist biological psychologists, for example Crick (1994), believe that all psychological events can be explained in the context of neurochemical activity.
More importantly, changes in synaptic activity and neuronal function can cause one’s behaviour, mood or cognitive function to be altered. The main neurotransmitter systems are the noradrenaline system, the serotonin system and the cholinergic system. Thus, alcohol and cocaine alter synaptic activity—they interfere with cognitive functioning. Some people, having drunk large quantities of alcohol, suffer memory loss, while others loose their inhibitions, and perform acts which would be previously feared. Cocaine blocks the reuptake of dopamine and leaves the neurotransmitter in the synaptic gap for a longer period of time. Cocaine users experience a ‘high’ when influenced by the drug; however, the dopamine depletion after a period of time can lead to an acute, but transitory, depression (Toates, 2007).
Prozac, a selective serotonin reuptake inhibitor (SSRI) blocks the reuptake of the serotonin being taken back into the neuron from which it was released, thus increasing its activity at the receptors. It is important to note that, like the monoamine neurotransmitters (MAOIs) and the tricyclic antidepressants (TCAs), prolonged use of SSRIs may not be effective and can lead to homeostasis or even down regulation (Sampson, 2001; Leykin, Amsterdam, DeRubeis et al, 2007). Nevertheless, antidepressants have continued to be used in the treatment of depression and, to some extent, monoamine neurotransmitter abnormalities—be they dopaminergic, serotonergic or noradrenergic—are involved in and related to depressive syndromes (McNeal and Cimbolic, 1986). Further, extreme stress and prolonged hypothalamic-pituitary adrenal axis mediated dysfunction can lead to depression and a downstream of pathophysiological self-regulation (Anisman and Zacharko, 1982; Mello, Mello, Carpenter and Price, 2003).
Finally, in addressing the question, ‘how human behaviour is mediated by the nervous system’, having given some examples pertaining to neural regulation and transmission, it is important to clarify and draw attention to two branches of the CNS—namely, (1) the somatic nervous system (SNS) and (2) the autonomic nervous system (ANS). The SNS is responsible for and controls skeletal muscles and voluntary behaviour. The neurons of (normally) the frontal cortex communicate with the motor neurons of the peripheral nervous system, causing muscle contraction (Toates, 2007). Any damage to the frontal cortex due to cerebrovascular accidents or severe head injury can lead to impaired motor control.
By contrast, the ANS is connected with involuntary, unconscious movement and response. The ANS controls emotion (crying, laughing), the production of saliva, breathing, heart rate, sweating amongst other things. Stress, again, can cause fatty substances to sit in our circulatory pathways, producing high levels of cortisol and increased heart rates (Toates, 2007). As a result, in these situations, people sweat and get increasingly anxious: these physiological responses are all the result of autonomic, defensive mechanisms.
Anisman H & Zacharko RM (1982). Depression: the predisposing influence of stress. The Behavioural and Brain Sciences, 5: 89-137.
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Kalat, JW (2000). Biological Psychology (Pacific Grove: California/Brooks Cole).
Lefkowitz RJ, Caron MC, Stiles GL (1984). Mechanisms of membrane-receptor regulation. Biological, physiological and clinical insights derived from studies of the adrenergic receptors. New England Journal of Medicine, 310: 1570-79.
Leyton Y, Amsterdam JD, DeRubeis RJ, Gallop R, Shelton RC & Hollon SD (2007). Progressive resistance to a selective serotonin inhibitor but not to cognitive therapy in the treatment of depression. Journal of Consulting and Clinical Psychology, 75 (2): 267-276.
McNeal ET & Cimbolic P (1986). Antidepressants and biochemical theories of depression. Psychological Bulletin, 99 (3): 361-374.
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Toates, F (2007). Biological processes and psychological explanation. In D Miell, A Phoenix & K Thomas (eds.) Mapping Psychology: Book 1 Introduction and Chapters 1-5 (Milton Keynes: Open University Press): 225-283.
Part 2: Methods Exercises
(a) The control for perceived harmfulness—the fact that all the creatures were harmless and were found injured in the wild—was not entirely successful, because most participants considered rats as potentially threatening, injured or not.
(b) The two variables were (1) ‘Ugliness’ (a subjective mean score of how ugly the animals were), and (2) ‘Rated Distance’, (how far away the participants would keep away from the animal).
(i) The scatterplot shows that there is a strong correlation coefficient.
(ii) As the value of the variable on the x-axis increases, generally, the value of the variable on the y axis increases.
(iii) Generally, the more ugly the animal, the greater distance, on average, participants would stay away from each animal. Note ugliness was measured on subjective responses from individuals on a scale from 1-10 (1=least ugly; 10=most ugly).
(d) 0.723 is a strong correlation coefficient.
(e) The researcher would design a field experiment. In the first instance, he would ask a veterinary surgeon permission to use five injured/sedated animals for an experiment and would position these animals in the centre of a small forest. The researcher would then, one by one, measure the participants’ willingness to approach each animal (in metres). Each participant, having approached each animal will be required to rate each animal on ugliness. This will be done using a scale from 1-10 (1=least ugly; 10=most ugly). Each test will be recorded at the same time each day, and each animal will show no movement. Rats will not be used in the experiment.
(i) This experiment is a ‘Between-Participants Design’.
(ii) The ‘Within-Participants Design’, also called ‘repeated measures’, is an experiment which requires each participant to take part in two separate conditions; the Piliavin, Rodin & Piliavin (1969) experiment cited measures the occurrence of altruism in 103 different trials on the 8th Avenue in New York. Different participants are involved in each trial. This is an independent sample design.
(b) The participants were the passengers travelling on the subway.
(c) The dependent variable is the number of participants who helped the victims on each occasion. The researchers measured this variable to see how it was affected by the independent variable. Perhaps, a suitable label would be, ‘level of altruism’.
(d) Non-intrusive female observers recorded whether or not one or more passengers helped them (although the text intimates that passengers were either helped or not at all).
(e) The independent variable is the type of victim—an apparently disabled person or a person pretending to be drunk. A suitable label would be, ‘disabled/drunk’.
(f) This was a random allocation exercise.
(g) Other independent variables that could be considered would be correlation between altruism and race—that is to say, the likelihood of people helping an African American versus a Caucasian American.
(h) The researchers did not consider any distress or inconvenience that this might have caused the unknowing participants. No feedback was given, and this might have affected some participants’ willingness to travel on the subway in the future.
(a) The researchers controlled the experiment by randomly allocating the children to groups, but by telling them that they were assigned to a specific group for a reason—that is to say, that they preferred abstract painting A or B.
(b) All the children were the same age (aged 10-11).
(i) The fact that one school was a mixed independent school and the other was a girls’ school could be a confounding variable.
(ii) There were, probably, more girls in the experiment than boys. Some girls might have favoured responses from girls in both schools (over the boys in one school). In addition, some pupils might have favoured students’ responses from their own school over the other school.
(iii) The researchers could design two separate experiments—the first for the mixed school, and the second for the girls’ school.
(i) Pupils could favour the responses from pupils in their own school.
(ii) Again, the researchers would reduce confounding data by limiting each experiment to one school.