The human nervous system

2.1 Introduction
2.2 Divisions within the human nervous system
2.3 Divisions and functions of the peripheral nervous system
2.3.1 Somatic nervous system
2.3.2 Autonomic nervous system
2.4 Meninges and cerebrospinal fluid
2.4.1 The dura mater
2.4.2 The arachnoid mater
2.4.3 The pia mater
2.5 Motor and sensory pathways in the spinal cord
2.5.1 Sensory tracts
2.5.2 Motor tracts
2.6 Hindbrain
2.6.1 The medulla oblongata
2.6.2 The pons
2.6.3 The cerebellum
2.7 Midbrain
2.7.1 Tectum
2.7.2 Tegmentum
2.8 Forebrain
2.8.1 Thalamus
2.8.2 Hypothalamus
2.8.3 Basal ganglia
2.8.4 Limbic system
2.9 Cerebral cortex lobes
2.9.1 The frontal lobes
2.9.2 The temporal lobes
2.9.3 The parietal lobes
2.9.4 The occipital lobes

2.1 Introduction
We have been looking at some fundamental biological processes that will enable us to gain a greater understanding of human behaviour. In the previous unit we concentrated our attention on the neuron, looking at its various structures as well as exploring the ways in which neurons function. You might be wondering why we are studying these basic biological structures and functions, questioning how they are relevant to psychology, which is the study of human behaviour (Weiten, 2013: 22). The answer lies in the fact that all human behaviour is controlled by the nervous system, so it makes sense that we should study how the brain functions in order to understand human behaviour at a more global level. Is it not fascinating that we as humans are made up of the same most basic components (such as neurons) yet we are each incredibly unique in our behaviours?
In this unit we will be looking more intently at the broader structures and functions of the human nervous system now that we have explored neurons, the most elemental building blocks making up our nervous system. The nervous system is extremely complex and comprises various subsystems and structures (Kalat, 2016: 66). This unit will therefore seek to identify and describe the most significant structures of the nervous system in a clear and concise way. Furthermore, the unit will include information about the various functions of each of these central structures.
To ensure that the above is achieved, the following learning outcomes for Unit 2 have been conceptualised:
‘ Identify and describe the divisions within the human nervous system
‘ Discuss and compare the divisions and functions of the peripheral nervous system
‘ Define and discuss the meninges and the cerebrospinal fluid
‘ Compare motor and sensory pathways in the spinal cord
‘ Discuss the three structures of the hindbrain and their respective functions
‘ Describe the two structures of the midbrain and their respective functions
‘ Describe the four structures of the forebrain and their respective functions
‘ Explore the nature of the four lobes of the cerebral cortex in the human brain

The nervous system is the control system of the body and a useful analogy to enhance our comprehension of this system is that of a computer. Think of the brain as the software of a computer which is responsible for making decisions while the nerves are similar to the hardware or wiring of a computer necessary for transmitting messages and communicating decisions to the rest of the body. The primary function of the nervous system, together with the endocrine system, is communication between the brain and the body, thereby controlling all activities which occur within the body. This means that the human nervous system’s primary function is the conveyance of information. Based on this, three other important functions can be extrapolated (Muller, 2004):

1. receiving sensory information from both the internal and external environments (from within the body and from outside of the body);
2. assimilation of this sensory input; and
3. responding to this sensory information.

2.2 Divisions within the human nervous system
How is the human nervous system organised? One of the most effective ways of understanding the different parts of the human nervous system is to visualise it. The diagram below can therefore be used as a starting point for our identification and discussion on the various divisions within the nervous system.

Figure 2.1 The organisation of the human nervous system
As you can see by looking at the diagram above, the nervous system can be divided into two main components: the central nervous system (CNS) and the peripheral nervous system (PNS).The central nervous system consists of the brain and spinal cord. The CNS is responsible for effecting decisions based on sensory information received from the internal and external environment. The CNS is therefore responsible for integrating this sensory information and responding accordingly. For example, the brain integrates sensory information and then coordinates bodily responses, at both a conscious (voluntary) and unconscious (involuntary) level. The spinal cord functions as a pathway for signals between the brain and the rest of the body and also controls simple musculoskeletal reflexes minus contribution from the brain.

The peripheral nervous system comprises motor and sensory nerves that are dispersed throughout the body (Rains, 2001: 47) and can be divided into the somatic nervous system and the autonomic nervous system. A clue to understanding the function of the peripheral nervous system is found in its name, ‘peripheral’, meaning the outer regions of an area. This explanation helps us understand that the PNS is vital for the transmission of messages from the brain and spinal cord (CNS) to the rest of the body or everything outside of the CNS (Kalat, 2016: 67) and vice versa. The PNS is therefore critically important due to its information gathering abilities.

2.3 The divisions and functions of the peripheral nervous system
As we have just read, the peripheral nervous system’s primary function is to connect the brain and spinal cord (CNS) to the remainder of the body via an elaborate network of nerves. The PNS consists of 31 pairs of spinal nerves which transmit sensory and motor information to and from the spinal cord as well as 12 pairs of cranial nerves that exit the brain bypassing the spinal cord entirely (Coon and Mitterer, 2016: 54). This network of nerves making up the PNS is specified into an afferent branch of incoming sensory nerves which transmits impulses from the periphery of the body inwards into the CNS (peripheral body can include internal organs, muscles, glands and somatic areas). The PNS is also specified into an efferent branch of outgoing motor nerves which conducts impulses outward from the CNS to the periphery of the body (Applegate, 2011: 171). This efferent branch can be further subdivided into the somatic nervous system and the autonomic nervous system (Applegate, 2011: 195).

2.3.1 The somatic nervous system
The somatic nervous system consists of nerves which end in skeletal muscles or sensory receptors (Plotnik and Kouyoumdjian, 2014: 72) and these nerves function to transmit motor commands from the CNS to the body, specifically to the muscles (Coon and Mitterer, 2016: 81; Kalat, 2016: 67). A simplified example would be if you want to move your hand to take notes from this unit: a motor command will leave your brain and travel along the somatic branch of the PNS until it reaches the muscles in your hand; the muscles will then follow the directive given from the brain and your hand will begin moving in order for you to write notes. This conscious control of muscles means that the somatic nervous system generally controls voluntary behaviours and activities (Plotnik and Kouyoumdjian, 2014: 72).

2.3.2 The autonomic nervous system
The autonomic nervous system consists of nerves which end in the internal organs, blood vessels and glands, termed the visceral motor nerves (viscera meaning internal organs). These are responsible for transmitting messages from the CNS to these internal organs, such as the heart and lungs, smooth muscles in blood vessels and the digestive tracts, as well as the glands (Coon and Mitterer, 2016: 335). The autonomic nervous system thereby controls activities such as digestion, heart rate and perspiration. These activities are involuntary and out of our conscious control thus the autonomic nervous system is said to be autonomous (Weiten, 2013: 91). An example would be the dilation of your pupils if you are reading this chapter in poor lighting, something you cannot consciously control. The autonomic nervous system can be further subdivided into the sympathetic and parasympathetic branches.
The sympathetic nervous system is vital in that it activates the body’s resources during times of activity. It is this ‘energy- expending system’ which is responsible for initiating the body’s fight or flight responses when an individual is in a stressful or threatening situation (Applegate, 2011: 197). Contrastingly, the parasympathetic nervous system conserves the body’s energies. The table below is a visual representation of the various components of the body and how they are impacted by the sympathetic and parasympathetic nervous systems (Weiten, 2013: 91):

Sympathetic nervous system
Parasympathetic nervous system
Eyes Dilation of pupils, inhibit tear formation Constriction of pupils, activate tear production
Mouth Saliva production decreases Saliva production increases
Skin Vessels constrict, skin cold and has goose bumps Vessels dilate, increase blood flow and lack of goose bumps
Palms Sweaty Dry
Lungs Dilation of airways to take in greater oxygen Constriction of airways, breathing is relaxed
Heart Increase in heart rate Decrease in heart rate
Blood Majority of blood directed towards muscles Majority of blood directed towards internal organs
Adrenal glands Increase in adrenal action Decrease in adrenal action
Digestion Repressed, blood rather flows to muscles Stimulated

Table 2.1: Table reflecting changes to the sympathetic and parasympathetic nervous systems

Exercise 2a
Imagine that you are out walking in the bush when you turn a corner and suddenly come face to face with a lion.
Required:
(i) State which system of the autonomic nervous system will be activated in this situation.
(ii) List the main effects of this system on the body.
(iii) Consider why these changes might occur (for bonus points).

Proposed solution (in separate Word document)
The sympathetic nervous system will be activated when coming face to face with a lion.
The pupils of the eyes will dilate and tear production will cease ‘ this is to let more light in so that one can focus, see better and pay attention to the danger at hand.
The mouth will become dry as saliva production is decreased
Digestion is repressed which redirects the body’s attention to focus on the most essential processes necessary in order to flee/face danger.
Heart rate increases to circulate fuel and oxygen to the vital organs so that you can flee/face danger.
Blood flow is redirected towards muscles (redirecting necessary fats and sugars to the muscles) so that the muscles can be activated for movement and respond efficiently to the danger.
Airways in lungs dilate in order to allow maximum flow of oxygen to the vital organs and less to the extremities.
Palms and skin become sweaty as blood vessels constrict.

Exercise 2b
Now imagine that you have chosen the ‘flight’ reaction in the fight/flight response and you have safely escaped the lion.
Required:
(i) State which system of the autonomic nervous system will now be activated considering there is no immediate danger or threat.
(ii) List the main effects this system has on your body.

Proposed solution (in separate Word document)
The parasympathetic nervous system will now kick in once the threat of the lion is no longer there.
The pupils of the eyes will constrict and tear production will begin again ‘ the eyes will now function normally as there is no need for focused vision to assist with immediate danger.
The mouth will become moist as saliva production is commenced again.
Digestion is commenced since the body is now in a rest state and can begin to focus energies on everyday essential (but not vital in the presence of danger) functions. ‘Rest and digest’ versus ‘fight or flight’.
Heart rate returns to a normal rate and fuel and oxygen are circulated to the whole body not just to the vital organs.
The majority of blood flow is redirected back towards the internal organs now that the muscles do not need most of the blood for activation. Rather the internal organs require blood flow so that they can return to homeostatic functioning.
Airways in lungs constrict back to normal state transporting oxygen optimally to all cells.
Palms and skin are no longer sweaty as blood vessels dilate.

2.4 The meninges and cerebrospinal fluid
Considering the indispensability of the CNS to both basic and higher level functions of the body, not only is the brain protected by the cranium and the spinal cord protected by the vertebrae, further mechanisms have developed in order to offer enhanced protection. In addition to bone, the CNS is surrounded by the meninges and cerebrospinal fluid. The meninges refer to three layers of fibrous membranes that encompass the CNS and therefore function to protect the brain and spinal cord. The outer most layer of the meninges is called the dura mater, the middle layer is called the arachnoid mater and the inner most layer, the pia mater (McCaffrey, 2015).

2.4.1 The dura mater
In Latin, ‘dura’ means hard and ‘mater’ means mother, so if taken literally it means ‘hard mother’ referring to both the protective and nourishing nature of this layer The dura mater is the thick, yet flexible, outermost meningeal layer located just inside the cranial bones and also lines the spinal canal. The resiliency of this layer protects the brain from displacement and functions to connect the meninges to the cranium. The dura mater can be further subdivided into two layers: an outer layer known as the endocranium, or periosteal layer, and a deeper inner layer referred to as the inner meningeal layer (Clark, 2005: 189). At some points in the cranium the inner and outer layers of the dura mater separate from one another forming a cavity, called the dural sinuses. The dural sinuses permit blood flow, transporting deoxygenated venous blood which has provided oxygen and nutrients to the brain, and returning it back to the cardiovascular system (Applegate, 2011: 179).

2.4.2 The arachnoid mater
The arachnoid layer is attached to the inner most layer, the pia mater, by arachnoid trabeculae which have a spider web like appearance, hence the name for this meningeal layer. The space under the arachnoid mater, the subarachnoid space, is filled with cerebrospinal fluid and contains blood vessels (Applegate, 2011: 179) which ultimately join and stabilise the arachnoid mater to the pia mater. The arachnoid layer is important as it provides a space through which cerebrospinal fluid can circulate and is also necessary for the absorption of the cerebrospinal fluid back into the bloodstream.

2.4.3 The pia mater
The delicate pia mater is the inner most layer of the meninges and is in direct contact with the neural tissues of the brain and spinal cord. The pia mater acts as an intermediary between the neurons of the CNS and the cerebrospinal fluid, preventing their direct contact (Clark, 2005: 189).

2.4.4 Cerebrospinal fluid
Cerebrospinal fluid, which is made up of glucose, proteins, electrolytes and some cells, is produced and located within a number of cavities in the brain, termed ventricles (Applegate, 2012: 184). The four ventricles within the brain contain a layer of dense capillary networks and supporting cells, called the choroid plexus. It is this choroid plexus which is responsible for producing the cerebrospinal fluid (Applegate, 2012: 184). Once the cerebrospinal fluid has been produced by the choroid plexus which lines the upper portions of the ventricles, it travels down the fourth ventricle, to openings at the base of the brain on towards the brain stem. As previously discussed, some of the cerebrospinal fluid flows through the subarachnoid space surrounding the brain and spinal cord and is reabsorbed back into the bloodstream by the arachnoid mater. Cerebrospinal fluid functions most importantly to protect the CNS from injury, cushioning the brain during concussions (Polzin, 2005).
The volume of cerebrospinal fluid in adults is usually around 100-150 ml and the fluid needs to be preserved at this capacity since there is restricted space in the CNS. Not only does the volume of cerebrospinal fluid need to be invariable but the pressure also needs to remain constant as an increase in pressure can result in compression to the surrounding neural tissue. This can be the case with meningitis which is diagnosed when swelling to the meningeal layers occurs. Meningitis can sometimes subsequently causes hindrances to the flow of cerebrospinal fluid or an uncharacteristic accumulation of cerebrospinal fluid, termed hydrocephalus (Polzin, 2005).

Exercise 2c
Study the simplified diagram of the layers of an orange below:

Thick outer peel: exocarp
Fleshy interior: mesocarp
Thin, almost translucent layer covering segments: endocarp

Figure 2.2: Simplified cross-section of an orange
Required:
(i) Apply your knowledge of the three meningeal layers linking each layer of the orange with an appropriate and comparable meningeal layer.
(ii) Also include a function of each meningeal layer.

Proposed solution (in a separate Word document):
The outer peel or exocarp is relatable to the dura mater. Function: protective, connective (connecting the meninges to the cranium), dural sinuses permit blood flow.
The fleshy interior or mesocarp is relatable to the arachnoid mater. Function: subarachnoid space contains cerebrospinal fluid and is therefore important for circulation of cerebrospinal fluid and also important because it allows the cerebrospinal fluid to be reabsorbed back into blood stream. Subarachnoid space also contains blood vessels which serve a connective function, joining this layer (subarachnoid layer) to pia mater.
The thin translucent layer or endocarp is relatable to the pia mater. Function: protective, nourishment and a safeguard function preventing direct contact of CNS and cerebrospinal fluid.

2.5 Motor and sensory pathways in the spinal cord
Sensory information is being conveyed to the CNS consistently while motor information is being conveyed from the CNS to the peripheral body. This motor information activates the body, causing it to respond in numerous ways to the given directives. The sensory and motor information is conveyed to and from the brain by way of different channels, or tracts, in the spinal cord. Ascending tracts are sensory and transmit information to the CNS. Names of these sensory tracts begin with the prefix ‘spino-‘. Conversely, descending tracts are motor and are responsible for transmitting information to the periphery. The names of these tracts end in the word ‘spinal’ (Bassett, 2012).

2.5.1 Sensory tracts
There are three main sensory tracts in the spinal cord namely: the posterior column tract, the spinothalamic tract and the spinocerebellar tract. The posterior column tracts are responsible for the following sensations: proprioception which is defined as ‘denoting stimuli produced and perceived within an organism, especially those relating to position and movement of the body’ Soanes and Stevenson, 2006: 1152), fine touch, pressure and vibration intensities. The spinothalamic tracts are responsible for the following sensations: pain and temperature perceptions, basic touch and pressure. The spinocerebellar tracts are responsible for the sensation of proprioception (Bassett, 2012)

2.5.2 Motor tracts
The CNS conveys motor commands as a reaction to sensory information it receives from the internal and external environment. Considering we have previously discussed the divisions and functions of the peripheral nervous system, we know that motor information is transmitted along the efferent branch of the PNS via the somatic nervous system (which ends in the skeletal muscles) and the autonomic nervous system (which connects to internal organs, smooth muscles and glands) (Bassett, 2012).
There are two primary motor tracts: the corticospinal tract and the subconscious tract. The former is responsible for the voluntary control of skeletal muscles while the latter is responsible for the involuntary control of balance, muscle tone, eye, and upper limb placement (Bassett, 2012). More specifically, the corticospinal tract comprises three descending tracts which function to regulate the voluntary control over skeletal muscles as well as regulation of the eye and facial muscles (Basset, 2012). The subconscious motor tracts are responsible for posture and balance, subconscious reactions of the head and upper limbs to auditory and visual stimuli in addition to control over involuntary eye movements, respiratory and skeletal muscles (Bassett, 2012).

Exercise 2d
The following short questions are based on the subheading ‘motor and sensory pathways in the spinal cord’:
(i) What type of information (sensory/motor) is conveyed from the CNS to the peripheral body?
(ii) What type of information (sensory/motor) is conveyed to the CNS?
(iii) Considering that sensory and motor information are transmitted via different tracts in the human nervous system, state what information is transmitted via ascending tracts and what information is transmitted via descending tracts.
(iv) Name the main sensory tracts in the spinal cord.
(v) Now state what each main sensory tract is responsible for.
(vi) Fill in the blanks:
a. Motor information is transmitted along _______ branch of the PNS via the somatic nervous system which ends in ___________.
b. Motor information is also transmitted via the autonomic nervous system of the PNS which ends in ____________.
(vii) Name the main motor tracts in the spinal cord.
(viii) Now state what each main motor tract is responsible for.

Proposed solution (in separate Word document)
(i) Motor
(ii) Sensory
(iii) Ascending tracts are sensory and transmit information to the CNS
Descending tracts are motor and transmit information from CNS to peripheral body
(iv) There are 3 main sensory tracts: posterior column tract, spinothalamic tract and the spinocerebellar tracts
(v) The posterior column tract. Responsible for ‘ proprioception, fine touch, pressure and vibrations
The spinothalamic tract. Responsible for ‘ perception of pain and temperature, basic touch and pressure
The spinocerebellar tract. Responsible for ‘ proprioception
(vi) a. Motor information is transmitted along the efferent branch of the PNS via the somatic nervous system which ends in the skeletal muscles.
b. Motor information is also transmitted via the autonomic nervous system of the PNS which ends in the viscera (internal organs, smooth muscles and glands).
(vii) There are 2 main motor tracts: the corticospinal tract and the subconscious tract.
(viii) The corticospinal tract. Responsible for ‘ voluntary control of skeletal muscles
Subconscious tract. Responsible for ‘ involuntary control of balance, muscle tone, eye and upper limb placement.

2.6 The Hindbrain
The posterior part of the brain is called the hindbrain and is evolutionarily the most primitive part of our brains. The three main structures of the hindbrain are the medulla oblongata, pons and cerebellum.

2.6.1 Medulla Oblongata
The medulla oblongata, often referred to as just the medulla, is positioned just above the spinal cord and is sometimes considered an appendage of the spinal cord. The medulla is primarily responsible for the control of unconscious involuntary body responses or reflexes. These bodily responses are initiated in order to maintain homeostasis of the body, which is defined as ‘the maintenance of a stable equilibrium, especially through physiological processes’ (Soanes and Stevenson, 2006: 681). In other words, the medulla functions to maintain balance within the body. It achieves this by controlling the intercostal muscles and diaphragm which are essential for breathing, regulating heart rate and blood pressure in addition to circulating blood through the management of the diameter of arterioles (Muller, 2004).

2.6.2 Pons
The pons, lying just in front of the medulla, serves as a bridge between the medulla and other areas of the brain. It is critical in controlling reflexes that regulate breathing and is also responsible for the control and balance of the sleep-wake cycle (Weiten, 2013: 105).

2.6.3 Cerebellum
The word ‘cerebellum’ literally translated means ‘little brain’ and is the second largest part of the human brain, positioned anterior to the pons and medulla. The cerebellum is a profoundly folded structure in its appearance and is most commonly recognised for its necessity in motor skills which include, the coordination of body movements and physical balance. Importantly to note is that while the directives for muscular movements do not originate in the cerebellum itself (but rather from systems higher in the brain); the cerebellum is critical in regulating the motor information that conducts these movements (Weiten, 2013: 98).

2.7 The Midbrain
The midbrain is often thought of as a communication point as it is situated between the hindbrain and forebrain and is important in linking pathways from the brainstem to higher brain areas. It is also critical as a communication point for motor information of the brain. It can be divided into two main structures, namely the tectum and tegmentum (Rains, 2001: 67).

2.7.1 Tectum
The covering of the midbrain is called the tectum, which is made up of two enlargements situated on each side: the superior colliculus and the inferior colliculus. These structures are responsible for mediating sensory information with the superior colliculus focused on visual information and the inferior colliculus serving as a communication point for information from the ears to the auditory cortex as well as modulating auditory reflexes (Kalat, 2016: 72).

2.7.2 Tegmentum
The tegmentum comprises the reticular formation which is important in activating the forebrain and therefore plays a role in attention and alertness. Situated just below the tegmentum is the substantia nigra, an area of the midbrain that generates dopamine producing cells and is therefore conceptualised as a ‘reward or pleasure centre’ (Plotnik and Kouyoumdjian, 2014: 73). These dopamine producing cells also play a function in the development of voluntary movements (Weiten, 2013: 98) and their degeneration is implicated in the development of Parkinson’s disease.

2.8 The forebrain
The forebrain is the most substantial structure of the human brain and comprises four main structures: the thalamus, hypothalamus, basal ganglia and limbic system. One of the most notable features of the forebrain is that it is divided into two main hemispheres, the left cerebral hemisphere and the right cerebral hemisphere, which are visible when looking at the brain. Information transmitted to the hemispheres is organised in a contralateral manner with the left hemisphere mostly receiving information from the right side of the body and vice versa (Rains, 2001: 47).

2.8.1 The thalamus
Together, the thalamus and the hypothalamus are referred to as the diencephalon with the thalamus itself being divided into a pair of oval shaped structures, one situated in the right hemisphere of the brain and the other in the left hemisphere. The thalamus is often considered a transmission point for the cerebral cortex, mediating inbound and outbound sensory information. The thalamus is therefore critical in that the majority of sensory information is initially sent first to the thalamus and from there directed on to the cerebral cortex (Coon and Mitterer, 2015: 74). This applies to all sensory information except our sense of smell (or olfactory information), which is transmitted to olfactory sensors and then directly on to the cerebral cortex. According to Weiten (2013: 100) the thalamus should not be conceptualised as simply consisting of passive relay nuclei but is moreover an active structure in the processing of sensory information.

2.8.2 The hypothalamus
The hypothalamus is situated just below the thalamus and has a wide range of vital functions. Firstly, it is essential in the maintenance of fundamental internal physiological needs, including the fight or flight responses, feeding and procreating through its maintenance of homeostasis (Weiten, 2013: 100). Homeostasis is defined by Rains (2001: 65) as ‘the internal biological steady state that every living organism must constantly maintain in order to remain alive’. Other visceral biological processes include thirst, body temperature, circadian rhythms as well as sexual drive and arousal. Another inner physiological state that the hypothalamus exerts control over is the automatic nervous system (ANS), which is responsible for driving the heart, lungs, smooth muscles and digestive tract. Furthermore, the hypothalamus is critical in the regulation of the endocrine system, conveying information from the pituitary gland in the brain to the rest of the endocrine system (Kalat, 2016: 298 ‘ 314).

2.8.3 The basal ganglia
The basal ganglia are a group of grey matter structures linked to the thalamus (Rains, 2001: 60). The basal ganglia is essential for regulating movement and muscular coordination (Plotnik and Kouyoumdjian, 2014: 60) and are also especially critical for spontaneous and voluntary behaviours (Kalat, 2016: 244). Although voluntary movements do not originate in the basal ganglia themselves, but rather in the cortex, basal ganglia remain critical to the relay of motor information from the motor cortex to the necessary structures in the nervous system (Rains, 2001: 60).
In order to function optimally, the basal ganglia must receive sufficient amounts of the neurotransmitter dopamine. Parkinson’s disease, which is characterised by languidness or lack of movement, rigidity and tremors, is associated with the gradual degeneration of dopamine neurotransmitters in the basal ganglia (Applegate, 2012: 182).

2.8.4 The limbic system
The term limbic translates to ‘border’ and refers to the position of the limbic system which is located along the boundary between the forebrain and brain stem. The limbic system is not a well-defined structure in the brain and the literature lacks consensus on which structures actually constitute this system. When broadly conceptualised, the limbic system includes the hypothalamus, hippocampus, the amygdala, the olfactory bulb and the cingulate gyrus (Weiten, 2013: 100). The hippocampus plays a role in memory, particularly in the consolidation of memory which involves converting information into long lasting information (long term memory). When there is damage to both sides of the hippocampus, anterograde amnesia is most likely to occur, which is defined as an inability to create new memories. The amygdala is critical in the experience and processing of emotion, especially fear, and is also involved in social behaviour (Rains, 2001: 63).

2.9 The cerebral cortex lobes
The characteristic folds of the cerebral cortex create four divisions which are referred to as the cerebral lobes. Specifically these are the frontal lobe, parietal lobe, temporal lobe and occipital lobe. Various parts of these lobes are implicated in the ability for us to see, hear, taste and move, among others. As such the cerebral lobes, and the cerebral hemispheres, provide us with a sort of ‘map’ for human behaviours, referred to as laterality of functioning (Ricker, 2015).

2.9.1 The frontal lobes
Exploration of the functions of the frontal lobes can be perplexing due to the wide range of functions attributed to them. In spite of this, most literature narrows down the responsibility of the frontal lobes to higher mental processes and is also related to an individual having a sense of self (Coon and Mitterer, 2016: 69). Moreover, they are strongly implicated in the organisation and execution of movement (Rains, 2001: 57). The frontal lobes are also imperative in executive functions such as cognitive and social-emotional behaviours, especially the ability for self-control which includes impulse control as well as the self-regulation of emotions and sexual impulses (Plotnik and Kouyoumdijan, 2014: 76). The frontal lobes are divided into the primary motor cortex and prefrontal cortex.
The primary motor cortex
The primary motor cortex is associated with voluntary motor movement in terms of controlling the body’s muscles, specifically fine motor movements. Importantly to note is that this control is contralateral, meaning that the left hemisphere of the brain controls the right side of the body while the right hemisphere controls the left side of the body (Kalat, 2016: 84). Different body parts have varying degrees of agility. Consider how much easier it is to use your hand to write compared to using your foot. The varying degrees of agility are based on the proportion of primary motor cortex associated with that particular body part such that, the hands have a greater proportion of the primary motor cortex devoted to them than the feet. Luckily due to neuroplasticity these proportions are not fixed and over time individuals can learn to enhance their abilities over various body parts (Coon and Mitterer, 2016: 69).

(Source: Woelker 2012)
Figure 2.3 Motor homunculus

The prefrontal cortex
The prefrontal cortex is instrumental in higher-order control of movement, specifically the organisation and adjustment of behaviour in line with responses to this behaviour (Rains, 2001: 57). It is also associated with making decisions and planning movements based on response predictions (Kalat, 2016: 85).
Broca’s area
Finally, an area in the frontal lobe called Broca’s area, usually situated in the left frontal lobe, plays an important role in working memory (a combination of auditory and visual-spatial memory). Broca’s area is crucial for linking individual sounds into whole words and then ordering these words in such a way as to make meaningful sentences. Broca’s aphasia is characterised by diminished fluency or trouble in getting words out. Individuals with Broca’s aphasia also experience difficulty in naming words but are usually responsive when prompted with cues (Plotnik and Kouyoumdjian, 2014: 78).

2.9.2 The temporal lobes
Located near the temples in close proximity to the ears, the temporal lobes, found in both hemispheres of the brain, are implicated in our ability to hear. They are important to our perception of auditory properties including pitch, tone and rhythm. Unlike other information pathways in the cerebral lobes, the auditory pathway is not contralateral. This means that each ear projects auditory information to both hemispheres of the brain and each of the hemispheres receives auditory projections from both ears (Rains, 2001: 59). The temporal lobes can be divided into the primary auditory cortex and the auditory association area.
Primary auditory cortex
The primary auditory cortex is located at the top portion of each temporal lobe. This is the area that the initially receives auditory signals. At this point, the auditory signals remain ineffective as just sounds and clicks for example (Plotnik and Kouyoumdjian, 2014: 78). These meaningless signals must now be transmitted to the auditory association area.
Auditory association area
The auditory association area is the next location to which auditory signals are sent and it is vital for converting meaningless auditory signals into actual meaningful words or music (Plotnik and Kouyoumdjian, 2014: 78).
Wernicke’s area
Wernicke’s area is named after Karl Wernicke who first described it in 1873. It is an area located on the left temporal lobe and is important for the processing of language and the meaning of words. Damage to Wernicke’s area results in receptive aphasia, also known as Wernicke’s aphasia, whereby an individual is able to hear spoken words but struggles to understand or comprehend the meaning of these words (Coon and Mitterer, 2016: 72). Individuals diagnosed with Wernicke’s aphasia generally have little difficulty in speaking or writing, the difficulty lies in the fact that what they say or write is incomprehensible and other individuals have trouble understanding what they are trying to communicate. Individuals with Wernicke’s aphasia may also suffer from anomia, a decreased capacity for giving names to objects.

2.9.3 The parietal lobes
The parietal lobes are located above the occipital lobe and behind the frontal lobe. These lobes are responsible for a wide range of functions but are primarily important for processing tactile (touch) sensory information and allowing an individual to determine what an object is through size, texture and shape. The parietal lobes are also critical for processing information about taste, heat, cold, pain, pleasure and pressure. Moreover, they provide us with the ability to distinguish the space around one’s body, orient the body in space (proprioception) and direct attention to objects. So for example, an awareness of one’s limbs is largely attributed to the parietal lobes (Plotnik and Kouyoumdjian, 2014: 77). Interestingly, our different body parts experience varying sensitivities to external stimuli and this is nicely illustrated with what is termed a ‘sensory homunculus’. Again, like other sensory pathways in the cerebral lobes, the pathways for tactile information are contralateral, therefore the parietal lobes in the left hemisphere of the brain receive information from the right side of the body and vice versa. So that, for example, when the left parietal lobe is damaged, individuals often lose the ability to perceive or orient themselves to the space on the right side of their bodies, referred to as right-side neglect.

(Source: Woelker 2012)
Figure 2.4 Sensory homunculus

2.9.4 The occipital lobe
The smallest of all the cerebral lobes, the occipital lobe is located at the back of the brain and is the principal visual processing area. It is responsible for receiving and processing visual information, including discerning colours, lights, shadows and shapes. ‘Light falls on the eye, but you see with your brain,’ (Weiten, 2013: 139) such that although the retina of the eye is critical in receiving visual information, it can only be interpreted by the brain itself.
For ease of understanding, the process of seeing can be simplified into a two-step process. The first step occurs in the primary visual cortex, at the very back of the occipital lobe. Visual information is transferred to the axons of ganglion cells located at the back of each eye. This visual information registers as meaningless basic visual sensations such as light and shadows. The whole process is contralateral such that stimuli from the right side of space stimulate the left side of the retina in each eye and eventually travel to the left hemisphere of the brain (Rains, 2001: 58).These retinal ganglion cells eventually form the optic nerves of each eye and convene at the optic chiasm defined as the point where the optic nerves from the half of each eye cross over and then travel to the opposite side of the brain (Weiten, 2013: 139). See diagram below:

Left visual field Right visual field

Left hemisphere of brain Right hemisphere of brain

Figure 2.5 Diagram depicting visual processing from the eye to the brain

The next step in the process of seeing occurs in the visual association area whereby the meaningless basic visual sensations are transformed into whole meaningful visual perceptions (Plotnik and Kouyoumdjian, 2014: 79).

Exercise 2e

For each of the following conditions, state the area or areas of the brain responsible and also provide a brief explanation as to how these areas are implicated in the conditions:

(i) Right-side neglect
(ii) Wernicke’s aphasia
(iii) Parkinson’s disease
(iv) Broca’s aphasia

Proposed solution (in separate Word document)

(i) Left parietal lobe is most often associated with right-side neglect. The parietal lobe functions in a contralateral manner so that damage to the left parietal lobe can cause right-side neglect, characterised by an individuals’ inability to perceive or orient themselves in the space on the right side of their body. Parietal lobe is therefore critical in spatial orientation (proprioception).
(ii) Temporal lobe associated with Wernicke’s aphasia. Temporal lobe important for processing auditory information with Wernicke’s area specifically involved in processing of language and meaning making. Damage to Wernicke’s area in the temporal lobe does not result in a diminished capacity to hear, speak and write but rather in a diminished ability for meaningful language.
(iii) Basal ganglia and/or thalamus (the two are connected) are associated with Parkinson’s disease. Basal ganglia comprise dopamine neurotransmitters and this area is responsible for regulating movement and muscle coordination. Parkinson’s disease characterised by ridged movement and tremors, occurs when there is a reduction in dopamine neurotransmitters in this area.
(iv) Broca’s area in the frontal lobe is implicated in Broca’s aphasia. This area is important in executive functions such as working memory (ability to hold on to information while completing a task). Broca’s area therefore is responsible for integrating individual sounds, into words into meaningful sentences/music. It therefore assist the individual in holding onto information long enough to use it in a meaningful way. Broca’s aphasia is therefore characterised by inarticulateness.

Source: Essay UK - https://www.essay.uk.com/essays/science/the-human-nervous-system/


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