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2.1. Overview of the Brain
Welcome to the Brain
The human nervous system can be classified in one of two ways: structurally or functionally.
Structurally, the nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain, spinal cord and retina, and it’s responsible for most of the ‘processing’ of the nervous system. The PNS consists of the twelve pairs of cranial nerves and the 31 pairs of spinal nerves. Sometimes, the enteric nervous system (i.e. the independent nerve network of the gastrointestinal tract) is also classified as part of the PNS.
Functionally, the nervous system can be divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS). The SNS is under voluntary control and has sensory and motor components. For example, the motor nerves allow the muscles in the hands and fingers to contract when we want to type on a PC, and the sensory nerves allow us to feel the pressure and texture of the keys on the keyboard when we type. The ANS is involuntary and ensures that peripheral blood vessels constrict (‘vasoconstriction’) when we are cold, sweat is produced when we are hot, adrenaline is secreted in times of stress and our pupils constrict in bright light.
In this section, we will be focusing on the structural divisions of the nervous system, specifically the CNS.
Structurally, the nervous system can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain, spinal cord and retina, and it’s responsible for most of the ‘processing’ of the nervous system. The PNS consists of the twelve pairs of cranial nerves and the 31 pairs of spinal nerves. Sometimes, the enteric nervous system (i.e. the independent nerve network of the gastrointestinal tract) is also classified as part of the PNS.
Functionally, the nervous system can be divided into the somatic nervous system (SNS) and the autonomic nervous system (ANS). The SNS is under voluntary control and has sensory and motor components. For example, the motor nerves allow the muscles in the hands and fingers to contract when we want to type on a PC, and the sensory nerves allow us to feel the pressure and texture of the keys on the keyboard when we type. The ANS is involuntary and ensures that peripheral blood vessels constrict (‘vasoconstriction’) when we are cold, sweat is produced when we are hot, adrenaline is secreted in times of stress and our pupils constrict in bright light.
In this section, we will be focusing on the structural divisions of the nervous system, specifically the CNS.
Welcome to the Brain: the Hindbrain and Midbrain
The brain is a wonderful but complex organ that affects, in some way, almost every aspect of our experience. Before we look at its function, let’s get an understanding of its structure.
The brain can be divided into three main areas (see Figure 2.1.1. and Figure 2.1.2.) based on its development in the embryo and These are the:
1. Hindbrain
The hindbrain lies caudally in the brain overall and it consists of three structures: the medulla oblongata, the pons and the cerebellum. The medulla and pons, in combination with the midbrain, are referred to jointly as the brainstem. The brainstem is integral to our survival because it contains vital cardiac and respiratory centres that in part determine are heart rate and breathing rate. A loss in any activity in the brainstem due to illness or injury is extremely dangerous. The cerebellum, which is also a hindbrain structure, allows us to perform repeated, stereotypical activities, such as walking, and maintains our coordination and balance. Take a look at the two figures on this page and see if you can identify the medulla, pons and cerebellum.
2. Midbrain
As mentioned above, the midbrain is part of the brainstem. Like the cerebellum, it helps us to walk. The midbrain is also involved in the processing of eye reflexes, such as the pupillary light reflex (i.e. both your pupils constrict in bright light).
The brain can be divided into three main areas (see Figure 2.1.1. and Figure 2.1.2.) based on its development in the embryo and These are the:
1. Hindbrain
The hindbrain lies caudally in the brain overall and it consists of three structures: the medulla oblongata, the pons and the cerebellum. The medulla and pons, in combination with the midbrain, are referred to jointly as the brainstem. The brainstem is integral to our survival because it contains vital cardiac and respiratory centres that in part determine are heart rate and breathing rate. A loss in any activity in the brainstem due to illness or injury is extremely dangerous. The cerebellum, which is also a hindbrain structure, allows us to perform repeated, stereotypical activities, such as walking, and maintains our coordination and balance. Take a look at the two figures on this page and see if you can identify the medulla, pons and cerebellum.
2. Midbrain
As mentioned above, the midbrain is part of the brainstem. Like the cerebellum, it helps us to walk. The midbrain is also involved in the processing of eye reflexes, such as the pupillary light reflex (i.e. both your pupils constrict in bright light).
Welcome to the Brain: the Forebrain
3. Forebrain
The forebrain is well-developed in human compared to other mammals and has four main parts. First is the cerebral cortex. This is the outer layer of grey matter surrounding the cerebrum. The cerebral cortex is divided into hemispheres (left and right) and then subdivided into six distinct areas, or lobes. These are called the frontal lobe, temporal lobe, occipital lobe, parietal lobe, limbic lobe and insular cortex/insula. The cerebral cortex is a higher processing centre and, as such, it allows us to process and understand lots of sensations (e.g. pain, temperature, pressure), process and enact complex motor activities (such as the intricate finger work involved in painting) and understanding what we see in our visual field. See if you can identify these structures in the accompanying figures.
The second part of the forebrain includes subcortical structures, such as the basal ganglia, which is involved in controlling voluntary movements and inhibiting excess and unwanted movements. Note that the hippocampus and amygdala, although they appear subcortical, are actually folds of the cerebral cortex. The hippocampus is involved in the formation of memories and the amygdala is involved in regulating instinctive behaviours, such as hunger, fear, anger and sexual arousal.
The third part of the forebrain includes the large bundles of fibres that connect the two halves of the cerebrum. The largest bundle of fibres is called the corpus callosum, and it can be seen in Figure 2.1.2.
The fourth part of the forebrain is the diencephalon, which consists of the thalamus and hypothalamus. The thalamus is a major relay centre, connecting the brainstem with the cerebrum. The hypothalamus is home to the pituitary gland, and it is involved in the regulation of lots of nervous and endocrine responses, such as managing our temperature, sleep-wake cycles, thyroid hormones and the menstrual cycle. You can see the approximate location of the thalamus and hypothalamus in Figure 2.1.2.
The forebrain is well-developed in human compared to other mammals and has four main parts. First is the cerebral cortex. This is the outer layer of grey matter surrounding the cerebrum. The cerebral cortex is divided into hemispheres (left and right) and then subdivided into six distinct areas, or lobes. These are called the frontal lobe, temporal lobe, occipital lobe, parietal lobe, limbic lobe and insular cortex/insula. The cerebral cortex is a higher processing centre and, as such, it allows us to process and understand lots of sensations (e.g. pain, temperature, pressure), process and enact complex motor activities (such as the intricate finger work involved in painting) and understanding what we see in our visual field. See if you can identify these structures in the accompanying figures.
The second part of the forebrain includes subcortical structures, such as the basal ganglia, which is involved in controlling voluntary movements and inhibiting excess and unwanted movements. Note that the hippocampus and amygdala, although they appear subcortical, are actually folds of the cerebral cortex. The hippocampus is involved in the formation of memories and the amygdala is involved in regulating instinctive behaviours, such as hunger, fear, anger and sexual arousal.
The third part of the forebrain includes the large bundles of fibres that connect the two halves of the cerebrum. The largest bundle of fibres is called the corpus callosum, and it can be seen in Figure 2.1.2.
The fourth part of the forebrain is the diencephalon, which consists of the thalamus and hypothalamus. The thalamus is a major relay centre, connecting the brainstem with the cerebrum. The hypothalamus is home to the pituitary gland, and it is involved in the regulation of lots of nervous and endocrine responses, such as managing our temperature, sleep-wake cycles, thyroid hormones and the menstrual cycle. You can see the approximate location of the thalamus and hypothalamus in Figure 2.1.2.
Clinical Top Tip:
Brain Damage
As you can probably guess, the extent of disability after brain damage is dependent on where exactly the brain is injured. For example, a traumatic blow to the back of the head could damage the cerebellum and brainstem – if the cardiac and respiratory centres stop working, the person’s heart and lungs will stop working. This is referred to as ‘brainstem death’. Take another example: damage to the hippocampus in Alzheimer’s disease can lead to memory problems (see Section 4.1. Alzheimer’s Disease).
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