A2 The Human Brain

Question 1

How did the brain develop?

Answer 1

During the development of vertebrate embryos a neural tube forms along the whole of the dorsal side, above the gut near the surface, we learnt about this before in A1. Most of the neural tube becomes the spinal chord, but the anterior end expands and develops into the brain as part of a process called cephalization, the development of a head.

The human brain contains approximately 86 billion neurones.

The brain acts as the central control centre for the whole body, both directly from cranial nerves (a set of 12 nerves in the brain) and indirectly via the spinal chord and numerous signal molecules carried by the blood.

Question 2

What does the medulla oblongata do?

Answer 2

The medulla oblongata is used in autonomic controls of gut muscles, breathing, blood vessels and heart muscle.

Question 3

What does the cerebellum do?

Answer 3

The cerebellum coordinates unconscious functions such as posture, non-voluntary movement and balance.

Question 4

What does the hypothalamus do?

Answer 4

This is the interface between the brain and the pituitary gland, synthesising hormones secreted by the posterior pituitary, and releasing factors that regulate the secretion of hormones by the anterior pituitary.

Question 5

What does the pituitary gland do?

Answer 5

The posterior lobe stores and releases hormones produced by the hypothalamus and the anterior lobe produces and secretes hormones that regulate many body functions.

Question 6

What do the cerebral hemispheres do?

Answer 6

They act as the integrating centre for high complex functions such as learning memory and emotions.

Question 7

What are the methods of brain research?

Answer 7

• Animal experiments
• Autopsy
• Lesions
• fMRI

Question 8

How do lesion studies give us information about the brain?

Answer 8

They allow us to explore what would happen if parts of the brain were damaged and see what functions are missing or what has changed in the person. For example, a famous case was the railway construction worker Phineas Gage, who suffered severe damage to the frontal lobes of his brain in 1848 when an accident with explosives caused a large metal rod to pass through his forehead. He recovered from the wound but the brain damage radically and permanently altered his personality and particularly his capacity for social interaction.

Question 9

What is the advantage of having a brain and not just a spinal chord?

Answer 9

The advantage of having a brain is that communication between the billions of neurons involved can be more rapid than if control centres were more dispersed. The main sensory organs are here after all, eyes, ears, nose etc.

Question 10

How do autopsys help us learn about the brain?

Answer 10

Many lesions due to tumours, strokes or accidental damage have been investigated by carrying out an autopsy and relating the position of the lesion to observed changes in behaviour and capacities.

Question 11

How does animal research help us learn about the brain?

Answer 11

When lesions occur accidentally we can often look at where the damage has occurred, what the changes in behaviour are, and then see what functions those parts of the brain have. However rather than wait for these opportunities some neuroscientists have studied experimental animals. Removal of parts of the skull gives access to the brain and allows experimental procedures to be performed. The brain itself does not feel pain - even today some forms of neurosurgery are performed on fully conscious patients. The effects of local stimulation in an animal’s brain can be observed, as can long term changes in the animal’s temperament and capacities.

Question 12

What are the problems with animal research?

Answer 12

• The suffering it may cause to the animal and whether this is ethical.
• At the end the animal is often sacrificed.
• However the information gained if useful to understanding and therefore treating diseases such as Parkinson’s disease and multiple sclerosis.
• Increasingly genetic mutants and selective inactivation of genes, which are technically possible only in mice, are used to achieve similar experimental modification of brain structure and behaviour.

Question 13

How is fMRI used to give us information about the brain?

Answer 13

Magnetic resonance imaging (MRI) is a more modern and less controversial technique. Basic MRI is used to investigate the internal structure of the body, including looking for tumours or other abnormalities in patients. fMRI detects iron in the body, if a part of the brain is being worked particularly hard there will be lots of respiration occurring there, this means there will need to be lots of blood flow to that part of the brain, because of the sudden demand for more oxygen and the removal of carbon dioxide. This is brought in the red blood cells which contain iron. The fMRI scanner detects this iron and therefore which parts of the brain are particularly active.
You can give a subject a stimulus and see how the brain then reacts to it, when the subject is in the scanner.

Question 14

What does the visual cortex do?

Answer 14

• Each of the two cerebral hemispheres has a visual cortex, in which neural signals originating from light sensitive rod and cone cells in the retina of the eyes are processed. Although there is an initial stage in which a map of visual information is projected in a region called VI, the information is then analysed by multiple pathways in regions V2 to V5 of the visual cortex. This analysis includes pattern recognition and judging the speed and direction of moving objects.

Question 15

What does Broca’s area do?

Answer 15

It is part of the left cerebral hemisphere that controls the production of speech. If there is damage to this area an individual knows what they want to say and can produce sounds, but they cannot articulate meaningful words and sentences. For example, if we see a black and white striped animal Broca’s area allows us to say ‘zebra’ someone with a damaged Broca’s area knows it is a ‘zebra’ but cannot say the word.

Question 16

What does the nucleus accumbens do?

Answer 16

There is a nucleus accumbens in each of the cerebral hemispheres. It is the pleasure or reward centre of the brain. A variety of stimuli including food and sex cause the release of the neurotransmitter dopamine in the nucleus acumbens, which causes feelings of well-being, pleasure and satisfaction. Cocaine, heroin and nicotine are addictive because they artificially cause release of dopamine in the nucleus accumbens.

Question 17

What is the autonomic nervous system?

Answer 17

The autonomic nervous system controls involuntary processes in the body using centres located in the medulla oblongata.

The peripheral nervous system comprises all of the nerves outside the central nervous system. It is divided into two parts the voluntary and the autonomic nervous systems. Involuntary processes are controlled by the autonomic nervous system, using two centres in the medulla oblongata. The autonomic nervous system has two parts: sympathetic and parasympathetic. These often have contrary effects on the involuntary process. For example, parasympathetic nerves cause an increase in blood flow to the gut wall during digestion and absorption and sympathetic nerves cause a decrease in blood flow during fasting or when blood is needed elsewhere.

Question 18

What is the parasympathetic nervous system?

Answer 18

The parasympathetic nervous system is one of two parts of the autonomic nervous system located in the medulla oblongata and controls involuntary movement. Paired with the sympathetic nervous system. The parasympathetic nervous system controls homeostasis and the body at rest and is responsible for the body’s “rest and digest” function.

Question 19

What is the sympathetic nervous system?

Answer 19

The sympathetic nervous system is one of two parts of the autonomic nervous system located in the medulla oblongata and helps to control involuntary movement. The sympathetic nervous system controls the body’s responses to a perceived threat and is responsible for the “fight or flight” response.

Question 20

What activities are coordinated by the medulla?

Answer 20

• Swallowing, breathing and heart rate

Question 21

How does swallowing work and what controls it?

Answer 21

• The medulla
  The first phase of swallowing, in which food is passed from the mouth cavity to the pharynx, is voluntary and so is controlled by the cerebral cortex. The remaining phases in which the food passes from the pharynx to the stomach via the esophagus, are involuntary and are coordinated by the swallowing centre of the medulla oblongata.

Question 22

How does breathing work and what controls it?

Answer 22

• The medulla
  Two centres in the medulla control breathing: one controls the timing of inspiration; the other controls the force of inspiration and also active, voluntary expiration. There are chemoreceptors in the medulla that monitor blood pH. The carbon dioxide concentration in the blood is very important in controlling breathing rate, even more than oxygen concentration. If blood pH falls indicating an increase in carbon dioxide concentration, the medulla will pick up on this and increase the breathing rate or the deepness of each breath.

Question 23

How does controlling heart rate work and what controls it?

Answer 23

• The cardiovascular centre in the medulla.

The cardiovascular centre in the centre of the medulla regulates the rate at which the heart beats. Blood pH and pressure are monitored by receptor cells in blood vessels and in the medulla. In response to this information, the cardiovascular centre can increase or decrease the heart rate by sending signals to the heart’s pacemaker. Signals sent from the sympathetic nervous system speed up the heart rate and signals from the parasympathetic nervous system in the vagus nerve slow it down.

Question 24

What is the ‘pupil test’ used for?

Answer 24

• To assess brain damage.
  Doctors sometimes use the pupil reflex to test a patient’s brain function. A light is shone onto each eye. If the pupils do not constrict at once, the medulla oblongata is probably damaged. If this and other tests of brain stem function repeatedly fail the patient is said to have suffered brain death. It may be possible to sustain other parts of the patient’s body on a life support machine, but full recovery is extremely unlikely.

Question 25

How does your pupil constrict?

Answer 25

Muscles in the iris control the size of the pupil of the eye. Impulses carried to radial muscle fibres by neurons of the sympathetic system cause them to contract and dilate the pupil; impulses carried to the circular muscle fibres by neurons of the parasympathetic system cause the pupil to constrict.
The pupil reflex occurs when bright light suddenly shines into the eye. Photoreceptive ganglian cells in the retina perceive the bright light, sending signals through the optic nerve to the mid-brain, immediately activating the parasympathetic nervous system that stimulates circular muscle in the iris, constricting the pupil and reducing the amount of light entering the eye, protecting the delicate retina from damage.

Question 26

What differentiates our brains from animals brains?

Answer 26

We have a much larger cerebral cortex. The cerebral cortex forms a larger proportion of the brain and is more highly developed in humans than other animals.

Question 27

What part of the brain do only mammals have?

Answer 27

Only mammals have a cerebral cortex. Birds and reptiles have regions of the brain that perform a similar range of functions but they are structurally different, with cells arranged in clusters rather than layers. Among the mammals the cerebral cortex varies in size considerably. In humans it forms a larger proportion of the brain than in any other mammal.

Question 28

How has the cerebral cortex evolved?

Answer 28

The human cerebral cortex has become enlarged principally by an increase in total area with extensive folding to accommodate it within the cranium.

Question 29

What is the area of the brain?

Answer 29

The area of the brain is estimated to be about 0.18m2. It has evolved to be such a huge size. This is so large that the brain can only be accommodated inside a greatly enlarged cranium, forming the distinctive shape of the human skull. Most of the surface area of the cerebral cortex is in the folds rather than on the outer surface.

Question 30

What is the difference between human’s cerebral cortex and the cerebral cortex of mice?

Answer 30

In a human the cerebral cortex’s surface area is in the folds rather than in the outer surface. The cerebral cortex is so folded to fix its huge area into the cranium. In contrast however, mice and rats have an unfolded smooth cortex, in cats there are so folds and elephants and dolphins have more. Among the primates, monkeys and apes show a range of cortex size and degree of folding, with larger sizes in primates that are more closely related to humans.

Question 31

What do the cerebral hemispheres do?

Answer 31

The cerebral hemispheres are responsible for higher order functions.

They carry out the most complex of the brains tasks. These are known as higher order functions and include learning, memory, speech and emotions. These higher order functions involve association of stimuli from different sources including the eye and ear and also from memories. They rely on very complex networks of neurons that are still only partially understood by neurobiologists. The most sophisticated though processes such as reasoning, decision-making, and planning occur in the frontal and prefrontal lobes of the cerebral cortex. Using these parts of the brain we can organise our actions in a logical sequence, predict their outcomes, develop a sense of right and wrong and be aware of our own existence.

Question 32

Where are the sensory inputs to the cerebral hemispheres?

Answer 32

The left cerebral hemisphere receives sensory input from sensory receptors in the right side of the body and the right side of the visual field in both eyes and vice versa.

The cerebral hemispheres receive sensory inputs from all the sense organs of the body. For example, signals from the left ear pass to the left hemisphere and from the right ear to the right hemisphere. Inputs from the skin, muscles and other internal organs pass via the spinal chord to the somatosensory area of the parietal lobe. Perhaps surprisingly, the impulses from each side cross in the base of the brain so that the left hemisphere receives impulses from the right side of the body and vice versa.

Inputs from the eye pass to the visual area in the occipital lobe, known as the visual cortex. Impulses from the right side of the field of vision in each eye are passed to the visual cortex in the left hemisphere, while impulses from the left side of the field of vision in each eye pass to the right hemisphere. This integration of inputs enables the brain to judge distance and perspective.

Question 33

Where do inputs to the eyes pass?

Answer 33

If it is the right field of vision in each eye, it will go to the visual area in the occipital lobe, known as the visual cortex in the left hemisphere.

Question 34

Where is motor control controlled?

Answer 34

Voluntary movement is controlled by the cortex. The left cerebral hemisphere controls movement in the right side of the body and vice versa.

Regions in each of the cerebral hemispheres control striated (voluntary) muscles. The main region is the posterior part of the frontal lobe and is called the primary motor cortex. In this region there is a series of overlapping areas that control muscles throughout the body, from the mouth at one end of the primary motor cortex to the toes at the other end.

The primary motor cortex in the left hemisphere controls muscles in the right side of the body and that in the right side controls muscles in the left side of the body. So a stroke (or other brain damage) in the left side of the brain can cause paralysis in the right side of the body and vice versa.

Question 35

What is the sensory homunculus?

Answer 35

This is the model that shows the somoatosensory cortex and the motor cortex and the parts of the body they control relative to the size of sensory inputs. For example, the hands are really big in comparison to the hip which is represented tiny. This is because there are so many more inputs coming from the hands.

Question 36

What sort of energy is required by the brain?

Answer 36

Brain metabolism requires large energy inputs.

Energy released by cell respiration is needed to maintain the resting potential in neurons and to re-establish it after an action potential, as well as for synthesis of neurotransmitters and other signal molecules. The brain contains a huge number of neurons so it needs much oxygen and glucose to generate this energy by aerobic cell respiration. In most vertebrates the brain uses less than 10% of the energy consumed by basal metabolism but in the adult human brain it is over 20% and an even higher proportion in infants and small children.