12.1 Essential ideas
12.1.2 The human brain
- The human brain develops from expansion of the anterior portion of the embryonic neural tube.
- Expansion of the neural tube is followed by the development of two distinct hemispheres by inward folding of the outer membrane.
- The brain continues to fold into lobes and ridges, or gyri, thereby increasing the surface area of cerebral cortex.
- The posterior portion of the neural tube develops into the spinal cord.
Skill: Identifying parts of the brain from a scan, photograph, or diagram
Print the diagram of the brain from and compare it to Figure 12.1.2b.
Different parts of the brain have specific roles:
- Medulla oblongata
Coordinates autonomic nervous functions and homeostasis, including the action of smooth and cardiac muscle, e.g. swallowing, breathing, heart rate, digestion.
Coordinates unconscious actions such as balance, posture, movement.
- Acts as a bridge between the nervous and endocrine systems.
- Coordinates autonomic nervous functions and homeostasis, e.g. body temperature, thirst, sleep patterns.
- Synthesises hormones and releasing factors and releases them into the pituitary gland.
- Pituitary gland
- Synthesises hormones that regulate growth, development and the menstrual cycle.
- Secretes hormones in response to chemical cues from the hypothalamus
- Secretes hormones produced by the hypothalamus.
- Cerebral hemispheres
Coordinating centre for higher order functions including emotion, memory, learning, reasoning and language.
The cerebral cortex
- The cerebral cortex is the outermost layer of the cerebral hemispheres. It is about 2–3mm deep in humans.
- It contains a high density of neurons and synapses, and is the brain’s centre of conscious thought and action.
- The human cerebral cortex makes up a larger proportion and is more highly developed in humans than in other animals.
- The cortex has become enlarged by an increase in total area, with extensive folding to accommodate it within the cranium.
Skill: Correlation between body and brain size
- State the relationship between brain mass and body mass.
- Outline the trend in brain mass to body mass in the hominid species (A. africansus, H. habilis, H. sapiens).
- Discuss the hypothesis that fish and reptiles have smaller brains than mammals and birds.
- Estimate the brain mass to body mass ratio for humans and elephants.
The left and right cerebral hemispheres
- The left cerebral hemisphere receives 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 left cerebral hemisphere controls muscle contraction in the right side of the body and vice versa for the left side.
Figure 12.1.2e – The left and right cerebral hemispheres are ‘cross-wired’ to sensory and motor neurons.
- The brain looks symmetrical, but there is lateralisation of functions in the hemispheres, meaning that one side is more dominant than the other for certain functions.
- This explains, for example, why people are more commonly right-handed or left-handed and not ambidextrous.
Brainrequires large energy inputs
- Unlike the liver or muscles, the brain stores very little glycogen.
- It requires a constant supply of oxygen and glucose for cellular respiration.
- The brain uses up to 20% more oxygen than other tissues in the body. Part of the reason for this is that synapses are continually firing, even when the body is at rest.
- Much of the energy is used to maintain and re-establish resting potentials after a nervous impulse.
- The parts of the brain specialise in different functions.
- The autonomic nervous system controls involuntary processes using centres in the brainstem.
- The cerebral hemispheres are responsible for higher order functions.
- The cerebral cortex appears as ‘grey matter’ because it consists of a large density of nuclei and dendrites, while other parts of the brain and spinal cord appear as ‘white matter’ consisting mainly of myelinated axons.
Food for thought
- Although specific functions can be attributed to different areas of the brain, brain injury shows that some activities are spread in many areas and that neural plasticity allows the brain to re-organise itself following a disturbance such as stroke. See Page 12.2.1.
- Does studying make you hungry? It’s probably a psychological effect. Researchers are still not sure if thinking hard requires more energy, since the overall metabolic rate of the brain is relatively constant.
Nature of Science
Using models: The cortical homunculus is a pictorial model showing locations on the somatosensory and motor cortex where information from different parts of the body is processed. The ‘little man’ has distorted features to reflect the relative amount of innervation of the different body parts.
- Details on the structure and function of the hypothalamus and pituitary glands can be found in 15.1.5.