A.2 The human brain Flashcards
Development of the brain
In a mammalian embryo, the neural tube is initially a straight, linear structure running the length of the rear-side.
Most of the neural tube eventually becomes the spinal cord, but as early as the fourth week the anterior end develops into three distinct bulges known as the primary vesicles. The vesicles continue to develop and form the fore, mid and hind sections of the brain.
Metabolism of the brain
The energy demands of the brain are high: they account for at least 20% of a human adult’s energy consumption.
Studies indicate have linked higher cognitive functions to increased glucose demand.
Cell respiration produces
the ATP that is constantly required by neurons to:
Synthesise neurotransmitters
carry out the active transport needed to maintain resting potential.
Hypothalamus
Maintains homeostasis via coordination of the nervous and endocrine systems, produces hormones secreted by posterior pituitary
Pituitary gland
Produces and secretes hormones regulating many body functions, e.g. such as ADH
Medulla oblongata
Controls automatic and homeostatic activities, such as swallowing, digestion and vomiting, and breathing and heart rate.
Cerebellum
Coordinates unconscious functions, such as movement and balance.
Cerebral Hemisphere
Act as the integration centre for highly complex functions, such as learning, memory and emotion.
Identifying brain function – autopsy and animal experiments
Animal experiments can take different approaches to investigating brain function. Commons approaches are:
Autopsy, dissection of the brain
Stimulating regions of the brain with electrodes and then observing behaviour and movement – e.g. Ferrier work with dogs and monkeys
Lobotomy - removing regions of the brain and observing impairment of brain function, e.g. Flourens’ experiments on pigeons
Identifying brain function - lesions
Presence of a lesion is usually associated with damaged and loss of function in the affected area.
Lesions are
abnormal areas in brain tissue caused by accidents or present from birth
Identifying brain function - fMRI
fMRI is the primary tool used in modern research. It measures changes in blood flow through the brain. This can indicate which regions of the brain are most active.
Measurements can be made in real-time so regional brain activity can be correlated with a stimulus. Also the sequencing of brain activity, e.g. language comprehension followed by production can be observed.
The procedure is non-invasive and can be performed without injury, but it is an indirect measure and not all brain activity is detected.
fMRI works because
oxyhaemoglobin responds differently to a magnetic field than (deoxygenated) haemoglobin. Computers interpret the results to produce coloured brain activity images (different colours often represent different levels of activity).
The autonomic nervous system
The autonomic nervous system controls unconscious processes using the medulla oblongata.
The autonomic nervous system is part of the peripheral nervous system, which consists of all the nerves outside the central nervous system.
There are two distinct parts to the autonomic nervous system:
sympathetic and parasympathetic. These two parts have contrary effects, e.g. parasympathetic nerves cause an increase in blood flow to the gut, during digestion and absorption of food, whereas the sympathetic nerves cause a decrease in blood flow.
Activities coordinated by the medulla – breathing and heart rate
stimulus:
- blood pH falls as CO2 concentration increases
receptor:
- breathing and heart rate centres in the medulla oblongata contain chemoreceptors in their blood vessels.
nerves:
- sympathetic nerves stimulate …
responses:
- the Intercostal muscles and diaphragm (lung muscles) to increase the rate and size of contractions: increasing the rate and depth of ventilation.
heart’s pacemaker (sino-atrial node) to increase the heart rate
Activities coordinated by the medulla – swallowing
stimulus:
- bolus of food touches the walls of the pharynx
receptor:
- touch receptors in the walls of the pharynx
nerves:
- parasympathetic nerves send impulses via the swallowing centre in the medulla oblongata
responses:
- parasympathetic nerves send impulses via the swallowing centre in the medulla oblongata
The pupil reflex:
The pupil reflex originates in the brainstem and is under the control of the autonomic nervous system
Stimulus: Bright light
Receptor: photoreceptors on the retina detect potential damaging levels of light
Coordination: Sensory neurons in the optic nerve send impulses to the medulla oblongata. Relay and then motor neurons direct the impulse to the iris muscles.
Effect: Radial muscles relax and circular muscles contract to constrict the size of the pupil
The pupil reflex – assessing brain damage
Failure of the pupil reflex indicates the damage to medulla oblongata (brain stem).
The brain stem controls basic automatic brain functions. If the brain stem fails then the organism can no longer function and it is unlikely that higher order brain functions persist. Therefore brain stem death is used to test for whole brain death.
Following brain stem death life support can sustain other parts of the body, but it is very unlikely that a recovery can be made.
Enlargement and folding of the cerebral cortex
The cerebral cortex, which consists of the two cerebral hemispheres, is the outer layer of neural tissue in humans and other mammals. It is very thin (2 – 4 mm) and contains the neurons key to controlling complex behaviour.
The folds in the cortex enabling a large increase in surface area, without increasing the size of the cranium (skull).
Through evolution the human cranium has increased in size and the folding has become more extensive. Both changes allowed an increase in the number of neurons present in the cortex controlling complex behaviour, folding being the key change.
Is there a relationship between the size of an animal and the size of it’s brain?
There is a strong correlation (despite outliers such as the Alligator). Note the logarithmic scales, this relationship is not directly proportional.
Can you suggest reasons for a causal link between body size and brain size?
The larger the animal the larger the brain required to monitor and control it’s processes.
Brain size is limited by the metabolism of the animal.
n.b. when groups of animals, e.g. primates, are examined the correlation becomes very weak. Other factors, such as evolution and the ecological niche occupied, seem to have more effect on brain size than body size.
Evolutionary development of the cerebral cortex
In mammals certain brain areas, such as the cerebellum, which coordinates muscle movement have remained a constant proportion compared to the overall size of the brain.
The cerebral cortex, which is only present in mammals, varies greatly as to it’s proportion of the brain’s mass. For instance in shrews it is 20% of the mass, whereas in humans it accounts for 80%.
The cerebral cortex linked to the development of complex behaviours, but changes in the connections between the neurons, not just the number of neurons, is also key to behaviour development.
cerebral hemispheres
act as the integration centres for highly complex functions, such as learning, memory and emotion.
Higher order functions rely on input both from stimuli and and memories.
Sophisticated processes, such as reasoning, planning, self-awareness and morality, rely on a complex network of neurons. It is estimated that there are 1014 synapses in the brain.
Due to the huge complexity of these neural networks how these functions work is only partially understood.
Left and right cerebral hemisphere functions
Some functions are limited to a particular hemisphere, but most brain functions are bilateral, they are dealt with by both sides of the brain.
It is however true that the brain is “cross-wired”:
The right side of the brain receives stimuli from the left side of the body (and vice-versa)
The left visual field from both eyes is processed by the right side of the brain (and vice-versa)
The left side of the brain controls movement, muscle contraction, on the right side of the body (and vice-versa)
This explains why stroke victims lose sensation and/or the ability to move limbs on one side of the body as a stroke may cause a brain damage to a single hemisphere.
processing visual stimuli
both left and right eyes have a visual field
perception of visual stimuli begins in the retina, with edge enhancement
retinal ganglia carry nerve impulses through the optic nerve to the brain
The nasal ganglia cross over at the
optic chiasm
this means that both sides of the brain process images from both eyes and is called contra-lateral processing
the right side of the visual cortex processes images on…
both right retinas (therefore both left visual fields)
the visual cortex constructs images in the brain from
the stimuli received
stereoscopic vision
due to 2 eyes being apart, images are slightly different
leads to perception and 3D image
inverted image that hits the retina is also converted
Localised functions of the cerebral hemispheres
Certain regions of the cerebral hemispheres are specialised to carry out certain functions, for example
Visual cortex
processes stimuli received by light-sensitive rod and cone cells in the retina. The stimuli from both eyes are combined to allow distance, speed and size to be judged.
Broca’s area
involved in both language comprehension, but is most strongly associated with language production. If damaged a person has trouble forming words and sentences to express their thoughts
Nucleus accumbens
pleasure and reward centres. Selective stimuli, e.g. food and sex, cause the release of the neurotransmitter dopamine by the nucleus accumbens. Dopamine causes feelings of well-being and pleasure. The feeling of pleasure can also act as a reinforcement during learning.
Cortical homunculi
cortical homunculus is a physical representation of the human body, located within the brain
