A2 The Human Brain Flashcards
Brain formation
During embryonic development, the neural tube will enlarge and develop into different components of the nervous system:
The anterior part of the neural tube will expand to form the brain during cephalisation (development of the head)
The remainder of the neural tube will develop into the spinal cord
Cells that comprised the neural crest will differentiate to form most of the peripheral nervous system
The embryonic brain will initially be composed of three primary structures – the forebrain, midbrain and hindbrain
These structures will eventually give rise to the identifiable components of the developed brain
Purpose of the brain
The human brain acts as an integration and coordination system for the control of body systems
It processes sensory information received from the body and relays motor responses to effector organ
The human brain is organised into clearly identifiable sections that have specific roles
The major external structures include the cerebral cortex, cerebellum and brainstem
Internal structures include the hypothalamus, pituitary gland and corpus callosum
Cerebral cortex
composed of two hemispheres and several lobes:
frontal lobe
parietal lobe
temporal lobe
occipital lobe
Frontal lobe
controls motor activity and tasks associated with the dopamine system (memory, attention, etc.)
Parietal lobe
is responsible for touch sensation (tactility) as well as spatial navigation (proprioception)
Temporal lobe
is involved in auditory processing and language comprehension
Occipital lobe
is the visual processing centre of the brain and is responsible for sight perception
Cerebellum
The cerebellum appears as a separate structure at the base of the brain, underneath the cerebral hemispheres
It is responsible for coordinating unconscious motor functions – such as balance and movement coordination
Brainstem
is the posterior part of the brain that connects to the spinal cord (which relays signals to and from the body)
includes the pons, medulla oblongata (often referred to as the medulla) and the midbrain
The brainstem (via the medulla) controls automatic and involuntary activities (breathing, swallowing, heart rate, etc.)
Hypothalmus
The hypothalamus is the region of the brain that functions as the interface with the pituitary gland
As such, the hypothalamus functions to maintain homeostasis via the coordination of the nervous and endocrine systems
The hypothalamus also produces some hormones directly, which are secreted via the posterior pituitary (neurohypophysis)
Pituitary gland
The pituitary gland is considered the ‘master’ gland – it produces hormones that regulate other glands and target organs
The anterior lobe is called the adenohypophysis and secretes hormones such as FSH, LH, growth hormone and prolactin
The posterior lobe is called the neurohypophysis and secretes hormones such as ADH and oxytocin
Corpus Callosum
The corpus callosum is a bundle of nerve fibres that connects the two cerebral hemispheres
It is the largest white matter structure in the brain, consisting of roughly 250 million axon projections
Damage to the corpus callosum can prevent information exchange between left and right hemispheres (split brain disorders)
Brocas area
controls the production of speech
Located within the frontal lobe of the left cerebral hemisphere (not present in the right hemisphere)
Sensory cortex
recieves sensory inputs especially touch
Wernickes area
controls the understanding of speech
Nucleus accumbens
is deep in the frontal cortex
it acts as a pleasure of rewards centre in the brain
It secretes neurotransmitters responsible for feelings of pleasure (dopamine) and satiety (serotonin)
It communicates with other centres involved in the mechanisms of pleasure, such as the ventral tegmental area (VTA)
Ventricles
cavities containing cerebrospinal fluid which absorbs shock and delivers nutrients
Meninges
membrane covering which protects the brain hemispheres
Animal experiments - brain
Animal experimentation can be used to identify function by stimulating regions with electrodes or removing via lobotomy
Because such methods are highly invasive and potentially damaging, animal models are frequently used
Experimentation on animals involves less ethical restrictions than human studies (although ethical standards do exist)
Animal studies are limited by the differences between animal and human brains, making valid comparisons difficult
Example: Animal studies using mice and rats have been used to develop drug treatments for diseases such as MS
Lesions
Lesions are abnormal areas of brain tissue which can indicate the effect of the loss of a brain area
Lesions can be identified via post-mortem analysis (autopsy) or via scans of the brain (CT scans or MRI)
The effects of lesions can be difficult to identify, as many functions may involve multiple brain areas
Additionally, the brain has the capacity to re-learn certain skills by re-routing instructions to other areas (plasticity)
Example: Split brain patients have been used to identify specific roles of the left and right cerebral hemisphere
Autopsy
An autopsy is a post-mortem examination of a corpse via dissection in order to evaluate causes of death
Comparisons can be made between the brains of healthy and diseased corpses to identify affected brain areas
Example: Cadavers who suffered from aphasia (language impairment) in life demonstrate damage to specific areas
fMRI
Functional magnetic resonance imaging (fMRI) records changes in blood flow within the brain to identify activated areas
Oxygenated haemoglobin responds differently to a magnetic field than deoxygenated haemoglobin
These differences in oxygenation can be represented visually and reflect differences in the level of brain activity
fMRI is non-invasive and can be used to identify multiple brain regions involved in complex, integrated brain activities
Example: fMRI studies have been used to diagnose ADHD and dyslexia, as well as monitor recovery from strokes
Visual cortex
Located within the occipital lobe of the cerebrum and receives neural impulses from light-sensitive cells in the eyes
The visual cortex is the region of the brain responsible for visual perception (sight)
Evolution and the brain
Through evolution, the human cerebral cortex has been greatly enlarged in comparison to other brain structures
The disproportional enlargement of the cerebral cortex in humans is responsible for our capacity for cognitive thought
The increase in total area is mediated by extensive folding (gyrification) to form wrinkled peaks (gyrus) and troughs (sulcus)
This greatly increases surface area without increasing volume – allowing the brain to fit within the cranium
The extent of gyrification of the cerebral cortex is a reliable indicator of potential cognitive capacity
Primates and humans have a greater degree of folding compared to lower mammals (e.g. rats have a smooth cortex)
Human brain vs animals brain
The cerebral cortex is the outer layer of neural tissue found in the cerebrum of humans and other mammals
It is composed of grey matter and is involved in complex actions, such as memory, perception, consciousness and thought
The cerebral cortex is much more highly developed in humans than other animals and forms a larger proportion of the brain
The cerebral cortex can be externally classified according to four topographical lobes – frontal, parietal, temporal, occipital
Cerebral hemispheres
The left cerebral hemisphere is responsible for processing sensory information from the right side of the body (and vice versa)
Tactile sensation from the left side of the body is processed by the right side of the brain (at the somatosensory cortex)
Objects on the left side of the visual field in both eyes are processed on the right side of the visual cortex
The left cerebral hemisphere is also responsible for processing motor information for the right side of the body (and vice versa)
Muscular contractions are coordinated by the motor cortex (premotor cortex = preparation ; primary motor cortex = execution)
A consequence of this contralateral processing is that damage to one side of the brain affects the other side of the body
For instance, a stroke in the left hemisphere may cause paralysis to the right side of the body
Contralateral processing
The processing of information on the opposite side of the body
Tactile information from the left side of the body is transferred to the right side in the spinal cord or brainstem
Visual information from the left visual field is transferred to the right cerebral hemisphere at the optic chiasma
Subdivisions of the human nervous system
Firstly, the nervous system can be divided into the central nervous system (brain and spine) and peripheral nervous system
The peripheral nervous system (PNS) can be divided into the sensory (afferent) pathway or the motor (efferent) pathway
The motor pathway can be subdivided according to whether the response is voluntary (somatic) or involuntary (autonomic)
Sympathetic nerves
release noradrenaline (adrenergic) to mobilise body systems (‘fight or flight’ responses)
Parasympathetic nerves
release acetylcholine (cholinergic) to relax body systems and conserve energy (‘rest and digest’)
Pupil reflex
is an involuntary response originating at the brainstem and under the control of the autonomic nervous system
It involves the resizing of the iris to regulate the amount of light that reaches the retina (excess light can damage the retina)
Pupils constrict in bright light (to prevent overstimulation of photoreceptors) and dilate in dim light (to maximise light exposure)
In bright light, parasympathetic nerves trigger circular muscles to contract and cause the pupils to constrict
In dim light, sympathetic nerves trigger radial muscles to contract and cause the pupils to dilate
Brain death
Brain death is defined as the permanent absence of measurable activity in both the cerebrum and brainstem
The brainstem is responsible for involuntary autonomic responses and may function alone to maintain homeostasis
Hence, individuals with a non-functioning cerebrum but a functioning brainstem may be kept alive in a vegetative state
Brain death can be determined by medical professionals by testing the function of specific autonomic responses
The pupil reflex is one autonomic test used to assess brain death – brain dead individuals will not exhibit a pupil reflex
The Glasgow Coma Scale uses multiple tests to determine the neurological health of someone with suspected brain injury
Encephalization
the amount of brain mass relative to an animal’s body mass
here is a positive correlation between body size and brain size in different animals – larger animals have larger brains
This correlation follows a linear pattern of progression but is not directly proportional
Body mass increases disproportionately to an increase in brain mass as most tasks only require a fixed brain capacity
While there is a correlation between body size and brain size, there is not a correlation between brain size and intelligence
Brain metabolism
The human brain consumes ~20% of the body’s energy levels, despite making up only ~2% of the body’s mass
The brain’s rate of energy consumption varies little, regardless of the level of physical exertion by the body
The large amounts of energy required by the brain are used to sustain neurons and their processes
Energy is needed to maintain a resting potential when neurons are not firing (Na+/K+ pump uses ATP)
Energy is used to synthesise large numbers of neurotransmitters to facilitate neuronal communication
