A2 - THE HUMAN BRAIN

how will the neural tube enlarge and develop into different components of the nervous system during embryonic development?

• The anterior (front) 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

Formation of the Human Brain

What does the human brain act as and process

• 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

How is the human brain organized

The human brain is organised into clearly identifiable sections that have specific roles

• Internal structures include the hypothalamus, pituitary gland and corpus callosum

major external structures of the brain

The major external structures include the cerebral cortex, cerebellum and brainstem

cerebral cortex

The cerebral cortex is an outer layer of tissue organised into two cerebral hemispheres and composed of four distinct 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

responsible for touch sensation (tactility) as well as spatial navigation (proprioception)

temporal lobe

involved in auditory processing and language comprehension

occipital lobe

the visual processing centre of the brain and is responsible for sight perception

cerebellum - what does it appear as and what is it responsible for

• 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

• The brainstem is the posterior (rear) part of the brain that connects to the spinal cord (which relays signals to and from the body)

• The brainstem 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.)

major internal structures of the brain

• Hypothalamus

• Pituitary Gland

• Corpus Callosum

Hypothalamus

• 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)

how can the role of a specific brain part can be identified by either stimulating or removing the region to assess its effect

by either stimulating or removing the region to assess its effect

• Identification of brain roles can be made via the use of animal experiments, autopsy, lesions and fMRI

animal experiments

• 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 abnormalareas of braintissue 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 involvemultiple 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

• Example:  fMRI studies have been used to diagnose ADHD and dyslexia, as well as monitor recovery from strokes

examples of brain areas with clearly defined functions

visual cortex, Broca’s area and the nucleus accumbens

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)

Broca’s Area

• Located within the frontal lobe of the left cerebral hemisphere (not present in the right hemisphere)

• Is responsible for speech production (if damaged, the individual cannot produce meaningful speech despite intending to)

Nucleus Accumbens

• The nucleus accumbens is involved in the pleasure reward pathway and is found within each cerebral hemisphere

• 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)

What is the cerebral cortex

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

• is much more highly developed in humans than other animals and forms a larger proportion of the brain

result of evolution of human cerebral cortex and what is this responsible for

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

what is the increase in human cerebral cortex mediated by and what does this allow for

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

what does the extent of gyrification of the cerebral cortex indicate

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)

Brain Comparison – Human versus Rat

what is the cerebrum organised into

two hemispheres that are responsible for higher order functions and complex skills

functions of hemispheres of cerebrum (6)

• memory,

• speech,

• cognitive thought,

• problem solving,

• attention

• and emotions

are some activities localised to a single side of the cerebral hemispheres? example

yes - Not all complex tasks are equally represented by both cerebral hemispheres – some activities are localised to a single side

• Speech production is coordinated by Broca’s area, which is situated in the left frontal lobe of the brain

what is the left/ right cerebral hemisphere responsible for

• The left cerebral hemisphere is responsible for processing sensory information from the right side of the body

  • processed on the left side of the visual cortex

• The right cerebral hemisphere is responsible for processing sensory information from the left side of the body

  • processed on the right side of the visual cortex

contralateral processing

The processing of information on the opposite side of the body

how does contralateral processing work

• Tactile information from the left side of the body is transferred to the right side in the spinal cord or brainstem and vice versa

• Visual information from the left visual field is transferred to the right cerebral hemisphere at the optic chiasma and vice versa

how is motor information processesed on the diff sides of the cerebral hemispheres

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)

Contralateral Processing diagram

what sub-system is the nervous system divided into

the nervous system can be divided into the

• central nervous system (brain and spine) and

• peripheral nervous system

what sub-system is the peripheral nervous system divided into

The peripheral nervous system (PNS) can be divided into the

• sensory (afferent) pathway or

• the motor (efferent) pathway

how can the motor pathway of the peripheral nervous system be subdivided

The motor pathway can be subdivided according to whether the response is

• voluntary (somatic) or

• involuntary (autonomic)

what does the autonomic nervous system control - using what two things

The autonomic nervous system controls involuntary processes in the body using sympathetic and parasympathetic nerves

• 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’)

Divisions of the Nervous System diagram

what is the medulla oblangata

The medulla oblongata is a part of the brainstem responsible for coordinating many autonomic (involuntary) activities

• This includes the regulation of body activities such as swallowing, breathing and heart rate

what occurs during sympathetic Responses

‘Fight or flight’ mode

• Decreases salivary release and blood flow to the gut in response to swallowing

• Increasesventilationrate and dilates airways in response to a reduction in bloodpH (caused by increased levels of CO₂)

• Increasesheartrate by raising the normal sinus rhythm of the pacemaker of the heart

what occurs during parasympathetic responses

‘Rest & Digest’ mode

• Increases salivary release and blood flow to the gut in response to swallowing

• Lowers ventilation rate and constrictsairways in response to an increase in blood pH (caused by lower levels of CO₂)

• Reduces heart rate (via vagus nerve) by lowering the normal sinus rhythm of the pacemaker of the heart

Autonomic Control of Body Systems diagram

what is the pupil reflex

The 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)

how do pupils respond to diff. levels of light

Pupils constrict in bright light (to prevent overstimulation of photoreceptors) and dilate in dim light (to maximise light exposure)

how do the para and sympathetic nerve control pupil constriction

• 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

Overview of the Pupil Reflex diagram

What is brain death

Brain death is defined as the permanent absence of measurable activity in both the cerebrum and brainstem

when will a vegatative state occur and how

• 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

how can brain death be determined

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

correlation between body and brain size in diff. animals

There 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

brain:body ratio in larger animals explanation

While an increase in body size results in an increase in brain size, the brain:body ratio decreases in larger animals

• Body mass increases disproportionately to an increase in brain mass as most tasks only require a fixed brain capacity

what is Encephalization

the amount of brain mass relative to an animal's body mass

Encephalization graph

brain energy consumption

The human brain consumes ~20% of the body’s energy levels, despite making up only ~2% of the body’s mass

why are large amounts of energy needed/ what is it used for in the brain

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