Brain Structure and Function

Question 1

divisions of the nervous system

Answer 1

• CNS (brain and spinal cord)
  -Peripheral nervous system
• Autonomic nervous system
  -Sympathetic and parasympathetic divisions
  -Somatic nervous system
• sensory nervous system and motor system

Question 2

5 major structures of the brain

Answer 2

• 5 divisions are evident in the human brain from embryonic development through to adulthood
  
  • Myelencephalon- medulla- largely comprises tracts between brain and spinal cord
  • Metencephalon- pons and cerebellum
  • Mesencephalon- tectum and tegmentum
  • Diencephalon- thalamus and hypothalamus
    Telencephalon- cerebral cortex, limbic system and basal ganglia

Question 3

cerebral cortex

Answer 3

• Composed of small unmyelinated neurons
  
  • Grey matter (other layers are composed of large myelinated axons and are white matter)
  • Convolutions serve to increase surface area
  • Large convolutions = fissures
  • Small convolutions = sulci
  • Ridges between fissures and sulci = gyri
  • Longitudinal fissures separates hemispheres (it remains connected by cerebral commissure inc corpus callosum)
    Contains the neocortex and subcortical structures (hippocampus, limbic system and basal ganglia)

Question 4

neocortex

Answer 4

• It is the newest part of the cerebral cortex to evolve
  
  • Cerebral cortex- largest part of the telencephalon, composed of grey matter
  • Neocortex- largest part of the cerebral cortex (90%) of cerebral cortex is neocortex in humans)
  • Other part is allocortex (contains hippocampus)
  • Main difference is that the neocortex has six layers- the most developed in its number of layers and organisation of the cerebral tissues (specific to mammals)
  • Humans have large neocortex ratio, which correlates with complexity of behaviour
    ○ For a large neocortex to evolve, brain must evolve in size to support it
  • Central and lateral fissure divide each hempishere into 4 lobes (frontal, parietal, occipital and temporal)
    Lobes are not functional units

Question 5

4 lobes of the cerebral cortex/neocortex

Answer 5

• frontal lobe: motor cortex
• parietal lobe: somatic sensations
• temporal lobe: hearing and language
• occipital lobe: visual processing

Question 6

the rise and fall of phrenology

Answer 6

• An enduring question in biopsychology is the extent to which functions of the brain (language, thought movement etc) can be localised to specific areas of the brain
  
  • Franz Joseph Gall (1758-1828)- famous proponent of localisation theory- founded phrenology
    ○ Phrenology comes from the Ancient Greek ‘phren’ meaning ‘mind’ and ‘logos’ meaning ‘knowledge’
    ○ A pseudomedicine which attempted to divine individual intellect and personality from examination of skull shape- assumed the surface of the skull faithfully reflects the relative development of various regions of the brain
    ○ Borne out of observation of his schoolmate being able to recite long passages of prose who had bulging eyes- he reasoned the ability for verbal memory lay in the frontal region behind the eyes
    ○ His lectures on ‘cranioscopy’ offended religious leaders and were condemned in 1802 by Austrian government and banned. 3 years later he was forced to leave the country
    ○ Gall identified 27 cranial regions in total that correspond to distinct mental traits
    ○ He found regions responsible for murder and inclination to steal (by feeling criminals heads until he began to ‘detect patterns’
    Localised ‘destructiveness’ to above the ear- because he had a student who liked torturing animals who had a bump by his ear

Question 7

Galls positive contributions

Answer 7

• Although phrenology was discredited there were some positive contributions to research
  
  • He believed the brain was the physical organ of the mind which governed mental faculties and feelings
  • He proposed the idea that the cerebral cortex contains areas with localised functions
  • He was proved correct as later specific regions of the cerebral cortex were shown to be specifically involved in language (Broca located a speech centre in 1861) and movement (motor cortex)
  • Gall was the first to identify grey matter of the brain with active tissue (neurons) and white matter conducting tissue (ganglia)
    His views were modern for the time, and they inspired others to explore the brain e.g. Pierre Flourens (1794-1867)- the first scientist to use lesioning (the removal of tissue from the brain) as a means of experimentally studying the brains different regions

Question 8

Lesion studies: Broca’s Aphasia

Answer 8

• In 1860 Paul Broca was interested in Galls claims that language functions were located in frontal lobes of the brain
  
  • Broca was consulted about a 51 year old patient with multiple neurological problems who had been without speech for many years
    ○ Could only say the word ‘tan’
    ○ A test case for question about language localisation in the frontal lobes- since no productive language
    ○ Tan died several days later- autopsy revealed lesion on the surface of left frontal lobe
  • A second patient had reduced speech due to stroke 1 year previous
    ○ 85 year old patient could only say 5 words
    ○ At autopsy there was a lesion at approximately same region as Tan
    Photographs of the brains of Paul Broca’s first 2 aphasic patients
  • This speech deficit is known as Broca’s aphasia
    Inferior frontal gyrus on the left cerebral hemisphere- known now as Broca’s area

Question 9

Lesion studies- Wernickes Aphasia

Answer 9

• According to Broca- damage restricted to Broca’s area should disrupt speech production, but not comprehension
  
  • The next major event in cerebral localisation of language- Carl Wernicke’s 10 clinical cases of language comprehension (1874)
  • Wernicke suggested that selective lesions of Wernicke’s area produced a syndrome that is primarily receptive
    ○ Characterised by poor comprehension of written and spoken language and speech that is meaningless, but still retains superficial structure, rhythm and intonation of normal speech
  • Wernicke’s aphasia- word salad
    Localised by autopsy to the left temporal lobe

Question 10

Localisation- Brodmanns area

Answer 10

• So work was already being conducted to localise language to specific brain regions
  
  • Further progress was made by German neurologist Korbinian Brodmann (1868-1918)
  • Began to produce maps of the brain based on cytoarchitectural organisation of neurons in cerebral cortex using the Nissl method of cell staining
  • Identified 52 areas of the cerebral cortex that differ histologically (cells/structures)
    These are known as Brodmann functional areas of cerebral cortex

Question 11

Importance of Brodmanns area

Answer 11

• Brodmann’s areas were defined based solely on their neuronal organisation, but have since been correlated closely to diverse cortical functions
  
  • E.g. Broca’s speech and language areas were localised to BA 44 and 45
    Thus Brodmann provided a map based on collections of neuron types- which have been examined using lesion studies, experimental ablation and functional neuroimaging to map onto different brain functions

Question 12

Functional neuroanatomy

Answer 12

• With Brodmann’s findings of differing cell types located in clusters, this propelled the idea of functional localisation
  
  • Since then, lesion studies, case studies, experimental ablation in animals and functional neuroimaging have opened up our understanding of how the brain works
  • We move from basic naming of lobes and structures by location to naming areas by function e.g. motor areas and visual cortex- and that multiple areas contribute to behaviour- these brain regions connect with each other and pass information
  • General classification of three function areas: sensory, motor and association
    We are still figuring out how the brain makes connections and produces behaviour

Question 13

Prefrontal cortex

Answer 13

• Very developed in humans
  
  • Belies complex cognitive behaviour, conscious thought, social behaviour, personality, decision making
  • Executive functions- higher order cognitive functions- inhibitory control, updating memory, switching attention, word fluency
  • Working memory
  • Recall
    People with head injuries show deficits in these functions

Question 14

prefrontal cortex damage

Answer 14

• Early studies in humans and monkeys report large portions of the PFC can be removed without loss to mental capacity or changes in behaviour (Hebb, 1939)
  
  • This view contributed to widespread use of psychosurgery (lobotomy or leucotomy) for treatment of psychiatric disorders in first half of 20th century
  • Lobotomy- severing connections from PFC to other areas of the brain
  • Procedure introduced by Antonia Egas Moniz who won the noble prize for physiology and medicine (1949) for the ‘discovery of therapeutic value of leucotomy in certain psychosis’
  • Success described as ‘mixed’ with some patients being more docile, able to leave hospital or become more manageable- others committed suicide or were severely brain damaged
  • David Ferrier (1876): experimental ablation of frontal lobes of monkeys- sensory faculties of sight, hearing, touch, taste and smell unimpaired, however lost the faculty of attentive and intelligent observation
    Lesions (due to head injury or cancers) in humans have led to further localisation of functions

Question 15

Prefrontal subdivisions

Answer 15

• Dorsolateral prefrontal cortex: working memory, rule learning, planning, attention and motivation
• orbitofrontal cortex: inhibitory and emotional control and an inability to effectively function in the social domain
• ventrolateral: disparate functions including spatial attention, inhibitory control and language

Question 16

primary motor cortex

Answer 16

• Located in the precentral gyrus of the frontal lobe
  
  • 1937 Penfield and Boldrey mapped primary motor cortex of conscious human patients during neurosurgery with low intensity electrical stimulations to points on the cortical surface- they noted which body parts moved in response to stimulation
  • Each stimulation activated a contralateral muscle and produced simple movement- they found that the primary motor cortex is organised somatotopically
    Somatotopic layout referred to as motor homunculus

Question 17

primary motor cortex lesions

Answer 17

• Extensive damage to human primary motor cortex does not actually eliminate all voluntary movement
  
  • Large lesions to primary motor cortex do disrupt ability to move individual body parts e.g. one finger independently of others, and reduce speed, accuracy and force of movements
  • Other movements able probably due to association and secondary motor areas

Question 18

association motor areas

Answer 18

• Posterior parietal cortex- integrates orientation information about body parts, and external objects positions- provides spatial information prior to movement
  
  • DLPFC received projections from posterior parietal cortex and projects to secondary motor cortex, primary motor cortex and frontal eye field
    DLPFC responds in anticipation of motor activity

Question 19

sensory areas (hearing, touch, smell, taste, vision)

Answer 19

• Sensory areas of the cortex consist of primary, secondary and association areas
  
  • Primary areas receive input from thalamic relay nuclei
  • Secondary sensory cortex receives input from primary sensory cortex of a system or other areas of secondary sensory cortex
  • Association areas integrates info from more than one sensory system
  • Posterior parts of the brain behind central sulcus
  • Large parts of the brain dedicated to processing sensory stimuli
    Postcentral gyrus- location of primary somatosensory cortex

Question 20

primary somatosensory cortex

Answer 20

• Penfield et al (1937)- electrical stimulation to cortical surface- patients who were fully conscious described what they felt
  
  • Brodmann areas 1-3 (postcentral gyrus)- patients reported sensations in various parts of their body
  • Somatotopic organisation (sensory homunculus)
  • Medial parts= leg, lateral parts= face
  • Distribution biased towards areas where sensory discrimination is high (fingers, mouth)
    SII- ventral to SI in postcentral gyrus- receives input from S1 (secondary somatosensory cortex)

Question 21

somatosensory system and association cortex

Answer 21

• Damage to primary somatosensory cortex has surprisingly mild effects, due to having multiple pathways
  
  • Corkin et al (1970) unilateral lesion of S1 in epileptic- two minor contralateral deficits- ability to detect light touch and reduced ability to identify objects by touch- deficits were bilateral is lesion went into S2
    Somatosensory signals conducted to highest level of sensory hierarchy is association cortex (prefrontal and posterior parietal cortex)

Question 22

somatosensory agnosias

Answer 22

• Two types: astereognosia and asomatognosia
  
  • Astereognosia- inability to recognise objects by touch- these are rare in the absence of simple sensory deficits
  • Asomatognosia- failure to recognise parts of ones own body- usually unilateral affecting only left side of the body- associated with extensive damage to the right temporal and posterior parietal love (association cortex)
    Case of Aunt Betty

Question 23

visual cortex

Answer 23

• Vision is also represented in the brain in 3 major regions
  
  • Primary visual cortex- located in the posterior region of occipital lobes- receives most of input from visual relay of the thalamus
  • Areas of secondary visual cortex (prestriate and inferotemporal corices) receive input from primary visual cortex and visual association cortex
    Association cortex (posterior parietal cortex)

Question 24

damage to the primary visual cortex

Answer 24

• Damage to an area of the primary visual cortex produces a scotoma (area of blindness) in the corresponding area of the contralateral visual field
  
  • Contralateral- relating to the side of the body that is opposite to that on which a structure of the brain occurs
  • Many patients with scotomas are not consciously aware of their deficits- sometimes visual completion can occur
    “Talking with a friend, I glanced just to the right of his face wherein his head disappeared. His shoulders and necktie were still visible but the vertical stripes on the wallpaper behind him seemed to extend down to the necktie. It was impossible to see this as a blank area when projected on the striped wallpaper of uniformly patterned surface, although any intervening object failed to be seen”. (Lashley, 1941, p. 338)

Question 25

other areas of the visual system identified from fMRI

Answer 25

• So far 12 or so functional areas of human visual cortex have been identified
  
  • There are 30 in monkeys (24 secondary areas, and 7 in association areas)
    ○ The neurons in each area respond to different aspects of vision- colour, movement, shape etc)
  • Selective lesions produce different visual losses
    There are many connection pathways between these

Question 26

dorsal and ventral stream

Answer 26

• Information from primary visual cortex project to areas of secondary visual cortex and association cortex via two major streams
  
  • Dorsal stream projecting up to the posterior parietal cortex
  • Ventral stream which projects across to inferotemporal cortex
  • Dorsal stream- spatial stimuli (location of objects and their movement)
  • Ventral stream- characteristics of objects (colour/shape)
  • Damage to the posterior parietal cortex- can describe objects but cannot reach out to pick them up
    Damage to the inferotemporal cortex- difficulty describing but no difficulty reaching to pick them up

Question 27

damage to the secondary visual cortex -prosopagnosia (face blindnessO

Answer 27

• Term coined in 1947 by German neurologist Joachim Bodamer from the Greek prosipon (side) agnosia (not knowledge)
  
  • Difficulty recognising people that they have encountered many times
  • Documented cases usually from brain damage to right fusiform gyrus during head trauma, stroke and degenerative disease
  • Dr. P is described by Oliver Sacks in his book ‘the man who mistook his wife for a hat’
    ○ An eminent musician with a progressive cognitive failure- he would get confused between objects
    ○ At the end of an interview with Dr Sacks he confused the head of his wife for a hate and grabbed her in an attempt to put it on his head

Question 28

other sensory areas

Answer 28

• Auditory areas- primary auditory cortex: superior temporal lobe, inside lateral sulcus (BA 41)
  ○ Auditory association area: posterior to primary auditory cortex (BA22)- evaluates sounds (next to Wernicke’s area0
  
  • Gustatory (taste) cortex (BA43)- roof of lateral sulcus
  • Olfactory (smell) cortex- medial temporal lobe- connects to limbic system (emotions)
    Association areas are where primary inputs are processed and comprehended

Question 29

monamine pathways in the brain

Answer 29

• Neurotransmitter pathways have also been mapped in the brain
  
  • Dahlstrom and Fuxe (1964) used immunofluorescence staining techniques to visualise the monoamine neurotransmitter pathways of serotonin, noradrenaline and dopamine
    Monoamine neurotransmitters emanate from brain stem and project to the forebrain and beyond

Question 30

cerebral lateralisation of function

Answer 30

• Left and right cerebral hemispheres are separate apart from cerebral commissures connecting them
  
  • Dax (1836)- noted that he had 40 brain damaged patients with speech problems- all of which had damage in the left hemisphere
  • Both of Broca’s aphasia patients had left hemisphere legions in frontal cortex- then a further 7 patients all had lesions to Broca’s area- which was localised to left PFC
  • Hugo-Karl Liepmann- apraxia associated with left hemisphere damage- even though symptoms are bilateral
  • Much research then focused on finding out lateralised functions to varying degrees of success
  • fMRI, PET, unilateral lesions, split brain patients have been studied to observe lateralisation of function- language and motor abilities of left hemisphere are readily apparent
  • For many functions there are no substantial differences between hemispheres; and when they do exist it is only slight biases for one hemisphere
  • However lateralisation is statistical rather than absolute- language is the most lateralised
    Certain functions display a superiority for one hemisphere above the other

Question 31

advanced neuroimaging techniques

Answer 31

• Our understanding of brain anatomy is advanced- but we are still at a relatively early stage of relating anatomy to behaviour
  
  • Much of our understanding has developed from case studies, lesion studies and experimental ablation studies
    Structural and functional MRI can allow us to improve our understanding of structure- functional relationships in the brain in vivo

Question 32

functional mri

Answer 32

• With functional MRI we advanced MRI to figure out how not only to show structure of the brain but also activity
  
  • Blood flow and neuronal activation are tightly coupled i.e. blood is pumped to an area of the brain when it becomes ‘active’
  • fMRI detects changes in blood flow (i.e. an increase) due to blood being diamagnetic- therefore we can image increases in brain activity
  • fMRI is confirming and expanding our understanding of structure-function relationships
    ○ E.g. imaging word generation (dysfunctional in Broca’s aphasia)
    ○ A meta-analyses o all ‘word generation’ studies (66 papers, 197 experiments, 1552 coordinates), a widely used test of neuropsychological function
    The meta-analytic results revealed extensive convergence in large portions of the left inferior frontal gyrus, centring on BA44/45 (Broca’s area)
  • fMRI can help us understand the plasticity of the brain following injury
    ○ fMRI can identify brain plasticity changes during recovery from a stroke
    These patients recovered language following a debilitating stroke, and their fMRI data shows how structures in the right hemisphere developed to compensate for left hemisphere damage

Question 33

functional connectivity analysis

Answer 33

• Functional connectivity analysis is an fMRI analysis technique that allows to observe which brain region correlate with one another in terms of their activation during a specific activity
  
  • We can measure the BOLD signal from the entire brain during an interesting task which encompasses the reward system- for example a gambling task- and compare this in relevant groups e.g. substance users vs controls
  • We can examine the BOLD signal changes associated with the trials when we expect system level activation (winning trials)
    By selecting an appropriate seed area, such as the nucleus accumbens, we can use the BOLD signal from the rest of the brain to see which other regions correlate in activity and whether this differs between groups

Question 34

frontiers in neuroscience

Answer 34

Transcranial stimulation: TMS is a non invasive method of brain stimulation that relies on electromagnetic induction using an insulated coil placed over the scalp, focused on an area of the brain thought to play a role in mood regulation

Question 35

brain computer interface

Answer 35

Researchers e.g. at Stanford University are taking what we have learnt about the study of the brain, and localisation of function, and are developing technologies to help people with paralysis of the body communicate using electrical signals from their brains