01. Brain Structure

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

Cerebral Cortex

• Outermost layer of the brain, composed of grey matter and small unmyelinated neurones

• Contains convolutions.

• Contains the neocortex and subcortical structures (hippocampus, limbic system, basal ganglia)

Neocortex

• Newest and largest part of the cerebral cortex to evolve

• Has six layers

• Humans have a large neocortex ratio, correlating to complexity of behaviour

• Divided into four lobes - frontal, parietal, temporal and occipital

Fissure

Large convolution in the brain

Sulci

Small convolutions in the brain

Gyri/gyrus

Ridges between fissures and sulci

Franz Gall (1758-1828)

• Founded phrenology - a pseudo medicine that attempts to attribute intellect and personality to skull shape

• Identified 27 cranial regions corresponding to distinct mental traits

Gall’s positive contributions

• Believed the brain was the physical organ of the mind, governing mental faculties and feelings

• Proposed the idea that cerebral cortex contained areas with localised functions

• First to identify grey matter with active tissue (neurons) and white matter conducting tissue (ganglia)

Pierre Flourens (1794-1867)

First scientist to use lesioning - the removal of tissue from the brain - as a means of experimentally studying the brains different regions

Broca’s Aphasia

• Issues with production of language/speech

• Associated with the inferior frontal gyrus on the left cerebral hemisphere (Broca’s area)

• Speech issues occur following damage to Broca’s area

Wernicke’s Aphasia

• Issues with language comprehension

• Poor comprehension of written and spoken language, speech that is meaningless but still retains superficial structure, rhythm and intonation

• Localised to the left temporal lobe (Wernicke’s Area)

• Comprehension issues occur following damage to Wernicke’s area

Brodmann’s Areas

• Identified by Korbinian Brodmann (1868-1918)

• Brodmann produced maps of the brain based on organisation of neurons in the cerebral cortex using cell staining

• Identified 52 areas of the cerebral cortex that differ histologically

• Known as Brodmann’s functional areas of the cerebral cortex

Importance of Brodmann’s Areas

• Provided a map based on collections of neurone types, which have been examined to map onto different brain functions

• Propelled the idea of functional localisation

General classification of three functional areas

• Sensory

• Motor

• Association

Prefrontal Cortex

• Very developed in humans

• Controls complex cognitive behaviour, conscious thought, personality and decision making

• Executive functions, inhibitory control, switching attention, word fluency, memory and recall

Lobotomy

Severing connections from the prefrontal cortex to other areas of the brain

History of the lobotomy

• Early studies reported large portions of PFC can be removed without loss to mental capacity or changes in behaviour (Hebb, 1939)

• Contributed to widespread use of psychosurgery for treatment of psychiatric disorders

• Introduced by Antonio Egas Moniz

Lobotomy outcomes

• Mixed success

• Some patients more docile, able to leave hospital or become more manageable

• Other patients comitted suicide or severely brain damaged

Dorsolateral Prefrontal Cortex (DLPFC)

Working memory

Rule-learning

Planning

Attention

Motivation

Ventrolateral Prefrontal Cortex (VLPFC)

The human inferior frontal gyrus

Disparate Functions

Spatial attention, inhibitory control and language

Orbitofrontal Cortex (OFC)

inhibitory and emotional control

An inability to effectively function in the social domain

Primary Motor Cortex

• Located in the precentral gyrus of the frontal lobe

• Organised somatotopically, referred to as motor homunculus

Primary motor cortex lesions

• Disrupts ability to move individual body parts independent of others

• Reduces speed, accuracy and force of movements

Association Motor Areas

• Posterior parietal association cortex (PPAC) provides spatial information prior to movement

• DLPFC responds in anticipation of motor activity

Sensory Areas

• Primary, secondary and association areas

• Primary areas receive input from nuclei

• Secondary areas receives the input from the primary areas

• Association areas integrate info from more than one sensory system

Somatosensory System

• Touch and pain

• Sensations from the body

Penfield et al., 1937

Electrical stimulation to cortical surface

Patients who were fully conscious described what they felt

Reported sensations in various parts of their body

Somatotopic organisation

Corkin et al., 1970

• Damage to primary somatosensory cortex has surprisingly mild effects, due to having multiple pathways

• Unilateral lesion in epileptics

• Two minor deficits - ability to detect light touch and identify objects by touch