S8 L1: The Cerebral Cortex and Functions of the brain lobes

Question 1

What is the structure of the cerebral cortex?

Answer 1

• Fine structure
• 6 layers containing cell bodies and dendrites- grey matter
• Outputs via pyramidal neurones and project to widespread areas
• Inputs from thalamus and other cortical areas
• Interneurons connect inputs and outputs

Question 2

Where do the outputs project to?

Answer 2

• Projection fibres going down to brainstem and spinal cord
• Commissural fibres going between hemispheres
• Association fibres connecting nearby regions of the same cortex in the same hemisphere

Question 3

How have the functions of the cortex been identified?

Answer 3

Cortex defined into areas with specific roles

Most of what we know about lobe functions is derived from lesion studies resulting from natural experiments

Question 4

What is the function of the frontal lobe?

Answer 4

• Motor → Primary motor cortex and associated areas
• Expression of speech → Broca’s area usually in the left hemisphere
• Behavioural regulation / judgement → prefrontal cortex
• Cognition → prefrontal cortex
• Eye movements → frontal eye fields
• Continence → cortical areas responsible for maintenance of continence e.g. paracentral lobules

Question 5

What can damage to different areas of the frontal cortex result in?

Answer 5

Depends on area of lesion

• Frontal cortex damage can result in contralateral weakness
• Damage to left frontal lobe- expressive aphasia
• Impulsive, disinhibited behaviours e.g. sexual inappropriateness, aggression
• Difficulty with tasks such as complex problem solving including calculation
• Problems with conjugate gaze and other eye movement disturbances (however, diplopia without other cortical features would suggest brainstem/cranial nerve problems)
• Urinary incontinence

Question 6

What is the function of the parietal lobe?

Answer 6

• Sensory → Postcentral gyrus/ primary sensory cortex and associated areas
• Comprehension of speech → Wernicke’s area
• Body image and awareness of external environment
• Calculation and writing
• Although not a cortical function- superior optic radiation projects through the white matter

Question 7

What can damage to the parietal lobe result in?

Answer 7

• Contralateral anaesthesia affecting all modalities
• Receptive aphasia
• Neglect condition- normal visual fields but only aware of one half of the external environment
• Left parietal lobe → affect calculation ability
• Optic radiation damage→ contralateral inferior homonymous quadrantanopia

Question 8

What is the function of the temporal lobe?

Answer 8

• Hearing → Primary auditory cortex- Superior surface of temporal lobe
• Comprehension of speech → Wernicke’s areas (superior temporal lobe)
• Olfaction → Inferior medial aspect of temporal lobe
• Memory → Hippocampus - crucial structure for declarative memory
• Emotion → Contains number of limbic system structures such as hippocampus and amygdala
• Contains inferior optic radiation

Question 9

What are the effects of lesions in the temporal lobe?

Answer 9

• Number of complex hearing defects, auditory hallucinations
• Complex effects on smell, olfactory hallucinations
• Damage may lead to amnesia (two hippocampi), some pathologies such as temporal lobe epilepsy can trigger memories leading to deja vu
• Pathogenesis of some psychiatric disorders
• Superior homonymous quadrantanopia

Question 10

What is cerebral dominence?

Answer 10

Some functions are more prominent in one hemisphere than the other

• 95% left hemisphere dominant for language and mathematical/logical function → sequential processing
• 95% right hemisphere dominant for body image, visuospatial awareness, emotion and musical ability → whole picture processing

Question 11

How does each hemisphere communicate with each other?

Answer 11

• Through the corpus callosum
• Huge bundle of white matter between the hemispheres
• Destruction results in some interesting deficits → Alien hand syndrome (hands seem to work on their own without conscious control)

Question 12

What are the different language pathways?

Answer 12

• Primarily in dominant left hemisphere
• Wernicke’s area- parieto-temporal junction, sits near to primary auditory cortex in temporal lobe, responsible for the comprehension of speech/ Interpretation of language
• Broca’s area- infero-lateral frontal lobe, sits near to mouth/pharynx area of primary motor cortex, responsible for the production of speech
• Connected to each other via the arcuate fasciculus

Question 13

How can the language pathway be damaged?

Answer 13

Large middle cerebral artery infarcts can cause damage
Strokes
Neuronal degeneration - dementia

Question 14

What is Wernicke’s aphasia?

Answer 14

Damage to Wernicke’s area
→ fluent, nonsensical speech where the patient does not appear to understand
→ Speech is fine, can speak sentences fluently
→ Meaning is impairedq
Damage to Broca’s area→ Staccato speech, patient still understands what is being said to them

Question 15

What is Broca’s aphasia?

Answer 15

Damage to Broca’s area
→ Staccato speech, patient still understands what is being said to them
→ Struggle to speak and get words out
→ Fully understand what is being asked

Question 16

What is the language pathway for repeating a heard word?

Answer 16

1. Word heard in primary auditory cortex
2. Wernicke’s area interpret auditory information- turns it into language- understand pattern
3. Through arcuate fasciculus to Brocas area
4. Broca’s area communicates with primary motor cortex to coordinate action of muscle and produce sounds of word that we heard through facial and laryngeal msucles

Question 17

What is the language pathway for speaking a written word?

Answer 17

1. Reading uses visual system, recognition in visual cortex
2. Word sent from visual system to Wernicke’s area- interpret as word
3. Sent to Broca’s area via arcuate fasciculus
4. Broca’s area communicates with primary motor cortex- producing pattern of muscle contraction to produce sound

Question 18

What is the language pathway for speaking a ‘though’?

Answer 18

1. Thought collectively converse on Wernicke’s area
2. Wernicke’s area → interpret as word
3. Sent to Broca’s are via arcuate fasciculus
4. Broca’s are communicating with primary motor cortex- producing pattern of muscle contraction to produce sound

Question 19

Where is memory stored in the brain?

Answer 19

Believed to be stored across the brain
Explicit → within the cortex
Implicit → subcortical structures e.g. basal ganglia, and cerebellum

Question 20

What are the different types of memories?

Answer 20

Explicit/ declarative → factual information

Implicit/ nondeclarative → motor skills, emotions

Question 21

What is the difference between short and long term memories?

Answer 21

Short→ stored for seconds to minutes as a ‘reverberation’ or ‘echo’ in cortical circuits
Long→ stored for long periods in the cerebral cortex, cerebellum etc. (up to a lifetime) following consolidation

Question 22

What is consolidation?

Answer 22

Convert short term memories into long term memories

Question 23

What factors contribute to consolidation?

Answer 23

Emotional cortex → if an event has a strong emotional content, then it tends to be remembered better
Rehearsal → repetition
Association → if you associate a piece of knowledge with something you already know it tend to be more easily remembered

Question 24

What structure is responsible for consolidating declarative memories?

Answer 24

• Hippocampus → deep within the temporal lobe (rolled medial edge of temporal lobe)
• Multimodal inputs from many brain system (making it good at associating stimuli)
• It has a role as an ‘oscillator’, facilitating consolidation of memories in the cortex via its output pathways (primarily the fornix → mammillary bodies → thalamus → cortexs)

Question 25

What is long term potentiation?

Answer 25

• Key molecular mechanism for strengthening memories
• Causes change in glutamate receptors in synapses leading to synaptic strengthening
• New physical connection can also form between neurones to further strengthen connections (axonal sprouting) → ↑synaptic terminals, ↑NT release, more NT expressed in synapse