neuroanatomy and imaging

What is the basic anatomy of the brain split into

3 divisions

Forebrain

Midbrain

Hindbrain

Hindbrain

Oldest part of brain

Controls functions needed for life -autonomic functions

Includes cerebellum and most of brainstem

Why is this part of the brain important

Important for optometric practice as some px have “chiari malformations “ where the brainstem is forced through the magnum foramen -hole at the base of the skull

-these px often have issues with headaches ,diplopia, nystagmus and photophobia

Midbrain

Top of the brainstem

Responsible for saccades and managing visual attention -superior colliculus and eye movements , upper eyelid and pupil reactions (oculomotor nucleus)

Damage to part of midbrain

Can cause diplopia , nystagmus , vertical gaze palsy and tracking difficulties as well as impaired pupil reflexes

What does forebrain include

Includes cerebral cortex and limbic system

What does damage to this part cause

Affects how the brain interprets information

Px may experienced visual field loss, blurred vision, diplopia and poor spatial awareness

Cerebral cortex

Divided into two hemispheres each with 4 lobes

-folded surface provide large surface area for neurons , the more folded the brain the cleave re the animal

-bumps are “gyri” and the grooves are “sulci”

these two provide landmarks

Cerebral cortex image

Frontal lobe and its role

Largest of the 4 lobes

In charge of higher executive functions ,planning,problems ,solving ,reasoning , judgement

-responsible for personality and emotions and impulse control

includes brocas area which is main region for producing speech

What is the frontal lobe the most common area for

Brain damage , changes such as loss of speech, remembering words, executive dysfunction ,socially inappropriate behaviour

-also involved in neurodevelopment conditions ,ASD, ADHD

Parietal lobe

is the somatosensory part of the brain -interpretation of sensory information

Includes touch,pressure, temperature, and pain

What is parietal lobe responsible for

Creating a map of the world called spatial attention and orientation

Main part of the brain involved in reading and writing

-is also where visual perception and attention occurs

Sensory motor cortex

Area found at the join between the frontal and parietal lobes

Temporal lobe

Holds primary auditory cortex

Is in charge of oculo vestibular responses -balance and related eye movements

-home to wernikes area -responsible for the comprehension of speech and links to Broca’s area in the frontal lobe

Responsibility of temporal lobe

Memory, object and facial recognition

-px with temporal lobe defects can suffer with prosopagnosia (faces) or anomia (objects) as well as aphasia (language)

Occipital lobe

Completely devoted to visual processing

Links to parts of the temporal and parietal lobe to complete the pathway

Vision is thought to take up 30-50% of the brain

Dorsal and ventral pathways

Two anatomically distinct and functionally specialised pathways for processing different aspects of vision

-identified in 1982 by ungerleider and mishkin and build on by milner and goodale 1992 to form “perception and action” model of visual processing

Dorsal pathway

Beings in occipital lobe and travels to parietal

“Where” or “perception” pathway

Involved in locating objects in spacen

Ventral pathway

Beins in occipital lobe and travels to temporal lobe

“What” or “action” pathway

Involved in processing shapes ,textures ,patterns and colours

influence of dorsal and ventral “perception and action” pathways

remain highly influential

however there’s newer evidence suggesting huge numbers of of inter stream interactions which has changed the idea that we have functionally independent pathways

-complex visual functions likely to involve collaboration between the two specialised visual streams as well as other areas of the brain

What is the limbic system known as

Survival centre

Responsible for emotion and behavioural expression

What is the limbic system made up of

Thalamus - relay centre for sensory information

Hypothalamus - regulates hormones as well as controls body temperature, appetite and flight or fight response

Hippocampus - Houses emotions, learning and short term memory

Amygdala- also holds emotions, long term memory and fear

Pituitary gland - essential for hormones and reproduction

(Parts of limbic system run very cos to the optic nerve tract and chiasm)

Magnetic resonance imaging MRI

based on principles of nuclear magnetic resonance NMR

-uses very powerful magnet to align the hydrogen atoms in our body

-a brief radio signal then temporarily knocks the atoms out of alignment

-when they return to their aligned positions they emit a small amount of energy which can be measured

-different tissues and structures have different amounts of hydrogen atoms and so give different amounts of energy allowing us to distinguish them

Pros and cons of MRI

Good for imaging soft tissues of the body

Poor temporal resolution - delay in signal so wouldn’t pick up recent changes e.g haemorrhages

Contradicted for px with magnetic iron based foreign bodies

Imaging techniques - computerised tomography CT

Most similar to X ray but with greater spatial resolution

-multiple narrow beams of radiation sent through the body at different angles

-then received by a computer which uses an algorithm to create 2D or 3D image

What is CT scan good for

Imaging soft tissue , bones and blood vessels

-tissues or diff densities appear in different shades of grey , white areas habe maximum density

-similar to OCT to read , whiter areas let less light through

(Possible risk of cancer due to radiation exposure?)

MRI more detailed but CT more faster, better for trauma

Imaging techniques - positron emission tomography PET

Uses radioactive tracers as a radiation source introduced into the px body

• usually ingested but also IV if needed

• This radioactive substance can be detected in the blood by special CT scan (MRI also used occasionally)

PET scan

The more active parts of the brain use more blood and therefore show up on the scan

-the radioactive substance decays quickly (half life) and is not harmful

Mainly used for brain scans , blood vessels and tumours

Imaging technique MEG magnetoencephalography

Detects weak magnetic fields from above the surface of the head produced by electrical activity within the brain

When is MEG used

In a special room shielded from earths magnetic field and with coils kept at very low temps

Virtually silent, non invasive and quick

-well suited to clinical applications as it can be readily used in children and other populations who may not tolerate demanding procedures

-not as precise as other imaging techniques

Electroencephalography EEG

Electrophysiological technique for the recording of electrical activity arising from the human brain

How does EEG work

Microelectrodes placed on the scalp record normal electrical activity from the brain

Signal is amplified using software

Trace can be then monitored to check for abnormal activity

-excellent temporal sensitivity, can tell you when in the brain

When is EEG used

Primarily in epilepsy and brain disorders , but also in visual function research

Summary table of different imaging techniques