Session 4 Flashcards
What are the components of the CNS?
- Brain
- Spinal cord
- Cerebellum
What information do cranial nerves carry?
- Sensory and motor information
- Not all cranial nerves are mixed
- Carry special senses e.g. taste, vision, smell
What is the forebrain?
- Part of CNS that sits on top of the brainstem
- Cerebrum and diencephalon
Describe the structure of the cerebrum
- Largest portion of forebrain
- Can be split into 2 symmetrical hemispheres
Describe the structure of the diencephalon
- Hypothalamus sits in centre
- Thalamus x2 sit on either side and act as relay stations for information coming up towards the brain
Describe the structure of the brainstem
- Top = midbrain
- Middle = pons (bulbous structure)
- Bottom = medulla
What happens once the medulla passes through the foramen magnum?
- It becomes the spinal cord
Which components of the brain make up grey matter?
- Cortex
- Sulci and gyri
Why is grey matter grey?
- Due to high density of nerve cell bodies
What is the function of the grey matter of the brain?
- Necessary for conscious awareness
- All sensory menalities arise from grey matter and need to reach grey matter in order to be perceived
Describe the cortex of the brain
- Grey matter
- Outer surface of cerebrum
- A few mm thick
- Highest level at which motor system is represented
- Conscious decisions to move body originate here
What is the function of the sulci and gyri?
- Allows increase in surface area and the number of neurones that can be packed inside the brain
What are fissures?
- Deep furrows into surface of cortex
- Longitudinal fissure (falx cerebri found here)
- Lateral fissures x2
Why is white matter white?
- Colour is due to myelinated axons
- White matter is densely packed with axons that arise from cell bodies in grey matter
What are the 4 key lobes of the hemisphere?
- Frontal
- Parietal
- Temporal
- Occipital
Which sulci/fissures delineate the different lobes of the hemisphere?
- Central sulcus delineates frontal lobe from parietal lobe
- Lateral fissure delineates temporal lobe from frontal and parietal lobes
- Parieto-occipital sulcus delineates parietal lobe from occipital lobe
What is the corpus callosum?
- White matter connecting the 2 hemispheres of the brain
What is the ventricle of the brain?
- Cavity full of CSF
- Normally covered by septum pellucidum
What terms do we use when describing the orientation of the brain?
- Dorsal (superior surface of the brain)
- Caudal (posterior surface of brain)
- Ventral (inferior surface of brain)
- Rostral (anterior surface of brain)
What terms do we use when describing the orientation of the brain?
- Dorsal (superior surface of the brain)
- Caudal (posterior surface of brain)
- Ventral (inferior surface of brain)
- Rostral (anterior surface of brain)
What are the components of the brainstem?
- Midbrain
- Pons
- Medulla
What are the major functions of the midbrain?
- Many centres and cranial nerve nuclei important for coordinating eye movement found here
- Also important for coordinating reflexes of pupils
What are the major functions of the pons?
- Important role in feeding trigeminal nerve involved in mastication
- Contains centres for controlling sleep
What are the major functions of the medulla?
- Contains important centres for CVS and the respiratory centre
Why do pathologies involving the brainstem impact a significant number of functions?
- Brainstem contains lots of nervous tissue in a relatively small area
How do nerve signals enter the brainstem?
- Signals travel from forebrain to cranial or spinal nerves via the brainstem
- Sensory information passes to forebrain via the brainstem to be conciously perceived
What behaviours are controlled by the frontal lobe?
- Voluntary motor control
- Speech production
- Social behaviour
- Impulse control
- Higher cognition (planning, thinking)
What behaviours are controlled by the temporal lobe?
- Language
- Emotion
- Long-term memory
- Sense of smell
- Hearing
- Taste
What behaviours are controlled by the parietal lobe?
- Somatosensory perception
- Spatial awareness
What behaviours are controlled by the occipital lobe?
- Visual perception
What behaviours are controlled by the cerebellum?
- Co-ordination and motor learning
What is the clinical significance of the uncus?
- Can be pushed through tentorial notch when intracranial pressure rises - uncal herniation
- Uncus is located proximal to cranial nerve III
- When uncus herniates, it pushes against this nerve, causing dysfucntion
What is the clinical significance of the uncus?
- Can be pushed through tentorial notch when intracranial pressure rises - uncal herniation
- Uncus is located proximal to cranial nerve III
- When uncus herniates, it pushes against this nerve, causing dysfunction
What is the uncus?
- Where we receive olfactory information
What is meant by a topographical representation of the brain?
- Specific parts of the body are mapped onto specific regions of the cortex
- Certain parts of the body have greater representation in the cortex than others e.g. fingertips, tongue, lips
- Mapping helps localise where lesions might be in the brain because lesions will cause very specific neurological dysfunction
Outline the route taken by motor pathways in the brain
- Primary motor cortex is responsible for motor control in contralateral side of the body
- Pathways decussate (cross) to opposite side at level of lower medulla
- I.e. motor control of the left arm is initiated in the right side of the brain
Outline the pathway taken by sensory pathways in the brain
- Sensory information comes into the primary somatosensory cortex
- This pathway also crosses
- I.e. sensory information felt in right arm reaches and is perceived by left somatosensory complex
What do we mean by the statement that spinal nerves are mixed?
- Spinal nerves contain some axons of motor neurones and some axons of sensory neurones
What is meant by the statement that cranial nerves have dual cortical control?
- Pathways connecting primary motor cortex with cranial nerves controlling the muscles of neck/face decussate to the opposite side
- Crossing over occurs at level of nuclei onto which they synapse
- Cortical control of muscles is contralateral
- But most cranial nerves also have a back-up from the ipsilateral cortex
Give brief overview of the cranial nerves
- Supply tissues and structures if head and neck region including special sense organs
- Part of peripheral nervous system (except CNS I and II)
- Arise as pairs
- Most are associated with the brainstem due to their nuclei location
What kinds of fibres are carried in the cranial nerves?
- Some are mixed with motor and sensory fibres
- Some can be purely motor or sensory
- 4 cranial nerves carry parasympathetic fibres
- Nio sympathetic fibres
What can neurological signs of cranial nerve dysfunction arise due to?
- An injury or lesion involving
1. The cranial nerve during its route outside of the CNS
2. The brainstem where CN nuclei are located
3. The neurones within the forebrain/brainstem which connect other parts of the brain to cranial nerves
What is meant by 2 2 4 4?
- 2 cranial nerves arise from the forebrain
- 2 cranial nerves arise from the midbrain
- 4 cranial nerves arise from the pons
- 4 cranial nerves arise from the medulla
List the names and numbers of all the cranial nerves
- I = olfactory nerve
- II = optic nerve
- III = oculomotor nerve
- IV = trochlear nerve
- V = trigeminal nerve
- VI = abducens nerve
- VII = facial nerve
- VIII = vestibulocochlear nerve
- IX = glossopharyngeal nerve
- X = vagus nerve
- XI = accessory nerve
- XII = hypoglossal nerve
What is special about CN I and CN II?
- They arise as extensions of the forebrain
What can cause olfactory nerve lesions?
- Head/facial injury - shears olfactory neurones during passage through cribriform foramina
- Anterior cranial fossa tumours - compresses olfactory bulb/olfactory tract
- Parkinson’s, Alzheimer’s
- Commonest cause for anosmia is a common cold
How do we test for olfactory nerve lesions?
- Absence or reduced sense of smell (anosmia/hyposmia)
- Test one nostril at a time
- Not routinely tested
Outline the passage of the olfactory nerve
- Olfactory receptors within olfactory mucosa
- Olfactory nerves travel up through base of skull through cribriform foramina
- Travel to right or left olfactory bulb
Why is the optic nerve affected by raised intracranial pressure?
- It carries an extension of the meninges
How is the optic nerve tested?
Pupillary size and response to light - CNII forms sensory/afferent limb of the pupillary light reflex
- Visual acuity (Snellen chart) and visual fields
- Ophthalmoscopy - can directly visualise part of optic nerve
What will a patient with an optic nerve lesion report?
- Blurred vision or complete absence of vision in the eye supplied by the affected optic nerve
- On clinical examination, patient will have poor visual acuity, abnormalities in pupil size and response to light, evidence of pathology on ophthalmoscopy
Which diseases can cause optic nerve lesions?
- Optic neuritis
- Anterior ischaemic optic neuropathy
What changes might you see on opthalmoscopy of a patient with an optic nerve lesion?
- Papilloedema - swollen optic disc seen in cases of raised intracranial pressure
- Pale optic disc
Outline the route taken by the optic nerve
- Retinal ganglion cell axons converge at optic disc and form optic nerve
- Optic nerve exits back of orbit via optic canal
- Fibres from left and right optic nerve merge at optic chiasm (close to pituitary gland)
- Continue as left and right optic tracts
- Some fibres communicate from tract into brainstem to give information about light intensity and control pupil size
- Some continue visual pathway
Which pathologies can cause optic nerve lesions?
- Retinal detachment
- Optic neuritis
- Pituitary tumour - compress optic chiasm
- Strokes
Why can optic nerve lesions affect either one eye or both eyes?
- Depends on location of lesion along optic nerve
- Pathology affecting a retina or an optic nerve on one side will cause blurring/visual symptoms in that one eye
- Lesions involving optic chiasm onwards cause visual disturbances in both eyes
What do the oculomotor, trochlea and abducens nerve all have in common?
- Supply muscles in the orbital cavity that move the eyeball
- Have commonality in route after exiting brainstem at different levels
What is the common route taken by the oculomotor, trochlea and abducens nerve?
- Cavernous sinus
- Superior orbital fissure
- Orbital cavity
How do we clinically test the oculomotor, trochlea and abducens nerve?
- Observe patient’s resting gaze
- Ask patient to perform a series of eye movements
What fibres make up the oculomotor nerve?
- Contains motor and parasympathetic fibres
What are the targets of the oculomotor nerve?
- Somatic efferent fibres are motor
- Travel to skeletal muscle - all extraocular muscles (except 2)
- Also supply levator palpebrae superioris (opens eyelid) - Visceral efferent fibres are parasympathetic
- Supply muscles inside the eyeball: ciliary muscle (controls thickness of lens) and sphincter pupillae
Explain how we clinically examine the oculomotor nerve
- Check eyelid position - supplies levator palpabrae superioris that keeps eyelid retracted
- Eye movements - oculomotor nerve responsible for most muscles that move eyelid
- Pupils and pupillary light reflex - parasympathetic fibres supply muscles controlling pupillary constriction
What suggests an oculomotor nerve lesion?
- Pt reports double vision
- Ptosis
- Abnormal position of eye - ‘down and out’
- Pupil may or may not be dilated
- Signs arise due to involvement of somatic fibres
Where does the oculomotor nerve arise from?
- Midbrain
- Has close relationship to tentorium cerebelli edge as it travels to cavernous sinus
How are parasympathetic fibres arranged in a mixed cranial nerve?
- Around the periphery
What are the pupil sparing oculomotor nerve lesions?
- Microvascular ischaemia
- Risk factors include: age >50 years and diabetes/hypertension
What are pupil involving oculomotor nerve lesions?
- Compressive lesions
- E.g. aneurysmal, head injury, uncal herniation
What is the function of the trochlear nerve?
- Supplies superior oblique muscle
What fibres make up the trochlear nerve?
- Motor only
How do we examine the trochlear nerve?
- Inspect resting gaze - subtle abnormal eye position
- Test eye movements - pt will have difficulty moving eye downwards when eye is positioned inwards
- Pt will also report double vision
Where does the trochlear nerve arise from?
- Dorsal midbrain
How do trochlear nerve lesions arise?
- Congenital or acquired
- Acquired due to:
- microvascular ischaemia (risk factors include age >50, diabetes, hypertension)
- trauma (causes nerve to snap or stretch)
- intracranial tumour
From where does the abducens nerve arise?
- Caudal pons
- Takes a vertical upwards route to cavernous sinus - this makes it susceptible to stretch e.g. when intracranial pressure rises
What can result in abducens nerve lesions?
- Microvascular ischaemia (diabetes, hypertension)
- Head injury, tumour
- Raised intracranial pressure - results in false localising sign
Out of the oculomotor, trochlea and abducens nerve, which is most likely to be affected by raised ICP?
- Abducens nerve
- Has upwards vertical route
- Is fixed at point of brainstem exit and entry to cavernous sinus
What would you see on examination of a patient with abducens nerve lesion?
- Pt reports double vision
- Abnormal eye position at rest (one eye moves inwards when pt tries to look ahead)
- Difficulty/inability to move affected eye laterally
