M a s t i c a t i o n S a l i v a t i o n V o c a l i s a t i o n
B Fig. 7.4 Cortical function. A Features of localised cerebral
lesions. B Somatotopic homunculus.
The 12 pairs of cranial nerves (with the exception of the olfactory
(I) pair) arise from the brainstem (Fig. 7.5 and Box 7.4). Cranial
nerves II, III, IV and VI relate to the eye (Ch. 8) and the VIII nerve
to hearing and balance (Ch. 9).
The olfactory nerve conveys the sense of smell.
Bipolar cells in the olfactory bulb form olfactory filaments with small
receptors projecting through the cribriform plate high in the nasal
cavity. These cells synapse with second-order neurones, which
project centrally via the olfactory tract to the medial temporal
Bedside testing of smell is of limited clinical value, and rarely
performed, although objective ‘scratch and sniff’ test cards,
such as the University of Pennsylvania Smell Identification
Test (UPSIT), are available. You can ask patients if they think
their sense of smell is normal, although self-reporting can be
The posterior part of the frontal lobe is the motor strip (precentral
gyrus), which controls voluntary movement. The motor strip
is organised somatotopically (Fig. 7.4B). The area anterior
to the precentral gyrus is concerned with personality, social
behaviour, emotions, cognition and expressive language, and
contains the frontal eye fields and cortical centre for micturition
Frontal lobe damage may cause:
• personality and behaviour changes, such as apathy or
• loss of emotional responsiveness, or emotional lability
• cognitive impairments, such as memory, attention and
• dysphasia (dominant hemisphere)
• conjugate gaze deviation to the side of the lesion
• primitive reflexes, such as grasp
• focal motor seizures (motor strip).
The temporal lobe contains the primary auditory cortex, Wernicke’s
area and parts of the limbic system. The latter is crucially important
in memory, emotion and smell appreciation. The temporal lobe
also contains the lower fibres of the optic radiation and the area
Temporal lobe dysfunction may cause:
• focal seizures with psychic symptoms
• contralateral upper quadrantanopia (see Fig. 8.5(4))
• receptive dysphasia (dominant hemisphere).
The postcentral gyrus (sensory strip) is the most anterior part
of the parietal lobe and is the principal destination of conscious
sensations. The upper fibres of the optic radiation pass through
it. The dominant hemisphere contains aspects of language
function and the non-dominant lobe is concerned with spatial
Features of parietal lobe dysfunction include:
• cortical sensory impairments
• contralateral lower quadrantanopia (see Fig. 8.5(5))
• dyslexia, dyscalculia, dysgraphia
• apraxia (an inability to carry out complex tasks despite
having an intact sensory and motor system)
• focal sensory seizures (postcentral gyrus)
• visuospatial disturbance (non-dominant parietal lobe).
The occipital lobe blends with the temporal and parietal lobes
and forms the posterior part of the cerebral cortex. Its main
function is analysis of visual information.
Occipital lobe damage may cause:
• visual field defects: hemianopia (loss of part of a visual
field) or scotoma (blind spot) (see Fig. 8.5(6)).
• visual agnosia: the inability to recognise visual stimuli
• disturbances of visual perception, such as macropsia
(seeing things larger) or micropsia (seeing things smaller)
7.4 Summary of the 12 cranial nerves
Nerve Examination Abnormalities/symptoms
Optic disc and retinal changes
Strabismus, diplopia, nystagmus
Impairment, distortion or loss
Increase in upper motor neurone
IX Pharyngeal sensation Not routinely tested
X Palate movements Unilateral or bilateral impairment
ganglion, the V nerve passes to the pons. From here, pain and
temperature pathways descend to the C2 segment of the spinal
cord, so ipsilateral facial numbness may occur with cervical cord
There are three major branches of V (Fig. 7.6):
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