FUNCTION

Analysis of vision

LESIONS

Disinhibition

Lack of initiative

Antisocial behaviour

Impaired memory

Incontinence

Grasp reflexes

Anosmia

1 2

4 3

LESIONS

Homonymous hemianopia

Hemianopic scotomas

Visual agnosia

Impaired face recognition

(prosopagnosia)

Visual hallucinations

(lights, lines and zigzags)

A

Toes

Ankle

Knee Hip

Trunk

Shoulder

Elbow

Wrist

Hand

Little

Ring

Middle

Index

Thumb

Neck

Brow

Face

Eye

Lips

Jaw

Tongue

Swallowing

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.

Cranial nerves • 127

7

Cranial nerves

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).

Olfactory (I) nerve

The olfactory nerve conveys the sense of smell.

Anatomy

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

lobe and amygdala.

Examination sequence

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

surprisingly inaccurate.

Frontal lobe

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

(Fig. 7.4A).

Frontal lobe damage may cause:

• personality and behaviour changes, such as apathy or

disinhibition

• loss of emotional responsiveness, or emotional lability

• cognitive impairments, such as memory, attention and

concentration

• dysphasia (dominant hemisphere)

• conjugate gaze deviation to the side of the lesion

• urinary incontinence

• primitive reflexes, such as grasp

• focal motor seizures (motor strip).

Temporal lobe

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

of auditory perception.

Temporal lobe dysfunction may cause:

• memory impairment

• focal seizures with psychic symptoms

• contralateral upper quadrantanopia (see Fig. 8.5(4))

• receptive dysphasia (dominant hemisphere).

Parietal lobe

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

awareness.

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).

Occipital 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)

• visual hallucinations.

7.4 Summary of the 12 cranial nerves

Nerve Examination Abnormalities/symptoms

I Sense of smell, each

nostril

Anosmia/parosmia

II Visual acuity

Visual fields

Pupil size and shape

Pupil light reflex

Fundoscopy

Partial sight/blindness

Scotoma; hemianopia

Anisocoria

Impairment or loss

Optic disc and retinal changes

III Light and

accommodation reflex

Impairment or loss

III, IV

and VI

Eye position and

movements

Strabismus, diplopia, nystagmus

V Facial sensation

Corneal reflex

Muscles of mastication

Jaw jerk

Impairment, distortion or loss

Impairment or loss

Weakness of chewing movements

Increase in upper motor neurone

lesions

VII Muscles of facial

expression

Taste over anterior

two-thirds of tongue

Facial weakness

Ageusia (loss of taste)

VIII Whisper and tuning

fork tests

Vestibular tests

Impaired hearing/deafness

Nystagmus and vertigo

IX Pharyngeal sensation Not routinely tested

X Palate movements Unilateral or bilateral impairment

XI Trapezius and

sternomastoid

Weakness of scapular and neck

movement

XII Tongue appearance

and movement

Dysarthria and chewing/

swallowing difficulties

128 • The nervous system

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

lesions.

There are three major branches of V (Fig. 7.6):

• ophthalmic (V1): sensory

• maxillary (V2): sensory

• mandibular (V3): sensory and motor.

C3

3

2

1

C2