Mid-brain

The mid-brain is approximately 1.5 cm long and extends from the pons to the mamillary bodies. The ventral surface connects to the two superior cerebellar peduncles (S2.12). The roof of the mid-brain is composed of four colliculi, the superior colliculi associated with the visual system and the inferior colliculi related to the auditory system. The red nucleus is also contained within the mid-brain. The periaqueductal grey, an area of grey matter within the mid-brain, is important in the descending modulation of pain (S3.29).

The following cranial nerves emerge from the mid-brain:

Pons

The pons is about 2.5 cm long and is composed of transverse fibres which form ridges across the surface of the area. The middle cerebellar peduncles attach to the pons.

The following cranial nerves emerge from the pons:

Medulla oblongata

The medulla is about 3 cm long and forms the base of the brain stem, adjoining the spinal cord at the level of foramen magnum. The descending motor tracts passing through it form two pyramids on the ventral surface of the medulla between which is the anterior median fissure. This fissure is disrupted where the tracts cross over the midline, termed the ‘decussation of the pyramids’. The inferior cerebellar peduncle attaches to the medulla and behind it is the ‘olive’. The olive is a prominent oval swelling that marks the position of the inferior olivary nucleus (S2.12).

The following cranial nerves emerge from the medulla:

Motor nuclei

Five of the cranial nerves are entirely motor. Afferent input to the motor nuclei is from the cerebral cortex via the cortico-bulbar tract (S2.14). The output from the cranial nerve nuclei to the effector muscle is via a lower motor neuron.

Sensory nuclei

Three of the cranial nerves are entirely sensory. The afferent input is from the relevant sensory organ, e.g. the rods and cones of the retina for the optic nerve. They synapse in the brain stem in the relevant cranial nerve nuclei and finally decussate and ascend to other regions including the thalamus and ultimately the cerebral cortex.

Mixed nuclei

The remaining four nuclei are mixed, sensory and motor, nerves.

Assessment of the cranial nerves

Traditionally, this has been carried out by the medical team on initial contact and is not covered in this text. However, it is important for the therapist to understand the findings and to identify any changes that may occur. Symptoms associated with cranial deficits may be identified by the patient themselves during the subjective assessment (S3.16) or noticed by the therapist by general observation (S3.17). In either case, any suspicions should be followed up with a referral to an experienced clinician or the medical team.