Session 3 - The Brainstem Flashcards
What is the role of the midbrain?
The midbrain sits at the very top of the brainstem and is often difficult to see on an undissected specimen. The optic chiasm obstructs most of the view of the anterior brainstem. It is divided broadly into two parts, one ventral and one dorsal to the cerebral aqueduct.
What structures are in the midbrain?
• Tectum – this smaller part of the midbrain is located dorsally. It contains two pairs of rounded bumps, known as colliculi. The superior colliculi are involved in regulating eye movements and reflexes associated with visual stimuli, such as turning or moving the head quickly when something quickly enters our field of vision. The inferior colliculi are involved sound location, pitch discrimination and reflexes associated with auditory stimuli, such as turning our head on hearing a loud noise.
• Tegmentum – this larger part of the midbrain is located ventrally. It contains the substantia nigra which is important in motor control by producing dopamine. It also contains the cerebral peduncles on its most ventral surface which are large white matter bundles that connect the midbrain to the thalami, and the red nuclei which are involved in supporting motor control of the limbs. Surrounding the cerebral aqueduct is the periaqueductal grey (PAG) which is a collection neuronal cell bodies that plays a role in modulation of pain.
What does the midbrain contain?
The midbrain contains the nuclei of the oculomotor (CN III) and trochlear (CN IV) nerves, and the Edinger-Westphal nuclei (involved in the pupillary light reflex).
What is the pons like?
The pons is the large, rounded, middle part of the brainstem. On its ventral surface lies the basilar artery and on its dorsal surface, it has large white matter connections to the cerebellum called the middle cerebellar peduncles. The fourth ventricle of the ventricular system is situated dorsal to the pons, between the two middle cerebellar peduncles.
The pons contains the nuclei of the trigeminal (CN V), abducens (CN VI), facial (CN VII) and vestibulocochlear (CN VIII) nerves. It also contains the pneumotaxic and apneustic centres which are involved in the regulation of breathing.
What is the medulla like?
The medulla oblongata connects the pons to the spinal cord. It has a groove on its ventral surface known as the anterior median fissure. The medulla contains the two medullary pyramids, which form two distinct lumps of the ventral surface, separated by this fissure. Essential motor tracts known as the corticospinal tracts run inside the pyramids. Immediately lateral to the medullary pyramids are another pair of prominent ridges known as the medullary olives.
An important sensory pathway of nerves (the dorsal column medial lemniscus (DCML) pathway) runs in the dorsal part of the medulla within two pairs of nerve bundles: fasciculus gracilis and fasciculus cuneatus. Fibres of both corticospinal and DCML tracts cross over to the contralateral side within the medulla. This is covered in more detail later in this handbook.
The medulla contains the nuclei of the glossopharyngeal (CN IX), vagus (CN X), accessory (CN XI) and hypoglossal (CN XII) nerves. The medulla also contains vital centres responsible for regulating respiration, heart rate and blood pressure. Part of the medulla, known as the chemoreceptor trigger zone (CTZ) is important in the process of vomiting. It is able to detect certain blood-borne drugs, hormones or toxins and then communicate with the vomiting centre (also in the medulla) to initiate vomiting.
What is bulbar palsy?
This term refers to dysfunction of the cranial nerves that arise from the medulla (glossopharyngeal, vagus, accessory and hypoglossal). Symptoms include difficulty speaking and swallowing, excessive saliva production, wasting and fasciculations of the tongue and an absent gag reflex. Causes include diseases which affect peripheral nerves such as motor neurone disease and Guillain-Barré syndrome.
