Chapter 3 : The Forebrain and Somatosensory System

Brain: Contents Page


An Overview of the Brainstem

Anatomy of the Brainstem   Top

A diagramattic explanation of anatomical terms such as rostral, caudal, superior, inferior, dorsal, ventral, coronal, sagittal, etc can be found here: Some Anatomical Terms

The brainstem is the central stalk to which the cerebral hemispheres, the basal ganglia, thalamus and cerebellum are attached.

The brainstem is divided into three sections: above the cord is the Medulla, above that is the Pons, and the Midbrain is at the rostral (headwards) end of the brainstem. At the rostral end of the midbrain, the ventral surface divides into two cerebral peduncles which connect with the thalamus and internal capsule, and the aqueduct expands to become the third ventricle.

12 Cranial nerves originate from the brainstem and base of the brain.



Anatomical Terms concerning orientation

The diagram shows the way anatomists and radiologists describe the position of structures in the brain.

Anterior, Posterior, Superior and Inferior refer to the position of structures when the body is in the upright position.


Dorsal and ventral refer to the back and front of the body.

Rostral and caudal refer to the head and tail ends of the body.



Medulla and Pons   Top


The medulla is the caudal section of the brainstem and has a number of important surface features. In particular, on the ventral surface, are the Pyramids which consist of nerve fibres running from the cerebral cortex to the spinal cord. These form the cortico-spinal tract and cross the midline within the pyramids, so that neurones originating in the left cerebral cortex control muscles on the right side of the body.

Another visible structure is the olive which sends sensory information concerning muscle and joint position to the cerebellum.

Cranial nerves, VIII to XII originate in the medulla.

The medulla has an important role to play in initiating respiratory movements, controlling the cardiovascular system, and adjusting the tone of voluntary muscles, e.g. in maintaining balance and the upright posture.


The Pons lies above the medulla and connects with the cerebellum via the cerebellar peduncles on either side. The cerebellum therefore arches over the back of the pons, separated from it by the fourth ventricle.

Several cranial nerves have their cell bodies in nuclei within the pons (V, VI, VII, VIII), which are largely concerned with sensory and motor functions in the head and neck.


Cerebellum   Top


Top: Cerebellum

Bottom: Dorsal view of the brainstem with one half of the cerebellum attached.

The cerebellum is not part of the brainstem, but because of its close anatomical relationship with the brainstem is included briefly here. It consists of three lobes on each side of the midline vermis, and will be considered later.




When the cerebellum is removed it is possible to see the floor of the fourth ventricle, which is the dorsal surface of the brainstem.

The dorsal column nuclei (cuneate aned gracile nuclei) are a rostral continuation of the dorsal columns of the spinal cord. They can bee seen as small bumps on the surface of the loer medulla on eoither side of the Obex.


On either side of the medulla and pons are large diameter connections iwth the cerebellum, and on the left of the diagram it is possible to see the three divisions - the inferior, middle and superior cerebellar peduncles.

Rostral to the pons is the midbrain, and the features of its dorsal surface are the inferior and superior colliculi.

Midbrain   Top


At the top of the brainstem is the midbrain, which has a central canal called the aqueduct, dorsally situated colliculi concerned with eye movements, and ventrally a dark band called the substantia nigra, which is involved in the coordination of movements.

The substantia nigra on each side is part of a cerebral peduncle (or crus cerebri) which connects pathways to the nuclei of the thalamus and basal ganglia, and via the white matter of the internal capsule to the cerebral cortex.

The terms Tectum and Tegmentum refer to the areas dorsal and ventral to the aqueduct.

Cranial nerves III and IV have nuclei in the midbrain and are concerned with eye movements and the control of the iris and lens of the eye.


The Cranial Nerves

Cranial Nerve I

The Olfactory Nerve is a sensory nerve innervating the nose and responsible for the sense of smell.

Cranial Nerve II

The Optic nerve is a sensory nerve carrying information from the retina to the lateral geniculate nucleus of the thalamus

Cranial Nerve III

The Oculomotor Nerve is a motor nerve that innervates the extra-ocular muscles. It also has some parasympathic fibre that control the muscles of the iris and the shape of the lens

Cranial Nerve IV

The Trochlear nerve is a motor nerve that innervates certain extra-ocular muscles

Cranial Nerve V

The Trigeminal nerve is a mixed nerve. The sensory fibres innervate the skin of the face, mouth and teeth. It also controls some muscles, in particular the muscles of mastication.

Cranial Nerve VI

The Abducent Nerve innervates certain extra-ocular muscles

Cranial Nerve VII

The Facial nerve controls the muscles of facial expression and the secretion of salivary glands. It also conveys taste sensation from the front 2/3 or the tongue and sensation from the oral cavity.

Cranial Nerve VIII

The Auditory Nerve carries sensory information form the ear and vestibular apparatus

Cranial Nerve IX

The Glossopharyngeal nerve is mainly a sensory nerve, supplying the posterior 1/3 of the tongue, pharynx, the middle ear, the carotid sinuses and bodies. It also provides a motor supply to the stylopharyngeus muscle and causes secretion of the parotid gland.

Cranial Nerve X.

The Vagus nerve (meaning the 'Wanderer') provides  parasympathetic innervation to glands of mucous membranes of the pharynx, larynx, organs in the neck, thorax, and abdomen. It also innervates skeletal muscle of the pharynx and larynx. The majority of its axons are sensory, innervating the thoracic viscera, aortic arch and bodies, the foregut, the tympanic membrane, external auditory meatus, and epiglottis.

Cranial Nerve XI

The Accessory nerve is a motor nerve controlling certain skeletal muscles in the neck

Cranial Nerve XII

The Hypoglossal Nerve provides motor control of the intrinsic muscles of the tongue and also controls some muscles below the tongue.


Cranial Nerve Nuclei


This diagram shows the nuclei of many of the cranial nerves in the brainstem. Sensory nuclei are in pink on the left; motor nuclei in blue on the right.



Chapter 3 : The Forebrain and Somatosensory System

Brain: Contents Page

HumanPhysiology.Academy 2014-2015