The brainstem is divided into three sections: medulla, pons and midbrain, and consists of networks of neurones known as the Reticular Formation, together with
Ascending Pathways that carry information from the Spinal Cord pass through the brainstem to reach their target nuclei in the forebrain.
Descending pathways from the brain.
The nuclei of origin of many of the cranial nerves
2. Ascending pathways
Dorsal Column-Medial Lemniscal System :
Sensory neurones with cell bodies in the dorsal root ganglia project through the dorsal column-medial lemniscal pathway, which carries information about touch and vibration to the dorsal column nuclei (cuneate and gracile nuclei); there they synapse with second order neurones that carry this information to the thalamus, which in turn relays the information to the primary receiving ares of the cerebral cortex in the post-central gyrus.
Neurones with a cell body in the the spinal cord have long axons that ascend to the thalamus, cerebellum or brainstem, and therefore known as the spino-thalamic, spino-cerebellar or spino-reticular tracts. The anterolateral system carries information about pain and temperature to the thalamus: the spinothalamic tracts are the classical pathways, but the anterolateral system also includes other pathways that synapse in the medulla, pons and midbrain (the spino-reticular and spino-mesencephalic tracts), which also project to the thalamus, and are regarded as alternative pain pathways.
The spino-cerebellar tracts carry information about the position of limbs to the cerebellum. This pathway is used in the coordination of movements.
3. Descending Pathways:
The main descending pathway from the cortex to the motoneurones is the corticospinal tract, and this consists of bundles of axons that travel in the cerebral peduncles of the midbrain, through the reticular formation of the pons, and then cross the midline in the medulla. The decussation of the pyramids is the cross-over site before these axons enter the lateral columns of the spinal cord.
4. Reticular formation and its Projections
The substance of the brainstem, between the bundles of axons that form the ascending and descending tracts is known as the reticular formation. The reticular formation consists of groups of neurones with less well defined boundaries than the nuclei of the cranial nerves that contain characteristic histochemical properties and neurotransmitters. Examples are the basal forebrain cholinergic neurones and the serotoninergic raphe nuclei. Some of these project to different areas of the brainstem, the forebrain (the ascending reticular formation) or to the spinal cord (the descending reticular formation). Neurones in the ascending reticular formation are concerned with the sleep-waking cycle; and the descending reticular formation controls breathing, the cardiovascular system, muscle tone and nociceptive transmission in the dorsal horn.
5. Cranial Nerves:
Twelve cranial nerves innervate the head, neck and thoracic and abdominal viscera.
Key Words: Dorsal Columns, Dorsal column nuclei, medial lemniscus, thalamus: dorsal column-medial lemniscal system concerned with touch and vibration.
Spino-thalamic and Spino-reticular pathways concerned with pain and temperature sensation.
Spino-cerebellar pathways concerned with transmission of proprioceptive information to the cerebellum.
Twelve Cranial Nerves
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.
Medulla and Pons
The Brainstem is the central stalk of neural tissue, ahead of the spinal cord, 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 each side of the thalamus and internal capsule.
The aqueduct is the central canal containing CSF in the midbrain, and expands to become the third ventricle in the thalamus.
The brainstem consists of neurones of different types:
Axons passing between the brain and spinal cord in both directions.
Nuclei and axons of cranial nerves
The Reticular formation, which contains specialised areas of neural tissue concerned with different functions, but without distinct anatomical boundaries. These include areas of neural tissue controlling respiration, cardiovascular and gastrointestinal functions, micturition, sensitivity to pain, sleep and waking, and other functions.
Nerve cells and axons connecting the cerebellum with the remainder of the brain, including structures such as the olive in the medulla, and the cerebellar peduncles.
Structures such as the colliculi and substantia nigra of the midbrain, which have specific connections and functions.
The medulla is the point at which corticospinal axons fross the midline in the decussation of the pyramids.
Sensory fibres in the medial lemniscus also cross the midline in the medulla and pons to reach the contralateral thalamus.
Transverse Section of the Midbrain
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.
Image source: tutorvista.com
The image shows the ventral surface of the brain and the cranial nerves.
12 Cranial nerves originate from the brainstem and base of the brain, and have a wide range of functions, including:
Special senses (I: Smell; II: Vision; VIII: Hearing)
Eye Movements (III, IV, VI)
Sensory and Motor functions in the head and neck: (V, VII, XI)
Autonomic functions: (III, IX, X)
Contol of the tongue, larynx and speech (IX, X, XII)
CRANIAL NERVES : DETAILS
I The Olfactory Nerve
The Olfactory Nerve is a sensory nerve innervating the nose and responsible for the sense of smell.
II The Optic Nerve
The Optic nerve is a sensory nerve carrying information from the retina to the lateral geniculate nucleus of the thalamus
III The Oculomotor Nerve
The Oculomotor Nerve is a motor nerve that innervates the extra-ocular muscles. It also has some parasympathetic fibres that control the muscles of the iris and the shape of the lens
IV The Trochlear Nere
The Trochlear nerve is a motor nerve that innervates certain extra-ocular muscles
V The Trigeminal Nerve
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.
VI The Abducent Nerve
The Abducent Nerve innervates certain extra-ocular muscles
VII The Facial Nerve
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.
VIII The Auditory/Vestibular Nerve
The Auditory Nerve carries sensory information from the ear and vestibular apparatus
IX The Glossopharyngeal Nerve
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.
X The Vagus
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.
XI The Accessory Nerve
The Accessory nerve is a motor nerve controlling certain skeletal muscles in the neck
XII The Hypoglossal Nerve
The Hypoglossal Nerve provides motor control of the intrinsic muscles of the tongue and also controls some muscles below the tongue.