Upper and lower motor neurons
Below is a table showing differences between UMN and LMN cells.
| UMN |
LMN |
| Located in the primary motor cortex, premotor cortex, supplementary motor area, primary somatosensory cortex, and the superior parietal lobe |
Located in the anterior horn of spinal cord and cranial nerve nuclei, peripheral sympathetic and parasympathetic ganglia. |
| Neurotransmitter is Glutamate |
Neurotransmitter is Acetylcholine |
| Lesions cause spasticity, clasp knife rigidity, hyperreflexia, clonus, positive Babinsky sign, late onset atrophy seen from disuse. |
Lesions cause muscle atrophy, fasciculations, hyporeflexia, hypotonia, negative Babinsky sign, flaccid paralysis |
Cerebellum
The cerebellum is involved in maintenance of balance and posture, coordination of voluntary movements, motor learning and cognitive functions like language. It is a part of the hindbrain, located in the posterior cranial fossa. It is divided into 3 lobes -anterior, posterior and flocculonodular and a central vermis. The cerebellar cortex is made of three layers - external molecular layer, middle Purkinje cell layer and internal granular layer. The molecular layer is comprised of stellate and basket cells. Climbing and mossy fibres represent specific nerve fibre bundles in the cerebellum. The climbing fibres are terminations of the olivocerebellar tracts. They secrete excitatory neurotransmitter aspartate to activate the cerebellar Purkinje cells. The mossy fibres represent the terminal branches of all other cerebellar afferents. They secrete excitatory neurotransmitter glutamate to activate Purkinje cells. The vermis is involved in movements of the trunk, neck, shoulder, thorax, abdomen and hips. Intermediate zone in the cerebellar hemispheres is involved with movements of the distal extremities. Rest of the lateral area of cerebellar hemispheres is involved in planning of sequential movement of the entire body and to assess movement errors. The deep cerebellar nuclei are fastigial, globose, emboliform and dentate. It is supplied by superior cerebellar artery, anterior inferior cerebellar artery (AICA) and posterior inferior cerebellar artery (PICA).
Midbrain
The midbrain, pons and medulla collectively form the brainstem. Important structures in the midbrain are pairs of superior and inferior colliculi, cerebral aqueduct, cerebral peduncles containing corticopontine, corticospinal and corticobulbar tracts and substantia nigra which is located in the peduncle. The trochlear and oculomotor nerves emerge from the midbrain. The interpeduncular fossa is located between the crus cerebri. The superior colliculi are involved in conjugate gaze movements. The inferior colliculi are involved in the processing of auditory information relayed from the cochlear nuclei via the lateral lemniscus. The pretectal area is located near the superior colliculi and is involved in the pupillary light reflex. The substantia nigra is rich in melanin, so it looks grey on gross examination. It uses dopamine and GABA as neurotransmitters. Other important nuclei of the midbrain are the red nucleus, periaqueductal grey matter and raphe nucleus. The red nucleus is involved with movement. The periaqueductal grey matter plays a role in the suppression of pain. The raphe nucleus has serotonergic neurons. The ventral tegmental area contains dopaminergic neurons and is involved in reward mechanisms. The medial longitudinal fasciculus lies anterior to the periaqueductal grey matter.
Pons
It is located inferior to the midbrain. It is connected to the cerebellum by the middle cerebellar peduncles. Cranial nerve V (trigeminal) emerges anteriorly and superiorly from the pons while cranial nerves VI, VII and VIII emerge inferiorly in the groove between the pons and the medulla. The locus coeruleus is a member of the reticular activating system and is located in the posterolateral pons. This nucleus produces norepinephrine as a neurotransmitter. Pontine motor nuclei are located anteriorly.
Medulla oblongata
It continues below the level of the foramen magnum as the spinal cord. Cranial nerve XII emerges anteriorly from the medulla while cranial nerves IX, X and XI emerge posterior to the inferior olive. Anteriorly, two bulging masses called pyramids are seen. They contain the fibres of the pyramidal (corticospinal tract). The pyramidal decussation can also be seen in the medulla, which is the crossing of corticospinal fibres to the opposite side. Lateral to the pyramids are the olivary bodies which are part of the olivocerebellar system. The inferior cerebellar peduncles connect the medulla to the cerebellum. Dorsally, the gracile and cuneate tubercles can be seen which contain the gracile and cuneate nuclei respectively which are second order neurons of the dorsal columns-medial lemniscus system. The gracile nuclei, situated medially, carry fibres from the lower extremities and trunk while the cuneate nuclei, situated laterally, correspond to upper body above T6 except for face and ears.
Medial longitudinal fasciculus
It is located in the brainstem ventral to the fourth ventricle and cerebral aqueduct and connects the nuclei of the cranial nerve VIII with nuclei controlling eye movements i.e. cranial nerves III, IV and VI. It’s major role is the execution of conjugate ocular movements.
Lateral lemniscus
It carries auditory input from the cochlear nuclei to the superior olivary complex, nuclei of the lateral lemniscus, inferior colliculi and medial geniculate body, which then project to the auditory area in the temporal lobe.
