Neuroanat 2 Organisation of the central nervous system 2 PDF

Module 202: Organisation of the central nervous system 2: Tutor Notes. 202: Organisation of the central nervous system 2: Brainstem, ventricular system, and components of the basal ganglia and limbic system Learning Outcomes: In this prosection session, we will focus on the organisation of the cent...

Module 202: Organisation of the central nervous system 2: Tutor Notes. 202: Organisation of the central nervous system 2: Brainstem, ventricular system, and components of the basal ganglia and limbic system Learning Outcomes: In this prosection session, we will focus on the organisation of the central nervous system. The session consists of four workstations that will examine the brainstem, midline structures of the brain, the ventricles, and components of the basal ganglia and limbic system. By the end of the session, you should be able to: • Identify the components of the brainstem • Identify the origin of the cranial nerves • Locate the ventricles • Identify the midline structures of the brain • Identify the components of the basal ganglia • Identify the components of the limbic system Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 1 Module 202: Organisation of the central nervous system 2: Tutor Notes. Workstation 1. Brainstem In this workstation, you will examine the brainstem and origins of the cranial nerves. Ventral surface of the brainstem The ventral surface of the brainstem is shown below: Cerebral peduncle Midbrain Middle cerebellar peduncle Pons Medulla Pyramid Olive Trigeminal nerve (V) On the prosection, examine the ventral surface of the brainstem. 1. Locate the midbrain. The two cerebral peduncles will be visible on the anterior surface of the midbrain. These contain fibres travelling from the cortex to the pons and spinal cord. 2. Locate the pons. The anterior surface shows many transverse fibres that converge on each side as the middle cerebellar peduncle. The cerebellar peduncles connect the brainstem to the cerebellum. The basilar artery sits in the shallow groove along the midline of the anterior surface of the pons. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 2 Module 202: Organisation of the central nervous system 2: Tutor Notes. 3. Locate the medulla oblongata. The two pyramids will be visible, on the anterior surface of the medulla. The pyramids are composed of bundles of nerve fibres, the corticospinal tract, which originates in the cerebral cortex and pass into the spinal cord. The corticospinal tract is the voluntary motor pathway. Q1. In which part of the cerebral cortex do you think these fibres originate? Motor fibres mainly from the primary motor cortex (the voluntary motor pathway) but also the premotor cortex (involved with planning/programming voluntary movements). 4. Locate the olives, which sit lateral to the pyramids. These elevations are produced by the underlying inferior olivary nuclei. The olives are closely associated with the cerebellum and are involved in the control and coordination of fine movements. Dorsal surface of the brainstem The dorsal surface of the brainstem is shown below: Pineal gland Superior colliculus Medial and lateral geniculate nuclei Midbrain Inferior colliculus Middle cerebellar peduncles Pons Cuneate nucleus Medulla Gracile nucleus Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 3 Module 202: Organisation of the central nervous system 2: Tutor Notes. On the prosection, examine the dorsal surface of the brainstem. 1. Locate the midbrain. Observe the four colliculi (2 paired protrusions) on the posterior surface. The paired superior colliculi are part of the visual system and are involved in the regulation of eye movements. They receive information from the optic tracts and output to the cranial nerves that control the muscles that move the eye. The paired inferior colliculi are part of the auditory system. They receive inputs from the vestibulocochlear (CNVIII) nerve and output to the medial geniculate nuclei. 2. Locate the pineal gland sitting superior to the midbrain. This gland secretes melatonin and is involved in sleep cycles. 3. Locate pons. The middle cerebellar peduncles can be seen extending posteriorly. These form a link between the brainstem and cerebellum. 4. Locate the medulla. Observe the cuneate (lateral) and gracile (medial) tubercles on the posterior surface. These tubercles are formed from the underlying cuneate and gracile nuclei, which are where sensory fibres synapse from the upper and lower body respectively. They form part of a pathway involved in fine touch and proprioception. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 4 Module 202: Organisation of the central nervous system 2: Tutor Notes. On the prosection, examine the origins of the cranial nerves. The origins of the cranial nerves are shown below: Olfactory nerve (I) Forebrain Optic nerve (II) Oculomotor nerve (III) Midbrain/pons Trochlear nerve (IV) Midbrain (posterior) Trigeminal nerve (V) Pons Abducent nerve (VI) Facial nerve (VII) Pons/medulla Vestibulocochlear nerve (VIII) Glossopharyngeal nerve (IX) Medulla Vagus nerve (X) Accessory nerve (XI) Cervical spinal cord Hypoglossal nerve (XII) Medulla Whereas the olfactory and optic nerves are part of the central nervous system, cranial nerves III to XII, which have their origin from the brainstem, are part of the peripheral nervous system. 1. Identify the trigeminal nerve (CNV). This is the largest of the cranial nerves and can be easily observed projecting from the pons. 2. Using the figure above, try to identify other cranial nerves that have their origin from the brainstem. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 5 Module 202: Organisation of the central nervous system 2: Tutor Notes. Workstation 2. In this workstation you will examine the ventricles and midline structures of the brain. Medial structures The medial structures of the brain are shown below: Cingulate gyrus Corpus callosum Septum pellucidum Fornix Hypothalamus Thalamus Arbor vitae of the cerebellum These structures can also be seen on the MRI image below: Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 6 Module 202: Organisation of the central nervous system 2: Tutor Notes. Corpus callosum Arbor vitae of the cerebellum On the prosection, examine the medial structures. 1. Locate the corpus callosum, a large white matter tract. The posterior part of this tract is called the splenium. Q2. What is the function of the corpus callosum? It contains fibres connecting the cortices of the two cerebral hemispheres. 2. Locate the cingulate gyrus/cortex, which lies superior to the corpus callosum. The cingulate gyrus forms part of the limbic system. 3. Locate the septum pellucidum, which stretches between the corpus callosum and fornix. The septum pellucidum is a thin sheet of tissue containing white and grey matter. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 7 Module 202: Organisation of the central nervous system 2: Tutor Notes. 4. Locate the fornix, which is a semi-circular white matter tract that extends between the hippocampus and the mammillary bodies on the inferior surface of the brain. It is one of the connecting pathways associated with the limbic system. 5. Locate the thalamus, which is a paired mass of grey matter in the centre of the brain. It is functionally important, serving as a relay station for the main sensory systems (except the olfactory pathway). The interthalamic adhesion can be seen linking the thalami. 6. Locate the hypothalamus, which lies below the thalamus. Anatomically, the hypothalamus appears as a small area of the brain. However, it is functionally very important. It plays a major role in homeostasis, endocrine and autonomic functions, as well as in sleep, sexual and emotional behaviour. 7. Examine the arbor vitae of the cerebellum. This has arisen because the cerebellum, like the cerebral cortex, consists of an outer covering of grey matter (the cerebellar cortex) and the inner white matter. Embedded in the white matter are important nuclei. Ventricles The ventricular system is shown below: Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 8 Module 202: Organisation of the central nervous system 2: Tutor Notes. Lateral ventricle Interventricular foramen Third ventricle Cerebral aqueduct Fourth ventricle Use the prosections to examine the ventricular system. 1. Locate the lateral ventricles, which are separated by the septum pellucidum. Identify the anterior and posterior horns. Observe the corpus callosum in the roof of the lateral ventricles. 2. Identify the interventricular foramen (foramen of Monro), which provides a communication between the lateral and third ventricle. Q3. What would be the consequence of a blockage of the interventricular foramen? Non-communicating hydrocephalus, which is caused by obstruction of CSF flow and a subsequent increase in intercranial pressure. On MRI, the lateral ventricles would appear enlarged whereas the third and fourth ventricles would appear normal. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 9 Module 202: Organisation of the central nervous system 2: Tutor Notes. 3. Examine the narrow third ventricle, which is surrounded by the two thalami. Observe the fornix in the roof of the third ventricle. 4. Locate the cerebral aqueduct (aqueduct of Silvius) in the centre of the midbrain, which provides a communication between the third and fourth ventricle. 5. Locate the fourth ventricle, which is found between the pons and cerebellum. It communicates with the subarachnoid space of the spinal cord through the foramina of Luschka and Magendie. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 10 Module 202: Organisation of the central nervous system 2: Tutor Notes. Workstations 3 and 4. In workstations 3 and 4, you will examine horizontal (workstation 3) and coronal (Workstation 4) sections of the brain. Specifically, you will examine components of the basal ganglia and limbic system. Example sections through the brain are shown below: Anterior Head of caudate nucleus Claustrum Putamen Lentiform nucleus Internal capsule Thalamus Globus pallidus Tail of caudate nucleus An MRI (axial slice) of the above section is shown below: Head of caudate nucleus Putamen Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 11 Module 202: Organisation of the central nervous system 2: Tutor Notes. Cingulate gyrus Head of caudate nucleus Claustrum Putamen Globus pallidus Internal capsule Thalamus Tail of caudate nucleus Substantia nigra Hippocampus An MRI (coronal slice) of the above section is shown below: Head of caudate nucleus Hippocampus Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 12 Module 202: Organisation of the central nervous system 2: Tutor Notes. Basal ganglia The basal ganglia refer to a group of nuclei at the base of the cerebral hemispheres. Anatomically, the basal ganglia include the caudate nucleus and the lentiform nucleus. Collectively, these interconnected structures are referred to as the corpus striatum. They play an important part in the control of posture and movement. Components of the basal ganglia can be seen in the figure below: Body of caudate nucleus Head of caudate nucleus Lateral ventricle Lentiform nucleus Tail of caudate nucleus Amygdala On the horizontal and coronal slices, examine the components of the basal ganglia. Tip: Start by finding a horizontal or coronal slice that looks similar to the example sections shown above. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 13 Module 202: Organisation of the central nervous system 2: Tutor Notes. 1. Locate the two thalami. Observe the third ventricle sitting between the thalami. 2. Locate the lateral ventricles, which have a characteristic C-shape. They have an anterior horn, body, posterior horn and inferior horn. On each of the other sections, trace the shape of the lateral ventricles. 3. In the lateral wall of the lateral ventricle, locate the caudate nucleus. This nucleus is a large C-shaped mass of grey matter that follows the contours of the lateral ventricle. It has a head, body and tail. The head forms the lateral wall of the anterior horn of the lateral ventricle. The tail terminates in the roof of the inferior horn of the lateral ventricle. The amygdala can be seen immediately rostral to the tail of the caudate nucleus. On each of the other sections, trace the shape of the caudate nucleus. 4. Immediately lateral to the thalamus, observe the internal capsule, a large white matter tract that links the cortex with the thalamus and brainstem. 5. Lateral to the internal capsule, the lentiform nucleus can be seen. This lens-shaped nucleus is divided into two regions: the putamen (outer region) and the globus pallidus (inner region). Identify both regions. 6. Lateral to the putamen, observe a thin sheet of grey matter called the claustrum. This is separated from the putamen by the external capsule. 7. Lateral to the claustrum, the insula and lateral sulcus can be seen. 8. It may be possible to identify a black region within the midbrain. This is the substantia nigra, which also forms part of the basal ganglia. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 14 Module 202: Organisation of the central nervous system 2: Tutor Notes. Limbic system The limbic system is important in the control of emotion, memory, behaviour and drive. The limbic system is formed of many components that span the telencephalon and diencephalon. The components of the limbic system include the hippocampal formation, fornix, mammillary bodies (hypothalamus), thalamus, cingulate gyrus and amygdala. The hippocampus and fornix can be seen in the figure below: Hippocampus Columns of the fornix Inferior horn of lateral ventricle On the horizontal and coronal slices, and using the dissection of the hippocampus, examine the main components of the limbic system. 1. Examine the inferior horn of the lateral ventricle. On the floor of this horn, identify the hippocampus. This part of the hippocampus has a foot-like appearance (called the pes hippocampus). 2. Continuous with the hippocampus, the fornix can be identified. This white-matter tract has a semi-circular shape, extending from the hippocampus to the mammillary Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 15 Module 202: Organisation of the central nervous system 2: Tutor Notes. bodies. It curves posteriorly, medially and superiorly over the thalamus. At this point it sits in the roof of the third ventricle as the columns of the fornix. It then descends inferiorly over the thalamus to terminate on the mammillary bodies, which can be observed on the inferior surface. 3. On a coronal slice, identify the inferior horn of the lateral ventricle. In the floor, the hippocampus can be observed. The S-shaped gyri inferior to the hippocampus constitutes the dentate gyrus. 4. On the same section, locate the thalami and third ventricle. In the roof of the third ventricle, the columns of the fornix can be seen. 5. Now locate the corpus callosum, which is the large white matter tract that connects the cerebral hemispheres. Superior to this structure, the cingulate gyri can be inspected on the medial side of each hemisphere. 6. The amygdala can be difficult to identify. It may be visible in the temporal lobe, immediately rostral to the tail of the caudate nucleus. Organisation of the central nervous system 2 Prof. Andrew Dilley 10/19/2022 16