Neuroanat 1 Organisation of the central nervous system1_Student 2023.pdf

Module 202: Organisation of the central nervous system 1: Tutor Notes. 202: Organisation of the central nervous system 1: Spinal cord, meninges, and surface features of the brain Learning Outcomes: In this prosection session, we will focus on the organisation of the central nervous system. The sess...

Module 202: Organisation of the central nervous system 1: Tutor Notes. 202: Organisation of the central nervous system 1: Spinal cord, meninges, and surface features of the brain 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 spinal cord, meninges, venous sinuses, surfaces of the brain, brainstem, and cerebellum. By the end of the session, you should be able to: • Identify the features of the spinal cord • Identify the layers and folds of the meninges • Locate the dural venous sinuses • Identify the lobes of the brain, as well as the main sulci and gyri • Identify structures on the ventral surface of the brain • Identify components of the cerebellum • Identify components of the brainstem Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 1 Module 202: Organisation of the central nervous system 1: Tutor Notes. Workstation 1. Spinal cord In this workstation, you will examine the spinal cord, chorda equina, and spinal meninges. The features of the spinal cord are shown below: Anterior median fissure Posterior median sulcus Ventral root Dorsal root and dorsal root ganglia Epidural space Cervical enlargement Conus medullaris Lumbosacral enlargement Organisation of the central nervous system 1 Prof. Andrew Dilley Lumbar cistern 12/10/23 2 Module 202: Organisation of the central nervous system 1: Tutor Notes. The spinal meninges is shown below: Dura Arachnoid Pia On the prosection, examine the spinal cord and associated structures. 1. Identify the epidural space, which lies between the dura and the tissues that line the vertebral canal. It contains loosely packed connective tissue, fat and blood vessels. Epidural injections of local anaesthetic can be given into this space close to the spinal nerves. 2. Now examine the spinal meninges, which are described in more detail under Workstation 2. Identify the dura, which forms the outer layer of the meninges. The dura extends from the foramen magnum to S2. Laterally, it becomes continues with the epineurium of the spinal nerves. Directly below the dura, the arachnoid can be identified. Identify the subarachnoid space, which would contain cerebrospinal fluid. The pia can be seen adherent to the surface of the spinal cord. 3. Locate the lumbar cistern, which extends from the end of the spinal cord at the level of L2, to S2. The lumbar cistern contains the cauda equina. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 3 Module 202: Organisation of the central nervous system 1: Tutor Notes. 4. Locate the cervical and lumbosacral enlargements, which are formed at the C5-T1 and L1-S3 levels respectively. 5. Now examine the dorsal and ventral roots extending from the cervical and thoracic regions of the spinal cord. The dorsal roots contain sensory (afferent) nerve fibres, whereas the ventral roots contain motor (efferent) nerve fibres. The dorsal and ventral roots come together to form the spinal nerves. 6. Examine the cauda equina, which is formed by the lumbar and sacral dorsal and ventral roots. Identify individual dorsal and ventral roots that form the cauda equina. Follow some of these roots to where they exit as spinal nerves between the vertebrae. 7. Identify the conus medullaris, which forms the caudal end of the spinal cord. The pia continues as the terminal filum. Try to identify the terminal filum. 8. Examine the anterior and posterior spinal arteries that descend the length of the anterior and posterior surfaces of the spinal cord. These arteries arise from the vertebral arteries. The spinal cord also receives a blood supply from segmental spinal arteries that enter the vertebral canal through the intervertebral foramina at every level. 9. Identify the dorsal root ganglia, which sit close to the intervertebral foramina at each level. These ganglia contain the cell bodies of the primary sensory neurons. As such, they are continuous with the dorsal roots and the spinal nerves. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 4 Module 202: Organisation of the central nervous system 1: Tutor Notes. Using the laminated cross-section of the cervical spinal cord, examine the internal organisation of the cord. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 5 Module 202: Organisation of the central nervous system 1: Tutor Notes. Workstation 2. Meninges and dural venous sinuses In this workstation, you will examine the meninges and dural venous sinuses. The meninges are shown below: Dura Arachnoid Pia On the prosection, examine the meninges. The meninges comprise the dura, arachnoid and pia. 1. Identify the dura mater, which is the outer layer of the meninges. It is a dense fibrous membrane, formed of an outer endosteal (periosteal) layer and an inner meningeal layer. The endosteal layer adheres strongly to the internal surface of the skull. On the surface of the brain, the meningeal layer can be inspected. 2. Directly below the dura, the arachnoid mater can be inspected. The arachnoid mater is a thin avascular membrane that covers the brain but does not go into each of the sulci. 3. Identify the subarachnoid space below the arachnoid, which would contain cerebrospinal fluid. Projections of arachnoid, called trabeculae, extend between the arachnoid and the underlying pia mater. See if you can identify any of these trabeculae. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 6 Module 202: Organisation of the central nervous system 1: Tutor Notes. 4. Within the subarachnoid space, inspect the cerebral veins that drain blood from the brain into the venous sinuses. 5. On the surface of the brain, examine the pia mater. The pia is a thin vascular membrane the closely invests the brain. The middle meningeal artery is shown below: Middle meningeal artery 6. Re-examine the dura. If the endosteal layer is present, it may be possible to observe the middle meningeal artery running within this layer. The middle meningeal artery which is a branch of the maxillary artery (from the external carotid), supplies most of the dura. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 7 Module 202: Organisation of the central nervous system 1: Tutor Notes. The dural folds are shown in the figure below: Falx cerebri Tentorium cerebelli Falx cerebelli Tentorium cerebelli On the prosection, examine the dural folds. 1. Identify the falx cerebri, a fold of meningeal dura, which enters the longitudinal fissure. The falx cerebri separates the two cerebral hemispheres. Anteriorly, it is attached to the christa galli, a triangular process from the ethmoid bone, as well as the frontal bone. Posteriorly, it fuses with the tentorium cerebelli. Examine the christa galli on the inside of the skull. 2. Now identify the tentorium cerebelli, a horizontal projection of meningeal dura, which separates the cerebellum from the cerebral hemispheres. It is attached to the occipital bone, temporal bone and clinoid processes. Examine these attachments on the skull. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 8 Module 202: Organisation of the central nervous system 1: Tutor Notes. 3. Examine the falx cerebelli, which separates the cerebellar hemispheres. It is attached to the tentorium cerebelli and the occipital bone. The dural venous sinuses are shown in the figure below: Ophthalmic vein Sphenoparietal Cavernous Internal jugular vein Sigmoid Superior petrosal Transverse Inferior sagittal Straight Superior sagittal (confluence of sinuses) On the prosection, examine some of the dural venous sinuses. 1. Identify the superior sagittal sinus, which sits along the superior border of falx cerebri. The sinuses are spaces between the endosteal and meningeal layers of dura. 2. Within the superior sagittal sinus, inspect the arachnoid granulations, which are herniations of arachnoid membrane into the venous sinuses. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 9 Module 202: Organisation of the central nervous system 1: Tutor Notes. 3. Identify the transverse sinus that follows the lateral sulcus. The transverse sinus drains blood from the confluence of sinuses into the sigmoid sinus and internal jugular vein. 4. Using the skull, identify the location of some of the sinuses. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 10 Module 202: Organisation of the central nervous system 1: Tutor Notes. Workstation 3. Surface of the Brain In this workstation, you will examine the dorsal and ventral surfaces of the brain and the cerebellum. Dorsal surface The dorsal surface of the brain is shown below: Longitudinal fissure Central sulcus Parietal lobe Frontal lobe Occipital lobe Lateral sulcus Cerebral vessels Temporal lobe Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 11 Module 202: Organisation of the central nervous system 1: Tutor Notes. On the prosection, examine the dorsal surface of the brain 1. Identify the gyri (elevations) and sulci (grooves) of the cerebral hemispheres. Remind yourselves that below the relatively thin outer grey matter of the cerebral cortex lies an extensive network of white matter that connects the different regions of the brain. 2. Locate the longitudinal fissure, which divides the two hemispheres. 3. Identify the frontal, parietal, occipital and temporal lobes. Much of the frontal lobe is concerned with motor function and includes Broca’s “motor speech area”. In contrast, primary areas for processing sensory information are located in the parietal, occipital and temporal lobes. 4. Identify the central sulcus, which separates the frontal and parietal lobes. The central sulcus can be seen in the axial MRI image below (Note the prominent gyri either side of the sulcus): Anterior Posterior Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 12 Module 202: Organisation of the central nervous system 1: Tutor Notes. 5. Identify the lateral sulcus. An area of cortex called the insula lies deep to the sulcus and cannot be seen unless the lips of the sulcus are parted. Many functions have been attributed to the insula, including interoception, emotion and pain processing pain. The primary auditory area of the brain is situated within the inferior wall of the lateral sulcus. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 13 Module 202: Organisation of the central nervous system 1: Tutor Notes. Ventral surface The ventral surface of the brain is shown below: Olfactory tract and bulb Optic chiasma Pituitary stalk Mammillary bodies Hippocampal formation On the prosection, examine the ventral surface of the brain. 1. Identify the olfactory tract and bulb (CNI). The olfactory bulb is a narrow oval body that lies on the dura mater. The olfactory bulb is the main relay station in olfactory pathways. 2. Locate the optic chiasma, which is a flattened bundle of nerve fibres at the junction of the anterior wall and floor of the third ventricle. The anterolateral corners of the chiasma are continuous with the optic nerves (CNII), which will have been severed in removal of the brain. The optic tracts are found at the posterolateral corners of the chiasma. The optic tracts divide and pass to the thalamus and superior colliculus of the midbrain. Axons project from the thalamus to the primary visual cortex. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 14 Module 202: Organisation of the central nervous system 1: Tutor Notes. 3. Locate the pituitary stalk, also known as the infundibulum, which connects the hypothalamus to the pituitary gland. This endocrine gland is about the size of a pea. 4. Locate the mammillary bodies, which lie posterior to the pituitary stalk. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 15 Module 202: Organisation of the central nervous system 1: Tutor Notes. Workstation 4. Brainstem and cerebellum In this workstation, you will examine the brainstem and cerebellum. 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 1 Prof. Andrew Dilley 12/10/23 16 Module 202: Organisation of the central nervous system 1: 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. 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 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 Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 17 Module 202: Organisation of the central nervous system 1: Tutor Notes. 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. Cerebellum The cerebellum is shown below: Vermis Anterior lobe Flocculonodular lobe Posterior lobe On the prosection, examine the cerebellum The cerebellum, which is covered superiorly by the tentorium cerebelli, is the largest part of the hindbrain. The main functions of the cerebellum are the control of posture, coordinating and planning limb movements, and the control of eye movements. 1. Examine the two hemispheres, which are connected by the vermis in the midline. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 18 Module 202: Organisation of the central nervous system 1: Tutor Notes. 2. Identify the three lobes, namely the anterior, posterior and Flocculonodular lobes. The anterior and posterior lobes are separated by a horizontal fissure, called the primary fissure. Examine the lobules, which form the lobes. 3. Locate the superior, middle and inferior cerebellar peduncles, which connect the posterior aspect of the brainstem to the cerebellum. These connect the midbrain, pons and medulla respectively. The largest of these bundles of fibres is the middle cerebellar peduncle. 4. 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. Organisation of the central nervous system 1 Prof. Andrew Dilley 12/10/23 19