Support Systems of the Nervous System Lundy-Ekman –Chapter 1 Pp –Chapter 19

Meninges

Dura mater –Periosteal layer – attaches to skull –Meningeal layer – attaches to arachnoid Two layers attached except at dural venous sinuses Projections of inner layer –Falx cerebri – L and R cerebral hemispheres –Tentorium cerebelli – Cerebellum and posterior cerebrum

Arachnoid mater –Trabeculae – bars of tissue deep to arachnoid membrane Pia mater –Thin layer –Firmly attached to brain and spinal cord

Meningeal spaces Subarachnoid space –Space filled with CSF and blood vessels Subdural space –Between dura and arachnoid –Virtual space Epidural space –Between skull and dura –Virtual space

CSF System Ventricles –Lateral (2) –Third –Fourth Cerebral aqueduct

Telencephalon: Lateral ventricles Diencephalon: Third ventricle Midbrain: Cerebral aqueduct Pons/medulla/cerebellum: 4 th ventricle 4 th ventricle opens to subarachnoid space

Cerebrospinal Fluid Choroid plexus –All named ventricles CSF travels through the ventricles and out –Lat  3 rd  Cerebral aqueduct  4 th ventricle  Subarachnoid space CSF in subarachnoid space surrounds outside of brain and spinal cord

Absorption of CSF CSF travels through the arachnoid villi into the dural venous sinuses The CSF enters general circulation

Blood supply to the brain Our brain needs a constant supply of O 2 –10 seconds: lose consciousness –20 seconds: electrical activity stops –Few minutes: irreversible injury to brain starts Two main pairs of arteries –Vertebral arteries – posterior brain –Internal carotid arteries – anterior brain

Blood vessels (ventral view) p. 17

Vertebral arteries Superior spinal cord Brainstem Cerebellum Posterior occipital and temporal lobes of the cerebrum

Internal carotid arteries Most of the telencephalon Most of the diencephalon

Vertebral arteries Arise off subclavian arteries First 6 cervical vertebrae – Enter skull and travel along medulla Branches in medulla – page 17 –Posterior spinal arteries – posterolateral cord Posterior 1/3 rd of cord –Anterior spinal artery – anterior median cord Anterior 2/3 rd of cord –Posterior inferior cerebellar artery (PICA) Inferior cerebellum

Blood supply to the spinal cord The anterior and posterior spinal arteries do not receive enough blood to supply the whole spinal cord Radicular arteries enter the spinal cord with the nerve roots –Supplement vascular supply –Most are small, only a few are significant –Blockage of a radicular artery can damage the spinal cord at that level, but also axons passing through that region

Basilar artery The 2 vertebral arteries join at the pons Branches –Anterior inferior cerebellar artery (AICA) –Superior cerebellar artery Basilar artery and branches supply: –Pons –Most of cerebellum

Posterior Cerebral Artery At the rostral pons the basilar artery forks to form the 2 posterior cerebral arteries They supply –Midbrain –Occipital lobe –Medial and inferior temporal lobes

Internal Carotid artery Around optic chiasm it splits –Anterior cerebral artery Travels in longitudinal fissure Supplies medial frontal and parietal lobe –Primary motor and somatosensory cortex for LE –Middle cerebral artery Passes through lateral sulcus Supplies most of lateral cerebral hemispheres

Circle of Willis Contains all 3 pairs of cerebral arteries Communicating arteries connect: –Left and right anterior circulation Anterior communicating artery between the L and R anterior cerebral artery –Anterior and posterior circulation Posterior communicating arteries between the posterior cerebral artery and the internal carotid artery on each side Many variations on the standard pattern

Circle of Willis (cont) The circle is a common site for aneurysms –They can form at branch points in arteries Blood flow in the communicating arteries –Usually not a lot Blood vessels are small Similar pressure on each end of the artery –They arteries can expand to adapt to slowly developing occlusions in one of the main cerebral arteries

Watershed regions – p. 470 Regions where small anastomoses link the ends of the cerebral arteries With strokes that affect one artery, watershed regions are relatively spared When the blood supply in two adjacent arteries is affected, watershed regions are most affected

Watershed regions Events which can affect blood flow in two (or more adjacent arteries) –Severe drop in systolic blood pressure –Blockage or restriction in internal carotid artery Supplies both ACA and MCA

Watershed regions For ACA and MCA –Often includes the trunk and proximal limbs Man in a barrel syndrome –Transcortical aphasia syndromes Between MCA and PCA –Difficulties in higher order visual processing

Deep Cerebral Structures Branches of cerebral arteries –Dive deep into brain Anterior choroidal artery –Choroid plexus in lateral ventricles –Parts of optic tract, putamen, thalamus, internal capsule and hippocampus Posterior choroidal artery –Choroid plexus in third ventricle –Parts of hippocampus and thalamus

Brain Stem Syndromes K7d5A&nohtml5=Falsehttps:// K7d5A&nohtml5=False Medial Medullary Syndrome vftQwhttp:// vftQw

Veins Not paired with arteries Valveless –How might this affect the brain during infections? Frequent anastomoses Superficial and deep veins

Superficial veins On surface of cerebral hemispheres Most empty into superior sagittal sinus –Some connect to other sinuses located between the two layers of the dura Bridging veins connect these veins with the dural sinuses –What happens if these bridging veins are damaged?

Deep veins Drain deeper regions of telencephalon and diencephalon

Venous blood Returned to internal jugular vein Pathology –Usually few disorders –Low pressure system so occlusions and hemorrhages occur less frequently –Large number of functional anastomoses

Cerebral Blood Flow Index of brain activity Brain requires glucose and oxygen, but cannot store either Regions differ in their sensitivity to hypoxia –Cerebral cortex more sensitive than brainstem –Persistent vegetative state

Autoregulation of Blood Vessels Artery dilation –Low BP, Oxygen or pH –High CO 2 or lactate Artery constriction –High pH, Oxygen or BP –Low CO 2 or lactate Autoregulation important to ensure adequate blood flow and prevent edema