The brain

• General features

  From the external surface the most prominent features are the large cerebral hemispheres, named in relation to the bones of the skull they lie next to. Also apparent is the highly folded surface of the cerebellum and a small amount of the brain stem with its associated cranial nerves.

  
• The brain has 2 cerebral hemispheres so there is a space or fissure between the two hemispheres – the longitudinal fissure. The 2 hemispheres communicate with each other via the corpus callosum and resting on the superior surface of the corpus callosum are the anterior cerebral arteries that supply blood to the frontal and parietal lobes. Similarly, on the lateral surface of the brain (between the temporal lobe and the frontal/parietal lobes) is the lateral fissure which contains the middle cerebral artery.

  
• There is another important landmark that is visible – a groove that runs from the longitudinal to the lateral fissure – the central sulcus. This separates the frontal lobes from the parietal lobes.

  the motor cortex anterior to the sulcus and sensory cortex posterior to the sulcus.

  
• Cerebral hemispheres

  
• parts of the diencephalon (thalamus and hypothalamus) , the midbrain and the rest of the brainstem (pons and medulla). The varying parts of the brain must be able to communicate with each other and use white matter fibres/tracts to do this.

  
• Association tracts = between gyri of the same hemisphere

  Commisural tracts/fibres = from one hemisphere to the corresponding region of the other

  Projection fibres  = from cerebrum to thalamus, brainstem etc

  The communication can be via small fibres or collections of fibres called tracts

  
• Basal ganglia

  Main action = regulate initiation and termination of body movements. Also thought to be involved with memory, planning and emotional response (via the limbic system).

  Substansia nigra (in midbrain) communicates with the caudate and putamen. Sub-thalamic nuclei communicate with the globus palladus.

  . It is called the substansia nigra because it looks like a small black line crossing the tissue of the midbrain when seen in fresh brain tissue. There are many complex communication pathways between the basal ganglia themselves and the rest of the brain

  
• Basal ganglia

  The internal capsule is a white fibre pathway but looks black on the section due to the dye. It separates the globus pallidus and putamen from the caudate nucleus. The large ‘hole’ in the middle of the section is the lateral ventricles that are usually filled with cerebrospinal fluid. The choroid plexus that hangs into the ventricles acts to create this fluid

  
• Diencephalon

  The diencephalon lies deep in the brain and is composed of 3 areas – the largest is the thalamus that is involved with a large number of functional pathways.  The hypothalamus plays a role in autonomic regulation of the body and links the nervous system to the endocrine system via its communication with the pituitary gland. The epithalamus is also called the pineal gland. The connections between the diencephalon and the rest of the brain are complex

  
• Midbrain

  -Connects pons and medulla to diencephalon

  -Superior colliculi = visual tracking, scanning

  -Inferior colliculi = auditory startle reflex

  -Substansia nigra = control sub-conscious muscle action

  -Red nuclei = co-ordinate muscle movement

  The midbrain is classed as part of the brainstem and connects the diencephalon to the rest of the brainstem.

  
• Midbrain

  There are 2 structures that are easier to identify on the posterior surface of the midbrain – the superior and inferior colliculi. These small lumps of tissue are part of pathways associated with the visual and auditory systems. The other important fact about the midbrain is that CN III (oculomotor) and CN IV (trochlear) come from the midbrain. They are nerves that help to control the movements of the extra ocular muscles.

  
• Pons and Medulla

  -Pons

  Pontine nuclei = co-ordination and maximising voluntary motor output. Also tracts such as pneumotaxic which help to control breathing.

  origin of cranial nerves V, VI, VII, and vestibular part of VIII

  -Medulla

  Contains the pyramids (motor tracts); cardiovascular centres (rate/force heartbeat and diameter of vessels); breathing centres; nuclei concerned with touch, pressure and vibration. The olives = proprioception (joint and muscle position).

  origin of cranial nerves IX, X, XI, XII and cochlear part of VIII

  
• Pons and Medulla

  So the pons forms the trigeminal, abducent, facial and the balance-related part of the vestibulocochlear nerves. Whilst the medulla oblongata forms the hearing part of the vestibulocochlear nerve, the glossopharyngeal nerve , vagus nerve, accessory and hypoglossal nerves.

  
• Cerebellum

  -Vermis between lobes

  -Controls sub-conscious aspects of skeletal muscle movements and co-ordinates complex sequences

  -Regulate posture and balance

  -Superior cerebellar peduncle = midbrain to cerebellum. Middle cerebellar peduncle = pons to cerebellum. Inferior cerebellar peduncle = medulla to cerebellum

  Superior cerebellar peduncle = midbrain to cerebellum. Middle cerebellar peduncle = pons to cerebellum. Inferior cerebellar peduncle = medulla to cerebellum

  
• Pituitary gland

  -releases growth hormone

  -releases follicle stimulating hormone (FSH) and lutenising hormone (LH) – reproductive cycle

  This small gland has important communications with the hypothalamus. It sits under a small piece of dura covering the sella turcica (it is the same shape as a traditional Turkish-style saddle for a horse) and communicates with the rest of the brain via a tiny pituitary stalk.

  
• Circle of Willis

  The blood supply to the brain is from the internal carotid arteries and the vertebral arteries. They anastomose with each other to form the arterial circle of Willis. The vertebral arteries ascend the neck through the foramina in the transverse processes of the cervical vertebra and join close to the foramen magnum to form the basilar artery. The internal carotid is a terminal branch of the common carotid artery that passes into the carotid canal (one each side) and through the cavernous venous sinus.

  
• The circle of Willis is closely related to the inferior surface of the brain and the origins of the cranial nerves (shown in yellow on the diagram).

  
• Meninges

  The sub-arachnoid space lies between the pia and the arachnoid. It contains cerebrospinal fluid (CSF).

  
• The dura lines the skull, protects the brain and forms the dural venous sinuses. The arachnoid covers the surface of the brain and carries blood vessels. The pia adheres to the surface of the brain – going into the sulci and around the gyri.

  The dura is folded to create supporting structures around the brain. The falx cerebri lies between the cerebral hemispheres and carries the inferior sagittal dural venous sinus in its lower border (shown by the star).

  
• Blood supply to meninges

  -Anterior meningeal artery from the ethmoid branches of the internal carotid supplies the dura of the anterior fossa.

  -Middle meningeal artery from the maxillary artery passes through the foramen spinosum to supply the dura with blood. It is the major vessel supplying the dura and if damaged will lead to an extra-dural hematoma (compresses brain).

  -Posterior meningeal artery from the occipital artery supplies the dura of the posterior fossa.
• Nerve supply to the dura

  The nerve supply is mainly from the trigeminal nerve but nerves also ascend through the foramen magnum, hypoglossal canal and jugular foramen from C2 and C3 nerves of the neck. Some of these fibres are carried by the hypoglossal and vagus nerves.

  
• Dural venous sinuses

  -drain blood from the brain and channel it into the internal jugular vein

  These act as large veins related to the meninges and wall of the skull. There are communications between the cavernous venous sinus, the pterygoid venous plexus, the ophthalmic veins and the facial vein that allow the potential for spread of bacteria from superficial veins to the venous sinuses.

  
• -Each sinus is formed either between the periosteum of the skull and the dura (eg, sigmoid sinus) or between two layers of dura (eg, inferior sagittal sinus).

  -Communication between emissary veins of scalp and superior sagittal sinus, and between cavernous sinus and facial/inferior orbital veins are a possible route of infection into the meninges (bacterial meningitis).

  There is an area called the confluence of the sinuses (shown by the star) which collects the blood and passes it to the transverse and sigmoid sinuses. From there the blood from the brain drains into the internal jugular veins.

  
• Ventricles

  The brain and spinal cord is supported by cerebrospinal fluid (CSF).

  CSF is made within the brain by specialised cells called ependymal cells. To accommodate the movement of CSF there are ‘spaces’ within the brain called ventricles. There is one ventricle inside each of the cerebral hemispheres (lateral ventricles), one between the 2 parts of the thalamus (3rd ventricle) and one related to the cerebellum and brainstem (4th ventricle).

  
• Ventricles

  Small foramina and the cerebral aqueduct allow movement  of CSF between the ventricles and the spinal cord. The lateral ventricles are really ventricles 1 and 2 but are called ‘the lateral ventricles’ rather than being given a number.

  
• CSF circulates through the ventricles and into the sub-arachnoid space.

  Constantly re-cycled by loss through arachnoid villi (granulations) into the dural venous sinuses.

  The ependymal cells are clumped together as choroid plexus that hang inside the ventricles and create CSF. It is constantly created by these cells so must also be removed to prevent a build-up of CSF (hydrocephaly).

  
• Small pieces of tissue called arachnoid granulations communicate between the sub-arachnoid space and dural venous sinuses to provide a route to add the CSF back into the blood. These granulations usually cause small pits to form inside the cranial vault related to where the superior sagittal dural venous sinus lies in a living individual (dotted ring circles on the skull picture).