Ns1. Overview Of Brain Structure. Compressed

NS1 OVERVIEW OF BRAIN STRUCTURE by Robin O’Sullivan The walled city of Carcassonne from the River Aude, France

NS1 OVERVIEW OF BRAIN STRUCTURE

LEARNING OUTCOMES

Presentation of nervous system disease

Terminology

Cells of the nervous system; myelin

Main parts of the nervous system – lobes, brainstem, cord

What a nerve is – spinal/cranial nerves

Fibres, tracts

Spinal cord: levels of injury

DISEASES OF THE NERVOUS SYSTEM 1. NEURODEGENERATIVE DISEASES e.g. Parkinson’s Disease, Alzheimer’s Disease, Motor Neuron Disease 2. TUMOURS 3. DEMYELINATING DISEASES e.g. Multiple Sclerosis 4. VASCULAR DISEASE e.g. Stroke, Haemorrhage 5. PERIPHERAL NERVE DISORDERS e.g. Diabetic Neuropathy, Leprosy, Sj ö gren’s Syndrome 6. TRAUMA (damage may be transient or permanent) 7. INFECTION e.g. Encephalitis, Meningitis, Polio  In general, the location of the lesion determines the clinical presentation

MAIN PARTS OF THE NERVOUS SYSTEM CENTRAL NERVOUS SYSTEM (CNS) = BRAIN + SPINAL CORD PERIPHERAL NERVOUS SYSTEM (PNS) = ALL OF THE NERVES THAT EMERGE FROM THE CNS. ● MOTOR NERVES SOMATIC TO SKELETAL MUSCLES AUTONOMIC TO CARDIAC MUSCLE, SMOOTH MUSCLE AND EXOCRINE GLANDS ● SENSORY NERVES NERVE PLEXUSES CERVICAL (C1-C4) BRACHIAL (C5-T1) LUMBAR (T12-L4) SACRAL (L4-S3) COCCYGEAL (S4-Co1) The segmental nature of the innervation of the body wall is best seen in the intercostal nerves in the thoracic region.

BRAIN STEM CEREBRUM CEREBELLUM

These fossil endocasts of the interior of the cranal cavity reveal the approximate shape of the brain in an ancestral species, Homo erectus . A bulge on the surface of their brains which corresponds to the region of our speech centre suggests that they could speak. That bulge is missing in the brains of the great apes. The texture of these “brains” is rock hard. Living brains are extremely soft. They cannot be stitched (sutured) during neurosurgery. Bleeding is controlled by clips or cautery. This softness means that brains are prone to injury. The simple action of shaking our heads from side to side will give the brain sufficient momentum to result in serious damage, were it not for the protection given by the shape of the cranial cavity, the fibrous tissue meninges that cover the brain (dura mater, arachnoid mater, pia mater), and the fluid that surrounds it. THE SOURCE OF THIS PHOTOGRAPH IS NOT KNOWN

ANTERIOR CRANIAL FOSSA MIDDLE CRANIAL FOSSA POSTERIOR CRANIAL FOSSA FORAMEN MAGNUM PITUITARY FOSSA CRIBRIFORM PLATE THE CRANIAL CAVITY

FALX CEREBRI TENTORIUM CEREBELLI

In the living skull the cranial cavity is lined by fibrous tissue which is penetrated by various nerves and blood vessels. FALX CEREBELLI TENTORIUM CEREBELLI (CUT)

THE DURA MATER SKIN AND SUBCUTANEOUS TISSUES SKULL DURA MATER The DURA MATER (Latin = Hard Mother) is a layer of tough fibrous tissue fused to the periosteum lining the cranial cavity, and separated from it where the venous sinuses are located. It forms the folds which help to stabilise the position of the brain in the cranial cavity: the FALX CEREBRI between the two cerebral hemispheres, the FALX CEREBELLI between the two cerebellar hemispheres, and the TENTORIUM CEREBELLI between the cerebrum and the cerebellum. PERIOSTEUM VENOUS SINUS (This one is the Superior Sagittal Sinus) FALX CEREBRI

THE PIA MATER PIA MATER The PIA MATER (Latin = Good Mother) is a very thin layer of fibrous tissue fused to the surface of the brain. It closely follows all the contours of the brain and cannot easily be separated from it by dissection. Blood vessels which enter the substance of the brain pass through the pia mater. The connective tissue sheaths around those blood vessels are continuous with the pia mater.

THE ARACHNOID MATER ARACHNOID MATER The ARACHNOID MATER is a thin membrane that lies between the dura mater and the pia mater. It follows the course of the dura mater, and is separated from it by a very narrow space, the SUBDURAL SPACE . The arachnoid mater is joined to the pia mater by numerous delicate strands of connective tissue which resemble the strands of a spider’s web (Greek, ARACHNE = spider). Between the arachnoid mater and the pia mater is the SUBARACHNOID SPACE .

EXPOSED PIA MATER REMAINING ARACHNOID MATER GYRUS (Plural, GYRI) or RIDGE SULCUS (Plural, SULCI) or FISSURE

FOUR MONTHS AFTER FERTILISATION, THE CEREBRAL HEMISPHERES ARE SMOOTH. GYRI AND SULCI BEGIN TO APPEAR IN THE SEVENTH MONTH. ALL THE MAJOR ONES ARE PRESENT AT BIRTH. THE SOURCE OF THIS PHOTOGRAPH IS NOT KNOWN

CENTRAL SULCUS LATERAL SULCUS THE PATTERN OF FISSURES IS VARIABLE, BUT TWO ARE CONSTANT: THE CENTRAL AND LATERAL SULCI

PRE-OCCIPITAL NOTCH PARIETO-OCCIPITAL SULCUS FRONTAL LOBE (MOTOR) PARIETAL LOBE (SENSORY) TEMPORAL LOBE (HEARING) OCCIPITAL LOBE (VISION) THE CEREBRAL HEMISPHERE HAS SIX LOBES, OF WHICH FOUR ARE SEEN HERE

THE FIFTH LOBE LIES IN THE FLOOR OF THE LATERAL SULCUS INSULA

The sixth lobe is called the LIMBIC SYSTEM. It consists of a group of structures lying deep within the cerebral hemispheres, and which evolved at an early stage in the development of the mammalian brain. It is concerned with basic instincts including attacking, defending, feeding and breeding. It is closely related to the olfactory system.

The primitive NEURAL TUBE consists of a cavity surrounded by a wall. The wall gives rise to the various parts of the brain - the FOREBRAIN , MIDBRAIN and HINDBRAIN - and the spinal cord. The cavity persists, and develops into the VENTRICULAR SYSTEM of the brain and the CENTRAL CANAL of the spinal cord. FOREBRAIN MIDBRAIN HINDBRAIN

The tissues of the CNS can be divided into two big components. The difference between them can be demonstrated by special stains. The GREY MATTER contains the aggregated cell bodies of neurons. It is found as a superficial layer covering the cerebrum and cerebellum, the CORTEX , and as NUCLEI deep to the cortex. A nucleus in the CNS is the equivalent of a GANGLION in the PNS. CEREBRAL CORTEX CAUDATE NUCLEUS THALAMIC NUCLEI This slice of brain has been stained with MULLIGAN’S STAIN. The photograph is reproduced by kind permission of Ms Tracy Cuffe, Anatomy Department, University College, Cork, Ireland

The WHITE MATTER contains fat in the form of the insulating MYELIN SHEATHS associated with nerve fibres. A precise definition is difficult but in simple terms a nerve fibre can be defined as an axon with its myelin sheath. Groups of nerve fibres following the same course within the CNS are called TRACTS .

THERE ARE THREE MAIN CATEGORIES OF NERVE FIBRE TRACTS: 1. ASSOCIATION FIBRES ARE CONFINED TO ONE CEREBRAL HEMISPHERE. 2. COMMISSURAL FIBRES PASS FROM ONE CEREBRAL HEMISPHERE TO THE OTHER. 3. PROJECTION FIBRES PASS FROM THE CEREBRAL CORTEX TO UNDERLYING NUCLEI IN THE BRAIN OR SPINAL CORD.

Cells of the CNS: NEURONS, ASTROCYTES, OLIGODENDROCYTES and MICROGLIA

THE NEURON The fundamental cell type in the nervous system is the nerve cell or NEURON which can be associated with various types of SUPPORT CELLS . The support cells in the CNS are different to those in the PNS. Neurons are specialised for rapid communication. They are irritable cells and when appropriately stimulated their cell membranes conduct that irritability as a NERVE IMPULSE to other neurons, or to innervated cells in an effector organ such as an exocrine gland or a piece of muscle. Neurons consist of a cell body called a PERIKARYON or SOMA which has a number of branching cytoplasmic processes of varying lengths. The short processes are called DENDRITES . These typically receive incoming nerve impulses and transmit them down a long process, the AXON , whose branches terminate as small swellings adjacent to part of another neuron or effector organ. Neurons can be classified in a number of ways. For example, they can be classified according to their morphology, function, or the chemical nature of the neurotransmitter they secrete from the terminal swellings of the axon.

NEURONS can be classified morphologically by the arrangement of their processes. A UNIPOLAR neuron has a single process which branches into two, one giving rise to the dendrites and the other the axon. A BIPOLAR neuron has two processes, one forming the dendritic tree and the other being the axon. A MULTIPOLAR neuron has an axon and numerous dendrites arising directly from the perikaryon. Neurons develop from spherical cells called NEUROBLASTS . In the morphogenesis of the different types of neurons, neuroblasts first tend to differentiate into unipolar axons, which may go on to become bipolar and subsequently multipolar in form. NEUROBLAST UNIPOLAR NEURON BIPOLAR NEURON MULTIPOLAR NEURON

SUPPORT CELLS IN THE CNS The support cells in the CNS are traditionally called GLIA. The Greeks had two words for glue, kolla (as in collagen) and glia. It was formerly believed that the glial cells held the CNS together. 1. ASTROCYTES . These cells are present throughout the CNS and form a cellular framework or scaffolding throughout that region. They have many cytoplasmic processes radiating out from them. Some flattened extensions of the processes envelop the capillary blood vessels and constitute a BLOOD BRAIN BARRIER . More of them line the basement membrane deep to the innermost of the meningeal coverings of the CNS forming another barrier, the GLIA LIMITANS . Histochemically, astrocytes can be easily demonstrated because they contain Glial Fibrillary Acidic Protein, GFAP. 2. OLIGODENDROCYTES . These have fewer processes than the astrocytes. They envelop axons and can form a fatty MYELIN SHEATH around each axon which acts as an insulating material. An oligodendrocyte can myelinate up to fifty axons. An axon with its myelin sheath is termed a NERVE FIBRE . 3. MICROGLIA . These are small phagocytic cells. They are not very numerous but are found throughout the entire CNS. They become active in response to inflammation or injury. 4. EPENDYMA . These are the epithelial cells which line the fluid-filled VENTRICLES of the brain and the CENTRAL CANAL of the spinal cord.

THE ASTROCYTES KNOW EVERYTHING THAT HAPPENS WITHIN THE CNS!

MYELINATION. Left, An OLIGODENDROCYTE beginning to myelinate some axons in the CNS; Right, A transmission electronmicrograph showing SCHWANN CELLS in the PNS. A NERVE FIBRE is defined as an axon with its ensheathing Schwann cell or oligodendrocyte process

The stages of segregation and myelation in the PNS, from a fetal packet of axons (1) to a mature myelinated axon (4). 1 2 4 3

SEGREGATION AND MYELINATION SCHWANN CELL NUCLEUS COMPACT MYELIN SHEATH AXONS FUSED SCHWANN CELL MEMBRANE, THE ‘MESAXON’, WHICH WILL ENSHEATH THE AXON APPOSED LAYERS OF SCHWANN CELL MEMBRANE ABOUT TO FUSE

SPINAL NERVES DORSAL ROOT GANGLION DORSAL ROOTLETS VENTRAL VENTRAL ROOT ROOTLETS The MIXED SPINAL NERVE is composed of motor nerve fibres whose multipolar cell bodies are in the ventral horn of the spinal cord grey matter, and sensory nerve fibres whose unipolar cell bodies are in the dorsal root ganglia. MIXED SPINAL NERVE

DURA MATER VENTRAL ROOTLETS CAUDA EQUINA During the fetal period and in early infancy, the spinal cord grows more slowly than the vertebral column. In the adult the cord ends at L2. The ventral and dorsal roots are angled ever more steeply downwards, and the lowest roots are essentially vertical in their alignment. They constitute the CAUDA EQUINA (Latin = the horse’s tail).

SPINAL CORD LEVELS OF INJURY In general, neurologists define the level of injury as the first spinal segmental level that shows abnormal neurological loss. Thus, for example, if a person has loss of biceps, the motor level of the injury is often said to be C4. In contrast, physiotherapists or rehabilitation doctors tend to define level of injury as the lowest spinal segmental level that is normal. © WHO/P.Virot

THE END Carcassonne, France