Nervous system

Nervous system

The anatomy
The nervous system consists of the brain and spinal cord [central nervous systm[C.N.S] and peripheral nerves and muscles [peripheral nervous system [P.N.S].

The neurone has a cell body and axon terminating at a synapse and is the functioning unit of the nervous system ▪
Astrocytes provide the structural framework for for the neurones,control their biochmical environment and form blood-brain barrier. ▪

Micrglail cell are blood-derived mononuclear macrphage and have immune role. ▪

Inthe C.N.S,oligodendrocytes createe and mentain amyelin sheath round the axon while in the P.N.S the myelin sheath is produced by Schwann cells

The brain consists of two cerebral hemispheres each with four functionally specialized lobes [frontal, parietal,temporal and occipital] ,the brainstem and cerebellum The cerebral cortex over the cerebral hemispheres constitutes the highest level of function

The cerebral cortex constitutes the highest level of nervous function, the anterior half dealing with executive ('doing') functions and the posterior half constructing a perception of the environment ('receiving and perceiving'). and which of the two hemispheres is 'dominant', i.e. where language function is represented. In right-handed individuals this is almost always the left hemisphere while in left-handers either hemisphere may be dominant with about equal frequency

The brainstem consist of the midbrain,pons and medulla oblongata,contains all sensory and motor pathways entering and leaving the cerebral hemispheres ,together with nuclei of the cranial nerves
The cerebellum which lies below cerebral hemispheres and posterior to the brainstem,is responsible for co-ordination including gait and posture


Between the soft brain and the skull are three types of membranous covering called meninges ▪
The dura mater next to the bone ▪ The arachnoid in middle ▪ The pia mater next to the brain
Tentorium cerebelli divide the brain into two levels
Supratentorail level & infratentorail level

The spinal cord

▪ The spinal cord is 18 in (45 cm) long. It is continuous above with the medulla oblingata at the level of the foramen magnum and ends below at the lower level of the 1st, or the upper level of the 2nd lumbar vertebra. ▪ Tapers into the conus medullaris from which a prolongation of pia mater. ▪ At each intervertebral foramen the anterior and posterior nerve roots unite to form a spinal nerve which immediately divides into its anterior and ▪ posterior primary rami, each transmitting both motor and sensory fibres

NEUROLOGICAL INVESTIGATIONS

NEURORADIOLOGY
Skull X-rays ▪ fractures of the skull vault or base ▪ skull lesions (e.g. metastases, osteomyelitis, Paget's disease, abnormal skull foramina, fibrous dysplasia) ▪ enlargement or destruction of the pituitary fossa - intrasellar tumour, raised intracranial pressure ▪ Spinal X-rays show fractures, congenital bone lesions (e.g. cysts), destructive lesions (infection, metastasis) or spondylosis (degenerative change]

Imaging of the brain and spinal cord :

Brain CT is now widely available world-wide and MRI is rapidly becoming a standard test

Computed tomography:

A collimated X-ray beam moves synchronously across a slice of brain between 2 mm and 13 mm thick. ▪ attenuation (density) between bone, brain and CSF enable recognition of normal and infarcted tissue, tumour, extravasated blood, and oedema .
Enhancement with i.v. contrast media delineates areas of increased blood supply.
CT imaging of the spinal cord (myelography) and cerebral ventricles (ventriculography) is occasionally performed ▪
CT is safe apart from occasional systemic reactions to contrast; the irradiation involved is small.

: CT scanning demonstrates :

▪ cerebral tumours ▪ intracerebral haemorrhage and infarction ▪
subdural and extradural haematoma ▪ free blood in the subarachnoid space (subarachnoid haemorrhage) ▪
lateral shift of midline structures and displacement/enlargement of the ventricular system ▪
cerebral atrophy ▪
spinal trauma (with CT myelography( ▪
skull and scalp lesions . ▪
CT imaging, within its limitations, shows that the brain is anatomically normal, and gives reassurance .

Limitations of CT

▪ Lesions under 1 cm diameter may be missed . ▪ Lesions with attenuation close to that of bone may be missed if near the skull . ▪
Lesions with attenuation similar to that of brain are poorly imaged (e.g. MS plaques, isodense subdural haematoma( ▪
CT sometimes misses lesions within the posterior fossa . ▪ The spinal cord is not imaged directly by CT (contrast is necessary). ▪
Cooperation is required - a general anaesthetic is occasionally needed .

Magnetic resonance imaging: MRI

The hydrogen nucleus is a proton whose electrical charge creates a local electrical field. These protons are aligned by sudden strong magnetic impulses. Protons are then imaged with radiofrequency waves at right angles to their alignment. The protons resonate and spin, then revert to their normal alignment. As they do so, images are made at different phases of relaxation, known as T1, T2, T2 'STIR', diffusion-weighted imaging (DWI) and other sequences. Gadolinium is used as an intravenous contrast medium .

Advantages of MRI

MR distinguishes between brain white and grey
▪ Spinal cord and nerve roots are imaged directly . ▪ Pituitary imaging . ▪
MRI has greater resolution than CT (around 0.5 cm( ▪
No radiation is involved . ▪
Magnetic resonance angiography (MRA) images blood vessels without contrast . ▪ It is useful in muscle disease, e.g. myositis


Limitations of MRI are principally
▪ time and cost. Imaging one region takes about 20 minutes. ▪
Patients do need to cooperate. ▪
Claustrophobia is an issue but 'open' machines are available. ▪
Patients with pacemakers or with metallic bodies in the brain cannot be imaged. ▪
MR imaging for some days after lumbar puncture frequently shows diffuse meningeal enhancement with gadolinium .

Doppler studies B-mode and colour ultrasound are valuable in the detection of stenosis of the carotid arteries
Digital cerebral and spinal angiography, These techniques are little used. Contrast is injected intra-arterially or intravenously. These studies carry a mortality and risk of stroke (<1%). Sequences of the aorta, carotid and vertebral arteries demonstrate occlusion, stenoses, atheroma and aneurysms. Spinal angiography images cord arteriovenous malformations

Isotope bone scanning ▪ [99mTc]-pertechnetate is injected intravenously, is of value in detection of vertebral and skull lesions (e.g. metastases(
▪ Positron emission tomography (PET) and single proton emission computed tomography (SPECT) These techniques image function more than anatomy by tracking the uptake and metabolism of radioisotopes