THE DOUBLE ROW OF WALLS AROUND THE CITY OF CARCASSONNE, FRANCE NS9 Motor pathways by Robin O’Sullivan
<ul><li>NS9 Motor pathways </li></ul><ul><li>LEARNING OUTCOMES </li></ul><ul><ul><li>1. Corticospinal tracts </li></ul></ul><ul><ul><li>2. Internal capsule </li></ul></ul><ul><ul><li>3. Pyramidal and extrapyramidal fibres </li></ul></ul><ul><ul><li>4. Upper and lower motor neurons, and effects of lesions </li></ul></ul><ul><ul><li>5. Polio, multiple sclerosis, motor neuron disease </li></ul></ul>
The cerebral cortex contains about 50 X 10 9 neurons. These are arranged in various patterns of layers and columns which vary over the entire surface of the cortex. Among the characteristic features of the MOTOR CORTEX is the presence of large cells that have cell bodies whose shape is reminiscent of pyramids. Photograph reproduced by kind permission of Mary Ann Sullivan
Copyright Lutz Slomianka, by kind permission PYRAMIDAL CELLS (also known as BETZ CELLS) in the motor cortex
AXONS FROM THE PRECENTRAL GYRUS AND ASSOCIATED AREAS OF MOTOR CORTEX PROJECT TO MOTOR NUCLEI IN THE BRAIN STEM AS THE CORTICOBULBAR or CORTICONUCLEAR TRACT , AND SPINAL CORD AS THE CORTICOSPINAL TRACT . THESE NEURONS ARE TERMED THE UPPER MOTOR NEURONS . THE NEURONS IN THE BRAIN STEM AND SPINAL CORD WHICH INNERVATE THE MUSCLES ARE THE LOWER MOTOR NEURONS .
About one-third of the fibres in the corticobulbar and corticospinal tracts arise in the primary motor cortex and only 3% of those are associated with the pyramidal or Betz cells; another one third arise elsewhere in the frontal lobe, and the final one-third from the parietal lobe. In fact, to destroy all the axons in these descending motor pathways it would be necessary to decorticate, i.e. remove most of the cortex from, the cerebral hemispheres.
The nerve fibres in the corticobulbar and corticospinal tracts pass downwards and backwards through the INTERNAL CAPSULE , between the LENTIFORM NUCLEUS (composed of the putamen and globus pallidus) and the THALAMUS . The corticobulbar tract ends in the brainstem.
EXTERNAL CAPSULE INTERNAL CAPSULE : ANTERIOR LIMB GENU (LATIN, = ‘KNEE’, i.e. the bend in the knee) POSTERIOR LIMB LENTIFORM NUCLEUS : PUTAMEN GLOBUS PALLIDUS THALAMUS CAUDATE NUCLEUS N.B. IN REVISING THIS LECTURE, REFER TO THE PLASTIC MODELS OF THE BRAIN IN THE LIBRARY
The INTERNAL CAPSULE contains both descending motor fibres and ascending sensory fibres. Between the internal capsule and the cortex all of these fibres form a great radiating, fan-like assembly of nerve fibres called the CORONA RADIATA.
The internal capsule is a common site for a STROKE . This is commonly caused by a haemorrhage from one of the arterial vessels there, but it can also be caused by the blockage of one of the local arteries. The internal capsule is one of the parts of the brain supplied by the ANTERIOR PERFORATING ARTERIES which enter the brain through the ANTERIOR PRERFORATED SUBSTANCE , so named because in dissection the arteries can be pulled out of, or ‘unplugged’ from, the brain tissue. The small tunnels or perforations which are left can easily be seen by the unaided eye. ANTERIOR PERFORATED SUBSTANCE
The corticobulbar tract passes through the genu and accompanies the corticospinal tract into the brainstem where it ends on the motor nuclei of Cranial Nerves III, IV, V, VI, VII, IX, X, XI and XII. These nuclei largely receive bilateral innervation from the corticobulbar tract, i.e. each left and right nucleus is innervated by both the left and right corticobulbar tracts. The corticospinal tract passes through the posterior limb of the internal capsule and continues down, through the middle of the crus cerebri, into the pons and medulla oblongata where it lies in the swelling called the PYRAMID . At the lower end of the medulla oblongata the corticospinal tracts cross the midline. This is termed the DECUSSATION OF THE PYRAMIDS .
AT THE DECUSSATION OF THE PYRAMIDS THE CORTICOSPINAL TRACT DIVIDES INTO TWO MAIN TRACTS THAT PASS DOWN INTO THE SPINAL CORD 1. Nearly 90% of the fibres cross the midline and descend in the LATERAL CORTICOSPINAL TRACT . 2. Most of the remaining fibres continue in the ANTERIOR CORTICOSPINAL TRACT , crossing the midline at successively lower levels when they approach their termination. 3. A very small proportion do not cross the midline, and continue in the lateral corticospinal tract of their own side.
PYRAMIDAL AND EXTRAPYRAMIDAL PATHWAYS The corticospinal fibres that pass through the pyramids in the medulla oblongata, and the corticobulbar fibres that accompany them for part of their course, are referred to as the pyramidal fibres. The nerve impulses they carry constitute the PYRAMIDAL PATHWAY which is largely concerned with voluntary, skilled movements which require attention. There is another pathway by which motor impulses reach the motor neurons that innervate the muscles. This is called the EXTRAPYRAMIDAL PATHWAY . These fibres originate in a group of nuclei deep to the cortex called the BASAL GANGLIA and they descend into the spinal cord as a number of tracts which are anatomically distinct from the corticospinal tracts. The biggest of the basal ganglia are the CAUDATE NUCLEUS , the PUTAMEN and the GLOBUS PALLIDUS . The extrapyramidal pathway is largely concerned with movements of a routine nature which do not require close attention. In reality, the two pathways work very closely together and it is wrong to think of them as two separate and different systems.
LATERAL CORTICOSPINAL TRACT DIAGRAM OF A TRANSVERSE SECTION THROUGH THE SPINAL CORD TO SHOW THE PYRAMIDAL (red) AND EXTRA-PYRAMIDAL TRACTS (blue) ANTERIOR CORTICOSPINAL TRACT
FEATURES OF AN UPPER MOTOR NEURON LESION 1. The muscles in the affected limb or limbs are initially soft and floppy and the clinician can move them easily. This condition is called FLACCID PARALYSIS . 2. After some days or weeks, the stretch reflexes recover. The limb becomes stiff and shows SPASTIC PARALYSIS . On examination, there is considerable resistance to movement which suddenly gives way and the limb can be moved freely. This is described as ‘ CLASP-KNIFE RIGIDITY ’. 3. The stretch reflexes are typically exaggerated, HYPERREFLEXIA . If the clinician suddenly flexes the knee, or dorsiflexes the ankle, a sustained series of contractions of the flexor muscles can often be elicited. This is termed CLONUS . 4. BABINSKI’S SIGN is present: scraping the plantar surface of the foot causes dorsiflexion of the big toe and adduction of the other toes. The normal response to this stimulus is plantar flexion of the toes. 5. The ABDOMINAL REFLEXES are absent on the affected side. Normally, the abdominal muscles contract if the overlying skin is scraped.
FEATURES OF A LOWER MOTOR NEURON LESION 1. The affected muscles show FLACCID PARALYSIS . 2. After some weeks, progressive muscle wasting or ATROPHY is evident. This is much greater than the disuse atrophy seen in immobilised limbs where the motor nerves are intact. Motor neurons produce a trophic factor which is carried to the muscles along the axons. 3. The stretch reflexes are absent, AREFLEXIA . 4. FIBRILLATIONS are present: these are tiny spontaneous contractions of individual muscles fibres in response to the small amounts of acetylcholine circulating in the blood and tissue fluid. They are invisible and can only be detected by electromyography. 5. FASCICULATIONS , which are visible and uncoordinated contractions of motor units due to spontaneous activity in motor nerves, are evident in the early stages of the condition.
POLIOMYELITIS This highly infectious disease is caused by a virus in contaminated food or water. It enters the lining of the small intestine where it can be neutralised if the patient has been appropriately immunised. The virus multiplies in the intestinal wall, leading to a VIRAEMIA (= viruses in the blood). Most patients are unaware they are infected. Some develop a flu-like illness. Viral particles are shed in the faeces for several weeks. The virus tends to infect the cell bodies of motor neurons. A very small proportion of patients (about 0.5%) will manifest permanent damage, with paralysis and muscle atrophy most commonly affecting one or both lower limbs. Egyptian stele c. 3,500 years old showing the priest Rema with his wife and son
Polio victims and facilities at the Amar Jyoti Research and Rehabilitation Centre, Delhi Photographs reproduced by permission, copyright WHO / P.Virot
MULTIPLE SCLEROSIS In this condition axons within the CENTRAL NERVOUS SYSTEM undergo demyelination. It is the most common demyelinating disease. When axons myelinate, the POTASSIUM CHANNELS under the myelin largely disappear. After demyelination these parts cannot conduct nerve impulses efficiently. The areas of demyelination are called PLAQUES . They form in close proximity to veins and have a random distribution. They can be seen on MAGNETIC RESONANCE IMAGING but there is usually a poor correlation between the location of the plaques and the features of the disease in individual patients. An established plaque feels relatively firm ( Greek, SKLEROS = hard ). It has few oligodendrocytes and an abundance of astrocytes. The axons within the plaque survive for a long time but eventually degenerate. Multiple sclerosis often commences around the age of 30 years with visual disturbances being an early feature. Females are more commonly affected than males. It usually progresses slowly, with periods of remission and relapses, to a state of severe disability. It is more prevalent in temperate regions and is widely accepted to be an auto-immune disease, possibly a complication of an earlier viral disease in genetically-predisposed patients. There is no cure.
TRANSVERSE SECTION OF A SPINAL CORD, WITH TWO LARGE MS PLAQUES
SYMPTOMS OF MULTIPLE SCLEROSIS VISUAL DISTURBANCES : blurred vision, diplopia, blindness. OTHER SENSORY DISTURBANCES : numbness or parasthesia (“pins and needles”) in the hands or feet. Dizziness. MOTOR DISTURBANCES : Muscle weakness and incoordination. Problems with walking, balance and coordination. Loss of manual dexterity. Involuntary movements, especially of the lower limbs. Slurred Speech. PAIN : musculoskeletal, or due to ‘short circuiting’ between adjacent nerve fibres FATIGUE and DEPRESSION : lapses of memory, poor concentration. INCONTINENCE : an increased risk of urinary infection. SEXUAL PROBLEMS : erectile dysfunction, loss of libido. The symptoms very greatly between patients, and may be temporary due to the fluctuating nature of the disease.
MOTOR NEURON DISEASE This is not a single condition. The term covers a number of types of motor neuron diseases (MNDs). They are characterised by degeneration of upper or lower motor neurons. The precise features depend on which type of neurons are primarily affected. Some forms are familial and appear in young children. Other forms are acquired but the cause is unknown; they typically affect patients aged over 40 years. Common forms of MND include: AMYOTROPHIC LATERAL SCLEROSIS , characterised by weakness and spasticity of limb muscles mainly due to involvement of the descending motor tracts so the features will be largely those of upper motor neuron lesions. PROGRESSIVE MUSCULAR ATROPHY , characterised by weakness, wasting and flaccidity with fasciculations, mainly due to degeneration of the lower motor neurons in the ventral horn of the spinal cord. PSEUDOBULBAR PALSY , characterised by difficulties with swallowing and speech due to corticobulbar tract involvement. The features will be those of upper motor neuron lesions and lower motor neuron signs tend to be absent. PROGRESSIVE BULBAR PALSY , characterised by dysfunction of the cranial nerve nuclei in the lower brainstem, especially CNs IX, X and XII, so lower motor neuron signs are present.