Introduction to Neurology PDF

neurology Introduction to neurology Nerve tissue Nerve tissue is the main tissue component of the nervous system and is primarily composed of neurons and supporting glial cells. 1. neur...

neurology Introduction to neurology Nerve tissue Nerve tissue is the main tissue component of the nervous system and is primarily composed of neurons and supporting glial cells. 1. neurons  structure of a neuron  Types of neurons: 1  neurology Introduction to neurology 2. Supporting glial cells  Overview of glial cells N.B. Each myelinating Schwann cell insulates only ONE axon. Glial cells guard the axons of the nerve cells as COPS: CNS axons are myelinated by Oligodendrocytes; PNS axons are insulated by Schwann cells.  Blood-brain barrier 2  neurology Introduction to neurology Myelin  Insulating layer of modified plasma membrane that wraps around axons of nerve in a spiral structure  Increases the space constant and the conduction velocity of signals traveling down axons  Decreases membrane capacitance and increases membrane resistance  Node of Ranvier ▪ Unmyelinated regions between two adjacent myelinated segments of axons in the CNS and PNS ▪ Contain a large amount of Na+ channels: allows saltatory conduction increases the velocity of action potentials  Demyelination: ▪ It's a process in which myelin sheaths of nerves become damaged, which impairs electrical conduction ▪ Central demyelination occurs within the CNS (e.g., seen with multiple sclerosis, progressive multifocal leukoencephalopathy, leukodystrophies). ▪ Peripheral demyelination affects the PNS (e.g., seen with Guillain-Barre syndrome). 3  neurology Introduction to neurology organization of the nervous system o Brain, spinal cord o consists of the nerves and ganglia outside the brain and spinal cord, including the cranial nerves, spinal nerves, and their roots, peripheral nerves, and neuromuscular junctions. CENTRAL NERVOUS SYSTEM  The CNS is formed of 2 parts: 1. intracranial part: 1- Two cerebral hemispheres They are connected to each other by the corpus callosum. 4 neurology Introduction to neurology The surface of Each hemisphere is divided into 4 lobes: ✓ Frontal. ✓ Temporal. ✓ Parietal. ✓ Occipital. Outer gray matter Inner white matter cerebral cortex depth of the cerebral hemisphere composed of neural cell bodies & composed of myelinated axons unmyelinated axons It contains certain areas that control it conducts impulses to & from specific functions. the cerebral cortex. 5  Neurology Introduction to neurology 2- basal ganglia  The basal ganglia are a group of nuclei that lie beneath the cortex  Distributed over the telencephalon, diencephalon, and mesencephalon (midbrain).  Overview of the basal ganglia Basal ganglia Location Clinical significance Caudate nucleus Molecular changes in Huntington disease lead to neuronal loss Striatum Telencephalon Putamen in the striatum. Globus pallidus Wilson disease (hepatolenticular degeneration) leads to an Lentiform impaired function of the lentiform nucleus due to copper nucleus Putamen Diencephalon accumulation. Subthalamic nucleus Effect of lesion: contralateral hemiballismus Parkinson disease is characterized by neuronal degeneration in Substantia nigra Mesencephalon the substantia nigra. 6 neurology Introduction to neurology 7  neurology Introduction to neurology 3- brain stem part Cranial nerve motor nuclei Midbrain 3&4 Pons 5, 6, 7 Medulla 9, 10, 11, 12  NB: The 1, 2 & 8 cranial nerves have no motor nuclei, They are sensory nerves concerned with special sensation. 8  neurology Introduction to neurology 4- cerebellum  It lies at the back & the bottom of the cranium behind the brain stem, in The posterior cranial fossa. 9  neurology Introduction to neurology 2. spinal part:  It lies in the spinal canal.  It ends at the lower border of the 1st lumbar vertebra (L1). Cervical 8 segments Thoracic 12 segments Lumbar 5 segments Sacral 5 segments coccygeal 1 segment Conus medullaris: the lowermost 3 segments of the spinal cord (S 3, 4, 5). Epiconus: the 4 segments above the conus medullaris (L4, 5, S1, 2).  Spinal cord   a collection of lumbo–sacral roots that fills the lower part of spinal canal. Cauda  It starts at the lower border of the 1st lumber vertebra (L1). Equina 10 neurology Introduction to neurology 11  Neurology Introduction to neurology THE MOTOR SYSTEM ORIGIN TERMINATION CONTROL FUNCTIONS ✓ Initiation of the voluntary motor The Pyramidal in the cerebral at the AHCs of the it controls the cortex activity. System different levels of opposite side of the (motor area 4 & ✓ inhibition of the deep reflexes. (UMN): premotor area 6). the spinal cord body. ✓ inhibition of the muscle tone. ✓ Regulation of the voluntary motor The Extra at the AHCs of the it controls the activity. from the pyramidal different levels of opposite side of the ✓ regulation of the emotional & basal ganglia. System the spinal cord. body. associated movements. ✓ inhibition of the muscle tone. ✓ Co-ordination of the voluntary motor The Cerebellar at the AHCs of the activity initiated by the pyramidal from the it controls the same different levels of System cerebellum. the spinal cord. side of the body. system. ✓ maintenance of equilibrium. The Lower in the AHCs of the components: at the voluntary ✓ Transmission of the motor impulse from Motor Neuron different levels of AHCs, PN, NMJ & muscles. the AHCs to the voluntary muscles. (LMN) the spinal cord. Voluntary muscles. 20  Neurology Introduction to neurology Motor pathway ORIGIN: the order starts in the Cerebral cortex. TERMINATION: on the Voluntary muscle which will respond by movement. From the origin to the termination , the impulse passes through 2 neurons: ✓ Upper Motor Neuron (UMN): Pyramidal tract. ✓ Lower Motor Neuron (LMN): AHCS, Nerves, NMJ, Muscles. 1. Upper motor neuron (UMN)  Motor area "4" The voluntary motor impulse originates mainly in the large pyramidal cells (Betz cells) of motor area (4) & "cerebral cortex" to a lesser extent in the cells of the premotor area (6). The axons of these cells (carrying the impulse) descend in the depth of the Cerebral hemisphere in the  Internal capsule Corona radiata to pass in the Internal capsule, and continue their descent in the brain stem. In the Brain Stem, some of the descending fibers separate to supply the motor nuclei of the cranial nerves  Corticobulbar of BOTH sides except the lower 1/2 of the facial nucleus and all the hypoglossal nucleus which are supplied only from the opposite pyramidal tract. tract "brain stem" These fibers are known as the corticobulbar tract as they do not reach the spinal cord. The pathway from the cerebral cortex down to the AHCs in the spinal cord is known as corticospinal tract.  Corticospinal In the lower medulla: tract "spinal cord" ✓ 85 % of fibers cross (decussate) to descend in the white matter of the opposite side of the spinal cord. ✓ 15 % of the fibers descend directly in the white matter of the same side of the spinal cord. 21 Neurology Introduction to neurology 22  neurology Introduction to neurology 2. Lower motor neuron (LMN) They are special type of nerve cells situated anterior horns of the H- shaped gray matter of the spinal cord.  AHCS They receive the voluntary motor impulse from the corticospinal pyramidal tract. Their axons exit from the spinal cord as the anterior roots.  Peripheral They carry the motor impulse from the AHCs to the voluntary muscles. motor nerves  Neuromuscular junction.  Voluntary muscles. 23  neurology Introduction to neurology Muscle Tone  def: this is a spontaneous local axon stretch reflex.  mechanism: the length of any skeletal muscle is shorter than the distance between the origin and the insertion, So any muscle is always in a state of Persistent spontaneous slight stretch. spontaneous persistent stretch ↓ persistent activation the local axon reflex ↓ persistent (maintained) contraction of the muscle (tone).  N.B. The Muscle tone receives inhibition from the Pyramidal & Extrapyramidal systems.  EFFECT OF umnl & lmnl ON MUSCLE TONE: loss of inhibition on the muscle tone UMNL ↓ Increased muscle tone (spasticity) below the level of the lesion. interruption of the reflex arc LMNL ↓ Decreased muscle tone (flaccidity) at the level of the lesion. 24  neurology Introduction to neurology Deep Reflex (Tendon Jerk)  def: this is an Induced local axon stretch reflex.  mechanism: sudden stretch of the muscle by tapping the tendon with a hammer ↓ activate the local axon reflex (suddenly & temporarily) ↓ sudden transient contraction of the muscle (jerk)  N.B. The deep reflex receives inhibition from the Pyramidal system.  EFFECT OF umnl & lmnl ON MUSCLE TONE: loss of inhibition on the deep reflex UMNL ↓ Increased deep reflex (hyperreflexia) below the level of the lesion interruption of the reflex arc LMNL ↓ Decreased deep reflex (hyporeflexia) at the level of the lesion 25  neurology Introduction to neurology Clonus  Def: Rapid, Rhythmic, Regular contractions that result from Sudden sustained stretch of the muscle tendon.  It indicates Severe pyramidal lesion due to: loss of inhibition on the stretch reflex  It is elicited in: ankle, patella and wrist  It stops when: the stretch is stopped 26  neurology Introduction to neurology How would you clinically differentiate between UMNL & LMNL?? UMNL LMNL Paralysis or weakness below the Paralysis or weakness at the level of Muscle power level of the lesion. the lesion. No wasting, & if present it is late Early & marked wasting due to loss Muscle wasting and due to disuse atrophy. of muscle tone. Hypertonia (spasticity) below the Hypotonia (flaccidity) at the level of Muscle tone level of the lesion. the lesion. Deep reflexes Hyperreflexia below the level. Hyporeflexia at the level. Pathological May be present. Absent. deep reflexes Clonus May be present. Absent. Superficial Lost if the lesion is above the Lost if the lesion involves the supply reflexes segmental supply of the reflex. of the reflex. Positive, i.e. dorsiflexion of the big Plantar flexion of the toes, or, no toe ± fanning of the other toes response. Plantar reflex (Babinski) May be present in irritative lesion of Fasciculations Absent the AHCs. 27  neurology Introduction to neurology THE SENSORY SYSTEM  There are 3 types of sensations in the body: A. Somatic sensations: They are conducted to the CNS via the somatic nerves. They include: ✓ Pain Superficial sensations ✓ Temperature ✓ Touch Deep sensations ✓ Vibration sense ✓ Muscle sense (proprioceptive) ✓ Joint sense ✓ Nerve sense ✓ Tactile localization ✓ Two points discrimination Cortical sensations ✓ Stereognosis ✓ Graphosthesia B. Visceral sensations: They are conducted to the CNS via the "autonomic nerves. They include all sensations coming from the internal viscera, e.g. heart & intestines. C. Special sensations: They are conducted to the CNS via the cranial nerves. They include: Smell, Vision, Hearing. Somatic Sensations All somatic sensations (superficial or deep) pass through 3 order Neurons from receptors in the skin & deep structures to reach the Cortical sensory area of the opposite side: 1st order neuron Always Dorsal root ganglion (DRG) 2nd order neuron varies according to the type of sensation. 3rd order neuron always in the thalamus of the opposite side 28  neurology Introduction to neurology 1. Pathways of Superficial Sensations  Pathway of Pain & Temperature: 1st order neuron is: cell of the dorsal root ganglion (DRG). 1st order The afferent sensory nerve relays in the DRG. The efferent sensory nerve enters the spinal cord in Lissauer's tract to neuron relay in the cells of the Substantia Gelatinosa of Rolandi (SGR), at the posterior horn of the gray matter The 2nd order neuron is the cell of SGR & its axon. 2nd order This axon crosses to the OPPOSITE side & ascends in Lateral Spinothalamic neuron tract of the spinal cord, and then in Lateral Lemniscus of the brain stem, to relay in the thalamus. The 3rd order neuron starts in the cell of the thalamus. 3rd order Its axon ascends to pass through the internal capsule conducting the neuron impulse to the cortical sensory area in the parietal lobe.  Pathway of Touch: has the same pathway as pain, temperature, BUT: Crude touch: In the 2nd order neuron: it ascends ventral Spinothalamic tract of the OPPOSITE side of spinal cord. Fine touch has the same pathway as deep sensation. 29  neurology Introduction to neurology 2. Pathway of deep Sensations 1st order neuron is: cell of the dorsal root ganglion (DRG). The afferent sensory nerve relays in the DRG. The efferent sensory nerve enters the spinal cord & ascends in Gracile 1st order & Cuneate tracts within the posterior column of the SAME side (along with fibers carrying fine touch) to relay in Gracile & Cuneate nuclei in neuron the medulla: Gracile Tract carries fibers from Lower ½ of body & lies medially Cuneate Tract carries fibers from upper ½ of body & lies laterally. The 2nd order neuron is the cell of gracile & cuneate nuclei & its axon. 2nd order This axon crosses to the opposite side & ascends in the medial neuron lemniscus through the brain stem, to relay in the thalamus. The 3rd order neuron starts in the cell of the thalamus. 3rd order Its axon ascends to pass through the internal capsule conducting the neuron impulse to the cortical sensory area in the parietal lobe. 3. Pathway of cortical Sensations These are a mixture of refined superficial &deep sensations arriving to the thalamus via the 1st & 2nd order neuron conducted from the thalamus to the cortical sensory area (1,2,3) in the parietal lobe. 30  neurology Introduction to neurology Left image: the dorsal column (gracile fasciculus and cuneate fasciculus) carries information of fine touch, vibration, pressure, and proprioception (gracile fasciculus: primarily from the lower half of the body; cuneate fasciculus: primarily from the upper half of the body). Right image: the anterolateral column (anterior spinothalamic tract and lateral spinothalamic tract) carries information of crude touch, pressure, pain, and temperature. 1) Anterior spinothalamic tract: The first neuron is located within the spinal ganglia. Axons from the first neuron split and ascend or descend several segments (a). Having reached another segment, they synapse to the second neuron in the posterior horn of the spinal cord, and the axons decussate to the contralateral side of the spinal cord within the anterior white commissure (b). Further ascend in the anterior spinothalamic tract (c, d). Synapse to the 3rd neuron in the thalamus (e). 2) Lateral spinothalamic tract: The first neuron is located within the spinal ganglia. Synapse to the 2nd neuron in the gray matter on the same segment. The axon of the 2nd neuron decussates to the contralateral side of the spinal cord within the anterior white commissure (a). Ascend in the lateral spinothalamic tract (b, c, d). Synapse to the 3rd neuron in the thalamus (e). Both pathways end in the somatosensory cortex and transmit information that enters consciousness. 31