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THE NERVOUS SYSTEM Dr. Nabil Khouri Divisions of the nervous system Protection of the Brain: The Cranial Meninges • Cranium is covered with protective membranes called Meninges – Cranial meninges are continuous with spinal meninges ) composed of 3 layers) : – 1. An outer, fibrous dura mater – forms sheets (falx) that separate the cerebrum and the cerebellum into the hemispheres and the cerebellum from the cerebrum – comprised of an outer endosteal layer and an inner meningeal layer 2. middle Arachnoid mater – avascular layer – named for the spider-like struts (trabeculae) that connect the Arachnoid to the underlying Pia mater 3. inner, thin Pia mater – vascular connective tissue – makes direct contact with brain tissue – cells of the Pia mater are impermeable to the passage of many substances – this membrane is pierced by tiny capillaries that nourish the brain tissue – arise from the larger capillaries that travel within the Dura mater •large spaces for the circulation of blood can be found between the two dural layers called Sinuses e.g. superior sagittal sinus • also large veins run through the subarachnoid space e.g. cerebral veins There are spaces between these membranes A. Subarachnoid space: between the arachnoid and pia maters for the circulation of CSF B. Subdural space: between the arachnoid and the dura mater C. Epidural space – between the dura mater and the vertebral canal in the spinal column Protection of the Brain: CSF – CSF: 80 to 150 mL of clear, colorless liquid • Replaced completely up to three times per day • Components : glucose, proteins, lactic acid, urea, ions • Made by specialized cells in the lateral ventricles – Choroid plexus – Are networks of capillaries in the walls of the ventricles – Covered by ependymal cells (epithelialcells) that filter the blood plasma and produce CSF by secreting it – These cells are capable of allowing passage of certain substances from the blood through them into the CSF – inhibit the passage of others • Continually circulates through ventricles of the brain and central canal to subarachnoid space • functions: • 1. Chemical protection: provides an optimal chemical environment for neuronal signaling • 2. Mechanical protection: acts as a shock absorber, preventing direct physical contact between brain tissue and the bones of the cranium or vertebral canal • 3. Circulation: allows the exchange of nutrients and waste products between the blood and nervous tissue CSF formation • -CSF forms in the choroid plexi of the lateral ventricles and flows into the 3rd ventricle through the interventricular foramina • -the 3rd ventricle adds to the CSF volume • -the CSF then flows into the 4th ventricle via and cerebral aqueduct (passes through the midbrain) – contributes more volume • -then enters the subarachnoid space via openings in the 4th ventricle called apertures • -also enters the central canal of the SC • -circulation is driven by ciliary action and pressures provided by the blood and gravity – 10 mm Hg Circulation of the CSF •CSF is gradually reabsorbed into the blood through fingerlike projections into the dural venous sinuses called : Arachnoid granulations -absorbed at about 20ml/hr which equals its rate of formation •interfering with the drainage of CSF into the subarachnoid space can result in accumulation of CSF in the ventricles & CSF pressure rises = hydrocephalus Neuronal Organization Neural Organization: Pathways •A neural pathway is comprised of centers/cell bodies and tracts • Sensory pathway – Ascending – Information from sensory receptors to CNS • Motor pathway – – – – Descending Information from CNS to skeletal muscle or glands Direct pathways – cause precise, voluntary movements Indirect pathways – result in involuntary movement (from brain stem) Major Regions of the Brain Figure 15.1 Major Divisions of the Brain Major Regions of the Brain Major Regions and Landmarks • Cerebrum = largest portion -left and right cerebral hemispheres divided by the longitudinal fissure -connected by the corpus callosum -folded into ridges and grooves: ridges are caled Gyri (gyrus) and the grooves are called Sulci -sulci divide the cerebrum into lobes –Central sulcus •Frontal and parietal lobes Lobes of the Cerebrum • Five (5) lobes bilaterally: • Frontal lobe • Parietal lobe • Temporal lobe • Occipital lobe • Insula aka ‘Island of Reil’ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central sulcus Parietal lobe Occipital lobe Frontal lobe Insula Retracted temporal lobe (c) Functions of the Cerebral Lobes The Cerebrum Cerebrum is comprised of: 1. white matter - neurons with long, myelinated axons -organized into tracts A. Association tracts: conduct impulses between gyri within a hemisphere B. Commisural tracts: connects gyri in one hemisphere to others in the other hemisphere 1. corpus callosum 2. anterior commisure 3. posterior commisure C. Projection tracts: tracts that connect cerebrum to the lower parts of the CNS (e.g. Thalamus, brainstem) 2. gray matter – outer edge of the cerebrum = cerebral cortex (2-4 mm thick = billions of neurons) -localized areas of gray matter called the basal ganglia Major Regions and Landmarks -area for specific processing of sensation-area of voluntary movement, speech -areas for all “higher order” functions Cortex is comprised of primary and association areas e.g. primary visual, auditory & gustatory areas e.g. primary motor area (precentral gyrus -primary areas – areas where “raw” information is first received and raw commands are generated - association areas for integration and analysis of incoming info & help in making of “decisions” Basal Ganglia -nuclei found deep within the cerebrum - receives input from the cortex & provides output to the motor areas of the cortex via the thalamus -integrates motor commands provided by the cerebral cortex -regulates the initiation & termination of muscle movement. -anticipates body movements & controls subconscious contraction of skeletal muscle • comprised of the: • 1. Corpus striatum – caudate nucleus: controls movement of arms and legs when walking – putamen: precedes or anticipates body movements – nucleus accumbens • 2. globus pallidus: regulates muscle tone for movements • 3. Corpus claustrum: receives visual information • 4. substantia nigra: high concentration of dopanergic neurons • 5. subthalmic nucleus Major Regions and Landmarks • Diencephalon: Includes the thalamus, hypothalamus,, epithalamus and subthalamus The thalamus: 80% of the diencephalon • paired oval masses of gray matter organized into nuclei, interspersed with white matter • joined by the intermediate mass (gray matter) in about 70% of brains • major relay station for most sensory impulses from the SC, brain stem • crude perception of pain, heat and pressure (refined in cerebrum) • transmits motor information from cerebellum to the cerebrum Hypothalamus •Emotions, autonomic functions, hormone production •major functions: •1. Control of the ANS – integrates signals from the ANS (regulated smooth and cardiac muscle contraction) major regulator of visceral activities (heart rate, food movements, contraction of bladder) •2. Produces hormones & connects with pituitary to regulate its activity •-releasing hormones •-oxytocin •-vasopressin 3. regulates emotional and behavioral patterns – rage, aggression, pain and pleasure + sexual arousal 4. regulates eating & drinking – hypothalamus contains a thirst center which responds to a rise in osmotic pressure in the ECF (dehydration) 5. controls body temperature – monitors temp of blood flowing through the hypothalamus Epithalamus – consists of the pineal gland and habenular nuclei -pineal gland – part of the endocrine system -secretes the hormone melatonin -increased secretion in dark -promote sleepiness and helps set the circadian rhythms of the body (awake/sleep period) Subthalamus – works with the cerebrum and cerebellum to control body movements -majority is made of the subthamic nuclei -sends efferent connections to the caudate nucleus and putamen, to the medial and lateral nuclei of the thalamus and to the red nucleus and substantia nigra of the midbrain -also receives afferent connections from the substantia nigra Cerebellum Cerebellum – divided into hemisphere with lobes like the cerebrum • anterior and posterior lobes – involuntary motor activities • evaluates and coordinates motor activities initiated by the cerebrum and corrects problems by sending info back to the cerebrum • regulate posture & balance – has a superficial layer of gray matter called the cerebellar cortex - like the brain – deep to the gray matter are tracts of white matter = arbor vitae – also has nuclei = cerebellar nuclei (origin of neurons that connect the cerebellum to the brain and SC) – connected to the brain stem by three cerebellar peduncles • inferior – sensory information from the inner ear and body proprioceptors into the cerebellum • middle – carry motor commands for voluntary movements that originated in the cortex into the cerebellum for coordination of muscle movement • superior – connects to the red nuclei and the nuclei of the thalamus Midbrain Major Regions and Landmarks VIII IX X XII -associated with 5 pairs of cranial nerves VIII , IX, X, and XII BRAIN STEM • Medulla oblongata • Contains several nuclei also that regulate autonomic functions - reflex centers for regulating heartbeat and BP (cardiovascular center), respiration (respiratory center), plus vomiting, coughing, sneezing, and swallowing – nuclei in the posterior part are associated with sensations of touch, proprioception, pressure and vibration – Inferior olivary – Gracile – Cuneate nucleus Gray matter -nuclei: Reflex centers – e.g. cardiovascular & respiratory 1. Inferior olivary: part of the olive proprioceptors to the cerebellum – joint and muscle position 2. Gracile: ascending sensory tracts from SC synapse here -relayed into the thalamus -proprioception and touch from lower limbs 3. Cuneate: ascending sensory tracts from Spinal cored synapse here –relayed into the thalamus -prioprioception and touch from upper limbs – white matter – contains sensory/ascending and motor/descending tracts – some of the white matter form bulges called pyramids – white tracts that connect the cerebrum to the Spinal Cord • Pons BRAIN STEM = “bridge” - superior to the medulla and anterior to the cerebellum - connects the brain stem to the cerebrum – consists of nuclei connected by tracts • Pontine nuclei – control voluntary movements that originate in the cerebral cortex and are relayed through the pons into the cerebellum • Pneumotaxic area – controls breathing (with medulla) • Apneustic area – controls breathing (with medulla) Midbrain (Mesencephalon) Relay station between the cerebrum and the spinal cord, relay station with the cerebellum, controls visual reflexes & releases dopamine – extends from the pons to the diencephalon – relays motor tracts into the SC, medulla and pons & conducts sensory tracts into the thalamus – Anterior portion contains a pair of white tracts = cerebral peduncles • Connects the cerebrum to the brain stem (motor) – Posterior portion = tectum • white matter tracts = cerebellar peduncles (motor & sensory info) • four round elevations = superior and inferior colliculi • reflex centers for visual activities (tracking, scanning) pupillary reflex, shape of the lens • reflexes that mediate movements of the eyes, head and neck - the startle reflex • relays impulses from hearing receptors to the thalamus • -generates involuntary somatic motor responses • release of dopamine from substantia nigra (nuclei) - loss of these neurons = Parkinsons • red nuclei forms synapses with cerebellum to coordinate muscle movements Midbrain nuclei • colliculi – superior and inferior – Visual reflex centers • red nuclei – Connects the cerebellum to the motor cortex of the cerebrum – Connects the motor areas of the cerebrum to outgoing motor neurons for posture and balance • substantia nigra – Dopamine release • White matter tracts: cerebral peduncles, cerebellar peduncles I - Olfactory II - Optic III - Oculomotor IV-Trochlear V - Trigeminal VI - Abducens VII - Facial VIII - Acoustic IX - Glossopharyngeal X - Vagus XI - Accessory XII - Hypoglossal -cranial nerves – 12 pairs -considered part of the peripheral nervous system (PNS) -olfactory & optic contain only sensory axons = sensory nerves -remaining are either motor or mixed nerves – both motor and sensory axons “some say my mother bought my brother some bad beer, my my” Major Parts of the Brain 44 Spinal Cord • Slender column of nervous tissue continuous with brain and brainstem • Extends downward through vertebral canal • Begins at the foramen magnum and terminates at the first and second lumbar vertebrae (L1/L2) interspace Brainstem Foramen magnum Cervical enlargement Cervical enlargement Spinal cord Vertebral canal Lumbar enlargement Lumbar enlargement Conus medullaris Conus medullaris Cauda equina Filum terminale (a) (b) • length in adults = 16 to 18 inches • Cervical and lumbar enlargements – cervical = C4 to T1, nerves to and from upper limbs – lumbar = T9 to T12, nerves to and from lower limbs • Tapers to conus medullaris • filium terminale extension of the pia mater that anchors the SC to the coccyx • 31 segments each with – Dorsal root ganglia • Sensory neuron cell bodies – Pair of dorsal roots – Pair of ventral roots Spinal Cord Spinal Meninges • Specialized membranes • Provide physical stability and shock absorption • Three layers – Dura mater = dense irregular CT • continuous with the brain’s DM • above it is the epidural space – Arachnoid = continuous with brain • above it is the subdural space • below is the subarachnoid space • avascular – Pia mater = connective tissue • collagen and elastin bundles • well vascularized • The Pia Mater – Innermost meningeal layer – Bound firmly to underlying tissue – Denticulate ligaments bind pia mater to the arachnoid -spinal tap: under local anesthetic -long needle is inserted into the subarachnoid space and CSF is withdrawn or antibiotics or anesthetics are given -given between L3 & L4 or L4 & L5 Histology of the Spinal Cord • Central gray matter – Contains cell bodies of neurons and glial cells + unmyelinated axons – Gray matter projections are horns • Peripheral white matter – Myelinated and unmyelinated axons – Tracts or columns Organization of Gray Matter • Posterior gray horns – Somatic and visceral sensory nuclei • Anterior gray horns – Somatic motor control • Lateral gray horns – Visceral motor neurons • Gray commissures – Axons of interneurons crossing cordated and unmyelinated axons Organization of White Matter • Six columns (funiculi) – Anterior, lateral and posterior white columns – Contain tracts • Ascending tracts relay information from spinal cord to brain • Descending tracts carry information in the opposite direction Fasciculus gracilis Dorsal column Fasciculus cuneatus Posterior spinocerebellar tract Lateral corticospinal tract Lateral reticulospinal tract Rubrospinal tract Anterior spinocerebellar tract Lateral spinothalamic tract Anterolateral system Anterior spinothalamic tract Anterior reticulospinal tract Medial reticulospinal tract Anterior corticospinal tract General Components of a Spinal Reflex Spinal cord Interneuron Dorsal 1 Receptor 3 2 Sensory neuron Cell body of sensory neuron White matter Gray matter 4 Ventral Motor neuron 5 Effector (muscle or gland) (b) Central canal Medical Application: Alzheimer’s Disease -loss or reasoning, memory -11% of population over 65 (4 million people) -unknown cause – thought to be genetic factors + environmental & lifestyle -neuronal plasma membranes contain a protein = amyloid precursor protein (APP) abundant in presynaptic axon terminals -cleavage of APP yields a secreted product = sAPPa that is secreted by the presynaptic terminals normally -if APP is cleaved at the wrong site – beta-amyloid -two forms of beta-amyloid are possible based on cleavage site – the longer form (Ab40) is harmless -but the form Ab42 – 10% of the cleaved b-amyloid – aggregates to form plaques and is neurotoxic -underlying causes for Ab plaque formation remain unknown -about 15% of cases appear to have a genetic link – familial Alzheimer’s -mutations in 3 genes: prenisilin-1, -2 and APP lead to early onset forms (less the 15% of all cases) – prenisilins cleave APP -mutations in these genes can shift the balance of b-amyloid to the harmful form, -so can age -also mutations in gene coding for apolipoprotein E (ApoE) a protein that helps transport cholesterol in the blood -may account for 85% of the cases – late-onset Alzheimers -mutated genes for apoE = apoE4 – may increase risk of development -may predispose you to Ab plaque formation, or may hasten the onset - ????? Peripheral Nervous System • Cranial nerves arising from the brain • Somatic fibers connecting to the skin and skeletal muscles • Autonomic fibers connecting to viscera • Spinal nerves arising from the spinal cord • Somatic fibers connecting to the skin and skeletal muscles • Autonomic fibers connecting to viscera Structure of a Peripheral Nerve Fascicle Peripheral nerve Epineurium Motor neuron ending Axon Perineurium Endoneurium Node of Ranvier Schwann cell Sensory receptor Myelin sheath Neurilemma Nerve and Nerve Fiber Classification • Sensory nerves • Conduct impulses into brain or spinal cord • Motor nerves • Conduct impulses to muscles or glands • Mixed (both sensory and motor) nerves • Contain both sensory nerve fibers and motor nerve fibers • Most nerves are mixed nerves • ALL spinal nerves are mixed nerves (except the first pair) Nerve Fiber Classification • Special somatic efferent (SSE) fibers • Carry motor impulses from brain to muscles used in chewing, swallowing, speaking and forming facial expressions • Special visceral afferent (SVA) fibers • Carry sensory impulses to brain from olfactory and taste receptors • Special somatic afferent (SSA) fibers • Carry sensory impulses to brain from receptors of sight, hearing and equilibrium Cranial Nerves Olfactory bulb Olfactory (I) Olfactory tract Optic (II) Optic tract Oculomotor (III) Trochlear (IV) Trigeminal (V) Vestibulocochlear (VIII) Abducens (VI) Hypoglossal (XII) Facial (VII) Vagus (X) Glossopharyngeal (IX) Accessory (XI) I - Olfactory II - Optic III - Oculomotor IV-Trochlear V - Trigeminal VI - Abducens VII - Facial VIII - Acoustic IX - Glossopharyngeal X - Vagus XI - Accessory XII - Hypoglossal -cranial nerves – 12 pairs -considered part of the peripheral nervous system (PNS) -olfactory & optic contain only sensory axons = sensory nerves -remaining are either motor or mixed nerves – both motor and sensory axons “some say my mother bought my brother some bad beer, my my” Spinal Nerves • ALL are mixed nerves (except the first pair) • 31 pairs of spinal nerves: • 8 cervical nerves • (C1 to C8) • 12 thoracic nerves • (T1 to T12) • 5 lumbar nerves • (L1 to L5) • 5 sacral nerves • (S1 to S5) • 1 coccygeal nerve • (Co or Cc) C1 C2 C3 C4 C5 C6 C7 C8 T1 T2 T3 T4 Posterior view Cervical nerves T5 T6 T7 Thoracic nerves T8 T9 T10 T11 T12 L1 Cauda equina L2 L3 L4 L5 S1 S2 S3 S4 S5 Co Lumbar nerves Sacral nerves Coccygeal nerve Spinal Nerves • Ventral root (aka anterior root) • Motor root • Axons of motor neurons whose cell bodies are in the spinal cord Dorsal root Ventral branch of spinal nerve Dorsal root ganglion Dorsal root Ventral root Posterior median sulcus Paravertebral ganglion Posterior horn (b) Lateral horn • Spinal nerve • Union of ventral root and dorsal roots • Hence we now have a “mixed” nerve Dorsal branch of spinal nerve Anterior horn Central canal Anterior median fissure Ventral root (a) Visceral branch of spinal nerve Ventral branch of spinal nerve (ventral ramus) Dorsal branch of spinal nerve (dorsal ramus) Spinal nerve Paravertebral ganglion Visceral branch of spinal nerve Nerve Plexuses • Nerve plexus • Complex networks formed by anterior branches of spinal nerves • The fibers of various spinal nerves are sorted and recombined • There are three (3) nerve plexuses: • (1) Cervical plexus • Formed by anterior branches of C1-C4 spinal nerves • Lies deep in the neck • Supply to muscles and skin of the neck • C3-C4-C5 nerve roots contribute to phrenic nerves bilaterally •(2) Brachial plexus • Formed by anterior branches C5-T1 • Lies deep within shoulders • (3) Lumbosacral plexus • Formed by the anterior branches of L1-S5 roots • Can be a lumbar (L1-L5) plexus and a sacral (S1-S5) plexus • Extends from lumbar region into pelvic cavity Plexuses Posterior view Musculocutaneous nerve Axillary nerve Radial nerve Median nerve Ulnar nerve Phrenic nerve Cauda equina Femoral nerve Obturator nerve Sciatic nerve C1 C2 C3 C4 C5 C6 C7 C8 T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 L1 L2 L3 L4 L5 S1 S2 S3 S4 S5 Co Cervical plexus (C1–C4) Brachial plexus (C5–T1) Intercostal nerves Lumbosacral plexus (T12–S5) Autonomic Nervous System • Functions without conscious effort • Controls visceral activities • Regulates smooth muscle, cardiac muscle, and glands • Efferent fibers typically lead to ganglia outside of the CNS • Two autonomic divisions regulate: • Sympathetic division (speeds up) • Prepares body for ‘fight or flight’ situations • Parasympathetic division (slows down) • Prepares body for ‘resting and digesting’ activities Autonomic Nerve Fibers • All of the neurons are motor (efferent) Interneurons • Preganglionic fibers • Axons of preganglionic neurons • Neuron cell bodies in CNS • Postganglionic fibers • Axons of postganglionic neurons • Neuron cell bodies in ganglia Dorsal root ganglion Dorsal root ganglion Sensory neuron Sensory neuron Spinal cord Autonomic ganglion Preganglionic fiber Somatic motor neuron Postganglionic fiber Viscera Skin Skeletal muscle (a) Autonomic pathway (b) Somatic pathway Sympathetic Division • Thoracolumbar division – location of preganglionic neurons • Preganglionic fibers leave spinal nerves through white rami and enter paravertebral ganglia • Paraverterbral ganglia and fibers that connect them make up the sympathetic trunk Spinal cord Ventral root Sympathetic trunk Paravertebral sympathetic ganglion Dorsal root Dorsal root ganglion Pia mater Arachnoid mater Spinal nerves Dura mater Transverse process Vertebral notch (forms part of intervertebral foramen) Body of vertebra Sympathetic Division • Postganglionic fibers extend from sympathetic ganglia to visceral organs • Postganglionic fibers usually pass through gray rami and return to a spinal nerve before proceeding to an effector Preganglionic neuron Gray ramus Dorsal root ganglion Postganglionic neuron Spinal cord Dorsal root Posterior horn White ramus Sympathetic trunk Lateral horn Dorsal branch of spinal nerve Anterior horn Ventral root Spinal nerve Ventral branch of spinal nerve Paravertebral sympathetic ganglion Visceral effector (intestine) Collateral ganglion To visceral effectors (smooth muscle of blood vessels, arrector pili muscles, and sweat glands) Sympathetic Division Lacrimal gland Eye Parotid gland, submandibular and sublingual glands Blood vessels Heart Celiac and pulmonary plexuses Trachea Lungs Celiac ganglion Skin Fibers to skin, blood vessels, and adipose tissue Superior mesenteric ganglion Liver Gallbladder Stomach Pancreas Small intestine Spinal cord Inferior mesenteric ganglion Sympathetic chain ganglia Preganglionic neuron Large intestine Adrenal gland Kidney Urinary bladder Postganglionic neuron Ovary Uterus Penis Scrotum Parasympathetic Division • Craniosacral division – location of preganglionic neurons • Ganglia are near or within various organs • Terminal ganglia • Preganglionic fibers of the head are included in nerves III, VII, and IX • Preganglionic fibers of thorax and abdomen are parts of nerve X • Short postganglionic fibers • Continue to specific muscles or glands 70 Parasympathetic Division Sphenopalatine ganglion Cranial nerve III Ciliary ganglion Submandibular ganglion Cranial Nerve VII Cranial nerve IX Lacrimal gland Otic ganglion Eye Submandibular and sublingual glands Parotid gland Heart Cranial nerve X (Vagus) Trachea Lung Cardiac and pulmonary plexuses Liver Gallbladder Celiac plexus Stomach Spleen Pancreas Superior hypogastric plexus Small intestine Inferior hypogastric plexus Spinal cord Large intestine Kidney Pelvic nerves Preganglionic neuron Postganglionic neuron Urinary bladder Scrotum Penis Uterus Ovary