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Central Nervous System • Central nervous system (CNS) • brain and spinal cord enclosed in bony coverings • Functions of the spinal cord • spinal cord reflexes • integration (summation of inhibitory and excitatory) nerve impulses • highway for upward and downward travel of sensory and motor information • Brain functions • sensations, memory, emotions, decision making, behavior 14-1 Overview of Spinal Cord • Spinal cord - Information highway between brain and body • Each pair of spinal nerves receives sensory information and issues motor signals to muscles and glands • Spinal cord is a component of the CNS while the spinal nerves are part of the PNS 14-2 Functions of the Spinal Cord • Conduction • bundles of fibers passing information up and down spinal cord • connecting different levels of the trunk with each other and the brain • Neural integration - input from multiple sources, integrated, and executed output • Locomotion • repetitive, coordinated actions of several muscle groups • central pattern generators are pools of neurons providing control of flexors and extensors (walking) • Reflexes • involuntary, stereotyped responses to stimuli (remove hand from hot stove) • involves brain, spinal cord and peripheral nerves 14-3 Anatomy of the Spinal Cord • Spinal cord - cylinder of nervous tissue that arises from the brainstem at the foramen magnum of the skull • Passes through the vertebral canal (thick as a finger) • vertebral column grows faster so in an adult the spinal cord only extends to L1 • 31 pairs of spinal nerves arise from cervical, thoracic, lumbar and sacral regions of the cord • First pair passes between the skull and C1 • Rest pass through intervertebral foramina • Cord segment - part of the spinal cord supplied by each pair of spinal nerves 14-4 Surface Anatomy C1 Cervical enlargement Copyright © The McGraw-Hill CervicalCompanies, Inc. Permission required for reproduction or display. spinal nerves C7 Dural sheath Subarachnoid space Thoracic spinal nerves Spinal cord Vertebra (cut) Spinal nerve Lumbar enlargement T12 Spinal nerve rootlets Medullary cone Posterior median sulcus Lumbar spinal nerves Cauda equina Subarachnoid space Epidural space Posterior root ganglion L5 Rib Arachnoid mater Terminal filum Sacral spinal nerves Dura mater S5 Col (a) (b) Figure 13.1 13-5 Meninges of the Spinal Cord • 3 Fibrous layers enclosing spinal cord • Dura mater • tough collagenous membrane surrounded by epidural space filled with fat and blood vessels • epidural anesthesia utilized during childbirth • Arachnoid mater • layer of simple squamous epithelium lining dura mater and loose mesh of fibers filled with CSF (creates subarachnoid space) • Pia mater • delicate membrane adherent to spinal cord 14-6 Meninges of Vertebra and Spinal Cord 14-7 Cross-Sectional Anatomy of the Spinal Cord • Central area of gray matter shaped like a butterfly and surrounded by white matter in 3 columns • Gray matter = neuron cell bodies with little myelin • White matter = myelinated axons 14-8 Gray Matter in the Spinal Cord • Pair of dorsal or posterior horns • dorsal root of spinal nerve is totally sensory fibers • Pair of ventral or anterior horns • ventral root of spinal nerve is totally motor fibers • Connected by gray commissure punctured by a central canal continuous above with 4th ventricle 14-9 White Matter in the Spinal Cord • White matter of the spinal cord surrounds the gray matter • White column = bundles of myelinated axons that carry signals up and down to and from brainstem • 3 pairs of columns or funiculi • • • • Dorsal, lateral, and anterior columns Tracts or fasciculi - subdivisions of each column Ascending and descending tract head up or down Contralateral means origin and destination are on opposite sides while ipsilateral means on same side 14-10 Ascending Tracts • Ascending tracts carry sensory signals up the spinal cord Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Somesthetic cortex (postcentral gyrus) Third-order neuron Thalamus • Sensory signals travel across three neurons from origin in receptors to the destination in the sensory areas of the brain • First-order neurons: detect stimulus and transmit signal to spinal cord or brainstem • Second-order neurons: continues to the thalamus at the upper end of the brainstem • Third-order neurons: carries the signal the rest of the way to the sensory region of the cerebral cortex Cerebrum Midbrain Medial lemniscus Gracile nucleus Second-order neuron Cuneate nucleus Medulla Medial lemniscus First-order neuron Gracile fasciculus Cuneate fasciculus Spinal cord (a) 13-11 Receptors for body movement, limb positions, fine touch discrimination, and pressure Figure 13.5a Descending Tracts Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Descending tracts - carry motor signals down the brainstem and spinal cord • Involve two neurons • Upper motor neuron originates in cerebral cortex or brainstem and terminates on a lower motor neuron • Lower motor neuron in brainstem or spinal cord • Axon of lower motor neuron leads the rest of the way to the muscle or other target organ 13-12 Motor cortex (precentral gyrus) Internal capsule Cerebrum Midbrain Medulla Spinal cord Spinal cord Cerebral peduncle Upper motor neurons Medullary pyramid Decussation in medulla Lateral corticospinal tract Anterior corticospinal tract Decussation in spinal cord Lower motor neurons To skeletal muscles Figure 13.6 General Anatomy of Nerves and Ganglia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Rootlets Posterior root Posterior root ganglion Anterior root Spinal nerve • Nerve - a cordlike organ composed of numerous nerve fibers (axons) bound together by connective tissue • Mixed nerves contain both afferent (sensory) and efferent (motor) fibers • Blood vessels penetrate connective tissue coverings • Nerves have high metabolic rate and need plentiful blood supply Blood vessels Fascicle Epineurium Perineurium Unmyelinated nerve fibers Myelinated nerve fibers (a) Endoneurium Myelin Figure 13.8a 13-13 General Anatomy of Nerves and Ganglia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Direction of signal transmission Spinal cord Posterior root ganglion Anterior root Posterior root ganglion Somatosensory neurons Sensory nerve fibers Sensory pathway Figure 13.9 Spinal nerve Posterior root Epineurium Blood vessels Anterior root Motor nerve fibers Motor pathway To peripheral To spinal cord receptors and effectors • Ganglion—cluster of neurosomas outside the CNS • Enveloped in an endoneurium continuous with that of the nerve • Among neurosomas are bundles of nerve fibers leading into and out of the ganglion • Posterior root ganglion associated with spinal nerves 13-14 Spinal Nerves • 31 pairs of spinal nerves (mixed nerves) • 8 cervical (C1–C8); • C1 between skull and atlas • Others exiting at intervertebral foramen • 12 thoracic (T1–T12) • 5 lumbar (L1–L5) • 5 sacral (S1–S5) • 1 coccygeal (Co) 13-15 Spinal Nerves • Proximal branches forming spinal nerves • Each spinal nerve has two points of attachment to the spinal cord • Posterior (dorsal) root is sensory input to spinal cord • Posterior (dorsal) root ganglion—contains the somas of sensory neurons carrying signals to the spinal cord • Six to eight rootlets that enter posterior horn of cord • Anterior (ventral) root is motor output out of spinal cord • Six to eight rootlets that leave spinal cord and converge to form anterior root • These merge to form spinal nerve proper that enters intervertebral foramen • Cauda equina: formed from roots arising from L2 to Co • Distal branches of spinal nerves • Distal to vertebral foramen, the nerve divides into: • Anterior ramus: innervates the anterior and lateral skin and muscles of the trunk • Gives rise to nerves of the limbs • Posterior ramus: innervates the muscles and joints in that region of the spine and the skin of the back • Meningeal branch: reenters the vertebral canal and innervates the meninges, vertebrae, and spinal ligaments 13-16 Spinal Nerves Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Posterior Spinous process of vertebra Deep muscles of back Posterior root Spinal cord Posterior ramus Transverse process of vertebra Spinal nerve Posterior root ganglion Meningeal branch Anterior ramus Communicating rami Anterior root Sympathetic ganglion Vertebral body Anterior Figure 13.11 13-17 Nerve Plexuses • Anterior rami branch and anastomose repeatedly to form five nerve plexuses • Cervical plexus in the neck, C1 to C5 • Supplies neck and phrenic nerve to the diaphragm • Brachial plexus near the shoulder, C5 to T1 • Supplies upper limb and some of shoulder and neck • Median nerve—carpal tunnel syndrome • Lumbar plexus in the lower back, L1 to L4 • Supplies abdominal wall, anterior thigh, and genitalia • Sacral plexus in the pelvis, L4, L5, and S1 to S4 • Supplies remainder of lower trunk and lower limb • Coccygeal plexus, S4, S5, and Co 13-18 The Sacral and Coccygeal Plexuses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Lumbosacral trunk L4 Roots Anterior divisions Posterior divisions L5 S1 S2 Superior gluteal nerve Inferior gluteal nerve S3 S4 S5 Co1 Figure 13.18 13-19 Sciatic nerve: Common fibular nerve Tibial nerve Posterior cutaneous nerve Pudendal nerve Reflexes • Reflexes - quick, involuntary, stereotyped reactions of glands or muscle to stimulation • Automatic response to change in environment • Integration center for spinal reflexes is gray matter of spinal cord • Examples • somatic reflexes result in skeletal muscle contraction • autonomic (visceral) reflexes involve smooth & cardiac muscle and glands. • heart rate, respiration, digestion, urination, etc • Reflexes can be: • simple • involve peripheral nerves and the spinal cord • spinal reflexes • learned (acquired) • involve peripheral nerves and the brain 14-20 Reflex Arc • Specific nerve impulse pathway -5 components of reflex arc • Somatic Receptor • Registers stimuli • Sensory neuron - afferent nerve fibers • Carry information from receptors to posterior horn of spinal cord or the brainstem • Integrating center • Part of the CNS that processes the information and generates response • Motor neuron • Transmits the response to the effector • Effector • Muscle or gland • 4 important somatic spinal reflexes • Stretch reflex - when a muscle is stretched, it “fights back” and contracts • Tendon reflex - contraction of a muscle when its tendon is tapped • in response to excessive tension on the tendon 14-21 The Patellar Tendon Reflex Arc 14-22 Central Nervous System-The Brain • Evolution of the central nervous system shows spinal cord has changed very little while brain has changed a great deal • Greatest growth in areas of vision, memory, and motor control of the prehensile hand 14-23 The Brain • Longitudinal fissure - cerebral hemispheres. • gyri = folds; sulci = grooves • cortex = surface layer of gray matter • nuclei = deeper masses of gray matter • tracts = bundles of axons (white matter) 14-24 Median Section of the Brain • Three major portions of the brain • Cerebrum is 83% of brain volume; cerebral hemispheres, gyri and sulci, longitudinal fissure, corpus callosum • Cerebellum contains 50% of the neurons; second largest brain region, located in posterior cranial fossa • Brainstem is the portion of the brain that remains if the cerebrum and cerebellum are removed; diencephalon, midbrain, pons, and medulla oblongata 14-25 Gray and White Matter • Gray matter = neuron cell bodies, dendrites, and synapses • forms cortex over cerebrum and cerebellum • forms nuclei deep within brain • White matter = bundles of axons • forms tracts that connect parts of brain 14-26 Meninges of the Brain 14-27 Brain Ventricles • Ventricles - four internal chambers within the brain • Two lateral ventricles: one in each cerebral hemisphere • Interventricular foramen—a tiny pore that connects to third ventricle • Third ventricle: single narrow medial space beneath corpus callosum • Cerebral aqueduct runs through midbrain and connects third to fourth ventricle • Fourth ventricle: small triangular chamber between pons and cerebellum • Connects to central canal, runs down through spinal cord • Ependyma - neuroglia that lines the ventricles and covers choroid plexus • Produces cerebrospinal fluid • Choroid plexus - spongy mass of blood capillaries on the floor of each ventricle 14-28 Cerebrospinal Fluid • Fills ventricles and subarachnoid space • Brain produces and absorbs 500 ml/day • choroid plexus creates by filtration of blood • Functions • floats brain so it is neutrally buoyant • cushions from hitting inside of skull • chemical stability -- rinses away wastes • Escapes (4th ventricle) to surround brain • Absorbed into venous sinus by arachnoid villi 14-29 Ventricles and Cerebrospinal Fluid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Arachnoid villus 8 Superior sagittal sinus Arachnoid mater 1 CSF is secreted by choroid plexus in each lateral ventricle. Subarachnoid space Dura mater 1 2 CSF flows through interventricular foramina into third ventricle. 2 Choroid plexus Third ventricle 3 3 Choroid plexus in third ventricle adds more CSF. 7 4 Cerebral aqueduct 4 CSF flows down cerebral aqueduct to fourth ventricle. Lateral aperture 5 Choroid plexus in fourth ventricle adds more CSF. Fourth ventricle 6 5 6 CSF flows out two lateral apertures and one median aperture. Median aperture 7 CSF fills subarachnoid space and bathes external surfaces of brain and spinal cord. 7 8 At arachnoid villi, CSF is reabsorbed into venous blood of dural venous sinuses. Central canal of spinal cord Subarachnoid space of spinal cord 14-30 Blood Supply and the Brain Barrier System • Brain is only 2% of the adult body weight, and receives 15% of the blood - 750 mL/min • Blood is also a source of antibodies, macrophages, bacterial toxins, and other harmful agents • Brain barrier system—strictly regulates what substances can get from the bloodstream into the tissue fluid of the brain • Two points of entry must be guarded • Blood capillaries throughout the brain tissue • Capillaries of the choroid plexus 14-31 Hindbrain - Medulla Oblongata • Cardiac center • adjusts rate and force of heart • Vasomotor center • adjusts blood vessel diameter • Respiratory centers • control rate and depth of breathing • Reflex centers • for coughing, sneezing, gagging, swallowing, vomiting, salivation, sweating, movements of tongue and head • Most of the nerve fibers are crossing over • Left cortex controls right side of body 14-32 Pons • Bulge in brainstem, superior to medulla • Ascending sensory tracts • Descending motor tracts • Pathways in and out of cerebellum • Nuclei • concerned with posture, sleep, hearing, balance, taste, eye movements, facial expression, facial sensation, respiration, swallowing, and bladder control 14-33 Cerebellum • Two hemispheres connected by vermis • Cortex = surface folds called folia • Output comes from deep gray nuclei • granule and purkinje cells • White matter (arbor vitae) visible in sagittal section • Evaluation of sensory input • coordination and locomotor ability • spatial perception • Timekeeping center • predicting movement of objects 14-34 Midbrain - Cross Section • Tegmentum • connects to cerebellum and helps control fine movements through red nucleus • Substantia nigra • sends inhibitory signals to basal ganglia and thalamus (degeneration leads to tremors and Parkinson disease) • Central gray matter = pain awareness 14-35 Reticular Activating System • Scattered nuclei in medulla, pons & midbrain • Reticular activating system • alerts cerebral cortex to sensory signals (sound of alarm, flash light, smoke or intruder) to awaken from sleep • maintains consciousness & helps keep you awake with stimuli from ears, eyes, skin and muscles • Motor function is involvement with maintaining muscle tone • Somatic motor control • Cardiovascular control • Pain modulation 14-36 Diencephalon: Thalamus and Hypothalamus • Thalamus Functions • Relays signals from cerebellum to motor cortex • Emotional and memory functions • Hypothalamus Functions • hormone secretion • autonomic NS control • thermoregulation • food and water intake (hunger and satiety) • sleep and circadian rhythms • memory (mammillary bodies) • emotional behavior 14-37 Cerebrum -- Gross Anatomy • Cerebral cortex - 3mm layer of gray matter • extensive folds increase surface area - divided into lobes 14-38 Functions of Cerebrum - Lobes • Frontal lobe • Voluntary motor functions • Motivation, foresight, planning, memory, mood, emotion, social judgment, and aggression • Parietal lobe • Receives and integrates general sensory information, taste, and some visual processing • Occipital lobe • Primary visual center of brain • Temporal lobe • Areas for hearing, smell, learning, memory, and some aspects of vision and emotion • Insula (hidden by other regions) • Understanding spoken language, taste and sensory information from visceral receptors 14-39 Tracts of Cerebral White Matter • Most of cerebrum is white matter • Types of tracts • projection tracts • from brain to spinal cord, forms internal capsule • commissural tracts • cross to opposite hemisphere • corpus callosum • anterior and posterior commissures • association tracts • connect lobes and gyri within a hemisphere 14-40 Limbic System • Loop of cortical structures • amygdala, hippocampus and cingulate gyrus (arches over the top of the corpus callosum in the frontal and parietal lobes) • Role in emotion and memory • pleasure and aversion centers 14-41 Memory • Information management • requires learning, memory and forgetting • Amnesia • anterograde amnesia - no new memories • retrograde amnesia – can’t remember old ones • Hippocampus • organizes sensory and cognitive information into a new memory • Cerebellum • helps learn motor skills • Amygdala • emotional memory 14-42 General Properties of the Autonomic Nervous System • Autonomic nervous system (ANS)—a motor nervous system that controls glands, cardiac muscle, and smooth muscle • Also called visceral motor system • Primary organs of the ANS • Viscera of thoracic and abdominal cavities • Some structures of the body wall • Cutaneous blood vessels • Sweat glands • Piloerector muscles • Carries out actions involuntarily: without our conscious intent or awareness • Visceral effectors do not depend on the ANS to function; only to adjust their activity to the body’s changing needs • Denervation hypersensitivity - exaggerated response of cardiac and smooth muscle if autonomic nerves are severed Visceral Reflexes • Visceral reflexes - unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors and somewhat slower responses • Visceral reflex arc • Receptors: nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli • Afferent neurons: leading to the CNS • Interneurons: in the CNS • Efferent neurons: carry motor signals away from the CNS • Effectors: that make adjustments • ANS modifies effector activity 15-44 Visceral Reflexes • Example of homeostatic negative feedback loop • High blood pressure detected by arterial stretch receptors (1), afferent neuron (2) carries signal to CNS, efferent (3) signals travel to the heart, then (4) heart slows reducing blood pressure Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2 Glossopharyngeal nerve transmits signals to medulla oblongata 1 3 Vagus nerve transmits inhibitory signals to cardiac pacemaker Baroreceptors sense increased blood pressure Common carotid artery Terminal ganglion 4 Heart rate decreases Figure 15.1 15-45 Autonomic Nervous System • Visceral motor neurons control • • • • • heart rate breathing rate digestion blood pressure salivation • Nerve impulses of these motor neurons start in the CNS (medulla oblongata and pons) • Pathway through: • Spinal cord • Cranial nerves 14-46 Divisions of the ANS • Two divisions innervate same target organ • May have cooperative or contrasting effect • Prepares body for physical activity: exercise, trauma, arousal, competition, anger, or fear • Increases heart rate, BP, airflow, blood glucose levels, etc. • Reduces blood flow to the skin and digestive tract 15-47 Sympathetic Division • The sympathetic division is called the “fight or flight” system • when the body needs to generate energy • exercise, excitement, emergency, and embarrassment • Fight or flight response • increases heart rate, blood pressure, respiration rate, blood flow to skeletal muscles, glucose metabolism • decreases the activities that are not essential at the moment (digestive system organs are subdued- decreased blood flow to that system 14-48 Parasympathetic Division • The parasympathetic division is called the “rest and digest” • activated when the body needs to conserve energy • digestion, defecation, and diuresis (urination) • Promotes necessary changes during these activities • decreases heart rate, blood pressure, respiration rate, blood flow to skeletal muscles, glucose metabolism • increases the activity of and blood flow to the digestive system organs 14-49 Autonomic Output Pathways Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Somatic efferent innervation ACh Myelinated fiber Somatic effectors (skeletal muscles) Autonomic efferent innervation ACh Myelinated preganglionic fiber ACh or NE Unmyelinated postganglionic fiber Visceral effectors (cardiac muscle, smooth muscle, glands) Autonomic ganglion ANS—two neurons from CNS to effectors • Presynaptic neuron cell body is in CNS • Postsynaptic neuron cell body is in peripheral ganglion Figure 15.2 Efferent Sympathetic vs. Parasympathetic NE – Norepinephrine – Adrenergic fibers – adrenergic receptors Ach – Acetylcholine – Cholinergic fibers-cholinergic reseptors 14-51 Organization of the Sympathetic and Parasympathetic Division 14-52 Effects of Neurotransmitters of the Autonomic Nervous System • The cells of each organ controlled by the ANS have both ACh and NE receptors • organs are dually controlled • The response of the organ is determined by the identity of the neurotransmitter released • the binding of ACh to its receptor will cause the effector to respond in one way • the binding of NE to its receptor will cause the effector to respond in the opposite way • The effect of ACh and NE is effector specific • NE increases heart rate, ACh decreases heart rate • NE decreases the secretion of saliva, ACh increases the secretion of saliva 14-53