The Nervous System: An Overview PDF

The Nervous System Structural Organization CNS: brain and spinal cord, located in the dorsal body cavity. Functions as the command center. Integration center---Interprets sensory information and dictates responses based on past experiences, reflexes, and current conditions. PNS: Outside the CNS: s...

The Nervous System Structural Organization CNS: brain and spinal cord, located in the dorsal body cavity. Functions as the command center. Integration center---Interprets sensory information and dictates responses based on past experiences, reflexes, and current conditions. PNS: Outside the CNS: spinal and cranial nerves. The job is to carry impulses to and from the brain. Also ganglions (clusters of neuron cell bodies outside the brain and spinal cord). Functional Organization Sensory: (afferent): goes from PNS to CNS. (OH MY!! A TRAIN IS HEADING RIGHT FOR ME!!) Motor: (efferent): goes from CNS to PNS. Each has two subdivisions 1) Somatic (Voluntary)— between CNS and skeletal muscle 2) Autonomic (Involuntary)---Smooth muscle/cardiac muscle /glands 2  The Nervous System Histology of Nervous Tissue Two principal cell types Neurons—the structural unit of the nervous system; excitable cells that transmit electrical signals Neuroglia (glial cells)—supporting cells Supporting Cells Of The Nervous System: Totally 6 types of glial cells -- 4 in the CNS; 2 in the PNS. 50% of the mass of the brain is comprised of glial cells. CNS Glial Cells: Astrocytes, Microglia, Ependymal, Oligodendrocytes PNS Glial Cells: Satellite cells and Schwann cells (neurolemmocytes) 3 Glial Cells of the Central Nervous System (CNS) Most abundant K+ and neurotransmit 4  Glial Cells of the Peripheral Nervous System (PNS) Regulates chemical compositions of the interstitial fluid The Myelin Sheath and Related Structures In CNS cells: Oligodendrocytes; In PNS cells: Schwann cells. Insulates fibers so the speed of transmission is increased. Neurilemma: Schwann cell cytoplasm and nucleus are pushed to the periphery forming a neurilemma. Not in oligodendrocytes. After traumatic injuries, PNS axons can regenerate if the neuron cell body is intact and enough neurilemma remains. CNS axon regeneration is minimal Node of Ranvier: spaces between Schwann (neurolemmocytes)/Oligodendrocytes. From where Na+/K+ can move in/out of the axon. Saltatory conduction: the propagation of action potentials along myelinated axons. 6  Functional Classification of Neurons Sensory (afferent) neurons: bipolar and unipolar Motor (efferent) neurons: multipolar Interneurons (association neuron, most common): multipolar Structural Classifications Of Neurons Multipolar: many dendrites and 1 axon (most common) Bipolar: 2 processes: 1 axon and 1 dendrite-rare, special sense organs in the retina and olfactory area. Unipolar: a single short process that emerges from the cell body and divides into 2 branches. 7  The Synapse and EPSP/IPSP Synapse: a junction that mediates information transfer from one neuron: To another neuron, or To an effector cell Presynaptic neuron: releases neurotransmitters, sending signals out. Postsynaptic neuron: contains receptors of neurotransmitters, receiving signals. The graded potentials generated by the receptors on postsynaptic neurons are called postsynaptic potentials, and can be excitatory (EPSP) or inhibitory (IPSP). 8 Termination of Neurotransmitter Effects Within a few milliseconds, the neurotransmitter effect is terminated – Degradation by enzymes, for example, acetylcholinesterase – Reuptake by astrocytes or axon terminals – Diffusion away from the synaptic cleft Common Neurotransmitters 1. Acetylcholine: released at neuromuscular junctions and some ANS neurons 2. Biogenic Amines: norepinephrine; dopamine; serotonin; histamine. Broadly distributed in the brain. Play roles in emotional behaviors and the biological clock 3. Amino Acids: GABA (γ-aminobutyric acid) (Glycine, Aspartate, Glutamate) 9  CNS: Brain and Spinal Cord Major Brain Regions: 1) Cerebral Hemispheres 2) Diencephalon a. thalamus b. hypothalamus c. epithalamus 3) Brain Stem a. Midbrain b. Pons c. Medulla Oblongata 4) Cerebellum 10 Divisions of Brain  BRAIN REGIONS: Cerebral Hemispheres: Largest brain areas—can be divided into right and left hemispheres. Lobes: frontal, parietal, occipital, temporal and insula (inside) Hemispheres are separated by the longitudinal fissure. Hemispheres are separated from the cerebellum by the transverse fissure. Peaks: Gyri (pl) Gyrus (s) Valleys: Sulci (pl) Sulcus (s) Deep sulcus: fissure 12  Layers of cerebrum 1) Cerebral Cortex: Association fibers outermost-gray matter Commissural fibers 2) Cerebral White Matter: (corpus callosum) myelinated axons 3) Subcortical gray matter: clusters of gray matter NUCL EI embedded in the white matter. Gray matter If found in CNS, called basal White matter nuclei (also called ganglions) Projection fibers If found in PNS, called ganglion) (b) Gray matter: neuron cell bodies, dendrites, and unmyelinated axons White matter: myelinated axons only. 13 Cerebral Cortex Functions: “Executive Suite of the Brain”- houses the conscious mind. Center of intelligence and reasoning. Thought, memory, and judgment. Consists of gray matter—billions of neurons. (40% total brain mass—1/8” thick.) 3 Functional areas: Motor areas: voluntary motor function- Broca’s area-controls muscles of speech— (posterior part of) frontal lobes. Primary motor cortex— (precentral gyrus) frontal lobes. Motor homunculus Sensory areas: Awareness of sensation Primary somatosensory cortex- (postcentral gyrus of) parietal lobes. Sensory homunculus Primary Visual Cortex – occipital lobes. Primary auditory cortex & olfactory 14 cortex—temporal lobes 3 Functional areas-continued Integration areas: (association areas). General interpretation areas. Prefrontal cortex: complex thought, judgment, personality, planning, and deciding. Still developing in adolescence. Important distinction: if the primary sensory cortex is damaged, your sensation is affected; If the association areas are damaged, you can still sense, but you cannot explain what you’re sensing. 15 Each hemisphere controls the functions of the OPPOSITE (contralateral) side of the body. While the hemispheres are fairly symmetrical, they are NOT equal in function. Lateralization of cortical function: “division of labor” in the hemispheres. Cerebral dominance= the hemisphere that is dominant. 90% of the human population is left hemisphere dominant. The left hemisphere is associated with language, mathematics, and logic. The right hemisphere is associated with a more free- spirited approach—art, music, etc. *Used to be: bilaterally dominant may cause LD, dyslexia, etc.— NOT ANYMORE! 16 Cerebral White Matter: myelinated fiber bundles, deep to the cortex. Moves information from one place to another. They connect: 1) R & L sides of CNS organs, so they operate as a coordinated whole (Corpus callosum) (*Commissures) 2) different parts of the same hemispheres (*Association tracts) 3) the higher and lower brain regions (*Projection Fibers) Subcortical gray matter — deep within the cerebral white matter. It consists of the Basal Nuclei (or Basal ganglia). This area receives extensive neural input from the entire cerebral cortex and some other brain regions. It is involved in motor function---acts as a “switching” or “routing” station. 17  Diencephalon is the forebrain’s central core— surrounded by the cerebral hemispheres. There are three main regions: Thalamus; Hypothalamus; Epithalamus 18 Thalamus- 80% of the diencephalon—forms the upper side walls of the 3rd ventricle. The right and left sides are connected by the intermediate mass (commissural). “The Gateway to the Cerebral Cortex”. It receives extensive input from the body and lower brain regions and routers it to the appropriate cortical area. All inputs (*except olfactory) to the cortex go through the thalamus. (Sensory or Motor?) Epithalamus: the most dorsal part of the diencephalon. Contains the Pineal Gland: produces melatonin-regulates sleep/wake cycles (May have a role in the inhibition of very early sexual development, especially in males) Choroid Plexus (inside ventricles): the special capillary bed that forms CSF. 19 Hypothalamus: associated with the pituitary gland. The hypothalamus is the main visceral control center of the body—it controls overall body homeostasis. Has many functions: (A BEE, Food, H2O, and Sleep) a) Controls autonomic nervous system functions b) Regulation of body temp c) Controls emotions d) Regulation of endocrine function e) Regulation of food intake f) Regulation of thirst g) Regulation of urine output h) Regulates the sleep and wake cycle Limbic system: the emotional brain. Composed of multiple cerebral and diencephalic structures that process and experience emotions 21 Brain Stem: contains 3 areas with deep gray matter (control automatic behaviors necessary to survival) surrounded by white matter fiber tracts: Midbrain, Pons, Medulla Oblongata. Contains 10 of the 12 cranial nerves. 21 Reflex: A predictable (always the same outcome), automatic, involuntary response to a stimulus. 1) visual reflex center—coordinates head and eye movements. 2) auditory relays and the “startle” reflex (turn your head toward an unexpected sound) 3) Autonomic reflex centers: a) Respiratory Reflex Centers b) Cardiac Reflex - heart rate and strength c) Vasomotor Reflex center - blood pressure (by vessel diameter) b) and c), also called the cardiovascular center Substantia Nigra (in midbrain): high concentration of melanin, which is a precursor to the neural transmitter DOPAMINE Decussation Point in the Medulla Oblongata: just above the medulla/spinal cord junction, the tracts cross over so that each cerebral hemisphere controls the movement of muscles on the opposite side of the body. 22  Cerebellum: (11% total brain mass)—separated from the cerebral hems by the transverse fissure. Has R/L halves 2nd largest brain region-contains white and gray matter White: arbor vitae (tree of life) Gray: cell bodies, dendrites, unmyelinated axons. Coordination of skeletal muscle movement (muscle memory) 23  Spinal Cord The 2nd organ of the CNS. Extends from the foramen magnum to the 1st or 2nd lumbar vertebrae (lumbar puncture) Four parts: cervical, thoracic, lumbar, and sacral. 31 pairs of spinal nerves (PNS) attach to the cord by paired roots. 24  Spinal Cord-Cross Section Gray matter: posterior, anterior, and lateral (thoracic and lumbar) horns White matter: posterior, anterior, and lateral funiculi (white columns) Spinal cord contains a 2-way conduction pathway and spinal reflexes (i.e., knee-jerk) 25 Spinal Cord-continued AM PS Gray matter Anterior and lateral horns: cell bodies of motor neurons Posterior horns: cell bodies of interneurons that are getting sensory information White matter Lateral and anterior funiculi: sensory (ascending) and motor (descending) tracts Posterior funiculus: sensory tracts (axon bundles) Left and right anterior funiculi are interconnected Anterior Root: contains motor info only—from the brain to PNS. Posterior Root: contains sensory information only— moves toward the brain. The two roots fuse together into spinal nerves – both motor and sensory information. 26 Protective Structures common to the brain and spinal cord: 1) Bone (skull, vertebrae) 2) Meninges: CT membranes- provide 3 layers of protection: a) Dura Mater: outermost membrane—closest to the bone b) Arachnoid Mater: middle layer. Contains fibers that anchor it to the innermost layer c) Pia Mater: innermost layer: direct contact with brain/spinal cord d) Epidural space (spine only), subdural space, and subarachnoid space (between which two layers) (PAD-innermost to outermost) 3) Cerebrospinal Fluid: CSF: Fluid found within and around CNS organs (brain and spinal cord) 1. Within ventricles and central canal 2. Around subarachnoid spaces (acts as a cushion) CSF is derived from blood and is formed by a special capillary bed in the brain called the Choroid Plexus. 27  Meninges Cranial Not all labels are going to be tested. Please refer to the previous slide. Spinal 28 The Blood Brain Barrier: a protective mechanism that helps maintain a stable environment. Bloodborne substances within brain capillaries are separated from neurons. The barrier is selectively permeable: Freely permeable: fats, fatty acids, O2, CO2, and other fat-soluble molecules (alcohol, nicotine, and anesthetics) Passively transported via facilitated diffusion (also permeable): glucose, essential amino acids, some electrolytes (ions) Prevented from entering: metabolic wastes (urea, creatinine), proteins, certain toxins, and most drugs. Actively Pumped out: Non-essential amino acids and K+ * Formed by: Continuous endothelium of the capillary walls (tight junctions) Thick, basal lamina surround the external face of the capillary Feet of the astrocytes (signaling in tight junction formation) 29  Peripheral Nervous System (PNS) All neural structures outside the brain and spinal cord: sensory receptors, peripheral nerves, and associated ganglia, and motor endings Nerves Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers. Pure sensory (afferent) or motor (efferent) nerves are rare. Referred pain—inaccurate localization of sensory signals Signals from viscera perceived as originating from the skin, muscle Many somatic and visceral sensory neurons send signals via the same ascending tracts within the spinal cord Somatosensory cortex unable to determine the true source 30  Classification of Nerves Peripheral nerves classified as cranial or spinal nerves Based on the size of nerve fibers: Larger ones have more myelin, conduction speed is faster, and they are more likely to have saltatory conduction Smaller ones have less to no myelin, conduction speed is slower, and they are less likely to have saltatory conduction. a) Group A: largest diameter axon/most myelinated: fastest conduction speed found in skeletal muscles and joints b) Group B: smaller diameter/less myelin: slower conduction speed the viscera and internal organs. c) Group C: smallest diameter, mostly unmyelinated, slowest conduction speed, incapable of saltatory conduction. Non-essential organs. 31  Filaments of Cranial Nerves Frontal lobe olfac tory nerve (I ) Twelve pairs of Optic nerve Temporal lobe nerves associated (I I ) with the brain Fac ial Oc ulomotor Most are mixed in nerve (VI I ) Vestibulo- nerve (I I I ) T roc hlear function; two pairs c oc hlear nerve (VI I I ) nerve (I V) T rigeminal are purely sensory Glossopharyngeal nerve (V) Abduc ens nerve (I X) Vagus nerve (X) nerve (VI ) Ac c essory nerve (XI ) Olfactory –smell Hypoglossal nerve (XI I ) (a) Optic – vision Trigeminal – somatosensory of the face; chew Facial- facial expression (motor); taste Vestibulocochlear- hearing and movement/balance Vagus- only cranial nerve that passes the neck to enter the thoracic and abdominal cavities to control multiple organs. The major source of parasympathetic control. 32  Spinal Nerves 31 pairs of mixed nerves named according to their Cervical nerves point of issue from the C1 – C8 spinal cord – 8 cervical (C1–C8) Thoracic nerves – 12 thoracic (T1–T12) T1 – T12 – 5 Lumbar (L1–L5) – 5 Sacral (S1–S5) Lumbar nerves – 1 Coccygeal (C0) L1 – L5 Sacral nerves S 1 – S5 Coccygeal nerve Co1 33 Spinal Nerves - continued Nerve plexus: a network of interweaving anterior rami (branch) of spinal nerves. Four main plexuses occur bilaterally: cervical, brachial, lumbar, and sacral plexuses. Individual ramus branches repeatedly, so damage to one nerve or spinal segment does not deprive a muscle or skin region of all innervation. Cauda equina: a bundle of spinal nerves that control the lower limbs and the pelvic organs (the bladder, rectum, and internal genital organs ) Sciatic nerve: the thickest and longest nerve that controls the lower limb. Transection: cross sectioning of the spinal cord. Lose of both sensory and motor function Paraplegia— between T1 and L1. Lower limbs. Quadriplegia—in the cervical region. All four limbs.cross- 34 sectioning  Sensory Receptors Classification by Stimulus Type Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina) Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors—sensitive to pain-causing stimuli (e.g., extreme heat or cold, excessive pressure, inflammatory chemicals) Classification by Location Exteroceptors: respond to stimuli arising outside the body. Receptors in the skin for touch, pressure, pain, and temperature and most special sense organs Interoceptors (visceroceptors): respond to stimuli arising in internal viscera and blood vessels. Sensitive to chemical changes, tissue stretch, and temperature changes Proprioceptors: respond to stretch in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles. Inform the brain of one’s movements and balance 35  Homeostatic Imbalances of the Brain Degenerative brain disorders – Alzheimer’s disease (AD): a progressive degenerative disease of the brain that results in dementia, a general degeneration of the brain (starts with the regions for memory); the most common form of dementia – Parkinson’s disease: degeneration of the dopamine- releasing neurons of the substantia nigra – Huntington’s disease: a fatal hereditary disorder caused by the accumulation of the protein huntingtin that leads to degeneration of the basal nuclei and cerebral cortex 36  Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults Myelin sheaths in the CNS become nonfunctional sclerosis. Shunting and short-circuiting of nerve impulses occur. Impulse conduction slows and eventually ceases. Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence Amyotrophic Lateral Sclerosis (ALS) Also called Lou Gehrig’s disease Involves progressive destruction of motor neurons and fibers Linked to glutamate excitotoxicity, attack by the immune system, or both. Loss of the ability to speak, swallow, and breathe. Death typically occurs within five years. Transection of the spinal cord: lose of both sensory and motor function Paraplegia— between T1 and L1. Lower limbs. Quadriplegia—in the cervical region. All four limbs. 37 Control of Autonomic Functions by Higher Brain Centers 38  Autonomic Nervous System (ANS) The ANS consists of motor neurons that innervate smooth and cardiac muscle and glands. Sensory neurons are not included. Parasympathetic and sympathetic divisions. Parasympathetic Sympathetic Eye Eye Brain stem Salivary Skin* glands Cranial Salivary Sympathetic glands ganglia Heart Cervical Lungs Lungs T1 Heart Stomach Thoracic Stomach Pancreas Liver Dual innervations: almost all Pancreas L1 and gall- bladder visceral organs are served by Liver and gall- Lumbar Adrenal gland both divisions, but they cause bladder Bladder opposite effects Bladder Genitals 39 Genitals Sacral  Role of the Sympathetic Division Mobilizes the body during activity; is the “fight-or-flight” system Promotes adjustments during exercise or when threatened – Blood flow is shunted to skeletal muscles and heart – Bronchioles dilate Blood pressure, heart rate, and – Liver releases glucose respiratory rates are high GI and urinary system are slow Role of the Parasympathetic Division Promotes maintenance activities and conserves body energy Its activity is illustrated in a person who relaxes, reading, after a meal – Blood pressure, heart rate, and respiratory rates are low – Gastrointestinal tract activity is high – Pupils are constricted, and lenses are accommodated for close vision 40 Somatic and Autonomic Nervous Systems differ in: effectors; efferent pathways (and their neurotransmitters); target organ responses to neurotransmitters Effect + Stimulatory + Stimulatory or inhibitory, depending on neuro- transmitter and receptors on effector organs 41  Receptors for Neurotransmitters 1. Cholinergic receptors for Ach Two types of receptors bind ACh 1. Nicotinic 2. Muscarinic Named after drugs that bind to them and mimic ACh effects 2. Adrenergic receptors for NE (norepinephrine) Two types – Alpha () (subtypes 1, 2) – Beta () (subtypes 1, 2 , 3) Effects of NE depend on which subclass of receptor predominates on the target organ 42  medicines Receptor natural function condition agonist and antagonist side effects Type stimulate/ inhibit/ activate block α1 constrict blood hypertension antagonists vessels (BP increases) common cold agonists increased heart rate (stuffed nose, hypertension BV dilated too much) β1 increase heart rate hypertension/ antagonists breathing problem and blood pressure arrythmia (β blockers) for asthma/COPD β2 dilate blood vessels asthma/COPD agonists increased heart and bronchioles rate, hypertension agonists activate receptors; antagonists inhibit receptors Blood vessel constriction generally increases blood pressure, while dilation decreases blood pressure 43 44