T1 Phys - Topic 5 - Nervous - Apr 2020 PDF

Module #5 – The Nervous System Organization of the NS Central Nervous System (CNS)   brain spinal cord Peripheral Nervous System (PNS)  all of the nervous tissue outside of the CNS Functions of the NS 1) Sensory  detection and input 2) Integrative   processing/storing/analyzing of sensory info...

Module #5 – The Nervous System Organization of the NS Central Nervous System (CNS)   brain spinal cord Peripheral Nervous System (PNS)  all of the nervous tissue outside of the CNS Functions of the NS 1) Sensory  detection and input 2) Integrative   processing/storing/analyzing of sensory info decision‐making 3) Motor  output p. 72 p. 73 Types of Tissue ‐ Nervous Tissue   designed for communication 2 types of cells: 1) neuroglia 2) neurons Neuroglia    support nourish protect p. 74 Neurons   nerve cells 3 parts: dendrite, cell body, axon Dendrite    structure: multiple or single extensions off the cell body function: it is the input portion of the neuron contain lots of receptor sites for receiving chemical messages from other cells Cell Body (a.k.a. soma, perikaryon)   structure: contains the nucleus and other organelles function: cell processes (e.g. protein synthesis) Axon       a thin cylindrical process off the cell body function: it is the output portion of the neuron the end of an axon divides into many fine processes called axon terminals synaptic end bulb: the swollen ends of some axon terminals some axons are myelinated myelin: a multilayered lipid and protein covering that speeds up signal (‘action potential’) conduction (axons coated in myelin are called ‘myelinated’ axons p. 75 Neurons    possess electrical excitability  they can respond to a stimulus and convert it to an action potential (AP)  stimulus: any change in the environment (internal or external) that is strong enough to stimulate an AP  action potential: an electrical signal that travels along the cell membrane of a neuron the can communicate over very small or very great distances APs travel at different speeds Structural Classification of Neurons   neurons vary in size and shape structurally, neurons are classified according to the number of processes extending from the cell body 1) multipolar 2) bipolar 3) unipolar Multipolar   several dendrites and one axon most neurons in the brain and spinal cord and all motor neurons Bipolar  one main dendrite and one axon Unipolar    peripheral axon has sensory receptors central axon has axon terminals sensory neurons in the PNS are unipolar p. 76 Functional Classification of Neurons  functionally, they are classified according to the direction in which the AP is conducted with respect to the CNS 1) sensory neurons 2) motor neurons 3) interneurons Sensory Neurons      a.k.a. afferent neurons most are unipolar contain either sensory receptors at the dendritic ends or are located just after sensory receptors that are separate cells once activated by a stimulus, the AP forms and propagates along the axon and is conveyed into the CNS (via sensory or afferent neurons) sensory neurons travel together in spinal and cranial nerves p. 77 Motor Neurons     a.k.a. efferent neurons the AP propagates away from CNS to effectors (via motor or efferent neurons) effectors: the cells and organs (e.g. muscle fibres, glands) motor neurons are multipolar Interneurons (a.k.a. association neurons)     located primarily in the CNS between the sensory and motor neurons interneurons integrate/process information from sensory neurons if a motor response is required, the specifics of the response are formulated and relevant motor neurons are activated most interneurons are multipolar Neurons Classified by Function: Sensory (afferent) neurons conduct impulses from sensory receptors (in the skin, viscera, muscles) to the central nervous system; most cell bodies are in ganglia in the PNS. Motor (efferent) neurons transmit impulses from the CNS (brain or spinal cord) to effectors in the body periphery. Interneurons (association neurons) complete the communication pathway between sensory and motor neurons; their cell bodies reside in the CNS. p. 78 p. 79 Synapses   the site of communication between two neurons or between a neuron and another type of cell 2 types of synapses: electrical and chemical Electrical Synapses    APs conduct directly between the plasma membranes of adjacent cells through gap junctions faster (than chemical) allow for synchronization of function p. 80 Chemical Synapses Terminology      pre‐synaptic neuron: the neuron sending the signal post‐synaptic neuron: the neuron receiving the signal synaptic cleft: the space between the communicating neurons neurotransmitter (NT): a chemical released by the pre‐synaptic neuron to affect (excite or inhibit) the post‐synaptic neuron(s) or effector (muscle/gland) NT receptors are specific to the NT released (think lock and key) p. 81 Chemical Synapse Processes     when the AP reaches the end of the pre‐synaptic neuron, it causes the release of a neurotransmitter that diffuses across the synaptic cleft, binds to the post‐synaptic neuron if the neurotransmitter is excitatory, continuation of the AP is more likely if the neurotransmitter is inhibitory, continuation of the AP is less likely chemical synapses are slower (than electrical) Common Neurotransmitters     acetylcholine (ACh): PNS/CNS, excitatory at NMJ gamma‐aminobutyric acid (GABA): CNS, inhibitory dopamine: excitatory or inhibitory (depending on the receptor), emotional responses, addictive behaviours, skeletal muscle tone serotonin: excitatory or inhibitory (depending on the receptor), sensory perception, temperature regulation, mood, sleep, appetite p. 82 Neuroglia   support, nourish, and protect neurons with injury and/or disease, neuroglia multiply to fill the spaces formerly occupied by neurons Neuroglia of the CNS (4 types) 1) 2) 3) 4) astrocytes oligodendrocytes microglia ependymal cells p. 83 Astrocytes   help form the blood‐brain barrier (BBB) provide nutrients to neurons Blood‐Brain Barrier    a specialized barrier that prevents the passage of materials from the blood and the brain (and its surrounding fluid) it protects the brain from harmful substances thick basement membrane, tight junctions p. 84 Oligodendrocytes  form/maintain the myelin sheath of CNS neurons Microglia   phagocytotic cells (similar to tissue macrophages) they remove debris, phagocytize microbes Ependymal cells   they line the cavities of the brain and spinal cord they produce and assist in the circulation of cerebrospinal fluid ((CSF) a clear, colourless fluid that circulates around the brain and spinal cord ‐ it protects the brain and spinal cord from injury) p. 85 p. 86 Neuroglia of the PNS Schwann cells  cells that encircle axons in the PNS  they form the myelin sheath of PNS axons  are involved in the regeneration of PNS axons p. 87 Structures of the Nervous System (NS) CNS   brain spinal cord PNS      cranial nerves and their branches spinal nerves and their branches ganglia enteric plexuses sensory receptors p. 88 New Terminology (in general) nucleus: a cluster of neuronal cell bodies in the CNS (plural: nuclei) ganglion: a cluster of neuronal cell bodies in the PNS (plural: ganglia) tract: a bundle of axons in the CNS tracts interconnect neurons in the spinal cord and brain nerve: a bundle of axons in the PNS spinal nerves connect the spinal cord to the periphery cranial nerves connect the brain to the periphery grey matter: collections of cell bodies and unmyelinated nerve fibres in the CNS white matter: collections of myelinated axons in the CNS p. 89 p. 90 Central Nervous System (CNS) The Spinal Cord     encased in the vertebrae (bones of the spine) extends from the bottom part of the brain stem to the 2nd lumbar vertebra (L2) at L2, it tapers into a structure called the conus medullaris cauda equina: the roots of the spinal nerves below the conus medullaris p. 91 Structure of the Spinal Cord    white matter surrounding an inner core of grey matter anterior median fissure and posterior median sulcus: 2 grooves that divide the white matter into right and left sides central canal: a small tube in the centre of the SC that contains CSF Grey Matter     each side is divided into regions called horns posterior (a.k.a. dorsal) horn: axons of incoming sensory neurons and interneurons anterior (a.k.a. ventral) horn: motor nuclei lateral horn: present in thoracic, upper lumbar, sacral portions, they contain the sympathetic nuclei White Matter      each side is divided into regions called columns 3 regions: anterior (a.k.a. ventral) columns, posterior (a.k.a. dorsal) columns, lateral columns each column contains bundles of axons (tracts) that have a common origin or destination sensory (a.k.a. ascending) tracts consist of axons that conduct APs toward the brain motor (a.k.a. descending) tracts consist of axons that conduct APs away from the brain p. 92 p. 93 p. 94 The Brain Structure 1) 2) 3) 4) brain stem cerebellum diencephalon cerebrum Brain Stem     continuous with the SC, it is the part between the SC and the diencephalon 3 regions: medulla oblongata, pons, midbrain these regions also contain the nuclei of specific cranial nerves the net‐like reticular formation extends through the brainstem Medulla Oblongata  contains centres (nuclei) for the control of heart rate, blood pressure, breathing, swallowing, and vomiting  contains centres for the control of breathing Pons Midbrain   contain centres for reflex visual activities (e.g. tracking moving objects, scanning stationary objects), hearing contains nuclei called the substantia nigra – neurons that make dopamine extend from it p. 95 Reticular Formation   a net‐like formation of neural tissue that spreads throughout the brain stem contains the reticular activating system (RAS) which helps:     consciousness maintain attention prevent sensory overload by filtering out insignificant information regulate muscle tone Cerebellum    posterior to the brain stem smoothes and coordinates skeletal muscle contraction regulates posture and balance Diencephalon  includes: a) hypothalamus b) thalamus c) epithalamus a) Hypothalamus        major regulator of homeostasis controls and integrates the autonomic nervous system hormone production emotion and behaviour (with the limbic system) eating, drinking body temperature circadian rhythm (a 24 hour cycle in the biochemical, physiological and/or behavioural processes of living things) b) Thalamus  the major relay station for most sensory input to the cerebral cortex c) Epithalamus  contains the pineal gland  involved in smelling (especially emotional responses to smells) Cerebrum   2 halves or hemispheres (right and left) consists of: p. 96    an outer rim of grey matter (cerebral cortex); an inner region of white matter (association areas); and grey matter nuclei deep within the white (basal ganglia) Cerebral Cortex  contains sensory areas involved in perception  contains motor areas involved in the execution of voluntary movements Association Areas (white matter)  contain areas that deal with more complex functions like memory, emotions, reasoning, will, judgment, personality traits, intelligence Basal Ganglia a.k.a. basal nuclei helps to regulate the starting and stopping of movements helps control subconscious contraction of skeletal muscles helps suppress unwanted movement helps to set resting muscle tone functionally, the basal ganglia are linked to the substantia nigra p. 97 Other Brain Structures… The Limbic System   involved in emotion, smelling and memory it includes parts of the hypothalamus, the hippocampus, amygdala, and other nearby structures Hippocampus   functions in memory (encoding, consolidation, and retrieval) very important in converting short term memory into long term memory Amygdala  important in emotional function (esp. fear) p. 98 Connective Tissues of the CNS Meninges: three protective layers of CT that encircle the brain and SC 1) dura mater – tough outer layer 2) arachnoid membrane – middle layer 3) pia mater – inner layer     they cover the brain/SC up to the point where the nerves exit the spinal cord epidural space – the adipose and CT‐filled space between the wall of the vertebral canal and the dura mater (no epidural space in the brain) subdural space – interstitial fluid‐filled space between the dura mater and arachnoid membrane subarachnoid space – CSF filled space between arachnoid membrane and the pia mater p. 99 p. 100 p. 101 p. 102 Peripheral Nervous System (PNS)  all of the nervous tissue outside the CNS including:     cranial nerves spinal nerves ganglia sensory receptors Cranial Nerves   paths of communication between the brain and the periphery 12 pairs of nerves that exit from the base of the brain Spinal Nerves           paths of communication between the SC and the periphery there are 31 pairs of spinal nerves (C1‐C8, T1‐T12, L1‐L5, S1‐S5) parallel bundles of axons (and their associated neuroglial cells) wrapped in several layers of CT spinal nerves connect the SC to receptors, muscles, glands in all parts of the body 31 pairs that are numbered according to the region and level of the vertebral column from which they emerge the 1st pair of cervical nerves emerge from the spinal cord between the base of the skull and the 1st cervical vertebra (a.k.a. atlas) C1 – C7 exit the spine above their corresponding vertebra C8 exits between C7 and T1 vertebrae T1 – L5 exit below their corresponding vertebra not all are aligned with their corresponding vertebrae – lumbar, sacral and coccyeal nerves descend from ~ L2 to their respective levels p. 103 p. 104 p. 105 Spinal Nerves  nerve root: two bundles of axons that connect the spinal nerve to the SC  anterior (a.k.a. ventral) root: a bundle of motor axons  posterior (a.k.a. dorsal) root: a bundle of sensory axons  posterior/dorsal root ganglion: a swelling in the posterior root containing cell bodies of sensory (a.k.a. unipolar, primary afferent) neurons of the PNS Connective Tissue of a Spinal Nerve     endoneurium: innermost, covers the axon (whether it’s myelinated or not) perineurium: middle layer, covers fascicles (bundle of axons) epineurium: outermost layer, covers spinal nerve the perineurium and epineurium are highly vascularized p. 106 Subdivisions of the PNS    somatic NS (SNS) autonomic NS (ANS) enteric NS (ENS) Somatic Nervous System (SNS)    ‘the voluntary nervous system’ sensory neurons (a.k.a. 1st order neurons) convey information to the CNS (e.g. temperature, pressure, pain …) motor neurons conduct impulses (signals) from the CNS to skeletal muscles only Autonomic Nervous System (ANS)    monitors (sensory) and controls (motor) body activities (cardiac, smooth muscle, glands) automatically 2 divisions: sympathetic nervous system (SyNS) and parasympathetic nervous system (PaNS) SyNS and PaNS are terms used at S‐C but are not official Sympathetic Nervous System (SyNS)   fight or flight high sympathetic activity comes from being excited, scared, threatened Responses include:       pupil dilation increased heart rate, blood pressure airway dilation vasodilation of skeletal and cardiac mm. glucose release vasoconstriction in kidneys and the digestive tract Paraympathetic Nervous System (PaNS)    rest and digest conserve and restore energy systems increased digestive and urinary functions p. 107 Autonomic Tone       the balance between sympathetic and parasympathetic activity regulated by the hypothalamus most organs have dual innervation (PaNS and SyNS) a few structures receive only sympathetic innervation (sweat glands, arrector pilli, kidneys, most BVs, adrenal medullae) – for them, function is determined by increasing or decreasing sympathetic stimulation for normal functioning there needs to be a balance between sympathetic and parasympathetic activity the effects of sympathetic stimulation last longer and are more widespread than the effects of parasympathetic stimulation Autonomic Control  the major control and integration centre for the ANS is the hypothalamus Enteric Nervous System (ENS)   sensory neurons that monitor chemical changes within the gastrointestinal (GI) tract and the stretching of its walls motor neurons control contraction of GI tract smooth muscle and secretions of the GI organs (e.g. acid secretions by the stomach) p. 108 Processing of Sensory Input and Motor Output in the Spinal Cord Sensory Input   peripheral sensory receptor (e.g. unipolar neuron) detects stimulus (#1) sensory input (in the form of an AP) travels along the axon to the cell body in the dorsal root ganglion (DRG) (#2); from the DRG, the sensory axon may proceed in 3 ways: 1) the axon extends into the white matter and travels up to the brain as part of a sensory (ascending) tract (#3) 2) the axon enters the dorsal horn and synapses with an interneuron – its axon crosses over into the white matter of the opposite side and travels up to the brain as part of a sensory tract (#4) 3) the axon enters the dorsal horn and synapses with an interneuron which in turn synapses with a somatic motor neuron in the ventral horn as part of a spinal reflex pathway (#5) p. 109 Motor Output  motor output occurs via 2 ways: 1) somatic: axons from the motor (descending) tract (#6) synapse with the somatic motor neurons in the ventral horn on the contralateral side – these axons extend through the ventral root then the spinal nerve to innervate skeletal muscles (#7) 2) autonomic: autonomic motor neurons in the lateral horn send output along axons which sequentially pass through the lateral gray horn, the anterior gray horn and the anterior root to enter spinal nerve (#8). From the spinal nerve, the axons of autonomic motor neurons synapse with another group of autonomic motor neurons which will innervate cardiac muscles, smooth muscles or glands (#9) Metabolic Requirements of Nervous Tissues   high metabolic rate glucose is the primary energy substrate for the nervous system but neurons have no glycogen stores (they get it from the blood or from neuroglial cells) Regeneration and Repair of Nervous Tissues    although nervous tissue can readily adapt, it has limited ability to regenerate in the PNS: damage to dendrites and myelinated axons can be repaired if the cell body is intact and Schwann cells are active in the CNS: little or no repair of damaged neurons occurs p. 110