Chapter 48-49: The Nervous System & Neurons

Transcription

1 Invertebrates Chapter 48-49: The Nervous System & Neurons Radial Symmetry - Nerve net Cnideria Bilateral Symmetry double, ventral, solid nerve cord brain (cephalization) Vertebrates Dorsal, single, hollow, nerve cord Arrangement of Components of the Nervous System Central Nervous System (CNS) o Brain o Spinal Cord Peripheral Nervous System (PNS) o All sensory & motor neurons, including cranial & spinal nerves 1

2 Anatomy of Central Nervous System Brain o Covered by the meninges three concentric layers of tissue that also surround the brain. 2

3 o Brain stem the primitive brain Controls most of the basic reflexes of the body Midbrain o Serves mainly as a relay center between hindbrain and forebrain o Tectum contains nuclei involved in the integration of sensory info Superior colliculi are involved in regulation of visual reflexes Inferior colliculi are involved in regulation of auditory reflexes Pons o Major pathway for communicating info to and from cerebellum o Has nuclei involved in regulation of visceral activities like breathing o Relays information to and from higher brain centers Medulla Oblongata o Controls Respiration, heartbeat, swallowing, vomiting, digestion o Relays information to and from spinal cord and brain o Cerebellum Sensory coordinator especially touch and feeling Generalized timing timing body movements Mood and emotional control o Forebrain Thalamus Relays all sensory information to the cerebrum Contains one nucleus for each type of sensory information Relays motor information from the cerebrum Receives input from the cerebrum Receives input from brain centers involved in regulation of emotion and arousal Hypothalamus Center for many of basic drives of hunger, thirst, sex, and fear Connection between Nervous and endocrine systems Cerebrum Outer layer = cerebral cortex (gray matter) o Areas made of cell bodies, dendrites, and non-myelinated fibers o Greatly folded (Gyri = folds) Inner layer of white matter o Areas of myelinated fibers Subdivided into two hemispheres by the longitudinal fissure 3

4 Hemispheres functionally unique - lateralization o Right brain Specializes in pattern recognition, spatial relationships, nonverbal ideation, emotional processing, and parallel processing of info o Left brain analytical skills, speech, math Specializes in language, math, logic operations, and processing of serial sequences of information, and visual and auditory details Specializes in detailed activities required for motor control 4

5 Spinal Cord o Located along dorsal surface of vertebral column and covered by an archway formed by neural spines of each vertebra o Covered by the meninges o Consists of gray and white matter Anatomy of the Peripheral Nervous System (PNS) In humans, consists of: o 31 spinal o 12 cranial nerves 5

6 Subdivided into: o Somatic (Motor) Nervous system Sensory component receive information from the eternal environment Motor sends information to skeletal muscles o Autonomic Nervous System Sensory receive information from the internal environment Motor send information to smooth muscles of internal organs, cardiac muscle of heart, and the various glands of the body. Subdivided into two antagonistic groups: Sympathetic fight or flight o Nerves exit spinal cord in the region of thoracic and lumbar vertebrae 6

7 Parasympathetic veg out o Nerves exit spinal cord in region of brain (cranial nerves) and sacral vertebrae 7

8 Neuron Functional unit of Nervous System Anatomy o Cell body enlarged area that contains nucleus and a variety of other organelles o Dendrites thin, branched extensions from the cell body of the neuron o Axon single, slender and usually longer extension from the cell body typically branched only at its end o Input zones = cell body and dendrites; dendrites, in essence, extend the reach of a neuron o Output zone = axon o Trigger zone = area of plasma membrane at junction of axon (the axon hillock) Neuroglia o Astrocytes found in CNS Structural and metabolic support Induce formation of tight junctions in capillaries (help form blood-brain barrier) o Along Axons Oligodendrocytes in CNS Schwann cells in PNS Cells are rolled into several layers Secrete myelin; a fatty, white substance axon is said to be myelinated Nodes functions to insulate axon & speed up conduction of impulse 8

9 Types of Neurons o Sensory Neurons Receive stimulus from sense organ or another neuron Cell body located some distance from dendrites that lie at end of a single long fiber o Interneurons Lies completely in CNS Integrates information Receive from sensory, either sends to brain or to motor neuron Dendrites often incredibly branched o Motor Neurons Sends stimulus to effector (e.g. muscle or gland) 9

10 Propagation of Nerve Impulse Axon of a neuron at rest shows a weak electrical potential that can be measured o Electrical potential o Typically 70mV = resting potential (-60 to -80 mv) An impulse travelling along the axon produces an action potential o Reversal of polarity o Followed almost immediately by a return to resting potential o Self-propagating All-or-None Theory o Threshold stimulus o Once threshold reached get characteristic action potential o Increase stimulus does not alter action potential 10

11 How do we discriminate between weak and intense stimuli? o Though action potential does not change, the rate at which action potentials are produced in response to a continuous stimulus does o Each neuron has its own threshold, some respond to weak stimuli; some to strong stimuli o Answer to question: by the number of neurons firing and the rate at which each neuron is firing Cellular and Chemical nature of the nerve impulse o Intrinsic proteins of axon membrane o Like all cellular membranes the neuron membrane is polarized Sodium potassium pump maintains gradient Potassium channels: ungated and for leakage Sodium channels: ungated and for leakage (very few in number) Voltage-gated channels Sodium Potassium o How is resting potential established? K + concentration of 140 mm inside and 5mM outside establishes concentration gradient that favors K + out Na + Concentration 150 mm outside and 15 mm inside establishes concentration gradient that favors Na + in K + leakage channels allow K + to leak out down its concentration gradient at a rather rapid initial rate since these channels are quite numerous Leakage eventually checked by a second gradient that s established by initial diffusion As K + leaks out, the charge on inside of membrane becomes more negative since negatively charged ions Cl - and large organic anions are left behind o Establishes electrostatic gradient that s opposite to K + chemical gradient Eventually, rate of diffusion out due to concentration gradient equals rate of diffusion in due to electrostatic gradient o Inside of cell ends up more negative than outside establishing the resting potential which is typically -70mV Why doesn t the sodium move in the opposite direction? It does, BUT there are so few Na+ leakage channels that rate is slow Leakage actually keeps the resting potential from going below -60 mv 11

12 o How is action potential created? Presence of voltage gated channels for Na + and K + At rest these channels are closed (1) Initiation of action potential Reversal of polarity outside of membrane becomes relatively negative o When excitory stimulus applied, some sodium voltage gated channels begin to open (2) Potential begins to rise towards threshold (2) 12

13 o When threshold reached, more sodium gates open & sodium ions rushes in along strong charge & concentration gradient (3); reverses polarity Restoration of polarity (4) o Voltage gated channels for sodium close while those for potassium open o Potassium ions rush out in response to strong charge and concentration gradients restoring the polarity of the membrane 13

14 o Undershoot (5) more potassium channels open than at rest and the loss of potassium results in a greater negativity During this period (Refractory Period), potassium channels remain open, sodium channels remain closed and an action potential cannot be generated o Problem: potassium now on outside and sodium now on inside o Solution: potassium channels remain open at first and potassium leaks back in and sodium-potassium pump restores proper relationship Axon is now ready to conduct another impulse 14

15 o Action potential is self-propagation Change of membrane s polarity triggers opening of voltage-gated sodium channels in adjacent segment of axon (2) triggering action potential which opens next sodium gates (3) etc., all the way to the end of the axon Unidirectional because trailing edge is in refractory period and has to recover proper sodium-potassium balance o Saltatory conduction Mylenated axons action potential events only take place only in the nodes When action potential reaches end of node, gated channels at beginning of next node open almost instantly o Impulse appears to "jump" from node to node 15

16 Synaptic Transmission Synapse o Direct connection between presynaptic and postsynaptic neurons electrical synapse o Indirect connection between presynaptic and postsynaptic neurons chemical synapse Synaptic cleft Transmission across the synaptic cleft o Presynaptic events Action potential arrives at synaptic bouton Gated channels for calcium ions open Calcium rushes in (huge concentration gradient) Synaptic vesicles migrate to membrane Synaptic vesicles empty contents into synaptic cleft by exocytosis Contents = neurotransmitter o Removal of neurotransmitter from synapse enzymatic destruction of neurotransmitter, for example, acetylcholinesterase destroys the neurotransmitter, acetylcholine o Postsynaptic events Neurotransmitter binds to receptor on postsynaptic neuron Membrane response Postsynaptic potential graded potential (not all-or-none) o Inhibitory postsynaptic potentials hyperpolarize (more negative) o Excitatory postsynaptic potentials depoloarize (more positive) o Incoming messages are often combined 16

17 Neurotransmitters o Acetylcholine excitatory as a rule Excitatory to skeletal muscle Inhibitory to cardiac muscle Secreted by the CNS, PNS, and at vertebrate neuromuscular junctions o Norepinephrine (noradrenaline)/epinephrine (adrenaline) Can have excitatory or inhibitory effects Secreted by the CNS, PNS, and adrenal glands o Dopamine Generally excitatory; may be inhibitory at some sites Widespread in the brain Affects sleep, mood, attention, and learning Secreted by the CNS and PNS A lack of dopamine in the brain is associated with Parkinson s disease Excessive dopamine is linked to schizophrenia o Serotonin Generally inhibitory Widespread in the brain Affects sleep, mood, attention, and learning Secreted by the CNS 17

18 o GABA (gamma aminobutyric acid) Inhibitory reduces anxiety Secreted by the CNS o Amino acids glycine, aspartate, glutamate Neuromodulators influence events at synapse o Inhibiting release of neurotransmitter o Altering response by postsynaptic receptors Substance P - induces pain perception o Best studied endorphins met-enkephalin Relief of pain natural opiates - inhibits substance p Poisons and toxins o Botulism Clostridium botulinum toxin blocks release of acetylcholine from motor neurons o Curare arrow poison from South American plants blocks receptors for acetylcholine at neuromuscular junction o Organophosphate pesticides blocks release of acetylcholinesterase o Tetanus Clostridium tetani tetanus toxin (tetanospasmin) blocks release of GABA and causes a failure to inhibit pathways that cause muscles to contract. Muscles constantly stimulated eventually lock-up in a phenomenon known as tetany 18