Human Anatomy & Physiology I

Human Anatomy & Physiology I

Nervous System Chapter 9 Pages 211-257 Chapter 9 Wordbytes 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. af- = toward 11. -ferrent = carried arachn- = spider 12. gangli- = swelling astro- = star 13. -glia = glue auto- = self 14. mening- =membrane dendro- = tree

15. micro- = small di- = 2, through 16. neuro- = nerve ef- = away from 17. oid = similar to enter- = intestines 18. oligo- = few epen- = above 19. peri- = around encephalo- = brain 20. somat- = body Nervous System Overview

Master controller and communicator for the body Responsible for all behavior 3 functions: Sensory input monitors changes inside/outside of body Integration processes and interprets, then

decides what should be done Motor output causes a response in effector organs Organization2 main parts: 1. Central Nervous System (CNS) = brain and spinal cord 2.

Interprets incoming sensory info. and dictates motor responses Peripheral Nervous System (PNS) = nerves from brain & in spinal cord INPUT-Afferent or Sensory division OUTPUT- Efferent or Motor division Subdivided: Somatic (SNSfrom CNS to

skeletal muscles=voluntary) & Autonomic (ANSregulate smooth & cardiac muscle, glands=involuntary) Major structures: Histology Highly cellulardensely packed & tightly intertwined

2 types of cells: 1. Neuron= nerve cell 2. Specialized for signal carrying & information processing Neuroglia cells support, nourish & protect neurons

Neuroglia critical for homeostasis of interstitial fluid around neurons Supporting cells (Neuroglia) ~ half the volume of CNS Cells smaller than neurons Can multiply and divide and fill in brain areas Do not conduct nerve impulses

Supporting Cells in CNS Astrocytes most abundant and most versatile; blood-brain barrier Oligodendrocytes have fewer branches; produce insulating myelin sheath in CNS Microglia ovoid cells with thorny processes; provide defense (because immunity cells not allowed in CNS) Ependymal cells squamous/columnar cells with cilia; produce cerebrospinal fluid (CSF) Supporting Cells in PNS

Schwann cells PNS cell support; produce & maintain myelin sheath, regenerate PNS axons Satellite cells in PNS ganglia; support neurons in ganglia, regulate exchange of materials between neurons and interstitial fluid Neuron Characteristics They conduct nerve impulses from one part of the body to another They have extreme longevity

live/function for a lifetime They are amitotic lose their ability to divide They have a high metabolic rate = need O2 and glucose Neuronal Structure

Cell body nucleus, cytoplasm with typical organelles; most within CNS = protected by cranial bones & vertebrae Dendrites short, highly branched input structures emerging from cell body = high surface area to receive signals Axon may be short or long, only one per neuron; conducts away from cell body toward another neuron or effector Emerges at cone-shaped axon hillock

Axon terminals at end of axon with synaptic bulbs (Neurilemma) Figure 9.3 = impulse direction Pg. 216 Myelination

Axons covered with a myelin sheath Many layered lipid & protein creating insulations Increases speed of nerve conduction. Formed by: Schwann cells in PNS Oligodendrocytes in CNS Nodes of Ranvier= gaps in the myelin Nodes

are important for signal conduction Some diseases destroy myelin multiple sclerosis & Tay-Sachs Function choices: *use this to help with the Neuron Drawing on the back of the Support Cell WS

Send nerve impulse from neuron to other nerve

cells/effectors Portions of Schwann cells that contain cytoplasm and nuclei and surround the myelin sheath Gaps between myelin sheath that allow impulse to travel Network of fine threads; important in intracellular transport & maintaining cell shape & integrity Receive message from adjacent neurons & carries it to the cell body Insulates axons and increases impulse speed Synthesize proteins The point of contact of one neuron with another These produce and maintain myelin sheath in the PNS The control center of the cell

Multiple Sclerosis What is it? Gray and White Matter White matter- primarily myelinated axons Gray matter- nerve cell bodies, dendrites,

unmyelinated axons, axon terminals & neuroglia Spinal cord gray matter is centrally located Classification of Neurons 1.

Structural according to # of processes (Fig. 9.6): Multipolar 3 or more; most common, especially in CNS Bipolar 2 processes (an

axon and a dendrite) that extend from opposite sides; found in special sense organs Unipolar 1 process that divides like a T; found in ganglia in PNS 2. Functional according to the direction impulse travels (Fig. 9.7)

Sensory (afferent) neurons transmit impulses from sensory receptors toward or into the CNS; mostly unipolar, with cell bodies in ganglia outside CNS Motor (efferent) neurons carry impulses away from CNS to muscles and glands; multipolar, usually with cell bodies in CNS Interneurons (association neurons) between motor &

sensory neurons; most in CNS; 99% of neurons in body; mostly multipolar Neurophysiology Neurons are highly irritable = responsive to stimuli When stimulated, an electrical impulse (action potential) is conducted along its axon Action

potential underlies all functional activities of the nervous system Action Potentials Action potentials = nerve impulses Require a membrane potential electrical charge difference across cell membrane like a battery

Ion Channels allow ions to move by diffusion = current If no action potential then resting cell has resting membrane potential Ion Channels Allow specific ions to diffuse across membrane Move

from high concentration to low or toward area of opposite charge Leakage channels Gated channels- require trigger to open Voltage- Gated channels respond to a change in membrane potential Resting Membrane Potential Leakage channels Cytosol high in K+ & interstitial fluid high in

Na+ (sodium potassium pumps) Leakage lets K+ through easily and Na+ poorly inside is negative relative to outside actual value depends on the relative leakage channel numbers Figure 9.4 Graded Potentials Short-lived, local changes to membrane potential

Cause current flows that decrease with distance Magnitude varies with strength of stimulus Action Potential (AP) Generated by neurons and muscle cells Series of active events Channels actively open & close Some initial event is required to reach a voltage threshold (~ = - 55 mv)

Stimulus = any event bringing membrane to threshold Action Potential Resting state voltage-gated Depolarizing phase membrane

positive potential rises and becomes Repolarizing phase potential PK) channels closed

restored to resting value ( PNa, Undershoot Potassium permeability continues Figure 9.5 Active Events Stimulus to reach threshold Na+ channel opens=>

Na+ ions enter=> positive potential=> Causes K+ channel opening => repolarization All- or None Phenomenon This sequence is always the same If threshold then the same size of changes occur no larger or smaller APs Stimulus must reach threshold to start After one AP there is a short period before next can be triggered= absolute refractory

period each AP is a separate, all-or-none event; enforces one-way transmission of AP Conduction of Nerve Impulses Each section triggers next locally Refractory period keeps it going the right direction unmyelinated fiber- continuous conduction With myelin- saltatory conduction Can

only be triggered at nodes of Ranvier Myelinated fibers faster & larger neurons faster Figure 9.6a Figure 9.6b The Syanpse

Synapse (to clasp or join)- junction that mediates information transfer from 1 neuron to another or from a neuron to an effector cell Axodendritic or axosomatic synapses most synapses occur between the axonal ending of a neuron and the dendrites or cell body of other neurons Synaptic Transmission Electrical synapse

Sequence of events at synapse Triggered by voltage change of the Action Potential Sending neuron = presynaptic Receiving neuron = postsynaptic Space between = synaptic cleft Neurotransmitter carries signal across cleft Events at Synapse Chemical synapse AP arrives at presynaptic end bulb=> Opens voltage gated Ca2+ channels=>

Ca2+ flows into cell increased Ca2+ concentration => exocytosis of synaptic vesicles=> Neurotransmitter released into cleft Diffuse across and bind to receptors in postsynaptic cell membrane

Synaptic Transmission Binding at receptors Chemical trigger of ion channels May depolarize or hyperpolarize postsynaptic cell membrane If threshold reached at axon hillock then postsynaptic cell action potential results Synaptic Transmission Finally the neurotransmitter must be removed from the cleft Diffusion away Destroyed by enzymes in cleft

Transport back into presynaptic cell Neuroglia destruction Figure 9.7 Neurotransmitters AcetylCholine (Ach)- common in PNS Biogenic amines - Norepinephrine (NE), Dopamine (DA), serotonin, Histamine

Amino Acids Glutamate, Aspartate, gamma aminobutyric acid (GABA), glycine Neuropeptides endorphins Novel Messengers - ATP/ Nitric oxide (NO)/ Carbon monoxide (CO) Development of Neurons P. 422-424

Neuroblasts Growth cone Programmed cell death Web sites: tml gsaw_02/index.swf?startPosition=nervous n.cfm

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