Unit 6: Nervous System

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211 Terms

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Nervous System

  • Detects environmental changes that impact body, then works in tandem with endocrine system to respond to events

  • Responsible for all behaviours, memories, and movement

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Excitable Characteristics

Allows for generation of nerve impulses

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Two Divisions

  • Central nervous system

  • Peripheral nervous system

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CNS

Central nervous system

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PNS

Peripheral nervous system

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Peripheral Nervous System

  • Consists of cranial and spinal nerves that contain both sensory and motor fibers

  • Connects CNS to muscles, glands, and all sensory receptors

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Central Nervous System

Consists of brain and spinal cord

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Subdivisions of PNS

  • Somatic nervous system

  • Autonomic nervous system

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SNS

Somatic nervous system

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Somatic Nervous System

  • Voluntary

  • Neurons from cutaneous and special sensory receptors to CNS

  • Motor neurons > skeletal muscle tissue

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Autonomic Nervous Systems

  • Motor neurons to smooth and cardiac muscle and glands

  • Sensory neurons from visceral organs > CNS

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Autonomic Nervous Systems Divisions

  • Sympathetic

  • Parasympathetic

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Sympathetic Division

  • Tachycardia

  • Fight or flight

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Parasympathetic Division

  • Bradycardia

  • Rest and digest

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3 Steps of Nervous System

  • Sensory function

  • Integrative function

  • Motor function

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Sensory Function

Sense changes in internal and external environment through sensory receptors

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Sensory Neurons

Serve sensory function

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Integrative Function

Analyze sensory information, store some aspects, and make decisions regarding appropriate behaviours

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Interneurons

  • Association neurons

  • Serve integrative function

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Motor Function

Respond to stimuli by initiating action

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Motor Neurons

Serve motor function

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Neurons

  • Highly specialized

  • Intricate connections with other cells

  • Cannot divide

  • Functional unit of nervous system

  • Capacity to produce action potentials

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Action Potentials

Electrical excitability

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Neuroglia

  • Less specialized

  • Support, nourish, and protect neurons

  • Can divide

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Cell Body

  • Single nucleus with prominent nucleolus

  • Nissl bodies

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Nissl Bodies

Rough ER and free ribosomes for protein synthesis

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Neurofibrils

Give cell shape and support

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Microtubules

Move material inside cell

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Dendrites

  • Processed look like tree branches

  • Receive and transmit stimuli towards cell body

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Axon

  • Transmits info away from cell body

  • Forms branches > axon terminals store neurotransmitter

  • Many have myelin sheath

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Synaptic Knobs

Axon terminals

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Cell Processes

Dendrites and axons

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Multipolar Neurons

  • Many dendrites and only one axon

  • Located throughout brain and spinal cord

  • 99% neurons

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Bipolar Neurons

  • One main dendrite and one axon

  • Convey special senses of sight, smell, hearing, and balance

  • Retina of eye, inner ear, olfactory

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Unipolar Neurons

  • One process which extends from body and divides into a central branch that functions as an axon and a dendritic root

  • For sensory neurons that convey touch and stretching information from extremities

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Pseudounipolar Neurons

Unipolar neurons

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Afferent Neurons

  • Carry impulses from peripheral sense receptor to CNS

  • Skin, sense organs, visceral organs

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Efferent Neurons

  • Carry impulses away from CNS to effectors

  • Muscles, glands in PNS

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Association Neurons

  • Receive input for sensory neurons, communicate with one another or with motor neurons

  • Entirely in CNS

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Glia

  • Support, nourish, and protect neurons cells

  • Do not conduct nerve impulses

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CNS Neuroglia

  • Astrocytes

  • Oligodendrocytes

  • Microglia

  • Ependymal cells

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PNS Neuroglia

  • Schwann cells

  • Satellite cells

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Astrocytes Types

  • Protoplasmic

  • Fibrous

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Protoplasmic

Grey matter

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Fibrous

White matter

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Astrocytes Functions

  • Support neurons

  • Create a blood-brain barrier

  • Secrete chemicals that regulate growth, migration, interconnection among brain neurons

  • Role in learning and memory

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Oligodendrocytes

Form and maintain myelin sheath around CNS axons

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Microglia

Function as macrophages

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Ependymal Cells

  • Line ventricles of brain and central canal of spinal cord

  • Produce, monitor, assist in CSF circulation

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Schwann Cells

  • Encircle PNS axons

  • Form myelin sheath around PNS axons

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Satellite Cells

  • Surround cell bodies of neurons and PNS ganglia

  • Regulate exchanges of material b/w neuronal cells and interstitial fluid

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Ganglia

  • Small masses of neuronal cell bodies located outside brain and spinal cord, usually associated with cranial and spinal nerves

  • Somatic, autonomic, and enteric

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Myelination

Process of forming myelin sheath which insulates and increases nerve impulse speed

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Myelin Sheath

  • Multilayered lipid and protein that covers some axons

  • Increases from birth to maturity

  • Increases speed of nerve conduction

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Myelin Sheath Formation

Formed by oligodendrocytes in CNS and schwann cells in PNS

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Nodes of Ranvier

  • Are gaps in myelin sheath

  • Each schwann cell wraps around one axon segment b/w two nodes of raniver

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Nodes of Ranvier Length

Myelinated nodes are about 1mm in length and up to 100 layers

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M.S.

Autoimmune destruction of myelin

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White Matter

Lipid part of myelin imparts appearance

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Grey Matter

Colour is because it lacks myelin

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White Matter Formation

Formed from aggregations of myelinated axons from many neurons

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Grey Matter Formation

Formed from neuronal cell bodies and dendrites

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Electrical Signals in Neurons

  • Electrically excitable due to voltage difference across their membranes

  • In living cells, a flow of ions occurs through ion channels in the cell membrane

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2 Types of Electron Signals

  • Graded potentials

  • Action potentials

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Graded Potentials

Are used for short distance communication only

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Action Potentials

Allow communication over long distances within the body

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Neuron Cell Membrane

Separates charged particles on inside of cell from those on outside

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Extracellular Fluid

Dominated by Na+ ions and Cl- ions

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Intracellular Fluid

Dominated by K+ ions and negatively charged proteins (Pr-)

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Resting Cell

  • Polarized: ~70millivolts

  • Excess of “+” charges on outside of cell membrane and excess of “-” charges on inside

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Membrane Potential

Uneven distribution of charges

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Depolarization

Sudden change in membrane potential at one spot on surface, an action potential has developed

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Special Ion Channels

  • In plasma membrane

  • Open and allow sudden movement of Na+ ions across cell membrane

  • Excess + ions in ICF and - ions in ECF

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Reversal

  • Polarization to depolarization

  • Very short lived because ion channels close immediately however the change causes a chain reaction

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Chain Reaction

“Opening and closing” forces the action potential across the entire cell membrane

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Factors that Affect Speed of Propagation

  • Amount of myelination

  • Axon Diameter

  • Temperature

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Amount of Myelination

  • More myelin

  • The faster

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Axon Diameter

  • Larger diameter

  • The faster

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Temperature

Lower speeds when cooled

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Synapse

Communication b/w neurons occurs at a junction called the synapse

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Synapse Involves

  • Presynaptic neuron

  • Synaptic cleft

  • Postsynaptic neuron

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Presynaptic Neuron

Neuron sending impulse

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Synaptic Cleft

Space b/w presynaptic neuron and postsynaptic neuron

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Postsynaptic Neuron

Neuron receiving impulse

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Step 1 of Signal Transmission

Nerve impulse arrives at synaptic end bulb of presynaptic axon

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Step 2 of Signal Transmission

  • Depolarizing phase of nerve impulse opens voltage-gated Ca2+ channels

  • B/c Ca2+ ions more concentrated in extracellular fluid, Ca2+ flows through open channels

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Step 3 of Signal Transmission

  • Increase concentration of Ca2+ triggers exocytosis if synaptic vessels

  • Neurotransmitters are released into synaptic cleft

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Step 4 of Signal Transmission

They diffuse across synaptic cleft and bind to neurotransmitter receptors in postsynaptic neuron

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Step 5 of Signal Transmission

Binding of neurotransmitter molecules to receptors on ligand-gated channels opens channels to ion flow

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Step 6 of Signal Transmission

This causes voltage across the membrane to change

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Step 7 of Signal Transmission

When depolarizing postsynaptic potential reaches threshold, triggers nerve impulse in axon of postsynaptic neuron

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Action Potential Crossing Synaptic Cleft

  • Action potential arrives at end of axon called synoptic knobs

  • Inflow of Ca2+ caused by depolarizing phase of nerve impulse triggers presynaptic neuron to release a neurotransmitter

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Neurotransmitter Location

Stored in vesicles in synaptic knob

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Exocytosis

Active transport

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Neurotransmitter Release

Through exocytosis

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Neurotransmitter Crossing Synaptic Cleft

  • Diffuse across synaptic cleft until they reach receptors in membrane or receiving cell

  • They then bind to these receptors, which open ion channels and allow movement of Na+ ions across postsynaptic cell membrane

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Action Potential Triggered

Triggered by the movement of Na+ ions

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Neurotransmitter Removal

By diffusion out of synaptic cleft, enzymatic degradation, and re-uptake by cells

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Spinal Cord Anatomy

  • Protective structures, protect the spinal cord, and provide physical stability

  • Vertebral column

  • Meninges

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Meninges

  • Dura mater

  • Arachnoid

  • Pia mater