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Chapter 7 – Part 2 The Nervous System. Axons and Nerve Impulses. Axons end in axonal terminals Axonal terminals contain vesicles that contain chemicals called neurotransmitters

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Axons and Nerve Impulses
  • Axons end in axonal terminals
  • Axonal terminals contain vesicles that contain chemicals called neurotransmitters
    • When the impulses reach the axonal terminals, they stimulate the release of neurotransmitters into the extracellular space.
Axons and Nerve Impulses
  • Axonal terminals are separated from the next neuron by a gap
    • Synaptic Cleft – Gap between adjacent neurons
    • Synapse – Junction between nerves
  • Most long nerve fibers are covered with a whitish, fatty material called myelin
    • Has a waxy appearance
    • Protects and insulates the fibers
    • Increases the transmission rate of nerve impulses
  • Myelin sheaths – a tight coil of wrapped membranes that encloses the axon
Nerve Fiber Coverings
  • Schwann Cells– Produce myelin sheaths in jelly-roll like fashion in axons outside the CNS
  • Nodes of Ranvier– Gaps in myelin sheath along the axon
Multiple Sclerosis (MS)
  • The myelin sheaths around the fibers are gradually destroyed and converted to hardened sheaths called sclerosis.
  • As this happens, the circuit is short-circuited.
  • The affected person loses the ability to control his or her muscles and becomes increasingly disabled.
  • Is an autoimmune disease in which a protein component of the sheath is attacked.
Neuron Cell Body Location
  • Most neuron cell bodies are found insidethe CNS
    • Nuclei – Clusters of cell bodies within the white matter of the CNS
    • Well-protected location within the bony skull or vertebral column is essential
      • Neurons do not routinely undergo cell division after birth
      • If it is damaged the cell dies and is not replaced
  • Some neuron cell bodies are found outside the CNS
    • Ganglia – Collections of cell bodies outside the CNS
Nerve Fibers
  • Tracts – Bundles of nerve fibers (neuron processes) running through the CNS
  • Nerves – Bundles of nerve fibers (neuron processes) running through the PNS
Nerve Fibers
  • White matter – Consists of dense collections of myelinated fibers (tracts)
  • Gray matter – Contains mostly unmyelinated fibers and cell bodies
Functional Classification of Neurons
  • Sensory (Afferent) Neurons
    • Carry impulses from the sensory receptors to the CNS
    • Keep us informed about what is happening both inside and outside the body
    • The dendrite endings of the sensory neuron are usually associated with specialized receptors.
      • Cutaneous sense organs– Found in the skin
      • Proprioceptors – Detect stretch or tension in the muscles and tendons
Functional Classification of Neurons
  • Motor (Efferent) Neurons
    • Carry impulses from the CNS to the muscles and glands
  • Interneurons (Association Neurons)
    • Found in neural pathways in the CNS
    • Connect sensory and motor neurons
Structural Classification of Neurons
  • Multipolar Neurons – Many extensions from the cell body
    • Most common type: all motor and association neurons are multipolar
Structural Classification of Neurons
  • Bipolar Neurons – Neurons with two processes - one axon and one dendrite
    • Rare in adults
    • Found only in some special sense organs such as the ear and the eye, where they act as sensory receptor cells
Structural Classification of Neurons
  • Unipolar Neurons – Have a short single process leaving the cell body
    • The single process is short and divides almost immediately into central and peripheral fibers.
    • In this case, the axon conducts nerve impulses both toward and away from the cell body.
Functional Properties of Neurons
  • Neurons have two major functional properties:
    • Irritability – ability to respond to stimuli
    • Conductivity – ability to transmit an impulse
Plasma Membrane of a Resting Neuron
  • The plasma membrane at rest is polarized.
    • Fewer positive ions are inside the cell than outside the cell.
      • The major positive ions inside the cell are potassium (K+).
      • The major positive ions outside the cell are sodium (Na+).
    • As long as the inside remains more negative as compared to the outside, the neuron will stay inactive.
  • Many different stimuli excite neurons to become active and generate an impulse.
    • Light excites the eye receptors, sound excites some of the ear receptors, and pressure excites some cutaneous receptors of the skin.
    • Most neurons in the body are excited by neurotransmitters released by other neurons.
  • Regardless of what the stimulus is, the result is always the same – the “sodium gates” in the membrane open allowing an inward rush of sodium ions.
Starting a Nerve Impulse
  • A stimulus depolarizes the neuron’s membrane.
    • Depolarization – The loss of a negative charge inside the plasma membrane.
  • A depolarized membrane allows sodium (Na+) to flow inside the membrane.
    • The inside becomes more positive.
  • The exchange of ions initiates an action potentialin the neuron.
The Action Potential
  • If the action potential(nerve impulse) starts, it is propagated over the entire axon.
    • Almost immediately after the Na+ rush into the neuron, the membrane permeability changes again: It becomes impermeable to Na+, but permeable to K+.
  • K+ rush out of the neuron, which repolarizes the membrane
    • Repolarization - The outflow of positive ions, which restores the electrical conditions at the membrane to the resting state.
  • Until repolarization occurs, a neuron cannot conduct another impulse
    • Refractory Period– Period of repolarization of the neuron during which it cannot respond to a second stimulus
The Sodium Potassium Pump
  • The sodium-potassium pump restores the original configuration of sodium and potassium ions inside and outside the neuron.
    • This action requires ATP.
Nerve Impulse Propagation
  • The impulse continues to move toward the cell body
  • Impulses travel faster when fibers have a myelin sheath
    • Nerve impulses literally jumps or leaps from node to node along the fiber.
    • No current can flow across the axonal membrane where there is fatty myelin insulation.
Blocking Nerve Impulse Conduction
  • It is possible to block nerve impulses by reducing membrane permeability to sodium ions.
    • No sodium entry = no action potential
    • Alcohol, sedatives, and anesthetics all do this.
Blocking Nerve Impulse Conduction
  • It is also possible to hinder impulse conduction by interrupting blood circulation (interrupt the delivery of oxygen and nutrients).
    • Examples:
      • Cold – Fingers get numb when you hold an ice cube.
      • Continuous Pressure – When you sit on your foot, it “goes to sleep”
        • When you warm the fingers or remove the pressure from the foot, the impulses begin to be transmitted again, leading to an unpleasant prickly feeling.
Continuation of the Nerve Impulse between Neurons
  • Impulses are able to cross the synapse to another nerve
    • Neurotransmitter is released from a nerve’s axon terminal
    • The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter
    • An action potential is started in the dendrite