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Chapter 7 – Part 2 The Nervous System

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1 Chapter 7 – Part 2 The Nervous System

2 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. Axonal terminals are separated from the next neuron by a gap Synaptic cleft – gap between adjacent neurons Synapse – junction between nerves


4 Myelin 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

5 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

6 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.

7 Neuron Cell Body Location
Most are found in the 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 are found outside the CNS Ganglia – collections of cell bodies outside the CNS

8 Nerve Fibers Tracts – bundles of nerve fibers (neuron processes) running through the CNS Nerves – bundles of nerve fibers (neuron processes) running through the PNS White matter – consists of dense collections of myelinated fibers (tracts) Gray matter – contains mostly unmyelinated fibers and cell bodies

9 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

10 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

11 Neuron Classification

12 Structural Classification of Neurons
Multipolar neurons – many extensions from the cell body Most common type: all motor and association neurons are multipolar

13 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

14 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.

15 Functional Properties of Neurons
Neurons have two major functional properties: Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse

16 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.

17 Stimulus 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.

18 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 potential in the neuron.

19 The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon.

20 Repolarization 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

21 Repolarization 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

22 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.

23 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.

24 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. 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.

25 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

26 How Neurons Communicate at Synapses

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