1. Lecture packet 9 Reading: Chapter 7
The Nervous System
Lecture packet 9
Reading: Chapter 7
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Outline
▪ Nervous system function
▪ Central and peripheral nervous system
▪ Nervous system cells
▪ Myelinated neurons
▪ Nerve signal transmission
▪ Nerve synapse
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The Nervous System
▪ It integrates and coordinates all the body’s varied activities.
▪ It divides into two:
1. Central Nervous System (CNS)
- Brain and Spinal Cord
2. Peripheral Nervous System (PNS)
- Nervous tissue outside the brain and spinal cord
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The Nervous System
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Nervous Tissues
▪ There are two types of nervous tissues:
1. Neurons (nerve cells) are excitable cells that generate and transmit messages.
2. Neuroglial cells (also called glial cells) support and protect neurons.
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Nerve Cells
▪ Nerve cells function to conduct messages throughout the body.
▪ When nerve cells are stimulated, an electrical signal quickly travels through the never cell to the nerve ending, triggering events.
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Neuroglial Cells
▪ Microglia are immune system cells. They engulf bacteria and cellular debris.
▪ Astrocytes provide nutrients to neurons.
▪ Oligodendrocytes and Schwann cells form myelin sheaths.
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Neuron
▪ Cell body contains the nucleus (main body of the cell).
▪ Dendrites are projections from the cell body that carry messages to the cell body.
▪ An axon is one large projection that carry messages away from the cell body.
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Neuron
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Neuron
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Neuron
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Neuron
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Neuron
▪ Sensory (or afferent) neurons conduct information toward the brain and spinal cord.
- Generally extend from sensory receptors (information gatherers)
▪ Motor (or efferent) neurons conduct information away from the brain and spinal cord to an effector—either a muscle, which will contract, or a gland, which will secrete its product.
▪ Interneurons are located between sensory and motor neurons.
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Neuron
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Neuron
▪ The afferent, or sensory, neuron cell bodies are located in the dorsal root ganglion.
▪ The efferent, or motor, neuron cell bodies are located in the gray matter of the spinal cord. Their axons leave the CNS and go to the skeletal muscles.
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Reflex Arc
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Cell bodies of these neurons are in the dorsal root ganglia
Motor
Sensory
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These neuroglial cells provide nutrients to neurons
Microglia
Astrocytes
Oligodendrocytes
Schwann Cells
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Which of the following type of neuron would alert the brain that you had touched a hot object?
Afferent Neuron
Efferent Neuron
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Reflex Arc
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Myelinated Neurons
▪ Neurons that have axons covered with glial cells that contain the protein myelin are called myelinated neurons.
▪ Myelinated neurons are able to carry messages faster than non-myelinated neurons.
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Functions of Myelinated Neurons
Myelin sheaths increase the rate of conduction of a nerve impulse.
Myelin sheaths from Schwann cells also help regenerate injured PNS neuron axons.
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Myelinated Neurons
Outside of the brain and spinal cord, glial cells known as Schwann cells form neurons’ myelin sheaths.
In the CNS, oligodendrocytes form the myelin sheaths.
- Nodes of Ranvier are located in the spaces on the axon between adjacent glial cells.
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Myelinated Neurons
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Myelinated Neurons
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Saltatory Conduction
▪ With the myelin sheath in place, a nerve impulse can jump from one node of Ranvier to the next in a type of transmission known as saltatory conduction.
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Multiple Sclerosis (MS)
▪ It is caused by the destruction of the myelin sheath that surrounds axons found in the CNS.
▪ It can result in paralysis and loss of sensation, including loss of vision.
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Nerve
▪ Nerve is a bundle of neurons’ axons, blood vessels, and connective tissue.
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Nerve
▪ Nerve is a bundle of neurons’ axons, blood vessels, and connective tissue.
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Membrane Potential
▪ The difference in charge between the inside and outside of the neuron is the membrane potential.
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Resting Membrane Potential
▪ A neuron that is not conducting a message is said to be “resting.”
▪ The inside of the cell has a negative charge relative to the outside of the cell.
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Resting Membrane Potential
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Resting Membrane Potential
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Sodium Potassium Pump
▪ To maintain resting membrane potential, the neuron pumps Na+ out of the cell and K+ into the cell.
▪ The transport protein (Na+-K+ ATPase, or sodium-potassium pump) takes out 3 Na+ out for every 2 K+ into the cell.
▪ This is active transport. It requires ATP.
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Nerve Impulse
▪ A nerve impulse, or action potential, involves sodium ions (Na+) and potassium ions (K+) that cross the cell membrane through ion channels.
▪ Each ion channel is designed to allow only certain ions to pass through.
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Membrane Potential Changes
▪ Depolarization: Making the membrane more positive
▪ Repolarization: Going back to resting membrane potential
▪ Hyperpolarization: Making the membrane more negative
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Action Potential
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Steps of an Action Potential
1. The axon is depolarized when voltage-gated sodium ion channels open and Na+ comes rushing in, causing the inside of the neuron to be more positive (depolarized).
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Steps of an Action Potential
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Steps of an Action Potential
2. The axon is repolarized when voltage-gated potassium ion channels open up and allow K+ to go out of the axon.
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Steps of an Action Potential
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Steps of an Action Potential
▪ The sodium-potassium pump will restore the original conditions
▪ It pumps sodium out of the cell and potassium into the cell.
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The Nerve Impulse
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Action Potential Characteristics
▪ They are all or nothing responses. If it is not a great enough stimulation, the voltage-gated channels won’t open.
▪ The magnitude and shape of an action potential is always the same.
▪ The direction is always one way down the axon. The sodium channels are inactivated for a while after the action potential passes (refractory period).
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When a neuron is resting, sodium ions have a greater concentration
1. Inside the neuron
2. Outside the neuron
3. Concentration is the same both outside and inside
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When a neuron is depolarizing, which ions come into the neuron
Calcium
Sodium
Potassium
Chloride
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When a neuron is depolarizing, the inside of the neuron cell becomes
Positively charged
Negatively charged
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Nerve Synapse
▪ The junction between two neurons of between a neuron and a muscle is called a synapse.
▪ This is how message is passed from one point to another point.
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Components of a Synapse
Presynaptic neuron is the transmitting neuron. It contains neurotransmitters, or the chemical messengers.
Postsynaptic neuron is the receiving neuron or the muscle.
And the gap in between them is called the synaptic cleft.
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Synaptic Transmission
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Synaptic Transmission
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Synaptic Transmission
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Nerve Synapse
▪ The junction between two neurons of between a neuron and a muscle is called a synapse.
▪ This is how a message is passed from one point to another point.
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Transmission Across Synaptic Cleft
The action potential gets to the end of the presynaptic axon.
The action potential triggers calcium (Ca2+) to enter the presynaptic axon terminal.
Calcium triggers synaptic vesicles located at the axon terminal to merge with the neural membrane.
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Transmission Across Synaptic Cleft
4. The synaptic vesicles release the neurotransmitters into the synaptic cleft.
5. These neurotransmitters travel across the synaptic cleft to the postsynaptic neuron (or the muscle).
6. Neurotransmitter binds to receptors on the postsynaptic neuron (or muscle).
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Transmission Across Synaptic Cleft
7. These receptors are ligand-gated sodium ion channels, which allow sodium (Na+) to enter the postsynaptic neuron (or muscle) and triggers an action potential in the postsynaptic neuron (or muscle).
8. Once the neurotransmitters are released, they need to be destroyed or contained quickly or they will continue to stimulate the nerve.
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Neurotransmitters
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Myasthenia Gravis
▪ Acetylcholine is a neurotransmitter that acts in both the PNS and the CNS. It causes voluntary muscles to contract.
▪ Acetylcholinesterase hydrolyzes the neurotransmitter acetylcholine.
▪ Myasthenia gravis is an autoimmune disease that attacks the acetylcholine receptors, resulting in reduced muscle strength.
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