Nervous System - Neurons

Biol 105 Chapter 7 Outline

I. Nervous system function II. Central and peripheral nervous system III. Nervous system cells IV. Myelinated neurons V. Nerve signal transmission VI. Nerve Synapse

. Nervous tissue functions to conduct messages throughout the body.

. When nerve cells are stimulated, an electrical signal quickly travels through the nerve cell to the nerve ending, triggering events.

. Nervous system functions to: . Sense the environment – it receives information from both outside and inside the body. . Process the information it receives. . Respond to information – send out orders.

2. Peripheral Nervous System (PNS) . Nervous tissue outside brain and spine. . Sense organs.

Peripheral

. Two types of nervous tissue cells. . Neurons – The cells that are responsible for transmitting messages. . Neuroglial Cells – Cells that support the neurons.

. Microglia – Immune system cells, engulf bacteria and cellular debris.

. Astrocytes – Provide nutrients to neurons.

. Oligodenrocytes and Schwann Cells – Form myelin sheaths.

. Cell body – contains the nucleus, main body of cell.

. Dendrites – projections from the cell body that carry messages to the cell body.

. Axon – one projection that carries messages away from the cell body (can be very long).

Dendrites receive The cell body controls information from the cell’s metabolic other neurons or activities. from the environment. Axon endings release chemicals called neurotransmitters that affect the activity of nearby neurons or an effector (muscle or gland).

Nucleus

Cell body The cell body An axon conducts the nerve impulse away integrates input Axon from other neurons. from the cell body. endings

Receiving portion of Sending portion of neuron neuron

Copyright © 2009 Pearson Education, Inc. Figure 7.2 12-12 Copyright © 2009 Pearson Education, Inc. Neurons of the Peripheral Nervous System . Neurons in the PNS are either carrying messages to or from the CNS.

. Afferent = Sensory neurons = Neurons carrying messages to the CNS.

. Efferent = Motor neurons = Neurons carrying messages from the CNS.

Copyright © 2009 Pearson Education, Inc. Interneurons in the Central Nervous System . Interneurons are located between sensory and motor neurons within the CNS. . Interneurons integrate and interpret sensory signals.

. The afferent or sensory neuron cell bodies are located in 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.

50% 50% 1. Motor 2. Sensory

Motor Sensory

50% 50% 1. Motor 2. Sensory

Motor Sensory

Sensory Muscle receptor (effector) for pain

Impulse direction

Sensory Cell neuron body Motor neuron

Interneuron

1. Microglia 25% 25% 25% 25% 2. Astrocytes 3. Oligodenrocytes 4. Schwann cells

Microglia Astrocytes Schwann cells Oligodenrocytes

1. Microglia 25% 25% 25% 25% 2. Astrocytes 3. Oligodenrocytes 4. Schwann cells

Microglia Astrocytes Schwann cells Oligodenrocytes

50% 50% 1. Axons 2. Dendrites

Axons Dendrites

50% 50% 1. Axons 2. Dendrites

Axons Dendrites

50% 50% 1. efferent neuron 2. afferent neuron

efferent neuron afferent neuron

50% 50% 1. efferent neuron 2. afferent neuron

efferent neuron afferent neuron

50% 50% 1. Sensory 2. Motor

Motor Sensory

50% 50% 1. Sensory 2. Motor

Motor Sensory

. Neurons that have axons covered with neuroglial cells that contain the protein myelin are called myelinated neurons.

2. Myelin sheaths from Schwann cells also help regenerate injured PNS neuron axons.

. Schwann cells and Oligodenrocytes are wrapped around neuronal axons.

. Schwann cells are found in the PNS.

. Oligodendrocytes are found in the CNS.

. Nodes of Ranvier are spaces on the axon between the glial cells.

Nucleus

Dendrites

Cell body In saltatory conduction, the nerve impulses jump from one node of Ranvier to the next.

Node of Ranvier

Schwann cell

Myelin sheath (a)

. Caused by the destruction of the myelin sheath that surrounds axons found in the CNS.

. Can result in paralysis and loss of sensation, including loss of vision.

. Nerves contain Neuron axons that are bundled together. . These bundles contain: . Axons . Blood vessels . Connective tissue

Nerve

Connective tissue surrounding one nerve Blood supply Axons within a connective tissue sheath One axon

(d) The anatomy of a nerve

1. Neutron 33% 33% 33% 2. Proton 3. Electron

Proton Neutron Electron

1. Neutron 33% 33% 33% 2. Proton 3. Electron

Proton Neutron Electron

20% 20% 20% 20% 20% 1. Simple diffusion 2. Facilitated diffusion 3. Active transport 4. Both 2 and 3 5. All of the above

Both 2 and 3 Simple diffusion Active transport All of the above Facilitated diffusion

20% 20% 20% 20% 20% 1. Simple diffusion 2. Facilitated diffusion 3. Active transport 4. Both 2 and 3 5. All of the above

Both 2 and 3 Simple diffusion Active transport All of the above Facilitated diffusion

. 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.

Cross section

Axon membrane

Neuron plasma Extracellular membrane fluid

Cytoplasm

Continually open ion channels “Gated” ion channels Sodium-potassium pump

Ion channels Sodium-potassium pump Ion channels can be open continuously or opened and The sodium-potassium pump closed by a molecular gate uses cellular energy (ATP) to pump sodium ions out of the cell and potassium ions into the cell

. The difference in charge between the inside and outside of the neuron is the membrane potential.

Copyright © 2009 Pearson Education, Inc. Resting Membrane Potential . A neuron that is not conducting a message is said to be “Resting”. . When a neuron is resting there is more sodium (Na+) outside the neuron cell and more potassium (K+) inside the cell.

. The inside of the cell has a negative charge compared to the outside the cell.

. To maintain this resting membrane potential the neuron pumps Na+ out of the cell and K+ into the cell.

. The transport proteins take 3 Na+ ions out for every 2 K+ ions into the cell = Na+/K+ pump.

. This is Active Transport – requiring ATP.

. An electrochemical signal conducted along an axon. It is a wave of depolarization followed by repolarization.

. Depolarization is caused by sodium ions entering the axon.

. Repolarization is caused by potassium ions leaving axon.

Copyright © 2009 Pearson Education, Inc. 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 become positively charged.

Copyright © 2009 Pearson Education, Inc. Figure 7.5 (2 of 4) Steps of an Action Potential 2. The axon is repolarized when voltage gated potassium ion channels open and allow K+ to flow out of the axon.

. This returns the membrane potential to be negative on the inside of the neuron.

. The action potential travels down the axon.

. After the action potential, the sodium potassium pump restores the original conditions by pumping sodium (Na+) out of the cell and potassium (K+) back into the cell.

Copyright © 2009 Pearson Education, Inc. Figure 7.6 Action Potentials . It is an all or nothing response – if it is not a great enough stimulation the channels won’t open. The level of the action potential is always the same.

. The direction is always one way down the axon. The sodium channels are inactivated for awhile after the action potential passes = refractory period.

1. inside the neuron cell 33% 33% 33% 2. outside the neuron cell 3. concentration is the same

inside the neuron cell outside the neuron cell concentration is the ...

1. inside the neuron cell 33% 33% 33% 2. outside the neuron cell 3. concentration is the same

inside the neuron cell outside the neuron cell concentration is the ...

1. Calcium (Ca++) 25% 25% 25% 25% 2. Sodium (Na+) 3. Potassium (K+) 4. Chlorine (Cl-)

Sodium (Na+) Chlorine (Cl-) Calcium (Ca++) Potassium (K+)

1. Calcium (Ca++) 25% 25% 25% 25% 2. Sodium (Na+) 3. Potassium (K+) 4. Chlorine (Cl-)

Sodium (Na+) Chlorine (Cl-) Calcium (Ca++) Potassium (K+)

1. Positively charged 50% 50% 2. Negatively charged

Positively charged Negatively charged

1. Positively charged 50% 50% 2. Negatively charged

Positively charged Negatively charged

. How are messages passed from one nerve to the next or from the nerve to a muscle?

. The junction between two neurons or between a neuron and a muscle is called a synapse.

1. Presynaptic neuron is the transmitting neuron.

2. Postsynaptic neuron is the receiving neuron or the muscle.

3. And the gap in between them = synaptic cleft.

. Presynaptic neuron has synaptic vesicles that contain neurotransmitters.

Dendrites

Nucleus Axon

Cell Impulse body

Impulse

Step 1: The impulse reaches the axon ending of the Synaptic presynaptic membrane. knob

Step 2: Synaptic vesicles release neurotransmitter into the synaptic cleft.

Membrane of Synaptic Synaptic postsynaptic neuron cleft vesicle

Neurotransmitter Synaptic vesicle Step 3: Neurotransmitter diffuses across synaptic cleft.

Receptor (of sodium ion channel) on postsynaptic membrane

Step 4: Neurotransmitter molecules bind to receptors on the postsynaptic neuron.

Step 5: Sodium ion channels open.

Step 6: Sodium ions enter the postsynaptic neuron, causing depolarization and possible action potential.

1. The action potential gets to the end of the presynaptic axon. 2. The action potential triggers Ca2+ to enter the presynaptic axon terminal. 3. The Ca2+ triggers synaptic vesicles located at the axon terminal to merge with the neural membrane.

Copyright © 2009 Pearson Education, Inc. Transmission Across the 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).

7. These receptors may be ligand gated sodium ion channels which allow Na+ to enter the postsynaptic neuron (or muscle) and triggers an action potential in the postsynaptic neuron (or muscle contraction).

8. Once the neurotransmitters are released they need to be destroyed or contained quickly or they will continue to stimulate the nerve.

. Acetylcholine . Acts in both the PNS and the CNS as a neurotransmitter. . Causes voluntary muscles to contract. . Acetylcholinesterase.

. Myasthenia gravis is an autoimmune disease that attacks the acetylcholine receptors, resulting in reduced muscle strength.

Important Concepts

. Read Chapter 8

. What are the functions of nervous system

. What are the two types of cells in nervous tissue (neuroglial cells and neurons).

. What are the three types of neuroglial cells and their functions

. What are the two main divisions of nervous system (CNS, PNS) and where each is found

. What are the parts and functions of a neuron

. What are the three types of neurons (sensory, interneuron and motor neurons) and their functions, and where are they located

. Where are the cell bodies are located for motor and sensory nerve cells

. Where Schwann vs oligodendrocytes are found

. What is the cause and effects of multiple sclerosis

. What are the parts of a nerve

. What is the function of the sodium potassium pump

. What are the steps of messages being conducted through a neuron, starting with the resting stage and ending with the next neuron or muscle being stimulated.

Copyright © 2009 Pearson Education, Inc. Important Concepts . What ions enter and the leave the neuron during the depolarization and repolarization steps of action potential, what is the relative charge of the inside vs the outside of the neuron during these events, what is the order of events.

. Components of the synapse

. Function of neurotransmitters, how do they work, where do they work, know the ions involved and their functions.

. What is acetylcholine, where is it found, what effect does it have, how is acetylcholine removed from the synaptic cleft

. What is the cause and effect of Myasthenia gravis

Copyright © 2009 Pearson Education, Inc. Definitions . Afferent neurons, efferent neurons, dendrites, axons, sensory neurons, interneuron, motor neurons, myelin, myelin sheath, myelinated neurons, schwann cells, oligodendrocytes, nodes of ranvier, nerve, ions, ion channels, ligand gated ion channels, voltage gated ion channels, action potential, repolarization, depolarization, membrane potential, resting potential, sodium potassium pump, refractory period, synapse, synaptic cleft, synaptic vesicles, neurotransmitters, acetylcholinesterase, presynaptic neuron, postsynaptic neuron, stimulate, inhibit