KAAP430 Chapter Notes - Chapter 3: Axon Hillock, Neurotransmitter Receptor, Axon Terminal
21 Mar 2017
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Chapter 3: Neural Control of Exercising Muscle
Structure and Function of the Nervous System
Neuron
❖ Basic structural unit of the NS.
❖ Allows electrical impulses to be transmitted throughout the body.
3 Regions of a Neuron
❖ Dendrites.
➢ Many dendrites.
➢ Receives most impulses (APs) that enter neuron from sensory stimuli or from
adjacent neuron.
▪ Carries impulses cell body.
❖ Cell body (soma).
➢ Contains nucleus.
❖ Axon.
➢ 1 axon.
➢ Carries impulses away from cell body.
➢ Branches end branches axon terminals.
▪ Axon terminals contain vesicles filled with neurotransmitters.
▪ Neurotransmitters communication between a neuron & another cell.
Nerve Impulse
❖ Arises when a stimulus is strong enough to change the electrical charge of a neuron.
❖ Signal moves along neuron:
➢ Down axon.
➢ Toward end organ (another neuron, group of muscle gibers).
Resting Membrane Potential (RMP)
❖ Negative electrical potential of resting neuron: -70 mV.
➢ Negative relative to outside.
➢ Polarized.
❖ Caused by uneven separation of charged ions across membrane.
❖ Maintaining RMP: more positive outside than inside inside is negative relative to
outside.
➢ High K+ concentration inside the membrane.
▪ Membrane is permeable to K+ K+ goes outside membrane.
➢ High Na+ concentration outside the membrane.
▪ Membrane is not permeable to Na+ Na+ remains outside membrane.
➢ Sodium-potassium pumps (located in neuron membrane).
▪ Actively transport 2 K+ in.
▪ Actively transport 3 Na+ out.
Depolarization and Hyperpolarization
❖ Depolarization – inside: less negative relative to outside decreased membrane
potential (decreased polarization).
➢ Occurs any time charge difference more positive than RMP (-70 mV).
➢ Change in membrane permeability to Na+ depolarization.
❖ Hyperpolarization – inside: more negative relative to outside increased membrane
potential (increased polarization).
➢ Occurs any time charge difference more negative than RMP (-70 mV).
find more resources at oneclass.com
find more resources at oneclass.com
Chapter 3: Neural Control of Exercising Muscle
Graded Potentials (GPs)
❖ Localized changes in membrane potential:
➢ Depolarization.
➢ Hyperpolarization.
❖ Enough stimulation ion gates open allows more ions to move in & out of soma.
➢ Triggered by change in neuron’s local environment.
➢ Ion gates may open in response to:
▪ Transmission of impulse from anther neuron
▪ Sensory stimuli (changes in chemical concentrations, temperature, pressure).
❖ Most neuron receptors: located on dendrites.
➢ Impulse always transmitted from axon terminals.
➢ Impulse will be transmitted if and only if it is transmitted along the entire length
of a neuron.
▪ GP may cause depolarization but not necessarily strong enough travel
along entire length of neuron.
▪ In order to travel along the full length: impulse must be sufficiently strong
generate AP.
Action Potentials (APs)
❖ Rapid, substation depolarization of the neuron’s membrane.
❖ Membrane potential: -70mV +30 mV & then quickly returns to RMP.
➢ Lasts only about 1 ms.
❖ Depolarization threshold – membrane voltage: GP AP.
➢ Any depolarization that does not attain threshold no AP.
➢ All-or-none principle: as soon as depolarization reaches, exceeds threshold
AP.
❖ Absolute refractory period – segment of axons’ Na+ gates are open (in the process
of generating AP) cannot respond to another stimulus.
❖ Relative refractory period – segment of axons’ K+ gates are open (in the process of
repolarizing) may respond to another stimulus.
➢ Na+ gates are closed.
➢ Stimulus must be >>>> AP.
❖ Propagation of the AP: how it travels along the neuron.
➢ Myelination – axons are covered with myelin sheath (formed by Schwann cells).
▪ Not all neurons are myelinated.
▪ Myelin sheath: not continuous along length of axon.
• Nodes of Ranvier – gaps between myelinated segments of axon.
• Salutatory conduction – AP jumps from node-to-node faster conduction
that in unmyelinated axons.
▪ Occurs over 1st several years of life: children need time develop
coordinated movement.
➢ Diameter of the neuron: direct relationship with speed of propagation.
▪ Larger neuron diameter less resistance to local current flow increased
speed of propagation.
▪ Smaller neuron diameter more resistance to local current flow
decreased speed of propagation.
find more resources at oneclass.com
find more resources at oneclass.com
Chapter 3: Neural Control of Exercising Muscle
Synapse
❖ Site of AP transmission: axon terminal of 1 neuron dendrites of another neuron.
❖ Signal transmitted: electrical chemical electrical.
Components of a Synapse
❖ Axon terminals of presynaptic neuron – neuron sending AP.
❖ Receptors on postsynaptic neuron – neuron receiving AP.
❖ Synaptic cleft – space between structures.
Transmission of Action Potential (AP)
❖ Occurs in only 1 direction.
❖ Presynaptic axon terminals contain a lot of vesicles which contain neurotransmitters.
➢ Impulse reaches presynaptic axon terminals synaptic vesicles release
neurotransmitters into synaptic cleft.
➢ Neurotransmitters diffuse across synaptic cleft postsynaptic neuron receptors.
❖ Each neurotransmitter binds to specialized postsynaptic receptor.
➢ Sufficient binding series of GP depolarization.
➢ Depolarization reaches threshold AP.
▪ Impulse has been transmitted successfully to the next neuron.
❖ Depolarization of 2nd nerve depends on:
➢ Amount of neurotransmitter released.
➢ # of available receptor binding sites on postsynaptic neuron.
Neuromuscular Junction
❖ Motor unit – single-alpha motor neuron & all muscle fibers it innervates.
❖ Alpha-motor neuron communicates with muscle fibers at neuromuscular junction.
➢ Functions in the same manner as a synapse.
Components of a Neuromuscular Junction
❖ Axon terminals of presynaptic motor neuron.
❖ Motor end plates of postsynaptic muscle fiber.
❖ Space between motor nerve & muscle fiber.
Transmission of Action Potential (AP)
❖ Acetylcholine (ACh) released from alpha-motor neuron axon terminals.
❖ ACh diffuses across synaptic cleft binds to receptors on muscle fiber’s
plasmalemma.
❖ Binding depolarization.
➢ Opens Na+ ion channels.
➢ Depolarization reaches threshold AP.
▪ AP spreads across plasmalemma into T-tubules initiates muscle
contraction.
❖ Repolarization: Na+ gates close while K+ gates open.
➢ Refractory period where muscle cannot respond to another stimulus.
▪ Limits motor unit’s firing frequency.
➢ RMP completely restored fiber can respond to another stimulus.
Neurotransmitters
❖ Neurotransmitter binds to postsynaptic receptor neurotransmitter :
➢ Degraded by enzymes.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Allows electrical impulses to be transmitted throughout the body. Receives most impulses (aps) that enter neuron from sensory stimuli or from adjacent neuron: carries impulses cell body. Branches end branches axon terminals: axon terminals contain vesicles filled with neurotransmitters, neurotransmitters communication between a neuron & another cell. Arises when a stimulus is strong enough to change the electrical charge of a neuron. Toward end organ (another neuron, group of muscle gibers). Negative electrical potential of resting neuron: -70 mv. Caused by uneven separation of charged ions across membrane. Maintaining rmp: more positive outside than inside inside is negative relative to outside. High k+ concentration inside the membrane: membrane is permeable to k+ k+ goes outside membrane. High na+ concentration outside the membrane: membrane is not permeable to na+ na+ remains outside membrane. Sodium-potassium pumps (located in neuron membrane): actively transport 2 k+ in, actively transport 3 na+ out. Depolarization inside: less negative relative to outside decreased membrane potential (decreased polarization).
