Download Action potentials

chapter
3
Neural Control of
Exercising Muscle
Learning Objectives
• Learn the basic structures of the nervous system
• Follow the pathways of nerve impulses from initiation to
muscle action
• Discover how neurons communicate with one another
and learn the role of neurotransmitters in this
communication
(continued)
Learning Objectives (continued)
• Understand the functional organization of the central
nervous system
• Become familiar with the roles of the sensory and motor
divisions of the peripheral nervous system
• Learn how a sensory stimulus gives rise to a motor
response
• Consider how individual motor units respond and how
they are recruited in an orderly manner depending on
the required force
Organization of the Nervous System
Neurons
Insert Figure 3.2 b
Nerve Impulse
A nerve impulse—an electrical charge—is the signal
that passes from one neuron to the next and finally to
an end organ, such as a group of muscle fibers, or
back to the CNS
Resting Membrane Potential (RMP)
• Difference between the electrical charges inside and
outside a cell, caused by separation of charges across
the cell membrane
• High concentration of K+ inside of the neuron and Na+
on the outside of the neuron
• Cell is more permeable to K+, thus K+ ions can move
more freely
• In an attempt to establish equilibrium, K+ will move
outside the cell
• Sodium-potassium pump actively transports K+ into and
Na+ out of the cell to maintain the RMP
• RMP is maintained at –70mV
Changes in Membrane Potential
Depolarization occurs when inside of cell becomes
less negative relative to outside and is caused by a
change in the membrane’s Na+ permeability (>–70 mV)
Hyperpolarization occurs when inside of cell becomes
more negative relative to outside (<–70 mV)
Graded potentials are localized changes in membrane
potential (either depolarization or hyperpolarization)
Action potentials are rapid, substantial depolarizations
of the cell membrane (–70 mV → +30 mV → –70 mV in
1 ms)
What Is an Action Potential?
• All action potentials begin as graded potentials
• Requires depolarization greater than the threshold
value of 15 mV to 20 mV to be initiated
• The membrane voltage at which a graded potential
becomes an action potential is called the depolarization
threshold
• Once threshold is met or exceeded, the all-or-none
principle applies and an action potential results
Refractory Periods
不反應期
Absolute refractory period
– When a given segment of an axon is generating an
action potential, its sodium gates are open and it is
unable to respond to another stimulus
Relative refractory period
– When the sodium gates are closed, the potassium
gates are open, and repolarization is occurring, the
segment of the axon can respond to a new stimulus,
but the stimulus must be substantially greater to
evoke an action potential
Events During an Action Potential
1.
2.
3.
4.
5.
Resting state
Depolarization
Propagation of an action potential
Repolarization
Return to the resting state with the help of the
sodium-potassium pump
Voltage and Ion Permeability Changes
During an Action Potential
Fig. 8.9, p. 259 from HUMAN PHYSIOLOGY, 4th ed. By Dee Unglaub Silverthorn. Copyright © 2007 by Pearson
Education, Inc. Reprinted by permission.
The Velocity of an Action Potential
Myelinated fibers
• Saltatory conduction—action potential travels quickly
from one node of Ranvier to the next
• Action potential is faster in myelinated fibers than in
unmyelinated fibers
Diameter of the neuron
• Larger-diameter neurons conduct nerve impulses faster
due to less resistance to the current flow
The Nerve Impulse
Key Points
• A neuron’s RMP of –70 mV results from a separation of
Na+ and K+ ions and is actively maintained by the
sodium-potassium pump
• Changes in membrane potential occur when ion gates
in the membrane open, permitting ions to move from
one side to the other
- Depolarization (membrane potential becomes
less negative)
- Hyperpolarization (membrane potential becomes
more negative)
• If the membrane potential depolarizes by 15 mV to 20
mV, the threshold is reached, resulting in an action
potential
(continued)
The Nerve Impulse (continued)
Key Points
• In myelinated neurons, the impulse travels through the
axon by jumping between nodes of Ranvier in a
process called saltatory conduction
• Nerve impulses travel faster in myelinated axons and
in neurons with larger diameters
The Synapse
• A synapse is the site of an impulse transmission from
one neuron to another
• An impulse travels to a presynaptic axon terminal,
where it causes synaptic vesicles on the terminal to
release neurotransmitters into the synaptic cleft
• Neurotransmitters bind to postsynaptic receptors on the
postsynaptic neuron
A Chemical Synapse
The Neuromuscular Junction
• The site where an -motor neuron communicates with
a muscle fiber
• Axon terminal releases neurotransmitters which travel
across a synaptic cleft and bind to receptors on a
muscle fiber’s plasmalemma
• Neurotransmitter binding causes depolarization, and
once a threshold is reached, an action potential occurs
• The action potential spreads across the sarcolemma,
causing the muscle fiber to contract
The Neuromuscular Junction
Refractory Period
• Period of repolarization
• The muscle fiber is unable to respond to any further
stimulation
• The refractory period limits a motor unit’s firing
frequency
Neurotransmitters
• Categories of Neurotransmitters
– Small molecule, rapid-acting
– Neuropeptide, slow-acting
• Common Neurotransmitters
– Acetylcholine is the primary neurotransmitter for the
motor neurons that innervate skeletal muscle and
most parasympathetic nerve endings
– Norepinephrine is the neurotransmitter for most
sympathetic neurons
Synapses
Key Points
• Neurons communicate with one another by releasing
neurotransmitters across synapses
• Synapses involve a presynaptic axon terminal,
neurotransmitters, a postsynaptic receptor, and the
synaptic cleft
• Once sufficient amounts of neurotransmitter bind to
the receptors, depolarization (excitation) or
hyperpolarization occurs, depending on the specific
neurotransmitter inhibition the site to which it binds
• Neurotransmitters are destroyed by enzymes,
removed by reuptake into the presynaptic terminal, or
diffused away from the synapse
Neuromuscular Junctions
Key Points
• Neurons communicate with muscle cells at
neuromuscular junctions
• A neuromuscular junction involves presynaptic axon
terminals, the synaptic cleft, and motor end-plate
receptors on the plasmalemma (plasma membrane)
• The neurotransmitters most important in regulating
exercise are acetylcholine and norepinephrine
The Postsynaptic Response
• Excitatory postsynaptic potentials (EPSPs) are
depolarizations of the postsynaptic membrane
• Inhibitory postsynaptic potentials (IPSPs) are
hyperpolarizations of the membrane
• A summation of impulses is necessary to generate an
action potential and is monitored at the axon hillock
Central Nervous System
Brain: 4 Major Regions
• Cerebrum is the site of the mind and intellect
• Diencephalon is composed of the thalamus and
hypothalamus and is the site of sensory integration
and regulation of homeostasis
• Cerebellum plays a crucial role in coordinating
movement
• Brain stem is composed of the midbrain, pons橋, and
the medulla oblongata延 and connects brain to spinal
cord; it contains regulators of the respiratory and
cardiovascular systems
Central Nervous System
Spinal cord
• Afferent fiber carry neural signals from sensory
receptor, such as those in the skin, muscle, and
joints, to the upper levels of the CNS.
• Motor (efferent) fibers from the brain and upper
spinal cord transmit action potentials to end organ
(eg., muscle and glands)
Four Major Regions of the Brain and Four
Outer Lobes of the Cerebrum
Peripheral Nervous System
• Sensory division carries sensory information
from the body via afferent fibers to the CNS
• Motor division transmits information from CNS
via efferent fibers to target organs (fig 3.1)
Peripheral Nervous System:
Sensory Division
• Mechanoreceptors respond to mechanical forces such
as pressure, touch, vibrations, and stretch
• Thermoreceptors respond to changes in temperature
• Nociceptors respond to painful stimuli
• Photoreceptors respond to light to allow vision
• Chemoreceptors respond to chemical stimuli from
foods, odors, and changes in blood concentrations
Muscle and Joint Nerve Endings
• Kinesthetic receptors in joint capsules sense the
position and movement of joints
• Muscle spindles sense how much a muscle is
stretched
• Golgi tendon organs detect the tension of a muscle
on its tendon, providing information about the
strength of muscle contraction
Peripheral Nervous System:
Motor Division
Autonomic Nervous System
• Sympathetic Nervous System
• Parasympathetic Nervous System
The effects of the two systems are often antagonistic,
but the systems always function together
Sympathetic Nervous System
Fight-or-flight response prepares the body to face
crisis and sustains its function during that crisis
Effects of the SNS
• Increases heart rate and strength of heart contraction
• Increases blood supply to the heart and active muscles
• Increases vasoconstriction to inactive vascular beds
• Increases metabolic rate
• Increases glucose release from the liver
• Increases blood pressure
• Causes bronchodilation支氣管擴大作用 to improve gas exchange
• Improves mental activity and quickness of response
• Other functions not directly needed are slowed
Parasympathetic Nervous System
Housekeeping: digestion, urination, glandular腺體
secretion, and energy conservation
Actions oppose those of the sympathetic system:
• Decreases heart rate
• Constricts coronary vessels
• Bronchoconstriction in the lungs支氣管收縮
Peripheral Nervous System
Key Points
• The peripheral nervous system contains 43 pairs of
nerves: 12 cranial and 31 spinal
– Sensory
– Motor (includes autonomic)
• The sensory division carries information from the
sensory receptors to the CNS
• The motor division carries motor impulses from the
CNS to the muscles, organs, and other tissues
• The autonomic nervous system includes
– Sympathetic (fight or flight)
– Parasympathetic (housekeeping)
Sensory Motor Integration
1. A sensory stimulus is received by sensory receptors
2. The sensory action potential is transmitted along
sensory neurons to the CNS
3. The CNS interprets the incoming sensory information
and determines the most appropriate reflex response
4. The action potentials for the response are transmitted
from the CNS along -motor neurons
5. The motor action potential is transmitted to a muscle,
and the response occurs
Integration Centers
整合中心
Spinal cord controls simple motor reflexes
Lower brain stem controls more complex
subconscious motor reactions
Cerebellum governs subconscious control of
movement
Thalamus governs conscious distinction among
sensations
Cerebral cortex maintains conscious awareness of a
signal and the location of the signal within the body
The Sequence of Events in
Sensory-Motor Integration
Sensory Receptors and Their Pathways
Back to the Spinal Cord and Brain
Motor Control
• Motor responses can originate from any one of
three levels
– Spinal cord
– Lower regions of the brain
– Motor areas of the cerebral cortex
• Motor responses for more complex movement
patterns typically originate in the motor cortex
• A motor reflex is integrated by the spinal cord
without conscious thought
Muscle Spindles
• Lie between regular skeletal muscle fibers
• The middle of the spindle cannot contract but can
stretch
• When muscles attached to the spindle are stretched,
neurons on the spindle transmit information to the
CNS about the muscle’s length, and the rate at
which the length is changing
• Reflexive muscle contraction is triggered to resist
further stretching
Golgi Tendon Organs (GTOs)
• Encapsulated sensory organs through which a
small bundle of muscle tendon fibers pass
• Located proximal to the tendon’s attachment to the
muscle
• Sensitive to changes in tension
• Inhibit contracting (agonist) muscles and excite
antagonist muscles to prevent injury (muscle
contraction)
Muscle Spindles and GTOs
(a) A muscle belly, (b) a muscle spindle, and (c) a Golgi tendon organ
Page 368 from HUMAN PHYSIOLOGY, 4th ed. By Dee Unglaub Silverthorn. Copyright © 2007 by Pearson Education,
Inc. Reprinted by permission.
Higher Brain Centers
• Primary motor cortex: controls fine and discrete不連接的
muscle movement ( fig 3.6 frontal lobe)
– Premotor cortex: controls learned motor skills of a
repetitious pattern
• Basal ganglia大腦白質區: important in initiating movement
of a sustained and repetitive nature (walking and
running)
• Cerebellum: integration system that controls rapid and
complex muscular activity and facilitates movement
patterns by smoothing out the movement
– Receives visual and equilibrium input
Sensory-Motor Integration
Key Points
• Sensory-motor integration is the process by
which the PNS relays sensory input to the CNS;
the CNS interprets this information and then
sends out an appropriate motor signal to elicit the
desired motor response
• Sensory input can terminate at various levels of
the CNS (Spinal cord, Lower regions of the brain, Motor areas of the cerebral cortex)
• Reflexes are the simplest form of motor control
(not conscious responses)
(continued)
Sensory-Motor Integration (continued)
Key Points
• Muscle spindles trigger reflexive muscle action
when the muscle spindle is stretched
• Golgi tendon organs trigger a reflex that inhibits
contraction if the tendon fibers are stretched from
high muscle tension
• The primary motor cortex, located in the frontal lobe,
is the center of conscious motor control
• The basal ganglia大腦白質區help initiate some movement
and help control posture and muscle tone
• The cerebellum is an integration center that is
involved in all rapid and complex movement
processes
Control of Small vs. Large
Motor Responses
Muscles controlling fine movements, such as those
controlling the eyes, have a small number of muscle
fibers per motor neuron (about 1 neuron for every 15
muscle fibers) finger 1 neuron for 4 muscle fibers.
Muscles with more general function, such as those
controlling the calf muscle in the leg, have many fibers
per motor neuron (about 1 neuron for every 2,000
muscle fibers).