2. 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
3. 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
6. 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
7. 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
8. Resting Membrane Potential (RMP)
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
9. 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)
10. Changes in Membrane Potential
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)
11. 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
12. Refractory Periods
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
13. Events During an Action Potential
1. Resting state
2. Depolarization
3. Propagation of an action potential
4. Repolarization
5. Return to the resting state with the help of the
sodium-potassium pump
14. Voltage and Ion Permeability Changes
During an Action Potential
15. 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.
16. 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.
17. 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
18. 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.
20. The Neuro muscular 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
22. 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
23. 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
24. 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.
25. 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
26. 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
27. 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, and the
medulla oblongata and connects brain to spinal cord; it
contains regulators of the respiratory and cardiovascular
systems
28. 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)
30. 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
31. 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
32. 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
33. 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
34. 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 bronchodilationto improve gas exchange
• Improves mental activity and quickness of response
• Other functions not directly needed are slowed
35. Parasympathetic Nervous System
Housekeeping: digestion, urination, glandularsecretion,
and energy conservation
Actions oppose those of the sympathetic system:
• Decreases heart rate
• Constricts coronary vessels
• Bronchoconstriction in the lungs
36.
37. 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)
38. 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 a-motor neurons
5. The motor action potential is transmitted to a muscle, and
the response occurs
39. Integration Centers
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
42. 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
43. 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
44. 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)
45. Muscle Spindles and GTOs
(a) A muscle belly, (b) a muscle spindle, and (c) a Golgi tendon organ
46. Higher Brain Centers
Primary motor cortex: controls fine and discrete muscle
movement ( fig 3.6 frontal lobe)
– Premotor cortex: controls learned motor skills of a
repetitious pattern
Basal ganglia: 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
47. 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)
48. 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 gangliahelp 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
49. 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).
