Learning Objectives

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Learning Objectives
w Learn the basic structures of the nervous
system.
w Follow the pathways of nerve impulses from
initiation to muscle action.
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Learning Objectives
w Understand the functional organization of the
central nervous system.
w Become familiar with the roles of the sensory
and motor divisions of the peripheral nervous
system.
w Learn how a sensory stimulus gives rise to a
motor response.
w Consider how individual motor units respond and
how they are recruited in an orderly manner
depending on the required force.
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THE STRUCTURE OF A NEURON
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Nerve Impulse
An electrical charge that passes from one neuron
to the next and finally to an end organ, such as
a group of muscle fibers.
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Resting Membrane Potential (RMP)
w Difference between the electrical charges
inside and outside a cell, caused by separation
of charges across a membrane
w High concentration of K inside the neuron and
Na outside the neuron
w K ions can move freely, even outside the cell
to help maintain imbalance
w Sodium-potassium pump actively transports K
and Na ions to maintain imbalance
w The constant imbalance keeps the RMP at 70mV
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Changes in Membrane Potential
Depolarizationinside of cell becomes less
negative relative to outside (gt 70 mV)
Hyperpolarizationinside of cell becomes more
negative relative to outside (lt 70 mV)
Graded potentialslocalized changes in membrane
potential (either depolarization or
hyperpolarization)
Action potentialsrapid, substantial
depolarization of the membrane (70 mV to 30 mV
to 70 mV all in 1 ms)
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What Is an Action Potential?
w Starts as a graded potential

• Requires depolarization greater than the
  threshold value
• 15 mV to 20 mV increase needed

w Once threshold is met or exceeded, the
all-or-none principle applies
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RESTING STATE
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Events During an Action Potential
1. The 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
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AN ACTION POTENTIAL
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The Velocity of an Action Potential
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Excitatory and Inhibitory Impulses
Neuron responds based on the overall membrane
effect
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Excitatory and Inhibitory Impulses
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Key Points
The Nerve Impulse
w A neuron's RMP of 70 mV is maintained by the
sodium-potassium pump.
w Changes in membrane potential occur when ion
gates in the membrane open, permitting ions to
move from one side to the other.
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Key Points
The Nerve Impulse
w Impulses travel faster in myelinated axons and
in neurons with larger diameters.
w Saltatory conduction refers to an impulse
traveling along a myelinated fiber by jumping
from one node of Ranvier to the next.
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The Synapse
w A synapse is the site of an impulse
transmission between two neurons.
w An impulse travels to a presynaptic axon
terminal where it causes synaptic vesicles on
the terminal to release chemicals into the
synaptic cleft.
w Chemicals are picked up by postsynaptic
receptors on an adjacent neuron.
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THE CHEMICAL SYNAPSE
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The Neuromuscular Junction
w The junction is a site where a motor neuron
communicates with a muscle fiber.
w Motor axon terminal releases neurotransmitters
(such as acetylcholine or epinephrine) which
travel across a synaptic cleft and bind to
receptors on a muscle fiber.
w This binding causes depolarization, thus
possibly causing an action potential.
w The action potential spreads across the
sarcolemma causing the muscle fiber to contract.
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THE NEUROMUSCULAR JUNCTION
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Refractory Period
w Period of repolarization.
w The muscle fiber is unable to respond to any
further stimulation.
w The refractory period limits a motor unit's
firing frequency.
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Key Points
Synapses
w Neurons communicate with one another by
releasing neurotransmitters across synapses.
w Synapses involve a presynaptic axon terminal, a
postsynaptic receptor, neurotransmitters, and the
space between them.
w Neurotransmitters bind to the receptors and
cause depolarization (excitation) or
hyperpolarization (inhibition) depending on the
specific neurotransmitter and the site to which
it binds.
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Key Points
Neuromuscular Junctions
w Neurons communicate with muscle cells at
neuromuscular junctions, which function much like
a neural synapse.
w The refractory period is the time it takes the
muscle fiber to repolarize before the fiber can
respond to another stimulus.
w Acetylcholine and epinephrine are the
neurotransmitters most important in regulating
exercise.
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Key Points
The Postsynaptic Response
w Binding of a neurotransmitter causes a graded
action potential in the postsynaptic membrane.
w An excitatory impulse causes hyperpolarization
or depolarization.
w An inhibitory impulse causes hyperpolarization.
w The axon hillock keeps a running total of the
neuron's responses to incoming impulses.
w A summation of impulses is necessary to
generate an action potential.
The Behaving Brain
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Central Nervous System
Spinal cord
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Peripheral Nervous System
w 12 cranial nerves connected with the brain.
w 31 spinal nerves connected with the spinal cord.
w Sensory divisioncarries sensory info from body
via afferent fibers to the CNS.
w Motor divisiontransmits information from CNS
via efferent fibers to target organs.
w Autonomic nervous systemcontrols involuntary
internal functions.
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THE NERVOUS SYSTEMS
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Types of Sensory Receptors
Mechanoreceptorsrespond to mechanical forces
such as pressure, touch, vibrations, or stretch.
Thermoreceptorsrespond to changes in temperature.
Nociceptorsrespond to painful stimuli.
Photoreceptorsrespond to light to allow vision.
Chemoreceptorsrespond to chemical stimuli from
foods, odors, and changes in blood concentrations.
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Muscle and Joint Nerve Endings
w Joint kinesthetic receptors in joint capsules
sense the position and movement of joints.
w Muscle spindles sense how much a muscle is
stretched.
w Golgi tendon organs detect the tension of a
muscle on its tendon, providing information
about the strength of muscle contraction.
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SENSORY-MOTOR INTEGRATION
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Integration Centers
Spinal cordsimple motor reflexes such as pulling
your hand away after touching something hot.
Lower brain stemmore complex subconscious motor
reactions such as postural control.
Cerebellumsubconscious control of movement such
as that needed to coordinate multiple movements.
Thalamusconscious distinction among sensations
such as feeling hot or cold.
Cerebral cortexconscious awareness of a signal
and the location within body of the signal.
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SENSORY RECEPTORS AND PATHWAYS
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Motor Control
w Sensory impulses evoke a response through a
motor neuron.
w The closer to the brain the impulse stops, the
more complex the motor reaction.
w A motor reflex is a preprogrammed response that
is integrated by the spinal cord without
conscious thought.
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Muscle Spindles
w Lie between and are connected to regular
skeletal muscle fibers.
w The middle of the spindle cannot contract but
can stretch.
w When muscles attached to the spindle are
stretched, neurons on the spindle transmit
information to the CNS about the muscle's length.
w Reflexive muscle contraction is triggered to
resist further stretching.
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Lestienne Reading

• States that any theory needs to be able to
  address the posture-movement problem
• Why when we move away from an initial posture do
  we not have resistance from posture stabilization
  mechanisms
• CNS Organizes using Frames of Reference (FR) or
  systems of coordinates
• Changes in one FR impacts all others involved
• CNS can manipulate the parameters of the FR and
  thereby guide motor actions (affordances)

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Sub-Threshold Activation

• Highly important to motor control.
• Allows for responses to unexpected perturbations
• Feedforward preparation of the motor units allow
  for more rapid response to perturbations
• Reflexes play into this stretch reflex can
  operate at an increased sensitivity level
  (responds quicker)

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Central Pattern Generators and Reflexes

• Reflexes play an important role in movement
  generation
• Proprioceptive important questioned in
  deafferentation studies
• Complicated by sensory adaptation for lost
  system(s)
• When absent proprioceptions role can be seen in
  lost movement quality

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Theories of Motor Control

• Sherrington Reflex Theory
• States that movement are generated as a chain of
  reflexes
• Involuntary stereotypic responses
• Force Control Theories
• E.g., motor programming type theories
• Brain computes activation levels based on inverse
  dynamics
• Difficulty in explaining adaptation to
  perturbations and the inherent variability within
  the systems

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Theories of Motor Control

• Lamda Model
• Shifts muscle activation thresholds (resetting
  the muscle at a new posture) to
  accommodate/permit movements
• When error is produced error the contrast between
  the effector and the target
• Driving a car analogy
• Gas, brake, steering wheel are all factors
  various lamdas or muscle activation thresholds.

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Next Class

• Look further into the function of the CNS in
  processing sensory information and its effect on
  motor control
• Start to look at the influence of the spinal cord
  on movement generation
• Task for next class Find an abstract on (choose
  one)
• Spinal cord and movement control
• Cerebellum and movement control
• Basal Ganglia and movement control
• Motor or premotor cortex and movement control