Download Cellular and Systems Neurophysiology Part 13: The Motor

Biology for Engineers:
Cellular and Systems Neurophysiology
Christopher Fiorillo
BiS 521, Fall 2009
042 350 4326, [email protected]
Part 13: The Motor System
Reading: Bear, Connors, and Paradiso
Chapters 13 and 14
Anatomical Terms
•These terms are explained in chapter 7.
Memorize them.
•Rostral-Caudal (anterior-posterior)
•Dorsal-Ventral
•Medial-Lateral
•Coronal plane
•Horizontal plane
•Sagittal plane
Cortical Anatomy
•The frontal cortex (lobe) is greatly
expanded in humans relative to other
animals.
•Memorize the relative locations of all of
these structures. I will ask questions
about this on the exam. These questions
will be very easy if you can simply
memorize.
•You should also know the location of
the cerebellum. It is very important for
motor function (and very easy to find).
Cortical Anatomy
•Primary motor cortex (area 4)
is just rostral (anterior) of the
central sulcus
•It receives many inputs from
primary somatosensory cortex
and from prefrontal cortex
•Somatosensory information is
highly relevant to motor output,
even without much processing
•Prefrontal cortex contains highly
processed information from all
sensory modalities, and its
information is highly relevant to
motor output
•Thus it may be efficient to have
motor cortex located between
these two areas, and further from
primary auditory and visual
cortices, in order to keep
“wires” (axons) short
Basal Ganglia
•The basal ganglia are an interconnected set of motor-related subcortical nuclei
•Basal ganglia receive input from the cortex
•They send their output to the thalamus, which then sends the information back
to the cortex
•The basal ganglia receive a lot of dopamine from substantia nigra in the midbrain
•Loss of dopamine in the basal ganglia causes Parkinson’s disease
•Dopamine in the basal ganglia is probably important for learning “action values”
•The striatum is the largest nucleus within the basal ganglia
The Spinal Cord
•The spinal cord is part of the Central Nervous System
•It contains both somatosensory and motor neurons, and many
interneurons
Anatomy of the Spinal Cord
• The Dorsal Root Ganglion contains primary somatosensory neurons
•The Dorsal Horn is somatosensory
•The Ventral Horn is motor, and contains motor neurons
•The Ventral Root contains the axons of motor neurons
Inputs to Motor Neurons
• Excitatory and inhibitory interneurons of the spinal cord
• Primary proprioceptive neurons (excitatory)
–
Cell bodies are in dorsal root ganglion
• Descending inputs (excitatory)
Inputs to Motor Neurons
• Descending inputs
(excitatory) to motor
neurons
–
Corticospinal
•
–
–
–
–
From layer 5 of motor
cortex
Rubrospinal
Tectospinal
Vestibulospinal
Reticulospinal
Motor Neuron Output to Muscle
The Alpha Motor Neurons
These neurons directly control body
movements
The Gamma motor neurons assist in
proprioception
One muscle fiber gets input from
one motor neuron
One motor neuron may innervate
multiple muscle fibers. One motor
neuron plus its muscle fibers is
called a “motor unit.”
This is the smallest unit of motor
output
All the motor neurons that
innervate the same muscle are
called a “motor neuron pool”
Pathways from Sensory Input to Motor Output
A single motor neuron gets inputs, indirectly,
from a very large part of the nervous system
There are long paths and short paths that
connect sensory inputs to motor outputs
A long path is needed when sensory inputs, such
as light intensity, carry very little information
about future reward
A motor neuron must “decide” what firing rate
(and contraction of muscle) will maximize
reward.
What unprocessed sensory information should be
most relevant in “deciding” upon a motor
neuron’s optimal output (and thus have a short
path)?
The answer: Proprioceptive information indicating the
position of the relevant limb, and muscle tension
Vestibular information is also important, indicating
head position, velocity, and acceleration
Sudden pain in the same region of the body
Each of these types of sensory information takes a
short path to the motor neuron
The Myotatic (Stretch) Reflex
A 1a proprioceptive neuron is activated by stretch of a muscle
Stretch of a muscle provides information about the position of the limb that the muscle controls
The 1a axon directly excites motor neurons that innervates the same muscle
This is the shortest sensory-motor path in the central nervous system (at least for control of muscles,
maybe not for hormone outputs from the hypothalamus)
One effect of this would be to stabilize the limb position when the external
force on the limb changes. This would usually be beneficial.
Inhibition tends to occur at the same time as excitation in motor neurons
• The data below are from motor neurons that are mediating repetitive scratching behavior in a
turtle
• It seemed strange to most people that inhibition and excitation should occur at the same time
• That is why this was published in Science
• This observation fits well with the theory
•
•
Inhibitory inputs predict the excitatory inputs
This could be accounted for by anti-Hebbian plasticity of inhibitory inputs
• The same phenomena has been observed in other types of neurons, including retina, visual cortex,
and auditory cortex
Berg et al., Science, 2007
Motor Patterns
• Not all reward information needs to
come from the senses
• Patterns could be internally generated,
and then tested to see if they predict
reward
• Evidence that patterns (information)
may be internally generated
– Motor systems generate patterns (and coordinated
behaviors) prior to development of sensory
systems
– Central Pattern Generators
Central Pattern Generators
A “CPG” generates a pattern even without
receiving any patterned input
Thus a pattern can be generated in the complete
absence of sensory inputs, or when sensory input is
constant
For example, a simple pattern could be a
sinusoidal oscillation
If the pattern is very regular, like a
heartbeat, then it is called a “pacemaker”
If there are several action potential at high
frequency, followed by a pause, that is
called “bursting”
Simple CPG patterns are useful for
repetitive events. The are involved in:
Walking and swimming
Respiration (breathing)
Gastrointestinal movements
CPGs have been studied extensively in
invertebrates, such as the stomatogastric
ganglion of the lobster shown at right
Marder and Bucher, 2007
Cellular Mechanism of a CPG
As described in the textbook, this example is from a type of neuron that is important for swimming in the
lamprey
Glutamate is necessary to generate this oscillatory pattern
However, glutamate concentration can remain constant, with no pattern
Other types of neurons oscillate with no external (synaptic) input at all
In the constant presence of glutamate, NMDA receptors function as depolarization-activated cation
channels. They depolarize the membrane, eliciting multiple action potentials. Calcium concentration
increases, via NMDA receptors and other calcium channels that open during each action potential.
Calcium then activates calcium-gated K+ channels, which hyperpolarize the membrane. Once it is
hyperpolarized, NMDA receptors and other calcium channels close, calcium concentration decreases, K+
channels close, and the membrane depolarizes again to start a new cycle.
Bistability in Striatal Neurons
• The membrane voltage
is stable at an “upstate” and a “downstate,” but not
inbetween
• A large EPSP will
cause the neuron to
enter the up-state. The
up-state will last for a
few hundred ms after
the EPSC has finished