Physiology 3120 Lecture Notes - Lecture 3: Somatotopic Arrangement, Brainstem, Biceps
9 May 2018
School
Department
Course
Professor
Physiology 3120
Dr.Hore
Lecture 11
Motor Cortex
Techniques that have been used to study the motor cortex
- Stimulation
o Electrical
o Magnetic
o Epilepsy
▪ Abnormal firing of neurons in the brain that can spread and stimulate nearby
areas
- Lesions
o Electrical
o Chemical
o Stroke
▪ Can be rupture of artery
▪ Can be something that blocks the artery so that blood cant flow)
o Degeneration of neurons in brain
- Recording
o Single neurons
o Evoke potentials
o EEG (electroencephalography)
o PET (positron emission tomography)
▪ Inject radioactive substance and positron are emitted from cortex in relation to
how active those areas are in terms of blood flow
o fMRI (functional magnetic resonance imaging)
- specialized areas of cerebral cortex:
o somatosensory
o visual
o auditory
o motor
- using fMRI, it was discovered that there are 180 separate areas/hemispheres with different
functions all these areas interlink with each other
Voluntary movement requires…
- Selection, Planning, Execution.
o Execution of movement coming from efferent movements
- This occurs across multiple structures in a distributed system that is in parallel and in series
that’s the planning of it
- These structures project to Motor Cortex (these are all just one step/link/axon away from the
motor cortex and they then instruct the motor cortex in terms of setting up the motor final
command that comes from the motor cortex to produce the movement
o Parietal Cortex
o Cerebellum
o Basal Ganglia via SMA (Supplementary Motor Area/Cortex)
o Pre-Motor Cortex
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Define Motor Cortex
- Area of Cerebral cortex from which low intensity electrical stimulation produces skeletal muscle
contractions which are contralateral (opposite side) for arm and leg
- Located on Precentral Gyrus
- You could go to the somatosensory cortex and put an electrode on it and put the volts up and
you will evoke a movement. Why? Bc its spreading to the output axons of the motor cortex and
generating a movement so key point is its very low threshold
- If you take that electrode (low threshold and low intensity) and put it anywhere else on brain,
you wont get a movement
- One of the major features of the motor cortex (same with somatosensory cortex) is that there is
somatotopic organization
- Somatotopic Organization:
o Distorted Motor
o Homunculus
o Femunculus
- Fingers and hand and face have a much larger representation that any other
part
- Its distorted in this way bc these areas have a lot of axons going out to the
finger/face region so they need a bigger area in the motor cortex
Evidence for somatotopic organization (distorted homunculus)
- March of epileptic seizure (Jackson ~1870)
o First piece of evidence
o He would have patients that have a twitch would go from finger to hand to shoulder to
face to leg
o He called it a march of epilepsy that seemed to recreate in the order of the body
o This led to the idea that somewhere up in brain there must be a representation of the
body bc epileptic seizure was moving across
o A few years later, in animal experiments it was confirmed that there was a motor cortex
that could be excited with electrical stimulation to produce movements
- Electrical stimulation (Penfield ~1950)
o He would take a piece of skull out to reveal precentral gyrus and would do experiments
o He would take the epileptic focus out from one region
o He then did Precise electrical stimulation of precentral gyrus and mapped it the
precentral gyrus of the human
- Lesions (strokes)
o If you made a lesion in the leg area, the person would be weak or paralyzed in leg
movement in opposite side of body
- Recordings of neural activity
o If you recorded someone who was flexing and extending their toes and you recorded it
from the leg area on the contralateral side, if you were recording from a neuron that was
going to the flexor muscles of toes , it would discharge AP
o But if you took electrode up into arm area, you wouldn’t find any
correlation with any sort of leg movement
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find more resources at oneclass.com
Motor Representation
Is it a representation of muscle or of movements?
- If you had a muscle representation, from an area where you get biceps contraction - There is an
exact muscle representation in the motor cortex
- Cortex row shows cortical neurons
- MN= motor neurons
- Turns out that this is not the case
- You are able to specifically excite one or a small region of one
- When you look at the neurons that come out of the motor cortex
- When you excite a group of descending axons from motor cortex and its going to a particular
alpha motoneuron, you can have a widely dispersed site within the finger area, still within its
own somatotopic area – there can be a significant dispersal of axons from these regions that will
affect a particular alpha motoneuron
- On the other hand, there are also single cortical neurons, which can project to diff motor
neurons
o These would all be related to a similar type of movement (functional relationship)
o This doesn’t fit the idea of a muscle representation
Summary of Results
1. Wide motor cortical sites project to a single muscle
2. Single cortical neurons make monosynaptic connections with motoneurons innervating several
different muscles
3. Motor cortex microstimulation (for 1 second) produces purposeful-like movements
o e.g.,food to mouth and mouth opening, reaching, defensive postures)
o mouth opening:
▪ if you put an electrode into arm area of right motor cortex (precentral gyrus)
and stimulate for about 1s at very low current strength
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Techniques that have been used to study the motor cortex. Stimulation: electrical, magnetic, epilepsy, abnormal firing of neurons in the brain that can spread and stimulate nearby. Lesions areas: electrical, chemical, stroke, can be rupture of artery, can be something that blocks the artery so that blood cant flow, degeneration of neurons in brain. Recording: single neurons, evoke potentials, eeg (electroencephalography, pet (positron emission tomography) Inject radioactive substance and positron are emitted from cortex in relation to how active those areas are in terms of blood flow fmri (functional magnetic resonance imaging) specialized areas of cerebral cortex: somatosensory, visual, auditory, motor. Using fmri, it was discovered that there are 180 separate areas/hemispheres with different functions all these areas interlink with each other. Selection, planning, execution: execution of movement coming from efferent movements. This occurs across multiple structures in a distributed system that is in parallel and in series (cid:523)that"s the planning of it(cid:524)
