Lecture notes Introductory Psychology and Brain &
Cognition: Part B - INTERIM II
Lecture 8: CH8 - Action 25/11/2024
Top: Cortical association, premotor, and
supplementary motor areas translating
intentions and goals into action plans and
movement patterns.
Middle: Primary motor cortex, brainstem
structures, basal ganglia and cerebellum
converting movement patterns into
commands to the muscles
Bottom: Spinal motor neurons innervating
muscles, together with spinal sensory
neurons capable of producing simple
reflexes.
Muscle innervation
• Muscles consist of fibers attached to the skeleton
• Come in antagonist pairs: flexor and extensor (if one is flexed/contracted, the other is
extended/relaxed)
• Muscle contraction through acetylcholine released by firing of alpha motor neurons
• Firing frequency of alpha motor neurons (and number of muscle fibers) determines the
force generated by muscle
• Alpha motor neurons originate in spinal cord, exit through ventral root
• Gamma motor neurons adjust muscle spindles (important for sensing the stretch of the
muscle)
1
,• Sensory neurons entering the spinal cord through dorsal root send input from muscle
spindles to alpha motor neurons via spinal interneurons
• If stretch is unexpected, alpha neuron activation: stretch reflex
• Reflexes serve postural stability and protective functions without any help from the brain
Effector: Action, or motor movement, is generated by stimulating the skeletal muscle fibers
of an effector. An effector is a part of the body that can move.
Muscles are activated by motor neurons, which are the final neural elements of the motor
system. Alpha motor neurons innervate muscle fibers and produce contractions of the
fibers. Gamma motor neurons adjust muscle spindles (important for sensing the stretch of
the muscle) are part of the proprioceptive system. Motor neurons originate in the spinal cord,
exit through the ventral root, and terminate in the muscle fibers. Transmitter for alpha motor
neurons is acetylcholine, it makes the muscle fibers contract.
If a stretch is unexpected, alpha neuron activation: stretch reflex. These reflexes serve
postural stability and protective functions without any help from the brain.
Alpha motor neurons receive peripheral input from muscle spindles (sensory receptors in
the muscles that provide information about how much the muscle is stretched). The axons of
the spindles form an afferent nerve that enters the dorsal root of the spinal cord and
synapses on spinal interneurons that project to alpha motor neurons. If the stretch is
unexpected, the alpha motor neurons are activated, causing the muscle to return to its
original length; this response is called the stretch reflex. Reflexes allow postural stability to
be maintained without any help from the cortex. They also serve protective functions; e.g.
reflexes can contract a muscle to avoid a painful stimulus well before pain is consciously
perceived.
Spinal interneurons are innervated by afferent sensory nerves from the skin, muscles, and
joints, as well as by descending motor fibers (upper motor neurons) that originate in several
subcortical and motor cortical structures.
The spinal cord alone (without help from the brain) is capable of producing rhythmic
movements such as walking in four-legged animals. This indicates that the brain does not
need to specify patterns of muscle activity but can simply activate spinal pattern generators.
2 main subcortical components of the motor pathways:
Cerebellum:
- Contains 75% of all neurons of the CNS
- Has a key role in error correction (forward models)
- Damage = ataxia (difficulty maintaining balance and coordinated
movements
Basal ganglia:
- 5 nuclei with outputs to cortex via the thalamus: caudate nucleus,
putamen (together = striatum), globus pallidus, subthalamic nucleus, substantia
nigra
- Key role in movement selection and initiation (gating function)
- Damage = Parkinson’s and Huntington's
2
, Many nuclei within the brainstem, including the
vestibular nuclei, the reticular formation nuclei,
and the substantia nigra (which figures
prominently in Parkinson’s disease), send
direct projections down the spinal cord. These
motor pathways are referred to collectively as
the extrapyramidal tracts (controls posture,
muscle tone, movement of speed), meaning
that they are not part of the pyramidal tract, the
axons that travel directly from the cortex to the
spinal segments. Extrapyramidal tracts are a
primary source of indirect control over spinal
activity modulating posture, muscle tone, and
movement speed; they receive input from
subcortical and cortical structures.
Cortical motor regions:
Primary motor cortex (M1/BA4)
- Key area for motor initiation, activation of lower levels
- Main output: pyramidal (corticospinal) tract
Secondary motor areas
- Key areas for movement planning and control
- Premotor cortex (lateral part of BA6)
- Supplementary motor area (medial part of BA)
Additional key cortical regions
- Broca’s area (BA44/45)
- Inferior (BA39/40) and superior parietal lobule (BA5/7)
- Frontal eye fields (BA8)
Primary motor cortex (M1)
Key area for motor initiation:
- Control of movement on opposite side of the body via corticospinal tract (similar
contralateral control for most other tracts
- Hemiplegia after M1 lesion (stroke): loss of voluntary movements on contralateral
side of the body
Somatotopic organization
- Different M1 regions represent different body parts
- But organization is coarser than somatosensory cortex (S1) i.e. not such a clear
homunculus
- Representations of effects correspond to their importance for movement and level of
control
- Can be mapped with TMS and fMRI
- Representations of fingers correspond to how often they are used together
Secondary motor areas
- Coarse somatotopic organization similar to M1
- Key for planning and control of movement
3
Cognition: Part B - INTERIM II
Lecture 8: CH8 - Action 25/11/2024
Top: Cortical association, premotor, and
supplementary motor areas translating
intentions and goals into action plans and
movement patterns.
Middle: Primary motor cortex, brainstem
structures, basal ganglia and cerebellum
converting movement patterns into
commands to the muscles
Bottom: Spinal motor neurons innervating
muscles, together with spinal sensory
neurons capable of producing simple
reflexes.
Muscle innervation
• Muscles consist of fibers attached to the skeleton
• Come in antagonist pairs: flexor and extensor (if one is flexed/contracted, the other is
extended/relaxed)
• Muscle contraction through acetylcholine released by firing of alpha motor neurons
• Firing frequency of alpha motor neurons (and number of muscle fibers) determines the
force generated by muscle
• Alpha motor neurons originate in spinal cord, exit through ventral root
• Gamma motor neurons adjust muscle spindles (important for sensing the stretch of the
muscle)
1
,• Sensory neurons entering the spinal cord through dorsal root send input from muscle
spindles to alpha motor neurons via spinal interneurons
• If stretch is unexpected, alpha neuron activation: stretch reflex
• Reflexes serve postural stability and protective functions without any help from the brain
Effector: Action, or motor movement, is generated by stimulating the skeletal muscle fibers
of an effector. An effector is a part of the body that can move.
Muscles are activated by motor neurons, which are the final neural elements of the motor
system. Alpha motor neurons innervate muscle fibers and produce contractions of the
fibers. Gamma motor neurons adjust muscle spindles (important for sensing the stretch of
the muscle) are part of the proprioceptive system. Motor neurons originate in the spinal cord,
exit through the ventral root, and terminate in the muscle fibers. Transmitter for alpha motor
neurons is acetylcholine, it makes the muscle fibers contract.
If a stretch is unexpected, alpha neuron activation: stretch reflex. These reflexes serve
postural stability and protective functions without any help from the brain.
Alpha motor neurons receive peripheral input from muscle spindles (sensory receptors in
the muscles that provide information about how much the muscle is stretched). The axons of
the spindles form an afferent nerve that enters the dorsal root of the spinal cord and
synapses on spinal interneurons that project to alpha motor neurons. If the stretch is
unexpected, the alpha motor neurons are activated, causing the muscle to return to its
original length; this response is called the stretch reflex. Reflexes allow postural stability to
be maintained without any help from the cortex. They also serve protective functions; e.g.
reflexes can contract a muscle to avoid a painful stimulus well before pain is consciously
perceived.
Spinal interneurons are innervated by afferent sensory nerves from the skin, muscles, and
joints, as well as by descending motor fibers (upper motor neurons) that originate in several
subcortical and motor cortical structures.
The spinal cord alone (without help from the brain) is capable of producing rhythmic
movements such as walking in four-legged animals. This indicates that the brain does not
need to specify patterns of muscle activity but can simply activate spinal pattern generators.
2 main subcortical components of the motor pathways:
Cerebellum:
- Contains 75% of all neurons of the CNS
- Has a key role in error correction (forward models)
- Damage = ataxia (difficulty maintaining balance and coordinated
movements
Basal ganglia:
- 5 nuclei with outputs to cortex via the thalamus: caudate nucleus,
putamen (together = striatum), globus pallidus, subthalamic nucleus, substantia
nigra
- Key role in movement selection and initiation (gating function)
- Damage = Parkinson’s and Huntington's
2
, Many nuclei within the brainstem, including the
vestibular nuclei, the reticular formation nuclei,
and the substantia nigra (which figures
prominently in Parkinson’s disease), send
direct projections down the spinal cord. These
motor pathways are referred to collectively as
the extrapyramidal tracts (controls posture,
muscle tone, movement of speed), meaning
that they are not part of the pyramidal tract, the
axons that travel directly from the cortex to the
spinal segments. Extrapyramidal tracts are a
primary source of indirect control over spinal
activity modulating posture, muscle tone, and
movement speed; they receive input from
subcortical and cortical structures.
Cortical motor regions:
Primary motor cortex (M1/BA4)
- Key area for motor initiation, activation of lower levels
- Main output: pyramidal (corticospinal) tract
Secondary motor areas
- Key areas for movement planning and control
- Premotor cortex (lateral part of BA6)
- Supplementary motor area (medial part of BA)
Additional key cortical regions
- Broca’s area (BA44/45)
- Inferior (BA39/40) and superior parietal lobule (BA5/7)
- Frontal eye fields (BA8)
Primary motor cortex (M1)
Key area for motor initiation:
- Control of movement on opposite side of the body via corticospinal tract (similar
contralateral control for most other tracts
- Hemiplegia after M1 lesion (stroke): loss of voluntary movements on contralateral
side of the body
Somatotopic organization
- Different M1 regions represent different body parts
- But organization is coarser than somatosensory cortex (S1) i.e. not such a clear
homunculus
- Representations of effects correspond to their importance for movement and level of
control
- Can be mapped with TMS and fMRI
- Representations of fingers correspond to how often they are used together
Secondary motor areas
- Coarse somatotopic organization similar to M1
- Key for planning and control of movement
3
