qweq

1. Lecture 12: Motor SystemsLecture 12: Motor Systems
Reading:
Ch 7, section: somatic nervous system
Ch 5, sections on motor cortex, cerebellum, spinal cord
and spinal reflexes

2. Spinal Cord - each segment contains motor neurons that project
to specific skeletal muscles via the ventral root.

3. Spinal Cord - each segment contains motor neurons that project
to specific skeletal muscles on the same (ipsilateral) side of the
body.

4. Somatic Motor System - final
common pathway for neural
control of the skeletal
musculature.
Axons of these neurons project
out the ventral root of the spinal
cord and make synaptic contact
with skeletal muscles at the
neuromuscular junction.

5. Neuromuscular Junction (NMJ):
ACh increases the membrane permeability to Na+
and K+
leading to an EPSP called the end-plate-potential (EPP).
Acetylcholinesterase (AChE) - enzyme localized in the
synaptic cleft that degrades ACh.
Specialized synaptic contact, called the motor end plate, between
spinal motorneurons and skeletal muscle fibers
Acetylcholine (ACh) - neurotransmitter used by motorneurons.

6. Neuromuscular Junction (NMJ)
Sequence of Events (1-7):

7. Myasthenia Gravis:
An Autoimmune Disease of the Neuromuscular Synapse
Symptoms - muscle weakness, particularly during sustained activity
- commonly affects muscles controlling the eyelids, facial
expression, swallowing
- in the laboratory, decreased size of end plate
potentials in muscle afflicted with Myasthenia gravis
Underlying Cause - autoimmune disease where the body generates
antibodies that attack nicotenic ACh receptors
Treatment - ACh-esterase inhibitors (Neostigmine). By reducing
the rate of ACh degredation, ACh persists in the NMJ
longer and has the increased potential to “find” healthy
ACh receptors.

8. Spinal Reflexes
Simple neuromuscular circuits that mediate reflex responses to
sensory stimuli.
stretch reflex - simple reflex circuit that mediates muscular
contraction following stretch of the homonymous muscle.
withdrawal reflex - reflex circuit that mediates withdrawal
from a painful stimulus.

9. Components of a reflex circuit – the Withdrawal Reflex

10. Components of a reflex circuit – the Withdrawal Reflex coupled with
the Extensor Reflex

11. Tetanus Botulism
Contracted: via a wound which
becomes contaminated
with the bacteria
Clostridium tetani
via ingesting the spores from
bacteria (Clostridium
botulinum) allowed to grow in
an anaerobic environment
*** The 2 bacteria generate toxins that share ~ 40% homology
Question: How do two toxins that share tremendous similarity,
produce such different symptoms?
Mechanism: For both diseases, the toxins from the 2 bacteria block the
release of neurotransmitter
Symptoms: Muscle weaknessStiffness (lockjaw),
rigidity

12. Question: How do two toxins that share tremendous similarity,
produce such different symptoms?
- Botulinum toxin blocks the release of excitatory neurotransmitter.
- Tetanus toxin blocks the release of inhibitory neurotransmitter.
Answer:

13. Central Motor System - system of neural structures that carry out
specific controls of the skeletal musculature.
- Spinal Cord
- Primary Motor Cortex
- Supplementary Motor and Premotor cortices
- Cerebellum
- Basal Ganglia
Neural Structures:

14. Primary Motor Cortex - located on the precentral gyrus, and contains
a somatotopic map of the skeletal musculature. A subset of the neurons
project directly to the spinal cord forming the corticospinal tract.

15. Supplementary Motor and Premotor cortices - motor cortical areas
that project to and are located anterior to the primary motor cortex.
Involved in complex motor sequences and movement preparation.

16. Cerebellum - large neural lobe located on the lower posterior region
of the brain. Involved in numerous functions, and is particularly
important in the control of motor coordination. Heavily interconnected
with the cerebrum, particularly the cerebral cortex.

17. Cerebellum - compares the intended movement with the actual
movement and makes corrective adjustments
Figure 5-23 in 5th
edition only

18. Basal Ganglia - large nuclei in the center of the brain interconnected
with the cerebral cortex and the thalamus. Involved in motor planning
and the control of motor sequences.

19. Parkinson’s Disease
- disease that destroys dopaminergic neurons in the basal ganglia
- difficulty initiating movements
- resting tremors
Neurodegenerative Diseases of the Motor System
Multiple Sclerosis
- autoimmune disease that causes demyelination of neurons in
the cerebellum
- difficulty making precise movements
- action tremors

20. Summary
) Neuromuscular Junction
1) Synapse between a motor neuron and a muscle cell
2) ACh is the neurotransmitter, ACh-esterase degrades ACh
3) Functional organization
4) Sequence of events for synaptic transmission
) Spinal Reflexes
1) Stretch Reflex
2) Withdrawal Reflex
a) finger
b) leg
) Diseases of the motor system
1) Myasthenia Gravis
2) Tetanus and Botulism
3) Multiple Sclerosis
4) Parkinson’s Disease
D) Central Motor System: Spinal Cord; Primary, Supplementary, and
Premotor Motor Cortex; Cerebellum; Basal Ganglia