CNS 4 Flashcards
Motor commands from the
higher centers of the brain are transmitted through the
Brianstem to the spinal cord
Sensory signals form sensory afferent neurons mediate
Spinal reflexes and ascend to the brianstem, cerebellum and cerebral cortex, where they provide feedback of the evolving movement
Feedback Control
• The brain, cerebellum and brainstem issue a motor command (Desired limb position) to neuronal networks in the spinal cord
Sensory receptors in muscles, joints and skin signal the actual position back to the spinal cord, which compares this to the desired position and generates an output to the
muscles so that the difference
between actual and desired states is minimized
Muscle spindles are located in parallel with the force-producing muscle fibers, so their
sensory endings, which give rise to group Ia and II sensory afferents, respond to changes in muscle length (Ia and II refer to axonal conduction velocities: Ia: ~100 m/s, II: ~50 m/s).
Tendon organs are in the
tendinous fascicles at the ends of the muscle fibers. Their sensory endings (Group Ib) respond to muscle force (Ib conduction velocity is similar to Ia conduction velocity).
Muscle spindle group Ia afferents also respond to
muscle or tendon vibration.
Prolonged vibration is used in physiotherapy to alleviate
spasticity (Overactive stretch reflexes). This may be because over time, Ia afferents activate interneurons that inhibit transmission in the stretch reflex pathway.
Intrafusal
Inside muscle spindle
Extrafusal
-outside muscle spindle
Alpha motoneurons (MNs)
activate the main (Extrafusal) muscle fibres to contract
- muscle spindle afferents (Ia and II) —> signal muscle length changes
Gamma MNs → activate tiny
intrafusal muscle fibers at each end of
the muscle spindle, stretching the non-contractile middle part of the spindle where the sensory endings are located. The sensory afferents then fire more rapidly, particularly if the muscle is stretched
(ie. Gamma MN activity increases the sensitivity of muscle spindles to length changes.)
Golgi tendon organ afferents (Ib)
→
signal muscle force
Alpha-gamma Coactivation
• a, b:
No gamma activity: spindle afferent
firing increases with muscle stretch and decreases with shortening.
Alpha-gamma Coactivation
C:
Descending signals from the brain co-
Action potentials activate alpha and gamma MNs. The alpha MNs activate extrafusal muscle fibers, causing muscle shortening. The gamma MNs activate intrafusal muscle fibers.
Compared to when a muscle is contracting, passive stretch of the
relaxed muscle produces
less stretch of the tendon and fewer action potentials form GTO
Stretch reflex Stimulus: muscle stretch
Response:
1) Spindle-afferent-mediated monosynaptic excitation of agonist MN’s & disynaptic inhibition (via interneurons) of antagonist MN’s
– Opposes change in muscle length
Stretch reflex
Stimulus: muscle stretch
AND, simultaneously:
2) GTO-mediated disynaptic inhibition of agonist MN’s and disynaptic excitation of antagonist MN’s. Opposes change in muscle force.
During muscle stretch, spindle and GTO reflexes “compete” (Spindle feedback resists increase in length by reflexly
activating MNs, GTO feedback resists
increase in force by reflexly inhibiting
MNs). This results in muscles
resisting stretch in a spring-like manner (The stronger the spindle reflex, the stiffer the spring, the stronger the GTO reflex, the more compliant the spring)
Flexor Withdrawal Reflex
Noxious stimulus evokes flex ion and I ipsilateral leg and extension of contralateral leg
The primary motor cortex is also called the
“sensorimotor cortex,”
Somatotopic Map
- First established by electrical stimulation with a probe during brain operations
- Somatotopic (topographic) map
- A somatotopic representation similar to that of the somatosensory cortex exists in the primary motor cortex
- As in the sensory cortex, hand and facial representations are largest
Neurons in the primary motor cortex can be activated by
transcranial magnetic stimulation (TMS)
By moving the coil in TMS over the different somatotopic representation
areas,
movements of different parts of the body are elicited
Corticospinal Tract (CST)
• Axons from neurons in
sensorimotor cortex form the CST
CST neurons make monosynaptic
connections with
spinal alpha motoneurons, whose axons in turn
CST neurons are only one CNS
synapse away from
muscle, the “final common path” for commands from brain to muscle
CST lesions (stroke, cerebro-vascular accident (CVA), brain attack) result in
spastic hemiplegia
spastic hemiplegia
1) Weakness (paresis) or paralysis of extremities
2) Exaggerated stretch reflexes (hypertonus, spasticity)
3) Spasms
4) Speech deficits (dysarthria)
5) Attentional deficits (aphasia, apraxia, hemi-neglect)
CST lesions are also called
upper motoneuron lesions
upper motoneuron lesions
This derives from the idea that CST neurons are the final output pathway from the brain, and synapse directly onto spinal alpha motoneurons, which are called the “lower” motoneurons
Broca’s area:
- Motor aspects of speech
2. Lesions result in motor aphasia (Slurring speech)
Wernicke’s area:
- Comprehension of language
- Association of visual, auditory and tactile input with words
- Lesions result in sensory aphasia & dyslexia
Sensory aphasia
– eg. A person is handed a pen. When asked what it is, she says it’s a spoon. When asked to use it, she writes her name. This indicates that the sensory to motor transformation is separate from sensory to cognitive transformation.
