CNS Lecture 4 - Bodily movement, muscles, reflexes, brain motor centers, corticospinal tract, speech

Sensory input use

Used by the brain to control bodily movement

Flow of motor commands

1. Motor commands generated in higher brain centers

2. Sensorimotor cortex

3. Signals from sensorimotor cortex descent the brainstem and to the spinal cord motor neurons which innervate muscle fibers

Flow of information from sensory receptors

1. Information from sensory receptors

2. Sensory pathways ascend to the brainstem, cerebellum, and the thalamus

3. Transmits sensory information to the sensorimotor cortex

4. In the sensorimotor cortex combined with input from other parts of the brain to form output commands to the brainstem, basal ganglia, and cerebellum

5. Basal ganglia and the cerebellum send their outputs to the thalamus and the brainstem

Supraspinal Centers

Involved in generating motor commands (supraspinal - above the spinal cord)

• Sensorimotor cortex, brainstem, cerebellum, cerebral cortex, thalamus, basal ganglia (basal nucleus)

What parts of the brain issue motor commands via the brainstem and spinal cord

Sensorimotor cortex, cerebellum, and basal ganglia

Feedback control

involves receptors which send sensory information back to the central nervous system to generate the desired movement.

Feedback control steps

1. Motor command is a desired limb position

2. Spinal cord transmtsts command to muscles by alpha motal neuronal axons

3. Causes muscle contraction

4. Receptors in muscles, joints and skin signal posiiton in limb, report back to spinal cord via afferent axons

5. In Spinal cord - actual position of limb is sibttracted from desired position

6. Difference drives motor neurons to contract muscle to shorten the difference

7. Moves the muscle from the current position to the desired position

With external load - deflects arm form desired position

• Spinal cord will correct from external load by minimizing the difference

Two receptors that carry the most relevent movement sensory information

Muscle spindles: Receptors signaling change in muscle length

Golgi tendon organs: Receptors signaling change in muscle force

Muscle Spindles

Sensory receptor that signals change in muscle length. Composed of connective tissue capsule, intrafusal muscle fibers, and stretch receptors

Muscle spindle function

Sensory endings (give rise to group 1 and group 2 afferents) respond to changes in muscle length

Muscle spindle sensory endings anatomy

Spiral around intrafusal muscle fibers (fibers inside muscle spindle)

Golgi tendon organs

Tendons at the end of muscles, sensory endings respond to force produced by the muscle. Sensory endings give rise to group 1 B sensory afferents

Muscle spindle group 1A afferent function

Respond to muscle/tendon vibration, as well as responding to muscle length changes

Muscle spindle group 1A physiotherapy

tendon vibration (activates muscle spindle afferents) alleviates spasticity (overactive stretch reflex)

Muscle receptor recordings

The muscle spindle signals the length of a flexor muscle and the tendon organ signals the force in an extensor muscle.

Muscle spindle composition

• Intrafusal muscle fibers, surrounded by connective tissue and stretch receptors

• Only generate tiny amounts of force

Extrafusal muscle fibers

Main muscle fibers found outside muscle spindle - produce measurable force

2 types of motor neurons that innervate a muscle

Alpha + Gamma motor neurons

Alpha motor neurons

Activate main muscle extrafusal (outside muscle spindle) fibers to contract

• Axons have conduction velocities as muscle spindle 1A and tendon organ 1B

  

Gamma motor neurons

Activate intrafusal (inside muscle spindles) muscle fibers at each end of muscle spindle

• 25-40 m/s velocity

Muscle sensory ending location

Middle part of the spindle, which is non-contractile

Activation of gamma motor neurons

Intrafusal muscle fibers activate, stretching non-contractile middle part (sensory ending location)

• Causes rapid firing during muscle stretch

• Gamma motor neurons increase sensitivity of muscle spindles to length changes

  

Golgi tendon organ afferents function

Signal muscle force

Alpha gamma co-activation theory

gamma motor neurons are coactivated with alpha motoneurons to keep the muscle spindle afferents firing during muscle shortening

• If intrafusal muscle fibers are activated at the same time as alpha motor neurons, keeps muscle taught as muscle shorten

• Elastic, non-contractile region remains the same length

• Compensates for extrafusal shortening, allowing afferents to maintain/increase firing

Activation of golgi tendon organs

• Passive stretching causes Golgi tendon organs to respond with small increases in their rate of firing

• Contraction of extrafusal muscle fibers causes golgi tendon organs to fire more rapidly

Golgi tendon organs respond to force produced by a muscle, especially producing during active contraction

Muscle spindle activation by stretch reflex

Rapid increase in length of quad muscle activates muscle spindle afferents. Afferent APs travel to spinal cord, actiivgating quadricep motor neurons.

Muscles shorten again - reflex acts to counteract stretch - resist change and maintain desired state. Stretch reflex automatically performs substation

Afferent excitation of inhibitory neurons

Inhibits motoneurons that innervate antagonistic muscle

Golgi tendon activation by stretch relfex

Signals from Golgi tendon travel to spinal cord and have reverse reflex action than muscle spindle afferents

• Activates an interneuron that inhibits extensor motor neuron

• Activates interneuron which activates antagonist flexor motor neuron

Competition by golgi tendon/muscle feedback in stretch reflex

• Muscle spindle feedback resists increase in length by activating extensor motor neurons

• Golgi tendon organ feedback resists increase in force by inhibiting extensor neurons

Results in muscle resisting stretch in a spring like manner

• CNS allows control of “springiness”

Flexor withdrawal reflex

Afferent signals from nociceptors cause reflex activation of the flexor motor neurons and reflex inhibition of extensor motor neurons, resulting in removing leg from pain

• Occurs through spinal cord

  

Major motor areas of the cerebral cortex

Pre-motor cortex, supplemntary cortex, primary motor (sensorimotor) cortex, somatosensory cortex, parietal lobe association cortex

Somatotopic maps

Stimulating specific parts of the brain causes body part movement

• Somatotopic representation of the primary motor cortex and the parts it controls

• Primary motor cortex = sensorimotor cortex

• Primary sensory cortex = somatosensory cortex

• Hand and feet representation are large compared to other body parts

  

TMS

Transcranial magnetic stimulation - neurons in primary motor cortex can be activated

• Move magnetic coil over different somatotropic representation areas causes movement of body parts (mostly hands and feet)

• Research tool used in spinal cord operations to check for conduction block (thoracic surgery - make sure legs are still working)

Corticospinal Tract (pyramidal tract)

Conveys signals from the sensorimotor cortex through the brainstem to the spinal cord.
- Axons from neurons in the sensorimotor cortex form the CST

CST crossing

Crosses to the the contralateral side of the nervous system at the brainstem level

What do CST neurons connect with

Makes monosynaptic connections with spinal alpha motoneurons, whose axons activate muscles
- Only one neuronal synapse away from muscles

CST Lesions

Stroke, Cerebrovascular accident (CVA), brain attack
- Cannot move limb on the contralateral side of the body (hemiplasia)
- Motor control problems occur on the side of the body opposite to the site of the brain damage

Symptoms of CST Lesions

- Weakness (paresis) or complete paralysis of extremities
- Exaggerated stretch reflex in antigravity muscles
<Hypertonus - excessive level of skeletal muscle tension/activity
< Spasticity is a state of increased muscular tone with exaggeration of tendon reflexes

- Muscle spasm
- Speech deficits - seen when lesion on left side of brain
- Attention deficits
<Aphasia - inability to understand meaning of sensory inputs/defect in language.
<Apraxia - problem using day-to-day objects
<Hemineglect - Occurs when patients are not aware of items on one side of their body

CST lesions other name:

Upper motoneuron lesions

Why are CST lesions called upper motoneuron lesions

Final output pathway of the brain, synapsing onto spinal alpha motoneurons (lower motoneurons)

Which side of the brain controls speech

Left side

2 main speech areas:

Broca's area + Wernicke's area

Broca's area function

Motor aspects of speech (production)
- Control larynx, tongue and lips

Broca's area lesion

Slurring speech

Wernicke's area function

Comprehension of language + Association of visual, auditory, and tactile input with words

Werenicke's area lesion

Sensory aphasia - difficulty of understanding meaning of words

Dyslexia - Results in difficulty in reading aloud fluently

Example of sensory aphasia

o A person is handed a pen and says it is a spoon. When asked to use it, they write their name
o A problem in producing the correct linguistic response, yet the appropriate motor response is made
o Indicates that the sensory to motor transformation is separate from the sensory to linguistic transformation

PET Scan of language based activities

o Distinct areas of the brain are specialized for hearing, seeing, speaking and generating words