Proprioception and Muscle Sensors Quiz

Proprioception (Kinesthesia)

• The body's ability to sense movement, action, and location.

Golgi Tendon Organs (GTOs)

• Located at the junction of tendons and muscle fibers.
• Respond to the tension a muscle puts on a tendon.
• Composed of free nerve endings that wind between collagen fibers inside a connective tissue capsule.
• When a muscle contracts, its tendons act as a series elastic element during the isometric phase.
• Constantly monitor tendon tension, sending sensory information to the CNS.
• Sensory neuron stimulates interneuron in spinal cord.
• Interneuron inhibits motor neuron.
• This reduces tension in the tendon.

Muscle Spindle Apparatus

• Responds to muscle length changes.
• Muscles needing more control possess more spindles.
• Stretching a muscle stretches the spindles.
• Contains intrafusal fibers (thin muscle cells).
• Two types of intrafusal fibers:
  • Nuclear bag fibers: Nuclei in loose central aggregates.
  • Nuclear chain fibers: Nuclei in rows.
• Two types of sensory cells wrap around the fibers:
  • Primary (annulospiral): Most stimulated at the beginning of the stretch.
  • Secondary (flower-spray): Responds more during sustained stretch.

Muscle Spindles

• Stretch receptors encoding signals about muscle length and changes in length.
• Capsule encloses a group of small muscle fibers known as intrafusal fibers.
• Innervated by gamma motor neurons.
• Stretch reflex: A contraction response triggered by muscle stretch.
• Alpha-gamma coactivation: Simultaneous activation of alpha and gamma motor neurons.
• Alpha motor neurons fire, muscle shortens, tension released.
• Gamma motor neurons fire, intrafusal fibers contract, maintaining stretch.
• Result: Spindle remains active.

Alpha and Gamma Motor Neurons

• Alpha: Innervate extrafusal fibers (contracting muscle fibers).
• Gamma: Innervate intrafusal fibers (active stretch).
  • Contraction doesn't shorten the muscle, but increases sensitivity to stretch.
  • Provides enough tension during relaxation to maintain muscle tone.
• Both types are stimulated by upper motor neurons simultaneously through coactivation.

Pain and Reflexes

• Nociceptors detect pain.
• Ipsilateral flexor muscles are stimulated, opposite limb extends (crossed-extensor reflex).
• To support body weight, the opposite limb extends.
• Painful skin stimulation activates flexor muscles, inhibiting extensor muscles (ipsilateral leg).
• Withdrawal reflex: Moves affected limb away from harmful stimulus.
• Contralateral leg supports body weight.

Cells of Betz

• Giant pyramidal cells (neurons) in the fifth layer of the primary motor cortex grey matter.
• Largest neurons in the CNS, reaching 100 μm in diameter.
• Axons descend to the spinal cord via the corticospinal tract.
• Synapse directly with anterior horn cells, which in turn synapse directly with their target muscles.

Basal Ganglia

• Deep brain nuclei involved in motor control (start, stop, modulate).
• Several masses of gray matter located deep within the cerebral white matter
• Inhibits muscle tone throughout the body.
• Selects, maintains purposeful motor activity.
• Suppresses unwanted patterns of movement.
• Coordinates slow, sustained contractions.
• Starts, stops, and modulates movement.

Neurotransmitters

• Glutamate: Most prevalent excitatory neurotransmitter, facilitating memory/learning.
• GABA: Inhibitory neurotransmitter, stopping cells from firing. ~40% of synapses work with GABA.

Parkinson's Disease Symptoms

• Bradykinesia (slowed movement).
• Muscle rigidity.
• Resting tremors.

Parkinson's Disease Possible Causes

• Protein misfolding, aggregation, and toxicity.
• Defective proteolysis.
• Mitochondrial dysfunction.
• Oxidative stress.

Nigrostriatal Dopamine System

• Sends dopaminergic neurons from the substantia nigra to the corpus striatum.
• Crucial for movement control and initiation.
• Degeneration of these neurons causes Parkinson's disease.
• Treatment: L-dopa and MAOIs (monoamine oxidase inhibitors).

Cerebellum

• Contains over 50% of the brain's neurons.
• Three lobes: Anterior, Posterior, Flocculonodular.
• Coordinates movements (smoothness, balance, posture, tone).
• Maintains posture and equilibrium.
• (~10% brain mass, ~50% neurons).
• Regions: Outer cortex (gray), Arbor Vitae (white), Deep cerebellar nuclei.
• Receives input: Equilibrium & proprioceptive information.

Cerebellar Subdivisions & Functions

• Vestibulocerebellum: Balance and eye movements.
• Spinocerebellum: Enhances muscle tone and coordinates skilled movements.
• Cerebrocerebellum: Plans & initiates voluntary activity, storing procedural memories.

Cerebellar Cortex Neurons

• Primarily inhibitory: Purkinje, basket, stellate, Golgi.
• Primarily excitatory: Granule, unipolar brush.
• Additional type found in floccular lobe and vermis: Unipolar brush cell.

Cerebellum's Role in Movement

• Influences posture and movement via brainstem nuclei and thalamus to sensorimotor cortex.
• Receives information from sensorimotor cortex, vestibular system, eyes, skin, muscles, joints, tendons.
• Provides timing signals for precise movement execution.
• Coordinates movements, involved in "muscle memory."
• Plans movements, integrating intended movement and spatial information.
• Compares intended vs. actual movement (corrects if needed).
• Cerebellar disease: Uncoordinated movements, trouble starting/stopping, combining movements.

Muscle Tone

• Resistance to stretch in a relaxed muscle.
• Due to passive elastic properties of muscle/joints, and ongoing alpha motor neuron activity.
• Hypertonia: Abnormally high muscle tone (spasticity, rigidity).
• Hypotonia: Abnormally low muscle tone (weakness, atrophy).

Stretch Reflex

• Monosynaptic reflex: Muscle stretch leads to muscle contraction.
• Stretching intrafusal fibers increases action potentials, rapid muscle contraction.
• Contraction releases tension on spindle.
• Decreases action potentials and neuronal messages.
• Example: Knee-jerk reflex (patellar tendon/ligament stretch).
• Quadriceps femoris muscle contraction, knee extension.
• Stretch receptors in the quadriceps muscles.
• Important for balance & posture, part of walking mechanism.
• Only stretch reflexes are monosynaptic.

Sensorimotor Cortex

• Network of connected neurons (frontal and parietal).
• Involved in movement planning and control.
• Highest and middle levels of motor control hierarchy.
• Primary motor cortex (motor cortex) and premotor area in posterior frontal lobe.
• Somatotopic map (similar to somatosensory cortex).
• Other areas: Supplementary motor cortex, somatosensory cortex, parietal-lobe association cortex.
• Neuronal network: Multiple representation sites for muscles/movements.

Corticospinal Pathways

• Nerve fibers from sensorimotor cortex to spinal cord.
• Pyramidal tracts/pyramidal system.
• Most fibers cross (decussate) in the medulla oblongata.
• Control rapid, fine movements of distal extremities.

Brainstem Descending Pathways

• Coordinate large muscle groups (trunk, proximal limbs) for posture, locomotion, head/body movements.