somatic nervous system (proprioception)

• Muscle spindles have 4-8 intrafusal muscle fibres surrounded by a connective tissue capsule. They are distributed among and in a parallel arrangement with extrafusal fibres (force-producing fibres).
• Sensory afferents are coiled around the central part of the intrafusal spindle. When the muscle is stretched, the tension on the intrafusal fibres activates mechanically gated ion channels in the nerve endings, triggering action potentials.
• Innervation of muscle spindles from two classes of fibres: primary and secondary endings. Primary endings arise from the largest myelinated sensory axons (group Ia afferents) - rapidly adapt responses to changes in muscle length. Secondary endings (group II afferents) produce sustained responses to constant muscle lengths.

  Primary endings are thought to transmit information about limb dynamics—the velocity and direction of movement. Secondary endings provide information about the static position of limbs.

  Piezo2 is expressed by proprioceptors and is required for functional proprioception.
• The intrafusal fibres are contractile and controlled by γ motor neurons in the ventral horn of the spinal cord. Intrafusal fibres don't add appreciably to the force of muscle contraction, but changes in the tension of intrafusal fibres have significant impacts on the sensitivity of the spindle afferents to changes in muscle length. Thus, in order for central circuits to provide an accurate account of proprioception, the level of activity in the γ system must be taken into account
• The density of spindles in human muscles varies. Large muscles that generate coarse movements have relatively few spindles; in contrast, extraocular muscles and the intrinsic muscles of the hand and neck are richly supplied with spindles.
• Muscle spindles signal changes in muscle length, low-threshold mechanoreceptors in tendons inform the CNS about changes in muscle tension. These mechanoreceptors, called Golgi tendon organs, are formed by branches of group Ib afferents amongst the collagen fibres forming tendons. Each Golgi tendon organ is arranged in series with 10–20 extrafusal muscle fibres. The population of Golgi tendon organs for a given muscle is proportional to the tension in the muscle.
• The gain of the myotatic reflex refers to the amount of muscle force generated in response to a given stretch of the intrafusal fibres
• If the gain of the reflex is high, then a small amount of stretch applied to intrafusal fibers produces a large increase in the number of α motor neurons recruited and a large increase in their firing rates; this leads to a large increase in the amount of tension produced by the extrafusal fibers.
• If the gain is low, a greater stretch will be required to generate the same amount of tension in the extrafusal muscle fibers
• In general, the baseline activity level of γ motor neurons is high if a movement is relatively difficult and demands rapid and precise execution.
• With passive stretch, most of the change in length occurs in the muscle fibers, since they are more elastic than the fibrils of the tendon. Thus, activity increases in spindle afferents with stretch while there is little change in the firing rate of Golgi tendon organ afferents
• In active muscle contraction the generated force is transmitted to the tendon and transduced by the Golgi tendon organ, leading to an increase in the firing rate of Ib afferents. Thus, Golgi tendon organs are sensitive to increases in muscle tension from muscle contraction but insensitive to passive stretch.
•  The action of inhibitory neurons ensures that muscle contraction does not occur simultaneously in antagonistic pairs

  (= prevents spasticity). Reciprocal inhibition is allowed for by local inhibitory (GABAergic) interneurons in spinal motor funciton
• Vestibular sensation is the sense of balance/head position, spatial orientation and coordination
• Vestibulospinal reflex: stops you falling over by innervating spinal cord LMNs.

  A neural pathway connecting the vestibular system in the inner ear to the spinal cord to control balance and posture. the reflex helps maintain stability by adjusting muscle tone and coordinating movements in response to changes in head position or movement detected by the vestibular apparatus.
• When the vestibular system detects changes in head position or movement, the vestibulospinal reflex is activated. This reflex then sends signals to the spinal cord, modulating muscle activity to adjust posture and maintain balance. For example, if the head tilts to one side, the vestibulospinal reflex would adjust muscle tone to prevent falling by activating the appropriate muscles to counteract the tilt and maintain stability.
• Disorders affecting the vestibular system can disrupt the vestibulospinal reflex, leading to difficulties in balance, postural instability, vertigo, and impaired coordination of movements. Rehabilitation and exercises aimed at retraining this reflex are often used to help individuals recover from vestibular-related balance problems.
• Vestibulocollic reflex: keeps your head up by innervating cervical LMNs.The vestibulocollic reflex (VCR) coordinates head and neck movements in response to the vestibular system, which senses changes in head position and movement.

  Specifically, the VCR involves the connection between the vestibular apparatus in the inner ear and the muscles of the neck, particularly the neck muscles that control head movements.
• The primary function of the vestibulocollic reflex is to stabilize and adjust head position relative to body movements and gravitational forces. For example, if your head suddenly tilts to one side, the VCR kicks in to quickly adjust the position of your head to maintain stability and prevent your gaze from becoming disoriented.
• The primary function of the vestibulocollic reflex is to stabilize and adjust head position relative to body movements and gravitational forces. For example, if your head suddenly tilts to one side, the VCR kicks in to quickly adjust the position of your head to maintain stability and prevent your gaze from becoming disoriented.
• Disruptions in the vestibulocollic reflex can lead to difficulties in stabilizing head movements, resulting in symptoms such as dizziness, unsteadiness, and impaired coordination between head and body movements. Rehabilitation techniques, including exercises that target the vestibular system and neck muscles, are often used to help improve the function of the vestibulocollic reflex in individuals with balance or vestibular-related issues.
• Vestibulo-ocular reflex: keeps your gaze fixed by innervating cranial nuclei of III, IV, and VI (extra-ocular nerves).The vestibulo-ocular reflex (VOR) is a fundamental reflex that maintains stable vision during head movements by generating eye movements that compensate for changes in head position. This reflex is crucial for stabilizing gaze and ensuring clear vision while the head is in motion.
• The VOR operates in milliseconds. It maintains a clear and stable visual field despite head movements, such as turning the head to look at something or moving the head while walking or running.

  A well-functioning VOR is essential for visual acuity, depth perception, and spatial orientation. Disorders affecting the VOR, such as vestibular dysfunction or inner ear disorders, can lead to symptoms like blurred vision, oscillopsia (illusory movement of the environment), and difficulties maintaining focus during head movements.
• The tectospinal and tectobulbar tracts are neural pathways involved in coordinating head and neck movements in response to visual and auditory stimuli. These pathways originate in the superior colliculus, a structure located in the midbrain involved in processing visual information, particularly in the context of reflexive eye and head movements.
• Tectospinal Tract:

  • Function: coordinating reflexive head and neck movements in response to visual stimuli, orientate head and eyes to visual stimulus.
  • Pathway: Nerve fibres from the superior colliculus descend through the brainstem and synapse with motor neurons in the cervical spinal cord which innervate neck muscles
  • Role: When the eyes detect a visual stimulus, such as sudden movement or a bright object in the periphery, the tectospinal tract helps orient the head and eyes quickly towards that stimulus
• Tectobulbar Tract:

  • Function: coordinate reflexive movements of the head and face in response to auditory and visual stimuli.
  • Pathway: Similar to the tectospinal tract, nerve fibers from the superior colliculus project to various brainstem nuclei involved in controlling facial and head movements, like cranial nerve nuclei.
  • Role: coordinate movements of facial muscles, especially those involved in head and facial orientation towards a stimulus, whether visual or auditory.
• Summary

  The vestibulospinal tract sends vestibular motor signals.

  The tectospinal tract (not illustrated) sends motor signals to coordinate with eye and head movements (neck and shoulder LMNs + nuclei of III, IV, and VI).

  Reticulospinal tracts send motor signals to coordinate (especially in relation to upright postural control).

  Pyramidal tracts, CB + lCS + mCS, control voluntary movements.