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Vestibulospinal System Vestibular Classics March 02, 2007 Zakir Mridha The function of Vestibulospinal Syestem is to control of proper body posture and movement. Specially, human upright vertical position is unstable. A continuous activation of postural muscles is therefore required to avoid falling. The function of Vestibulospinal Syestem is to control of proper body posture and movement. Specially, human upright vertical position is unstable. A continuous activation of postural muscles is therefore required to avoid falling. The problem is complicated by the reduced dimension of the support base (the feet) and by the articulated structure of the human skeleton. The function of Vestibulospinal Syestem is to control of proper body posture and movement. Specially, human upright vertical position is unstable. A continuous activation of postural muscles is therefore required to avoid falling. The problem is complicated by the reduced dimension of the support base (the feet)and by the articulated structure of the human skeleton. But surprisingly, upright posture is a capability, which is learnt in the first year of life. Normal steady position of the head is maintained by powerful set of reflexes that is known as vestibulocollic reflex Peripheral sensory signals (angular and/or linear accelerations) detected by vestibular receptors processed in the vestibular nuclei LVST MVST relayed to the spinal cord Modulates activity in muscles that rotate the head and upper torso and modulate adjustments pertinent to limb and body orientation in the gravitational field. >20 pairs of muscles that control reflex movements of the head & neck. 3 major groups: a) Neck Extensors b) Neck Flexors c) Neck Rotators Neck motoneurons are located mainly in C1, C2 and C3 segments Pathways of axons in the spinal cord: a) i-LVST b) MVST and c) c-LVST >20 pairs of muscles that control reflex movements of the head & neck. 3 major groups: a) Neck Extensors b) Neck Flexors c) Neck Rotators Neck motoneurons are located mainly in C1, C2 and C3 segments Pathways of axons in the spinal cord: a) i-LVST b) MVST and c) c-LVST Location of the Vestibulo-spinal Neurons in Vestibular Nuclei Type of HC nerve-activated Neurons Axonal pathways Projection level Conduction velocity Experimental Arrangements Experimental Arrangements Orthodromic: Propagation of an impulse along an axon in normal direction. Antidromic: Performing a nerve conduction study in such a manner that the nerve impulses is being propagated in a direction opposite to that in which the nerve fiber ordinarily conducts. Experimental Arrangements Orthodromic: Propagation of an impulse along an axon in normal direction. Antidromic: Performing a nerve conduction study in such a manner that the nerve impulses is being propagated in a direction opposite to that in which the nerve fiber ordinarily conducts. Three types of Neurons: VS neurons: Send axon to spinal cord but not to the OMN. VO neurons: Send axon to OMN but not to the spinal cord. VOS neurons: Send axon to both Spinal cord and OMN. Example of orthdromic and antidromic spikes C1/2 C3 OMN Axonal projection of the HC-activated Vestibular neurons to the spinal cord were examined using a collision test between orthodromic andantidromic responses. Antidromic response was confirmed by its fixed latency at a stimulus intensuty near the threshold, and by consistent responses during high-frequency stimulation. The antidromic spike evoked by spinal cord stimulus were blocked by a preceding spike evoked by HC stimulation when the timing of the HC stimulation was close to that of the spinal cord stimulation. Projection levels in the spinal cord Axonal Pathways of HC nerve-activated Vestibulospinal Neurons: Locations of HC nerve activated vestibular neurons: HC nerve-activated vestibulospinal neurons Saccular nerve-activated vestibulospinal neurons Sato et al. 1997 Axonal pathways & projection levels of SAC & UT-activated vestibulospinal neurons 72% 12% 16% 30% 63% 7% Sato et al. 1997 Projection of VS Neurons at different Levels of the Spinal Cord 80 70 70 60 50 40 30 20 -2 C1 Percentage (% ) 80 50 40 30 20 -2 C1 10 C3 60 T1 0 T1 i-LVST L3 10 C3 0 MVST i-LVST MVST L3 c-LVST c-LVST (Sato et al. 1997) (Sato et al. 1996) Posterior Canal Anterior Canal Horizontal Canal Percentage (% ) Sacculus Utriculus 80 80 80 70 70 60 60 20 C 1- 2 10 C3 0 T1 i-LVST L3 MVST c-LVST (Sugita et al. 2004) 40 30 20 C 1-2 10 C3 0 T1 i-LVST L3 MVST c-LVST (Kitajima et al. 2006) 50 40 30 20 C1 -2 10 C3 0 T1 i-LVST L3 MVST c-LVST (Kushiro et al. 2007??) Percentage (% ) 30 50 Percentage (% ) 40 60 Percentage (% ) 50 70 Neural connections and pathways underlying sacculocollic reflexes were studied by intracellular recording from neck extensor and flexor motorneurons. Extensor muscle: Biventer Cervicis and Complexus Muscle– Motoneuron located at C2 and C3 levels. Flexor Muscle: Longus Capitis Muscle– Motoneuron located at C2 levels Methods: a) Selective electrical stimulation of Saccular nerves. b) Action potentials recorded from antidromically identified neck motoneurons c) Pathways were determined by making transection of VST Stimulus-response curves of N1 Potential (Inset: Typical field potential) Synaptic Potential in Neck Motoneurons following Saccular Nerve stimulation Excitatory Post Synaptic Potential (EPSP): is a temporary increase in postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell. They are the opposite of Inhibitory Post Synaptic Potential (IPSP), which usually result from the flow of negative ions into the cell. Latency of Synaptic Potential in Extensors and flexors motoneurons Diagram of disynaptic and trisynaptic sacculo-collic motoneuron connections Extensors Flexors Diagram of the Utriculo-neck motoneuron conections Ikegami et al. 1994 Sternocleidomastoid muscle: origin, sternal head-manubrium, clavicular head-clavicle; insertion, mastoid process and superior nuchal line of occipital bone; innervation, accessory nerve and cervical plexus; action, flexes vertebral column, rotates head. Effect of Saccular nerve stimulation on ipsi- and contrlateral SCM motoneurons Effect of Utricular nerve stimulation on ipsi- and contrlateral SCM motoneurons Results from transecting experiments Schematic diagram of the Sacculo- and Utriculo-sternocleidomastoid pathways Longissimus Muscle Group: action, lateral flexion 1. Obliqus Capitis superior Muscle (OCS) 2. Splenius Muscle (SPL) 3. Longissimus Muscle (LONG) Semispinalis Muscle Group: action, extends, rotates vertebral column 1. Biventer Cervicis Muscle (BIV) 2. Complexus Muscle (COMP) Predominant Connections from Otolith & Canal to neck motoneurons Muscles Saccules Utricules HC AC PC Ipsi Contra Ipsi Contra Ipsi Contra Ipsi Contra Ipsi Contra Extensors EP EP EP IP IP EP EP EP IP IP Flexors IP IP EP IP n.t EP IP IP n.t EP Rotators IP 0 IP EP IP EP IP EP IP EP Red = LVST Green = MVST n.t. = Not Tested O = No Effect Muscles Saccules Utricules HC AC PC Ipsi Contra Ipsi Contra Ipsi Contra Ipsi Contra Ipsi Contra Extensors EP EP EP IP IP EP EP EP IP IP Flexors IP IP EP IP n.t EP IP IP n.t EP Rotators IP 0 IP EP IP EP IP EP IP EP Projection of VS Neurons at different Levels of the Spinal Cord 80 70 60 50 40 30 20 -2 C1 ) 80 70 50 40 30 20 -2 C1 10 C3 60 Percentage (% Red = LVST Green = MVST Black = Unknown n.t. = Not Tested O = No Effect 0 T1 i-LVST L3 10 C3 0 T1 MVST i-LVST MVST L3 c-LVST c-LVST (Sato et al. 1997) (Sato et al. 1996) Posterior Canal Anterior Canal Horizontal Canal Percentage (% ) Sacculus Utriculus 80 80 80 20 10 C3 0 T1 i-LVST L3 MVST c-LVST (Sugita et al. 2004) 50 40 30 20 -2 C1 Percentage (% ) 30 Percentage (% ) 50 60 10 C3 0 T1 i-LVST L3 MVST c-LVST (Kitajima et al. 2006) 50 40 30 20 -2 C1 10 C3 0 T1 i-LVST L3 MVST c-LVST (Kushiro et al. 2007??) ) 70 60 Percentage (% 70 60 40 -2 C1 70