Lab 12: Systems I Quick overview of material covered: Somatosensory: contains specific and nonspecific pathways specific: dorsal column medial lemniscal (DCML) tract neospinothalamic tract trigemino-thalamic tract nonspecific: spinoreticulotectothalamic, paleospinothalamic Somatomotor: contains dorsolateral and ventromedial pathways dorsolateral: corticospinal tract (forms medullary pyramids) corticorubrospinal tract (corticobulbar fibers) ventromedial: lateral vestibulospinal tract medial vestibulospinal tract pontine and medullary reticulospinal tracts ------end overview---- SOMATOSENSORY PATHWAYS: SPECIFIC (lemniscal like): DCML, neospinothalamic (NST), trigeminothalamic (TGT), spinocervicothalamic (SCTT) - large diameter, heavily myelinated, long axons. - few synaptic stations (projection nuclei, first somatosensory cortex) - neurons are place specific - neurons are submodality specific (respond to one kind of stimulus) - possess somatotopic organization - more important body parts are more heavily represented. (eg, fingers, prehensile tail) - possess submodality segregation - neurons responding to one kind of stimulus are grouped together. - exhibit rapid conduction of somesthetic information - highly organized to transmit detailed somesthetic information rapidly and accurately to the first somatosensory area of the cortex (S1) to allow sophisticated somatosensory discriminations to occur. DCML: hair movement, light touch, pressure, vibratory, joint position, muscle, tendon NST: all of DCML except tendon. Also mechanical and thermal nociception and non nociceptive thermal. cells of origin mainly within laminae I, IV, V, VII, and VIII. SCTT: all but muscle and tendon NONSPECIFIC (extralemniscal like): spinoreticulotectothalamic, paleospinothalamic - small diameter, finely myelinated (or unmyelinated), short axons. - many symaptic stations (reticular formation, medial and intralaminar nuclei of the thalamus [nonspecific thalamic nuclei], archicortex, paleocortex, neocortex [sparsely and diffusely] and basal ganglia) - neurons are NOT place specific - neurons are NOT submodality specific - there is little or no submodality segregation - exhibit slow conduction of sensory information. - permit somatosensory information to contribute to arousal of CNS - appear to be diffusely organized, and to function as central arousing system. spinoreticulotectothalamic: cells of origin mainly within laminae VII, VIII. What are they good for? They provide information about the external world to the CNS. This information is important for localization of stimuli and identification of types of stimuli impinging on skin and subcutaneous tissues, required for performance of sensory discrimination (exteroception). They also conduct information from more deeply positioned receptors about the position of the body and limbs in space (proprioception) and their movement through space (kinesthesis). What is submodality specificity? Receptors can be classified by the type of stimuli they respond to - that is, what submodality they represent. Mechanoreceptors respond to mechanical stimuli like hair movement, light touch, pinprick, pressure, vibration, pinch, joint movement, or changes in muscle length and/or tension. Thermal receptors respond best to changes in temperature. Chemoreceptors (a type of nociceptor) respond to chemical changes such as histamine release - things that are found in tissue usually as a result of trauma and inflammatory response. What is submodality segregation? Cells of the dorsal root ganglia (DRG) send their central processes into the spinal cord. These fibers carry specific information related to pericpheral receptive fields localized to specific parts of the body as well as specific receptor types - and thus specific submodalities of somesthetic information - eg, light touch, or joint movement. These fibers become segregated and terminate either within specific laminae of the cord, or ascend the dorsal funiculus directly to terminate in dorsal column nuclei (gracilis and cuneatus) What are projection nuclei? They are more than passive functioning relay nuclei - because they possess complex connections and integrative functions. Dorsal column nuclei (DCN) (nucleus gracilis, nucleus cuneatus) are projection nuclei which recieve input from DRG cells and interneurons as well as the red nucleus, brainstem reticular formation, and S1 somatosensory cortex. The DCN project into the inferior olive (as well as directly to the cerebellum), the reticular formation of the brainstem, the red nucleus, the tectum, the subthalamus, the posterior nuclear complex (which then projects into association cortex - those are all subsidiary connections. The "classical" connection of the DCML pathway is DRG --> DCN (projection nuclei)--> --decussates and travels in medial lemniscus--> VPL --> S1 cortex. Why is there a parallel arrangment of tracts? Each pathway conducts some information specific to that pathway, and some that is shared with other tracts. That which is shared seems to be the predominant functional organization of the somatosensory system. The parallel arrangment is responsible at least in part for much of the maintenance or recovery of somatosensory function following surgical or traumatic interruption of a single pathway. SOMATOMOTOR: Somatomotor: contains dorsolateral and ventromedial pathways dorsolateral: corticospinal tract (forms medullary pyramids) corticorubrospinal tract (corticobulbar fibers) ventromedial: lateral vestibulospinal tract medial vestibulospinal tract pontine and medullary reticulospinal tracts Corticospinal: controls segmental neurons in the spinal cord Corticobulbar: controls motor nuclei in the brain stem DORSOLATERAL PATHWAYS: CST, CRST, CBT These pathways terminate in the dorsolateral parts of the spinal grey matter and influence motor neurons controlling distal muscles of the extremities. They play a crucial role in steering the extremities and in the fine control required for manipulating objects with the fingers and hand. I guess they are probably important for being able to type :) :) They descend in the lateral quadrant of the spinal cord. The terminate (more specifically) in the lateral portion of the intermediate zone, and also among dorsolateral groups of motor neurons innervating distal limb muscles. These pathways terminate on a small number of spinal segments - unlike the ventromedial pathways, which send off large numbers of collaterals at different levels. The main dorsolateral tracts are rubrospinal (corticorubrospinal) - these fibers go from red nucleus of midbrain, cross the midline ventral to the red nucleus, and descend in the ventrolateral quadrant of the medulla. The corticospinal tracts are also dorsolateral. The dorsolateral brain stem and lateral corticospinal pathways control distal limb muscles. When the corticospinal tract is lesioned, but connections from motor cortex to red nucleus are spared, the brain is able to control distal limb movements through the corticorubrospinal pathway in lower animals. In man/higher primates, the rubrospinal pathway regresses somewhat, and the degree of functional recovery following cortical lesions is smaller. How does the motor cortex exert control over motor activity? I knew you'd ask. There are two main routes: - The corticobulbar tract controls the motor neurons innervating cranial nerve nuclei. These fibers originate in the cortex and terminate in the medulla. - the corticospinal tract controls the motor neurons innervating the spinal segments. These originate in the cortex and terminate in the spinal cord, forming the medullary pyramids as they travel through the medulla. These two systems act directly on the motor neurons, or on the interneurons closely related to them. They also act on the descending brain stem pathways, mainly the reticulospinal and rubrospinal tracts. VENTROMEDIAL PATHWAYS: LVST, MVST, P/MRST These are important for maintaining balance and for postural fixation. They terminate in the ventromedial part of the spinal grey matter, therefore they influence motor neurons which innervate proximal muscles and axial muscles. This group of pathways descends in the ipsilateral ventral columns of the spinal cord, and terminates on medial and ventral motor neurons and also on interneurons - including long propriospinal interneurons in the ventromedial part of the intermediate zone. These pathways send off large numbers of collaterals at different levels of the spinal cord. There are three main parts to the ventromedial pathway system: 1. lateral and medial vestibulospinal tracts: originate in lateral and medial vestibular nuclei and carry information for reflex control of equilibrium from the vestibular labyrinth. More specifically - the lateral vestibulospinal tract originates in LVN and projects to all levels of the ipsilateral cord in the ventral funiculus (doesn't use the MLF in the medulla). Synapses with extensor motor neurons in ventral horn for ipsilateral forelimb and hindlimb extensors - antigravity muscles. This is the main way the vestibular system effects changes to compensate for tilts and movements of the body. The medial vestibulospinal tract arises mainly from MVN, also a bit from the DVN, and projects bilaterally to the cervical spinal cord, using MLF and then ventral funiculus to synapse in the ventral horn on medial motor neuron pools for neck muscle control. This is responsible for postural changes in neck muscles, reorienting the head after a change in neck position (signalled by the vestibular receptors). 2. tectospinal tract: originates in the tectum of the midbrain (superior colliculus), which is important for the coordinated control of head and eye movements directed toward visual targets. 3. reticulospinal tract - originates in reticular formation of medulla and pons, which is composed mainly of interneurons and their processes, and can best be considered as a rostral extension of the spinal intermediate zone into the brain. VESTIBULAR SYSTEM: vestibular nuclei are related to other CNS regions - like CN nuclei III, IV, and VI, which control eye movements. The vestibular nerve transmits proprioceptive impulses from hair cell receptors in the saccule, utricle, and semicircular canals. Bipolar ganglion cell bodies lie in vestibular ganglion. Peripheral processes innervate the hair cells; central processes form the vestibular nerve, which joins the auditory nerve and enters the brainstem as the vestibulocochlear nerve at the pontomedullary border - lateral to the facial nerve. most incoming fibers divide to distribute to the vestibular nuclei. Vestibular nuclear complex: superior, lateral, medial, and inferior/descending. This complex occupies a large area of the dorsal medulla, just below the floor of the fourth ventricle. LVN-->lateral vestibulospinal tract fibers project from here to control extensor motor neurons of ipsilateral forelimb and hindlimb CN III, IV, VI send fibers bilaterally via the median longitudinal fasciculus (MLF) to the superior and medial vestibular nuclei. MVN then sends fibers via MLF to motor neurons in cervical cord - bilaterally Primary and secondary afferents (from vestibular system? and vestibular nuclei) project to the cerebellum via the inferior cerebellar peduncle, and from there they project to the flocculonodular lobe (vestibulocerebellum) and the vermis (& fastigial nucleus). These areas - flocculonodular lobe and vermis and fastigial nucleus - then project back upon the vestibular nuclei, influencing the output - this is so we can regulate posture and coordinate eye and head movements. Other vestibular nuclear complex projections: There is a projection from vestibular nuclei to the contralateral thalamus, near VMP/VPL, or possibly to a part of MGN lateral to VPL, with a possible relay in the midbrain (eg, we aren't sure of exact route), ultimately ending up in the contralateral parietal cortex area known as vestibular cortex. This pathway gives us awareness of body position and movement through space. There's a projection to reticular formation (esp nucleus ambiguus) which mediates visceral responses to excessive motion - vomiting, motion sickness. There's a projection to the contralateral vestibular nuclear complex so that the two sides function together as a coordinated pair. There's a projection back to the vestibular apparatus of the inner ear for modulation of sensory input - an example of descending modulation of afferent input which is common to many sensory systems. There are projections to motor neurons in the ventral horn of the spinal cord via the medial and lateral vestibulospinal tracts (see above). There are projections to the motor nuclei of the extrinsic eye muscles - CN III, IV, and VI. Mainly the MVN and SVN send ascending fibers, via MLF, to synapse bilaterally on motor neurons (excitatory) or interneurons (inhibitory) of the coulomotor, trochlear, and abducens nuclei. This helps to coordinate eye movement following changing of head position, and reorienting gaze. This ascending projection forms much of the vestibulo-ocular reflex (VOR) which allows an animal's gaze to stay fixed on an object even though the head is moving. What is up with nystagmus? Any rapid movement or rotation of head and body (with neck and trunk stabilized) results in involuntary rhythmic oscillations of the eyeball called nystagmus. This has predictable slow and rapid ocular excursions in opposite directions. The slow phase will be in the direction opposite the spin, and the fast phase will be toward the spin (an attempt to reestablish gase on a new fixed point in the new visual field). Clinically, nystagmus is named for the direction of the fast phase, but the slow phase is the primary physiological movement. When rotation is abruptly stopped, endolymph continues moving due to inertia - so there is a normal postrotatory nystagmus in which slow and rapid phases are reversed - slow excursion is in same direction as (the now over) spin. What is the point? Well, vestibular system disease can cause spontaneous nystagmus - this is always abnormal. In animals, signs of vestibular disturbance are only occasionally accompanied by vomiting. With unilateral lesions of the inner ear/peripheral vestibular system, the head and body tild TOWARD the side of the lesion. Also with lesions of central components of the vestibular system. Remember that stuff about old dog vestibular syndrome! Don't kill old, rolling dogs! How do sensory and motor systems relate? Motor systems need to know what's going on in the periphery. Three types of afferent events are important. 1. Exteroreceptors provide motor systems with information about the spatial coordinates of objects we encounter. 2. Proprioceptors relay information about the position of the body in space, angles of joints, and length and tension of muscles. Via proprioceptors, the motor systems get information about the condition of the muscles and joints which have to be moved. 3. Exteroceptors and proprioceptors both provide to the motor systems needed information about the concequences of the action taken. Motor systems are therefore intimately related to and functionally dependent upon sensory information. What's up with hierarchical organization of the motor system?? The motor system consists of neural circuits in four areas - the spinal cord, the brainstem (and reticular formation), the motor cortex, and the premotor cortex. Motor behavior can be classified on a continuum that rages from most automatic (eg, reflexes) to least automatic (eg, skilled voluntary movement). These behaviors are mediated by different parts of the motor circuitry as outlined below: LEVEL ONE: THE SPINAL CORD: - responsible for organizing the most automatic and stereotyped responses to stimuli - generates a variety of automatic behaviors known as *reflexes* even when disconnected from brain stem. - some reflexes include: knee jerk, pain withdrawal, alternating contraction of flexors and extensors as occurs with locomotion. - sensory input in the cord goes either directly to the motor neurons which innervate different muscles, or to motor neurons indirectly via interneurons. - all motor processing is focused on the motor neurons, which are the "final common pathway" of the motor systems. note: although motor neurons are the final common pathway, many actions are coordinated at the level of interneurons. Simple descending commands can produce complex effects by acting on interneurons. Alternating activity of flexors and extensors in locomotion is organized by networks of interneurons in the spinal cord. So - a given descending motor pathway exerts control of the motor response by acting directly on motor neurons, or via interneurons. Spinal interneurons can be excitatory or inhibitory. Activity of spinal motor neurons and interneurons reflects the sum of the inputs from periphery, supraspinal regions, and other interneurons or motor neurons - this is called CONVERGENCE. The convergence of peripheral and descending synapses on spinal neurons allows for a great deal of flexibility in the way that the CNS can influence motor neuron activity. LEVEL TWO: THE BRAIN STEM: The brain stem integrates motor commands which descend from higher levels. It also processes information that ascends from the spinal cord and comes in from the special senses. The brain stem is especially crucial for preocessing of afferent input related to cranial nerve nuclei and required for postural adjustment. For example: - vestibular nuclei which recieve information about the position of the head from the organs of the inner ear relay this information to the spinal cord through vestibulospinal tracts. This pathway allows the muscular adjustments needed to stabilize posture. - all descending motor pathwayys to the spinal cord originate in the brainstem with only one exception: the corticospinal tract. LEVEL THREE: THE MOTOR CORTEX (Brodmann's area 4) This is where the actions of the highest levels of cortical organization converge. This is also the area from which certain descending motor commands requiring cortical processing are issued to the brain stem, other subcortical structures, and to the spinal cord. These commands are mediated by the corticospinal system, which controls segmental neurons in the spinal cord, and by the corticobulbar system, which controls motor nuclei in the brain stem. LEVEL FOUR: PREMOTOR CORTEX (Brodmann's area 6) The premotor cortical regions are closely connected by short and long association fibers to the prefrontal and posterior parietal cortices. The premotor areas are responsible for identifying targets in space, for choosing a course of action, and for programming movement. These areas act mainly on the motor cortex (level three) but also act directly on the lower brainstem and spinal cord. --------PREMOTOR CORTEX | | | | | \/ | MOTOR CORTEX---- | | | | \/ | -------> BRAIN STEM | | | | | | | | \/ | |------->SPINAL CORD<--- | ----> out to muscles What are somatotopic maps? Each different component of the motor system contains a somatotopic map. In these maps, areas that influence adjacent body parts are adjacent to each other. Somatotopia is largely preserved in the interconnections of different levels. The area of motor cortex that controls the arm gets input from the premotor area that controls the arm, and in turn influences the arm areas of the descending brainstem pathways. The somatotopic maps are an important aspect of the hierarchical organization of the motor system! What else is important about the hierarchical system? Each level gets input from the periphery, so sensory input can modify descending commands. Also, higher levels can suppress or allow the transmission of afferent input to themselves. What else do we need to know? Two other parts of the brain are important for motor function. These are the cerebellum and the basal ganglia. What does the cerebellum have to do with it? - a lot! - adjusts actions of the brainstem motor structures and the motor cortex by comparing descending signals for intended action with sensory signals resulting from the actual action. - updates and controls movement when movement deviates from intended trajectory. What about the basal ganglia? - these are the caudate nucleus, and the putamen and globus pallidus (which together are called the striatum). Note that the amygdala is not part of the basal ganglia due to differences in structure and function! - thankfully, these are not as well understood. - they get input from all the cortical areas. - they focus their actions mainly on motor and premotor areas of the cortex - diseases of basal ganglia cause motor abnormalities, mainly dyskinesias (in people) and changes in muscle tonus (hypertonia and pacing in animals) THERE IS ALSO PARALLEL ORGANIZATION! --------PREMOTOR CORTEX | | | | | \/ | MOTOR CORTEX---- | | | | \/ | -------> BRAIN STEM | | | | | | | | \/ | |------->SPINAL CORD<--- | ----> out to muscles Note that each level is not only hierarchially (serially) connected to the level beneath it, but that each level is independently able to act on the final common pathway - to control spinal interneurons or motor neurons *directly*. It is this parallel organization which allows commands from higher levels to either modify or supercede lower order reflex behavior. There is also overlap between the different elements which is very important for the recovery of function after local lesions/trauma. THE BIG PICTURE: Incoming signals from the activation of peripheral sensory receptors are carried into the cord by primary afferent fibers. These axons act on segmental interneurons and motor neurons and generate reflex outputs, mediated by the spinal cord. The interneuron networks of each segment of cord also connect to other cord segments through propriospinal (aka spinospinal connection) neurons. There are also ascending pathways which convey the information to motor centers of the brainstem and via thalamic nuclei to the cerebral cortex. The brain stem and the cortical centers then project back to the segmental networks of the cord to control reflex activity and to produce skilled voluntary movements. Output of the supraspinal centers is influenced and ultimately integrated by the cerebellum and basal ganglia. What controls those pesky motor neurons in the spinal cord? The motor neurons in the spinal cord are subject to afferent input *and* descending controls. Want to hear more? Ok: Afferent fibers and motor neurons: axons of the DRG cells which enter the spinal cord carrying sensory information from the periphery immediately send terminal branches to almost all laminae of the dorsal horn. Some fibers continue within the intermediate zone, and a few of them have the temerity to continue further still, until they reach the groups of motor neuron cell bodies in the ventral horn, where they synapse (obviously these are the monosynaptic connections.) What's the deal with motor neurons anyway? - they lie in the ventral horn of the cord - a group of them innervating a single muscle are a "motor neuron pool" - the pools are segregated into "longitudinal columns" - the columns extend through 2-4 spinal cord segments - impulses from a particular afferent axon will be distributed to motor neurons in the same pool, or those in pools innervating muscles of similar function. - there are two main groups of motor neuron pools: medial and lateral - medial ones: project to axial muscles, innervate mm of neck and back - lateral ones: project to limb muscles - lateral ones further subdivided into ventral and dorsal - ventrolateral: innervates proximal limb mm - shoulder, pelvic girdle - dorsolateral: innervates distal limb mm - digits, etc. What about interneurons and propriospinal neurons? These tend to hang out in the intermediate grey area. Welcome to the interzone....(sorry to William S.B.) Lateral parts of the intermediate zone contain interneurons projecting ipsilaterally to the dorsolateral motor neuron pools (to innervate distal limb mm). Medial regions of the intermediate zone contain interneurons projecting bilaterally to ventrolateral and medial motor neuron groups which innervate proximal limb mm and axial mm on both sides of the body. Note that many descending pathways terminate on interneurons in the intermediate grey.