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Dental lecture: brain stem, ascending and descending pathways
 

Dental lecture: brain stem, ascending and descending pathways

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    Dental lecture: brain stem, ascending and descending pathways Dental lecture: brain stem, ascending and descending pathways Presentation Transcript

    • Neurophysiology lecture topics 1. Role of brainstem and reticular formation 2. Ascending and descending tracts muscle power 3. Maintenance of posture, equilibrium, coordination muscle tone 4. Functions of limbic system and basal ganglia
    • Brains stem • Midbrain • Pons • Medulla
    • Role of brainstem • • • • • • • • • • Intermediate centre in controlling motor functions Ascending and descending pathways cross brain stem Contains vital centres Contains reticular formation Plays a vital role in attention, arousal and states of consciousness Brainstem injuries easily cause loss of consciousness Most of the cranial nerves are connected to brainstem Contain pain pathways Involved in suprasegmental control of reflexes and muscle tone Extrapyramodal tracts strats from the brain stme
    • Reticular formation • Located in the core of the brainstem • Network of neurons • Main centre of ascending and descending tracts • Functions: consciousness, motor control, pain modulation, cardiovascular control, sleep centres
    • Ascending pathways • Somatosensory pathways – Dorsal column – medial lemniscus pathway – Spinothalamic tracts • Anterior spinothalamic tract • Lateral spinothalamic tract – Spinocerebellar tracts • Dorsal • Ventral
    • Sensory area in the brain Ascending Sensory pathway Central Connections Sensory nerve Touch stimulus Receptor Sensory modality
    • Two main ascending pathways • Dorsal column - medial lemniscus pathway fast pathway • Spinothalamic pathway slow pathway These two pathways come together at the level of thalamus
    • Spinothalamic pathway Dorsal column pathway Lateral Spinothalamic tract Anterior Spinothalamic tract
    • Dorsal column pathway • touch: fine degree • highly localised touch sensations • vibratory sensations • sensations signalling movement • position sense • pressure: fine degree Spinothalamic pathway • Pain • Thermal sensations • Crude touch & pressure • crude localising sensations • tickle & itch • sexual sensations
    • 3rd order neuron thalamocortical tracts internal capsule thalamus Medial lemniscus Dorsal column nuclei 2nd order neuron (cuneate & gracile nucleus) Dorsal column 1st order neuron
    • 3rd order neuron thalamocortical tracts internal capsule thalamus Spinothalamic tracts 2nd order neuron 1st order neuron
    • Proprioceptive pathways • Dorsal column – medial lemniscus – thalamocortical pathway (conscious proprioception) • Spinocerebellar pathway (unconscious proprioception)
    • Main descending pathways • Motor pathways • Corticospinal and corticobulbar tracts • Starts from the motor cortex
    • Motor cortex • Located in the frontal lobe • Precentral gyrus
    • Motor homunculus First discovered by Penfield
    • Corticospinal tract (Pyramidal tract) • Starts from large cortical cells (pyramidal cells) in the primary motor cortex • These cells are called Betz cells • From these cells starts the motor axon • Divided into – Lateral corticospinal tract • Major part of the CST, cross to the opposite side at the level of medulla – Medial corticospinal tract (or anterior CST) • Minor part, uncrossed tract, at the level of spinal cord cross to the opposite side
    • Course of the corticospinal tract • Descends through – internal capsule – at the medulla • cross over to the other side – descends down as the corticospinal tract – ends in each anterior horn cell – synapse at the anterior horn cell
    • internal capsule Upper motor neuron Medulla anterior horn cell Lower motor neuron
    • Motor system • Consists of – Upper motor neuron • Corticospinal tract (pyramidal tract) • Extrapyramidal tracts – Lower motor neuron • Alpha motor neuron • Gamma motor neuron
    • Lower motor neuron • consists of mainly • alpha motor neuron – and also gamma motor neuron gamma motor neuron alpha motor neuron
    • Arrangement at the anterior horn cell gamma motor neuron alpha motor neuron corticospinal tract
    • alpha motor neuron • this is also called the final common pathway • Contraction of the muscle occurs through this whether – voluntary contraction through corticospinal tract or – involuntary contraction through gamma motor neuron - stretch reflex - Ia afferent
    • Upper motor neuron • Consists of – Corticospinal tract (pyramidal tract) – Extrapyramidal tracts
    • Extrapyramidal tracts • starts at the brain stem • descends down either ipsilaterally or contralaterally • ends at the anterior horn cell • modifies the motor functions
    • Reticulospinal tract • Starts from the reticular formation • Maintain normal postural tone • Controls mainly gamma motorneurons (lesser extent alpha motor neurons) • Inhibit antigravity muscles (extensor) • End on interneurons • Inhibited by cerebral influence • Mainly ipsilateral
    • Reticular formation • Loosely arranged cell bodies in the central core of the brain stem midbrain pons • Pontine reticular area medulla • Medullary reticular area spinal cord
    • Vestibulospinal tract • Starts from the vestibular nuclei (present in the medullar region) • Excitatory to alpha motor neurons of antigravity muscles (extensor) • End on interneurons • Regulates posture and balance • Mainly ipsilateral • There are inputs from vestibular organs and cerebellum to vestibular nuclei
    • • Rubrospinal and tectospinal tracts are not functionally important in human nervous system
    • pyramidal tracts extrapyramidal tracts Upper motor neuron alpha motor neurone gamma motor neurone Lower motor neuron
    • Suprasegmental control of reflexes and muscle tone • Alpha motor neuron is the final pathway • Gamma motor neuron control • Alpha-gamma coactivation • Supraspinal control – Pyramidal tract: activation of alpha – Extrapyramidal: mixed effects on alpha and gamma motor neurons • Net effect: suppression of gamma motor neuron
    • Extrapyramidal tracts •Voluntary movement •Muscle tone Gamma motor neuron Corticospinal tract Alpha motor neuron Muscle spindle • • • There is a complex effect of corticospinal and extrapyramidal tracts on the alpha and gamma motor neurons (in addition to the effect by muscle spindle) There are both excitatory and inhibitory effects Sum effect – excitatory on alpha motor neuron – Inhibitory on gamma motor neuron
    • Clinical Importance of the motor system examination • Tests of motor function: – Muscle power • Ability to contract a group of muscles in order to make an active movement – Muscle tone • Resistance against passive movement
    • Basis of tests • Muscle power – Test the integrity of motor cortex, corticospinal tract and lower motor neuron • Muscle tone – Test the integrity of stretch reflex, gamma motor neuron and the descending control of the stretch reflex
    • Muscle tone • Resistance against passive movement – Gamma motor neuron activate the spindles – Stretching the muscle will activate the stretch reflex – Muscle will contract involuntarily – Gamma activity is under higher centre inhibition
    • Clinical situations • Muscle power – Normal – Reduced (muscle weakness) • muscle paralysis • muscle paresis • Muscle tone – Normal – Reduced • Hypotonia (Flaccidity) – Increased • Hypertonia (Spasticity)
    • Main abnormalities • Muscle Weakness / paralysis – Reduced muscle power • Flaccidity – Reduced muscle tone • Spasticity – Increased muscle tone
    • • Lower motor neuron lesion causes – flaccid paralysis (flaccid weakness) • Upper motor neuron lesion causes – spastic paralysis (spastic weakness)
    • Lower motor neuron lesion • • • • • • muscle weakness flaccid paralysis muscle wasting (disuse atrophy) reduced muscle tone (hypotonia) reflexes: reduced or absent spontaneous muscle contractions (fasciculations) • plantar reflex: flexor • superficial abdominal reflexes: present
    • Muscle wasting
    • Fasciculations
    • Upper motor neuron lesion • • • • • • • • muscle weakness spastic paralysis increased muscle tone (hypertonia) reflexes: exaggerated Babinski sign: positive superficial abdominal reflexes: absent muscle wasting is very rare clonus can be seen: – rhythmical series of contractions in response to sudden stretch • clasp knife effect can be seen – passive stretch causing initial increased resistance which is released later
    • Clonus Clasp knife effect
    • Stroke patient walking
    • Babinski sign • when outer border of the sole of the foot is scratched • upward movement of big toe • fanning out of other toes • feature of upper motor neuron lesion • extensor plantar reflex • seen in infants during 1st year of life (because of immature corticospinal tract)
    • positive Babinski sign
    • Site of lesions Cortex Internal capsule Brain stem Spinal cord Anterior horn cell Motor nerve Neuromuscular junction Muscle
    • Site of lesions quadriplegia (tetraplegia) all 4 limbs are affected cervical cord or brain stem lesion hemiplegia one half of the body including UL and LL lesion in the Internal capsule paraplegia both lower limbs thoracic cord lesion monoplegia only 1 limb is affected either UL or LL, lower motor neuron lesion
    • Conditions which cause increased muscle tone • Spasticity – Stroke • Rigidity – Parkinsonism • Lead pipe rigidity • Cogwheel rigidity • Brainstem lesions – Decerebrate rigidity – Decorticate rigidity
    • Reticular formation • A set of network of interconnected neurons located in the central core of the brainstem • It is made up of ascend-ing and descend-ing fibers • It plays a big role in fil-ter-ing incom-ing stim-uli to dis-crim-i-nate irrel-e-vant back-ground stim-uli • There are a large number of neurons with great degree of convergence and divergence
    • Functions • Maintain consciousness, sleep and arousal • Reticulospinal pathways are part of the extrapyramidal tracts • Several nuclei (PAG, NRM) are part of the descending pain modulatory (inhibitory) pathway
    • Basal ganglia • These are a set of deep nuclei located in and around the basal part of the brain that are involved in motor control, action selection, and some forms of learning • Purposeful movement
    • Basal ganglia • Caudate nucleus • Putamen • Globus pallidus –(internal and external) • Subthalamic nuclei • Substantia nigra International Basal Ganglia Society
    • (Ref. Guyton)
    • basal ganglia • caudate nucleus • putamen • globus pallidus • subthalamic nuclei • substantia nigra corpus striatum lentiform nucleus
    • • Interconnecting circuitry through these nuclei • These circuits start from the cortex and ends in the cortex • These circuits are very complex • Their effect is excitatory or inhibitory on motor functions • They also have a role in cognitive functions.
    • Cortex Thalamus Putamen globus pallidus
    • Functions • eg. – writing letters of alphabet, – cutting papers with scissors, – hammering nails, – passing a football, – Vocalisation – Cognitive control of movement
    • • Some of these circuits are excitatory and some inhibitory • This depends on the neurotransmitter involved. • Inhibitory: dopamine and GABA • Excitatory: Ach • Others: glutamate (from cortical projections) enkephalin etc
    • Following pathways are known: • Dopamine pathway from substantia nigra to caudate nucleus and putamen • GABA pathway from caudate and putamen to globus pallidus and substantia nigra • Ach pathway in the caudate and putamen
    • glutamate Thalamus Reticular formation + striatum Cortex Interneurons: Ach + Caudate Putamen Dopamine Thalamus GABA globus pallidus Subthalamic nucleus GABA Substantia nigra Reticular formation
    • Functions of Basal Ganglia • • • • • Motor control Learning Sensorimotor integration Reward Cognition
    • Basal Ganglia disorders • Basal ganglia disorders are also called extrapyramidal disorders • Classical disorder is ―Parkinsonism‖ • Other disorders: Athetosis, Chorea, Hemiballismus
    • Parkinsonism • due to destruction of dopamine secreting pathways from substantia nigra to caudate and putamen. – also called ―paralysis agitans‖ or ―shaking palsy‖ – first described by Dr. James Parkinson in 1817. • In the west, it affects 1% of individuals after 60 yrs Classical Clinical features: • Tremor, resting • Rigidity of all the muscles • Akinesia (bradykinesia): very slow movements • Postural instability
    • – expressionless face – flexed posture – soft, rapid, indistinct speech – slow to start walking – rapid, small steps, tendency to run – reduced arm swinging – impaired balance on turning – resting tremor (3-5 Hz) (pill-rolling tremor) • diminishes on action – cogwheel rigidity – lead pipe rigidity – impaired fine movements – impaired repetitive movements
    • Physiology of Posture Prof. Vajira Weerasinghe Dept of Physiology
    • Dynamic vs static nature of motor control • Static stability – is dependent on the position of the centre of gravity with respect to the base of support • whereas dynamic stability – is dependent more on the moment of inertia of the body
    • Adult vs child • In normal standing, a tall adult will have a much larger moment of inertia than a toddler • Once the centre of gravity moves outside the base of support the body will begin to fall – The adult with the large moment of inertia will fall much more slowly and will therefore have a longer time to react to prevent the fall – This is one of the reasons that young children fall more often than adults.
    • Postural control • Maintaining static nature of the body
    • maintenance of posture • mainly to maintain the static posture • necessary for the stability of movements • involve a set of reflexes • integrated at spinal cord, brain stem and cortical level
    • normal postural control • three inputs are required – Vision – Proprioception (joint position sense) – Vestibular Mechanism (balance mechanisms) – Cutaneous sensations
    • Postural reflexes • Spinal cord reflexes – – – – stretch reflex positive supporting reaction (magnet reaction) negative supporting reaction mass reflex • Brainstem refelxes – – – – – – tonic labyrinthine reflex (vestibular) tonic neck reflexes labyrinthine righting reflex neck righting reflex body-on-head righting reflex body-on-body righting reflex • Cortical reflexes – optical righting reflex – placing reactions – hopping reaction
    • • these reflexes are under higher centre inhibition • transection of spinal cord or brain stem at different levels release this inhibition • then the relevant reflexes are seen
    • Retina Occulomotor system vestibular nuclei cerebellum complex pathways vestibular system neck receptors pressure & other receptors postural adjustments
    • cerebellum • centre of motor coordination • cerebellar disorders cause –incoordination or ataxia
    • structure • Cerebellum is divided into 3 lobes by 2 transverse fissures – anterior lobe – posterior lobe – flocculonodular lobe
    • structure – anterior lobe (paleocerebellum) – large posterior lobe (neocerebellum) – flocculonodular lobe (archicerebellum is the oldest lobe)
    • • Anterior cerebellum and part of posterior cerebellum – receives information from the spinal cord • Rest of the posterior cerebellum – receives information from the cortex • Flocculonodular lobe – involved in controlling the balance through vestibular apparatus
    • • Functionally cerebellum is divided into 3 areas medial to lateral – lateral zone – intermediate zone – vermis
    • Inputs •Corticopontocerebellar (cortical input) •Olivocerebellar •Vestibulocerebellar (balance, muscle tone, posture) •Reticulocerebellar (muscle tone, posture) •Spinocerebellar Cerebellum •(proprioception) Outputs Through deep cerebellar nuclei Brain stem (extrapyramidal pathways) Thalamus -> Cortex Basal ganglia
    • Neuronal circuitry of the cerebellum • Main cortical cells in cerebellum are known as Purkinje Cells (large cells). • There are about 30 million such cells. • These cells constitute a unit which repeats along the cerebellar cortex.
    • Functions of cerebellum • planning of movements • timing & sequencing of movements • particularly during rapid movments such as during walking, running • from the peripheral feedback & motor cortical impulses, cerebellum calculates when does a movement should begin and stop
    • Motor Cortex Thalamus Cerebellum brain stem nuclei proprioceptive tactile feedback Muscles
    • ‘Error correction’ • cerebellum receives two types of information – intended plan of movement • direct information from the motor cortex – what actual movements result • feedback from periphery – these two are compared: an error is calculated – corrective output signals goes to • motor cortex via thalamus • brain stem nuclei and then down to the anterior horn cell through extrapyramidal tracts
    • • ‘Prevention of overshoot’ – Soon after a movement has been initiated – cerebellum send signals to stop the movement at the intended point (otherwise overshooting occurs) • Ballistic movements – rapid movements of the body, eg. finger movements during typing, rapid eye movements (saccadic eye movements) – movements are so rapid it is difficult to decide on feedback – therefore the movement is preplanned • Cerebellum perform motor learning (memory)
    • planning of movements • mainly performed by lateral zones • sequencing & timing – lateral zones communicate with premotor areas, sensory cortex & basal ganglia to receive the plan – next sequential movement is planned – predicting the timings of each movement
    • features of cerebellar disorders • ataxia – incoordination of movements – ataxic gait • broad based gait • leaning towards side of the lesion • dysmetria – cannot plan movements • past pointing & overshoot • decomposition of movements • intentional tremor
    • features of cerebellar disorders • Dysdiadochokinesis (adiadochokinesis) – unable to perform rapidly alternating movements • dysarthria – slurring of speech • nystagmus – oscillatory movements of the eye
    • features of cerebellar disorders • hypotonia – reduction in tone • due to reduction in excitatory influence on gamma motor neurons by cerebellum (through vestibulospinal tracts) • decreased reflexes • head tremor • head tilt • In unilateral cerebellar lesions, incoordination occurs in the ipsilateral side
    • • But what finally drives us to action??? •perhaps motivation •motivation is controlled by limbic system and hypothalamus
    • Limbic system
    • limbic system • nuclei – – – – amygdala septal nuclei mammillary body hypothalamus • cortical areas – – – – hippocampal gyrus cingulate gyrus dentate gyrus entorhinal, amygdaloid cortex • paralimbic structures • orbital gyrus, insula, nucelus accumbens, thalamic nuclei, superior temporal gyrus, • fibre tracts: fornix, medial forebrain bundle
    • limbic cortex • consist of 3 layered cortex (in contrast to 6 layered cortex of the neocortex)
    • • Limbic system is a link between the brain stem and neocortex • Limbic structures are connected to each other and with the association cortex and the brain stem
    • • Medial forebrain bundle is a major efferent connection of the limbic system: • projected to the hypothalamus, reticular formation. Influence on autonomic and endocrine activity • Amygdala receives inputs from olfactory pathways • Connections with the neocortex provide a synthesis of emotional and rational thought
    • Functions Limbic system is also referred to as the ‘emotional brain’ • Emotional (include motor activity) • Behavioural (Motivations, Drives: appetite, thirst, sexual behaviour, Reward system) • Memory – Utilizes the hypothalamus to effect the physical manifestations associated with emotions, etc.
    • Complex role of the limbic system • as an intermediary between – external events (carried to the CNS via afferents) – our processing of those events (involving cortical and subcortical brain areas) – our responses to those events (both behavioral and autonomic)
    • Role in memory storage • Working memory—short term – cortical phenomenon • Explicit (declarative)—factual knowledge – temporal events, stored in hippocampus • Examples: what innervates biceps femoris m.? • Implicit (procedural)—learned skills – unconsciously recalled—includes emotional responses—stored in amygdala (at least in part) • Examples: writing, playing a musical instrument
    • Hippocampus • is a part of the brain located inside the temporal lobe • plays a major role memory consolidation • responsible for spatial memory • might act as a cognitive map — a neural representation of the layout of the environment. • In Alzheimer's disease, the hippocampus becomes one of the first regions of the brain to suffer damage