Neuroanatomy 3 Instructor PDF

BMS Neuroanatomy Lecture 3 Spinal Cord Sensory Pathways (In-Person Class) Presented By: Dr. K. Lumsden; [email protected] (Toronto Campus) Dr. M. Doroudi; [email protected] Boucher Campus) Organization of the Nervous System: Central Nervous System: integration, decision making, and processing of sig...

BMS Neuroanatomy Lecture 3 Spinal Cord Sensory Pathways (In-Person Class) Presented By: Dr. K. Lumsden; [email protected] (Toronto Campus) Dr. M. Doroudi; [email protected] Boucher Campus) Organization of the Nervous System: Central Nervous System: integration, decision making, and processing of signals occurs Peripheral Nervous System: Signals from CNS traveling to/from targets in the periphery 2 The Central Nervous System Grey matter White matter Cell bodies (soma), dendrites, axon terminals of neurons Where synapsing occurs between cells Information processing / passing of signals Bundles of axons forming white matter pathways in the central nervous system Where signals travel from one location to another within the CNS Where can we find grey matter? Cerebral cortex Cortical nuclei/diencephalon (ex: Thalamus) Grey horns of spinal cord Where can we find white matter? Cerebral tracts (Corpus callosum) White columns of spinal cord Spinal pathways (coming next week!) Peripheral Nerves 3 RECALL: Anatomy of the Spinal Cord AXIAL CUT WHITE MATTER COLUMNS / FUNICULUS – where information travels from one area to another GRAY MATTER HORNS = where information synapses / transfers to another cell within CNS Posterior – ascending sensory signals Anterolateral – contains both ascending sensory and descending motor signals Posterior – sensory signals Lateral – autonomic (unconscious) signals Anterior – motor signals 4 Connection between CNS and PNS: Anatomy of the PNS: 1. 31 pairs of spinal nerves 2. 12 pairs of cranial nerves 3. Autonomic nerves Signals travelling in spinal nerves can enter enter/exit the spinal cord via connections known as nerve “roots” Dorsal root: provides pathway for afferent/ incoming information to flow from spinal nerve into the spinal cord Sensory information may synapse in dorsal grey horn OR ascend in white columns Ventral root: provides pathway for efferent/outgoing signals to leave spinal cord and enter spinal nerve Motor information may synapse in ventral grey horn or descend in white columns Neurons will pass the signal from one cell to the next by forming a synapse between adjacent axon terminal and the cell body 5 RECALL: Anatomy of the Spinal Nerves There are 31 pairs of spinal nerves exiting off spinal cord Nerves are WHITE matter pathways that provide a route for signals to travel between PNS and CNS Spinal nerves are IPSILATERAL and will carry signals to and from targets/receptors on the same side of the body Posterior/Dorsal root carries sensory/afferent information into CNS so it can ascend in the cord to reach the cortex Swelling/bulb contains the soma/cell bodies of incoming sensory cells Ventral/Anterior root carries motor/efferent information out of CNS that has descended from the cortex to the cord Ventral & Dorsal roots come together to form a spinal nerve in the intervertebral foramen 6 Signals of the CNS and PNS: Each spinal nerve has the ability to carry many different signals into the CNS and out of the CNS Nervous signals are classified based on the direction that they flow, the type of signal that is carried and the target/effector that carries out the response SOMATIC = “of the body” what you are aware of (conscious) à Sensory information from surface of the skin / innervation and control of skeletal muscles AUTONOMIC = what you are not aware of (unconscious) à feedback information from the internal organs, glands, smooth, cardiac muscle and innervation/control of autonomic muscles (smooth and cardiac) “CODES” of the Nervous system: GSE: General somatic efferent à Conscious/voluntary motor (body movement) GSA: General somatic afferent à General sensation (sensation on skin) GVE: General visceral efferent à Involuntary/subconscious motor to organs/viscera (beating of heart) GVA: General visceral afferent à Visceral sensation (stretch of vessel) Peripheral Nervous System Integration between PNS and CNS Axon relay The “running” of the race happens within white matter pathways (white columns and tracts of the CNS) The “passing” of the baton between neurons is a synapse which happens in grey matter (horns, nuclei of the CNS) Communication to and from the brain involves chains of neurons (like a relay race) and they are numbered according to position Decussate – when signals cross over from one side to another (left to right; or right to left). Occurs within the CNS The signals in the relay race can either be ascending (sensory) that travel to the cortex OR they can be descending (motor) that travel to a target in the periphery (Peripheral fiber) (CNS fiber) 8 Peripheral Nervous System Integration between PNS and CNS Receptors for sensory information are located at the end of peripheral nerves within the layers of the skin FINISH! The destination for most incoming/ascending sensory information in the post-central gyrus of the parietal lobe Gather information about the surrounding environment and send information about sensation back to CNS for processing Afferent/Sensory information comes into the CNS via the dorsal root of the spinal nerve Ascends in the spinal cord via posterior, lateral and anterior columns Majority of signals synapse in Thalamus Two types of signals are possible: General Sensory Signals “GSA” Visceral Sensory Signals “GVA” Touch, temperature, tickle, pain, proprioception, pressure and vibration Sensory signals from organs (stretch, pressure, chemical) START! Each one of these triggers a response in a specific peripheral receptor 9 Peripheral Nervous System START! PRE-CENTRAL GYRUS OF FRONTAL LOBE Integration between PNS and CNS Efferent/Motor signals originate in the pre-central gyrus of the frontal lobe (primary motor cortex) Descend in the spinal cord via anterior or lateral columns Exits the CNS via ventral root of the spinal nerve Target for motor signals are known as effectors Two types of signals are possible: FINISH! Voluntary Motor Signals “GSE” Involuntary Motor Signals “GVE” Effector: Skeletal muscle Effectors: Glands, Heart muscle, smooth muscle 10 Overview of Sensory Tracts (Pathways) of the Spinal Cord v The sensory tracts of the spinal cord relay the sensory information from sensory receptors to the brain’s sensory areas. v These tracts consist of three neurons (except for the spinocerebellar tracts, which consists of two neurons). The first-order neuron resides in the dorsal root ganglion of the spinal nerve and conducts impulses from sensory receptors to the spinal cord or medulla oblongata. v The second-order neuron is located either in the dorsal horn of the spinal cord or within the medulla oblongata and relays impulses to the thalamus. v The third-order neuron residing in the thalamus conveys impulses to the general sensory area (postcentral gyrus) of the parietal lobe of the cerebral cortex. Peripheral Nervous System Integration between PNS and CNS Ascending tracts are sensory, deliver information from PNS to CNS Major ascending, sensory tracts 1. Spinothalamic: pain and temperature 2. Spinocerebellar: proprioception 3. Posterior/Dorsal column: discriminative touch 4. Spinotreticular tract 5. Spinotectal tract 6. Spinomesecephalic tract 7. Spino-olivary tract All of these enter the spinal cord using the DORSAL root of the spinal nerve 12 Peripheral Nervous System Posterior/Dorsal Columns: Transmits: General sensation (GSA) for fine touch and discriminative touch information from receptors of the skin FINISH! POST-CENTRAL GYRUS OF PARIETAL LOBE Pathway: Neuron 1: 1st order neuron Originates in peripheral receptor Travels towards CNS via spinal nerve Enters CNS via dorsal root of spinal nerve and does NOT synapse Axon continues to ascend in dorsal columns of spinal cord Synapses in nuclei of medulla Neuron 2: 2nd order neuron Cell body originates in nucleus of medulla Axon decussates in medulla oblongata Ascends to thalamic nuclei via medial leminiscus pathway Synapses in thalamus START! Neuron 3: 3rd order neuron Cell body originates in thalamus Axon travels from thalamus to post-central gyrus of parietal lobe Synapses in primary somatosensory cortex Sensation is interpreted 13 Peripheral Nervous System Posterior/Dorsal Columns: F F F F C G G C Dorsal Columns can be further subdivided in order to categorize touch information: Upper limb = Fasciculus Cuneatus Lower limb = Fasciculus Gracilis FINISH! Neuron 1: 1st order neuron Originates in peripheral receptor in UPPER limb or LOWER limb Travels towards spinal cord via spinal nerve Enters CNS via dorsal root and ascends in dorsal columns (FC or FG) of spinal cord Synapses in one of the nuclei in medulla: POST-CENTRAL GYRUS OF PARIETAL LOBE NG NC FC (lateral columns) = Fasciculus cuneatus – transmitting touch information from upper limbs Information travelling in this column will synapse in the “nucleus cuneatus” (NC)of the medulla START! FG (medial columns) = Fasciculus gracilis – transmitting touch information from lower limbs Information travelling in this column will synapse in the “nucleus gracilis” (NG) of the medulla 14 Dorsal Column (Medial Lemniscus) Tract v The dorsal column (medial lemniscus) tract conducts the nerve impulses for conscious proprioception – collected from the joints and muscles except for the temporomandibular joint - and discriminative (fine) touch and vibration from the entire body (except most of the head region) to the cerebral cortex. v The first-order neuron of this tract is located in the dorsal root ganglion of the spinal nerve. The axon of this neuron ascends in the dorsal column of the spinal cord. Fibres from the lower part of the body (i.e. the legs) ascend in the fasciculus gracilis. In contrast, the fibres from the upper part of the body (i.e. the arm) ascend in the fasciculus cuneatus. v The axon of the first-order neuron synapses with the second-order neuron in the nucleus gracilis and nucleus cuneatus within the medulla oblongata. v The axon of the second-order neuron decussates (crosses the midline) and ascends through the brainstem as the medial lemniscus. v The medial lemniscus synapses with the third-order neuron in the thalamus. The axon of the third-order neuron passes through the internal capsule and projects to the somatosensory area of the cerebral cortex (postcentral gyrus) for conscious sensation. Peripheral Nervous System Spinocerebellar Tract: Transmits: General sensation (GSA) for proprioception (bodies ability to observe position in space) Pathway: Neuron 1: 1st order neuron Originates in peripheral receptor Travels towards CNS via nerve Axon enters dorsal root and synapses in dorsal grey horn upon entry FINISH! CEREBELLUM Neuron 2: 2nd order neuron Cell body originates in dorsal grey horn Remains ipsilateral Ascends in anterolateral white column of spinal cord Axon ascends to the level of the cerebellum Synapses in cerebellar hemisphere START! *Injury will always be considered ipsilateral 16 Dorsal Spinocerebellar Tract vThe posterior spinocerebellar fibres receive joint muscle information from the muscle spindles, tendon organs, and joint receptors of the trunk and lower limbs. The cerebellum uses this information concerning the tension of muscle tendons and the movements of muscles and joints to coordinate limb movements and maintain posture. v The first-order neuron of this tract, located in the dorsal root ganglion of the spinal nerve, synapses with the second-order neuron that resides in the dorsal horn of the spinal cord. These neurons are known collectively as the nucleus dorsalis (Clarke's column). v The axons of the second-order neurons form the dorsal spinocerebellar tract that travels in the spinal cord’s lateral column (funiculus). This tract ascends through the medulla oblongata and ends in the ipsilateral cerebellar cortex after passing through the inferior cerebellar peduncle. v Because the nucleus dorsalis (Clarke's column) extends only from the C8 spinal cord segment caudally to the L3 spinal cord segment, axons entering the spinal cord from the posterior roots of the lower lumbar and sacral segments ascend in the fasciculus gracilis in the posterior white column until they reach the L3 segment, where they enter the nucleus dorsalis. v It’s worth mentioning that unconscious proprioception from the upper limb is conveyed by a different tract – the cuneocerebellar tract – to the cerebellum. Anterolateral System The anterolateral system (ALS) comprises a set of fibres encoding for nondiscriminative touch, pain, and temperature. These fibres can be divided into several different tracts. Most fibres travel to the thalamus (spinothalamic tract) and play a key role in mediating conscious perception or awareness of pain and temperature. Other smaller tracts modulate these sensations and terminate in various targets in the brainstem and diencephalon. The ALS includes: v Spinothalamic tracts (lateral and anterior); for non-discriminative touch, pain, and temperature v Spinoreticular tract; ends in the reticular formation of the medulla, pons, and midbrain. v Spinotectal tract; travels to the superior colliculus in the midbrain v Spinomesencephalic or spinoperiaqueductal tract ends in the periaqueductal grey matter in the midbrain v Spino-olivary tract projections to the inferior olivary nuclei in the medulla FINISH! Peripheral Nervous System Spinothalamic Tract: Transmits: General sensation (GSA) for pain and temperature sensation from the body POST-CENTRAL GYRUS OF PARIETAL LOBE Pathway: Neuron 1: 1st order neuron Originates in peripheral receptor Travels towards CNS via nerve Axon enters dorsal root and synapses in dorsal grey horn upon entry Neuron 2: 2nd order neuron Cell body originates in dorsal grey horn Axon decussates at the level of entry Ascends in antero-lateral white columns of spinal cord Synapses in thalamus START! Neuron 3: 3rd order neuron Cell body originates in thalamus Axon travels from thalamus to postcentral gyrus Synapses in primary somatosensory cortex of parietal lobe 19 v The lateral spinothalamic tract conveys the impulses concerned with pain and temperature collected from the entire body (except the head region). v The first-order neuron of this tract is located in the dorsal root ganglion of the spinal nerve. The axon of this neuron synapses with the second-order neuron in the dorsal horn of the spinal cord. The neurotransmitter used at this synapse is thought to be Substance P. v The axon of the second-order neuron decussates (crosses the midline) and ascends through the lateral column of the spinal cord as the lateral spinothalamic tract. This tract synapses with the third-order neuron in the thalamus. v The axon of the third-order neuron projects to the somatosensory area of the cerebral cortex, where the information is processed into conscious sensation. v The anterior spinothalamic tract is another ascending tract with similar neuronal design – origin, course and termination – that conveys the impulses of non-discriminative (crude) touch. Spinothalamic Tracts r Somatosensory Map of Postcentral Gyrus (Review!) Relative sizes of cortical areas – proportional to number of sensory receptors – proportional to the sensitivity of each part of the body Can be modified with learning – learn to read Braille & will have larger area representing fingertips Gate Pain Theory Serotonin “rubbing it better”, Substance P “willing it away” or “mind over matter”. enkephalins and endorphins Peripheral Nervous System Spinoreticular Pathway = levels of consciousness Pathway: Neuron 1: 1st order neuron Originates in peripheral receptor Travels towards CNS via nerve Axon enters dorsal root and synapses in dorsal grey horn upon entry Neuron 2: 2nd order neuron Cell body originates in dorsal grey horn Axon remains ipsilateral Ascends in antero-lateral white columns of spinal cord Synapses in medulla and midbrain FINISH! START! 25 Spinoreticular Tract The axons enter the spinal cord from the posterior root ganglion and terminate on unknown second-order neurons in the gray matter. The axons from these second-order neurons ascend the spinal cord as the spinoreticular tract in the lateral white column mixed with the lateral spinothalamic tract. Most of the fibres are uncrossed and terminate by synapsing with neurons of the reticular formation in the medulla oblongata, pons, and midbrain. The spinoreticular tract provides an afferent pathway for reticular formation, which plays an important role in influencing levels of consciousness. Peripheral Nervous System Spinotectal Pathway = coordinates movements of eyes and head Pathway: Neuron 1: 1st order neuron Originates in peripheral receptor Travels towards CNS via nerve Axon enters dorsal root and synapses in dorsal grey horn upon entry Neuron 2: 2nd order neuron Cell body originates in dorsal grey horn Axon cross over midline Ascends in antero-lateral white columns of spinal cord Synapses in superior colliculus of midbrain Spinotectal Pathway = “reflexive turning of head and eyes towards a point of cutaneous stimulation” FINISH! START! 27 Spinotectal Tract The axons enter the spinal cord from the posterior root ganglion and travel to the gray matter, where they synapse on unknown second-order neurons. The axons of the secondorder neurons cross the median plane and ascend as the spinotectal tract in the anterolateral white column lying close to the lateral spinothalamic tract. After passing through the medulla oblongata and pons, they terminate by synapsing with neurons in the superior colliculus of the midbrain. This pathway provides afferent information for spinovisual reflexes and brings about movements of the eyes and head toward the source of the stimulation. It is sometimes considered the subdivision of the spinothalamic tract. Spinomesencephalic tract v Similar path to the spinoreticular tract, except that it terminates in the midbrain's periaqueductal gray matter (PAG) (the area surrounding the cerebral aqueduct). v This region then has connections to the limbic system, so this tract is likely responsible for the fear and aversion LL reactions associated with pain. v It also activates the descending analgesia system that arises from the PAG (will be discussed with the motor pathways of the spinal cord). Peripheral Nervous System Spino-olivary Pathway = Pathway: Neuron 1: 1st order neuron Originates in peripheral receptor Travels towards CNS via nerve Axon enters dorsal root and synapses in dorsal grey horn upon entry FINISH! Neuron 2: 2nd order neuron Cell body originates in dorsal grey horn Axon cross over midline Ascends in antero-lateral white columns of spinal cord Synapses in inferior Olivary nucelus of medulla Neuron 3: 3rd order neuron Cell body originates in nucleus Axon cross the midline again Enter the cerebellum Synapses in cerebellum Role in control of movements of the body and limbs START! 30 Spino-olivary Tract The axons enter the spinal cord from the posterior root ganglion and terminate on unknown second-order neurons in the posterior gray column. The axons from the second-order neurons cross the midline and ascend as the spino-olivary tract in the white matter at the junction of the anterior and lateral columns. The axons end by synapsing on third-order neurons in the inferior olivary nuclei in the medulla oblongata. The axons of the third-order neurons cross the midline and enter the cerebellum through the inferior cerebellar peduncle. The spino-olivary tract conveys information to the cerebellum from cutaneous and proprioceptive organs.