Spinal Cord and SNS

Prof ****

Lectures 7+8 (22-11-16)

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Introduction to the Spinal Cord

  • Made up of ~100 million neurons and even more neuroglia, extending from the brain
  • Contain reflex circuits that control our rapid reactions to environmental changes
  • Grey matter: site for integration of postsynaptic potentials (IPSPs --> makes postsynaptic neuron less likely to generate an action potential, EPSPs --> make postsynaptic neuron more likely to generate an action potential)
  • White matter: contains major sensory and motor tracts to and from the brain, white due to the myelin sheath
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External Cord Anatomy

  • Continuation of the medulla oblongata to the conus medullaris
  • Anatomically divided into regions which control different parts of the body: Cervical, Thoracic, Lumbar and Sacral
  • Cauda Equina/ Horses Tail: The roots of the lower spinal nerves which splay out at the end
  • Spinal Nerves: A mix of motor and sensory neurons unlike other nerves
  • Plexuses e.g. celiac (solar) plexus: Where nerves join together, if this region is hit, all nerves connected will be unable to produce action potentials

Coverings of the Spinal Cord

  • Axons are incapable of regenerations in spinal cord (and brain)- highly protected by tissue coverings- spinal meninges
  • Dura Mater (tough mother): forms a sac that encloses the entrie cord, the epidural space lies between the dura and bony vertebra
  • Arachnoid Mater: attached to inside of dura forming roof of the subarachnoid space in which cebral spinal fluid circulates which provides a buffer between cord and vertebrae
  • Pia Mater: pressed up against the cord and is filled with blood vessels that supply nutrients
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Spinal Nerves

  • Bundles of sensory and motor axons- these are divided into small bundles which are made up of only motor or sensory axons
  • Divide into the anterior (vetral) and posterior (dorsal) roots as they connect to the spinal cord
  • Ventral Roots: contains axons of motor neurons; CNS---> Effector
  • Dorsal Roots: contains sensory axons; terminals at sensory receptors, with cell bodies in the dorsal root ganglion (big group of cell bodies which causes a swelling in the root) and axons synapse in the spinal cord
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Needles into the Spine

  • Epidural Anaesthesia: Commonly adminstered to women going into labour. The needle is placed betwee the bones of the posterior spine until it penetrates the ligamentum flavum yet remains superfical to the dura mater. Local anesthetic for pain relief or complete for caesarean
  • Lumbar Punctures: Used to test for conditions of the brain, spin cord or nervous system. The needle is placed between the 3rd and 4th lumbar vertebrae- below thetermination of the actual cord in the region of the cauda equina
  • With the needle in the subarachnoid space, CSF can be sample and anaesthetics given but using 1/10 the dose of an epidural anaesthesia
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Internal Cord Anatomy

  • White matter is on the outside and grey matter on the inside (opposite for the brain)
  • White matter: consists of nerve axons (fibres) which transport electrical information between the limbs, trunk and organs of the body, and the brain. At the front---> anterior white column, middle---> lateral white column, back---> posterior white column
  • Grey matter: the internal butterfly-shaped central matter that is made up of nerve cell bodies, around the central canel (filled with CSF). At the front---> anterior grey horn, back---> posterior grey horn
  • Anterior (ventral) grey horn: somatic motor neurons
  • Posterior (dorsal) grey horn: sensory neurons, the site of synapse between first order sensory neurons coming in from the periphery, and second order neurons which either ascend in the cord or exit back out as parts of reflex arcs
  • Interneurons: foundk completely with the spinal cord grey matter
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Axons run in Tracts

  • Tracts are bundles of axons that are all located in a specific are of the cord and all travelling to the same place in the brain or cord
  • The name of the tract denotes its origin and where it ends
  • Spinothalamic Tract: from the spinal cord to the brain (thalamus)---> ascending= sensory tract from the spinal cord to the brain
  • Coricospinal Tract: from the cortex of the brain to the spinal cord---> descending= motor tract from the brain to the spinal cord
  • Different motor tracts convey nerve impulses from the specific areas of the brainsj to the body e.g. lateral corticospinal tract ---> controls precise movements of hands and feet e.g. anterior corticospinal tract ---> controls movement of the trunk and limbs
  • Nerve impulses for specific senses are carried in specific sensory tracts e.g. posterior (dorsal) column ---> mainly touch and proprioception e.g. lateral (anterolateral) tracts ---> mainly pain and temperature
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Dermatomes

An area of skin that is innverated by a single spinal nerve, indicated by the letter and number of a particular segmental nerve

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Cortex

  • Somatosensory input ascends in bundles to the cortex (relay station- allows you to disperse signals)
  • Dorsal column: mainly touch and propioception, first synapse in the dorsal column nuclei of the medulla
  • Anterolateral: mainly pain and temperature, synapse upon entering the spinal cord
  • Regional processing is maintained in the cortex
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Sensory 'homunculus'

  • Somatic sensory neurons and their axons are not distributed evenly in the body
  • The peripheral areas with the highest density are represented in the brain with the larges amount of grey matter in the sensory homunculus
  • Sensory nerve endings in the skin are mapped onto the brain e.g. there is a big area for the hand
  • The most sensitive areas are the tip of the tongue, lips and fingertips
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Damage to the Cord

  • Transection of the spinal cord: the ascending and descending tracts are partially or completely severed resulting in paralysis, the extent of which deoending on the level of injury
  • Asphyxiation: at the base of the skull 
  • Quadriplegia: transaction in the upper cervical area
  • Some form of paraplegia: transaction of the lower cord enlargements
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Sensory Nervous System

  • Part of the peripheral nervous system
  • Voluntary
  • From the CNS: Motor neurons to skeletal muscle tissue
  • To the CNS: Neurons from cutaneous and special sensory receptors to the CNS
  • Somatosensation: Levels and components of pathways that convery sensory nerve impulses from the brain to the body and the general sensations (somatic and visceral) that result
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Somatic Sensory Pathways 1

First Order 

  • Somatosensory neurons that transmit from the receptor (where the generator potential is set up) to the spinal cord
  • Cell body is loacted in the dorsal root ganglia (DRG) just outside the CNS
  • Other end of theaxon passes into the posterior grey horn of the cord
  • Either terminate in the spinal cord to form a synapse into a second order neuron e.g. anterolateral (spinothalamic) tract ---> pain and temperature
  • Or ascend the spinal cord to the synapse onto second order neurons in the medulla e.g. posterior (dorsal) column tract ---> touch and proprioception

Second Order

  • Conduct ascending impulses
  • Cross (decussate) in the brainstem so that sensory information from on side of the body reaches the brain on the other side
  • Terminate in the thalamus to form a synapse with third order neuron
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Somatic Sensory Pathways 2

Third Order:

  • conduct impulses from the thalamus to the primary somatosensory area of the cortex on the same side
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Somatic Motor Pathways

  • Motor activity begins in the primary motor areas of the cortex

Upper Motor Neuron (UMN)

  • Any motor neuron that is not directly responsible for stimulating target muscles
  • UMNs connect the brain to the appropriate level in the spinal cord
  • Affected in Amyotrophic Lateral Sclerosis (ALS)/ Motor Neurone Disease/ Lou Gehrig's Disease

Lower Motor Neuron (LMN)

  • All excitatory and inhibitory signals that control movement converge on LMNs that descend to innervate skeletal muscle
  • Second order motor neurons
  • Axons of LMNs extend through cranial nerves to the skeletal muscles of the face and head and through spinal nerves to innervate skeletal muscles of the limbs and trunk
  • Only LMNs provide output from the CNS to skeletal muscle fibres, for this reason they are also called the final common pathway
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Motor Neuron Disease

  • Progressive degenerate disease that attacks motor areas of the cerebral cortex, axons of the UMNs and cell bodies of the LMNs
  • Begins in sections of the spinal cord that serve the hands and arms but rapidly spreads to involve the entire body
  • Death typically occurs in 2-5 years
  • Inherited mutations account for 15% of cases
  • Non-inherited cases have several implicating factors e.g. excitotoxicity due to defective uptake
  • The drug riluzole redces damage by decreasing the excitoxic effects of glutamate (treats ALS but not stroke despit having the same mechanism)
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Reflexes

  • A fast, involuntary response to a stimulus

Spinal Reflex

  • Takes place in the spinal cord, not brain
  • Monosynaptic- 1 sensory neuron with 1 motor neuron
  • Polysynaptic- involves interneurons
  • Can go in and out on the same, or opposite side of the cord

Reflec Arc

  • Sensory receptor---> Sensory neuron---> Integrating centre---> Motor neuron---> Effector
  • A pathway that a nerve impulse follows to produce a reflex
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Patellar Reflex

  • Used to check neurons are functioning
  • Sensory receptors involved in reflex arcs are specifically adapted to perceive the stimulus and intiate an impulse
  • The motor neuone becomes stimulated without any processing in the brain

Paralysis

  • Lower motor neurons: produces flaccid paralysis, there is neither voluntary of reflec action of the innervated motor fibres, muscle tone decreases and is lost and the muscle remain limp and flaccid
  • Upper motor neurons: injury/diseas of the cerebral cortex removes inhibitory influences on the LMNs, causing ******* paralysis of muscles on the opposite side of the body, muscle tone increases and reflexes are exaggerated
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