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6a sensory pathways Ascending tracts, 3 ascending pathways, ascending…

- - - - from the axons of the 1st order neuron
        in posterior root ganglia
      - fasciculus gracilis
        
        -receives afferents frm lower half
        (lumbar & sacral segments) of the body
      - fasciculus cuneatus
        
        -receives afferents frm upper half
        (cervical & thoracic segments)
    - - after entering spinal cord,
        fasciculus gracilis fibers-run in dorsal(posterior)column of same segment
        occupying the whole of it;s width upto mid thoracic region
        
        above this level, its fibers are pushed-medially by fasciculus cuneatus.
        
        so in this ascending system,
        fibers are represented as
        sacral to cervical segments
        frm medial to lateral
        in the spinal cord
        
        majority of fibers end in
        medulla- in the nucleus gracilis & nucleus cuneatus respectively
        (1st relay station)
        
        2nd order neurons arises frm here
        whose axons are divided into 2 groups
        
        external arcuate fibers
        
        dorsal group
        
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        ventral group
        
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        internal arcuate fibers
        
        cross to the opposite side
        & pass through mid brain in the
        medial lemniscus to end in
        the ventroposterior nucleus.
        
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    - - conveys the following info
        frm various parts of the body
        to
        the higher center
        
        fine touch
        
        tactile localization & discrimination
        
        proprioceptive & kinethetic sensation
        (sensation of body position in space
        & joint movements)
        
        vibration
        
        sense of deep pressure
        -some kinesthetic impulses which are essential for control of body posture
        
        constitutes sensory pathway for some superficial reflexes
  - - - origin
        
        frm the axons of 1st order neurons
        in the posterior root ganglia
      - situation,extent,& termionation
        
        after entering the spinal cord,
        fibers of all segments end around nociceptive neurons
        ie cells in dorsal horn cells of grey matter of spinal cord
        
        most of the axons of 2nd order neurons
        cross in anterior white commisure obliquely
        to opposite side of the same segment
        & ascend in the lateral spinothalamic tract
        to end int the thalamus
        (venteroposterior nucleus)
        
        some axons of the 2nd order neurons before crossing,
        -run up n down for a few segments n then cross in
        anterior white commissure
        to reach the lateral column
        of the opposite side
        
        axons frm the sacral n lumbar segments of the body are pushed laterally
        by axons crossing the mid line
        at successive higher levels
        -so the fibers are represented as
        cervical to sacral segments
        from medial to lateral
        in the spinal cord
        
        at higher brain stem level,
        this tract sends several collaterals
        into the reticulatr formation n tagmentum
        b4 ending in the thalamus
        
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      - functions
        
        carries fibers of all types of
        pain & temperature
        (both hot n cold)
  - - - origin
        
        frm the axons of 1st order neurons
        in the posterior root ganglion
      - situation, extent n termination
        
        After entering the spinal cord
        the fibers end round the cells
        which are situated at
        the median part in the dorsal horn
        (1st relay station),
        from where 2nd-order neuron arise
        
        Most of the axons of 2nd-order neurons
        cross in anterior white commissure obliquely
        to opposite side
        of the same segment,
        and ascend in anterior column
        of spinal cord
        to end in the thalamus
        (venteroposterior nucleus).
        
        Small number axons of 2nd-order neurons
        are uncrossed and
        may ascend homolaterally
        as anterior
        spinothalamic tract;
        while ascending upwards
        ventral(anterior) spinothalamic tract
        runs in parallel with
        the lateral spinothalamic tract
        n together they constitute
        the spinal lemniscus or anterolateral system
        of ascending fibers.
        In upper medulla or lower ponds,
        spinal lemniscus joins with
        the fibers from the gracilis and cuneatus nucleus to form medial lemniscus.
        
        At higher brain stem level,
        this tract sends several
        collaterals into the reticular formation n tegmentum before ending in the thalamus.
        
        In thalamus, 3rd-order neuron starts
        n axons of this neuron
        via posterior limb of internal capsule
        ends in
        the sensory cortex (postcentral gyrus).
      - function
        
        it conveys
        
        gross(crude) touch
        ie. touch with high threshold
        
        tactile localization
- - - - dorsal column-medial lemniscal pathway
        (DCML)
        
        carries the sensory modalities of
        
        fine touch (tactile sensation),
        
        vibration and
        
        proprioception.
        
        Its name arises from the two major structures that comprise the DCML
        
        in the spinal cord-information travels via the dorsal (posterior) columns.
        
        In the brainstem,- it is transmitted through the medial lemniscus.
        
        There are 3 groups of neurones
        involved in this pathway
        
        2nd order neurons
        
        begin in the
        
        cuneate nucleus or
        
        gracilis nucleus
        
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        3rd order neurons
        
        transmit the sensory signals
        from the thalamus to the
        ipsilateral primary sensory cortex
        of the brain.
        
        1st order neurons
        
        carry sensory information regarding
        
        touch
        
        proprioception
        
        vibration
        -from the peripheral nerves
        to the medulla oblongata
        
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      - the anterolateral system.
        
        consists of 2 separate tracts
        
        Anterior spinothalamic tract
        
        carries the sensory modalities
        
        of crude touch
        
        pressure.
        
        Lateral spinothalamic tract
        
        carries the sensory modalities
        
        of pain
        
        temperature.
        
        like the DCML pathway,
        both tracts of the anterolateral system
        have 3 groups of neurones.
        
        2nd order neurons
        
        carry the sensory information
        from the substantia gelatinosa
        to the thalamus.
        
        After synapsing with the first order neurones,
        these fibres decussate within the spinal cord,
        and then form 2 distinct tracts:
        
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        3rd order neurons
        
        carry the sensory signals
        from the thalamus
        to the ipsilateral primary sensory cortex
        of the brain.
        
        They ascend from the ventral posterolateral nucleus of the thalamus,
        travel through the internal capsule
        n terminate at the sensory cortex.
        
        1st order neurons
        
        arise from the
        
        sensory receptors in the periphery.
        
        They enter the spinal cord,
        ascend 1-2 vertebral levels,
        and synapse at the tip of the dorsal horn
        – an area known as the substantia gelatinosa.
  - - - spinocerebellar tracts.
        
        Although we cannot physically acknowledge these signals,
        
        they help our brain co-ordinate and refine motor movements.
        
        They transmit information from the muscles to the cerebellum.
        
        They transmit information
        from the muscles
        to the cerebellum.
        
        Within the spinocerebellar tracts,
        
        there are 4 individual pathways:
        
        Posterior spinocerebellar tract
        
        Carries proprioceptive information
        from the lower limbs
        to the ipsilateral cerebellum.
        
        Rostral spinocerebellar tract
        
        Carries proprioceptive information
        from the upper limbs
        to the ipsilateral cerebellum.
        
        Cuneocerebellar tract
        
        Carries proprioceptive information
        from the upper limbs
        to the ipsilateral cerebellum.
        
        Anterior spinocerebellar tract
        
        Carries proprioceptive information
        from the lower limbs.
        The fibres decussate twice
        – and so terminate in the
        ipsilateral cerebellum.