Spinal Tracts Study Notes

Spinal tracts    Ascending tracts carry sensory information up the cord, whilst descending tracts carry motor impulses down. Several tracts undergo decussation, and are known as contralateral. Tracts that do not decussate, so the origin and destination of its fibres are on the same side, are known as ipsilateral. Somatosensory system  Carry sensory signals up the spinal cord.  Afferent axons travel within muscle or cutaneous fascicles of peripheral nerves towards the spinal cord.  Each segment of the spinal cord receives information from a particular part of the body.  Sensory signals typically travel across three neurons from their origin in the receptors to their destination in the sensory areas of the brain: o First-order neuron: detects a stimulus and transmits signal to spinal cord. o Second-order neuron: continues as far as the thalamus. o Third order neuron: carries signal the rest of the way to the sensory region of the cerebral cortex. Dorsal columns       Responsible for vibration, visceral pain, deep and discriminative touch, and proprioception. Myelinated axons for fast transmission of impulses. Gracile fasciculus. o Carries signals from the midthoracic and lower parts of the body. o Below T6, it composes the entire dorsal column, but joins up with the cuneate fasciculus at T6. o Made up of first-order neurons that travel up the ipsilateral side of the spinal cord and terminate at the gracile nucleus in the medulla oblongata of the brainstem. Cuneate fasciculus. o It occupies the lateral portion of the dorsal column and forces the gracile fasciculus medially. o Fibres end in the cuneate nucleus on the ipsilateral side of the medulla oblongata. Second-order fibres of the gracile and cuneate systems decussate and form the medial lemniscus, a tract that leads the rest of the way up the brainstem to the thalamus, synapsing at the ventral posterolateral nucleus of the thalamus. Third-order fibres go from the thalamus to the cerebral cortex. Spinothalamic tract  Responsible for pain, temperature, pressure, tickle, itch, and light and crude touch.  Unmyelinated afferent axons (C fibres) enter the dorsal roots of the spinal cord and synapse within the substantia gelatinosa of the dorsal horn.  Second-order neurons decussate to the other side at the anterior commisure, and travel up the anterolateral funiculus as the spinothalamic tract to synapse in the medial nucleus of the posterior complex of the thalamus.  Third-order neurons project to and synapse at the primary somatosensory cortex at the post-central gyrus.  Form the anterolateral system, which passes up the anterior and lateral columns of the spinal cord. Dermatomes  Each segment of the spinal cord receives information from a particular part of the body.  Cutaneous afferent innervation is arranged in dermatomes, with each spinal segment supplying a strip of skin. o Knowing the dermatomes is clinically important in defining the level of spinal cord injury. Descending tracts  Carry motor signals down the brainstem and spinal cord.  The motor pathway typically involve two neurons: o Upper motor neuron: begins with a soma in the cerebral cortex or brainstem and has an axon that terminates in the brain stem or spinal cord. o Lower motor neuron: continues its pathway from the upper motor neuron and leads the rest of the way to the muscle or other target organ. Corticospinal  Responsible for precise finely coordinated limb movement.  Myelinated efferents project down through the internal capsule to the pyramidal decussation in the medulla, where most decussate to the contralateral side. o Fibres that decussate (80%) travel down the lateral tract. o Those that do not decussate at the medulla (20%) form the ventral tract. However, these eventually decussate at lower levels of spinal cord, so control contralateral muscles as well.  Axons of upper motor neurons then make monosynaptic connections onto lower motor neurons within the ventral horn of the spinal cord.  Alpha motor neurons have a large diameter, and innervate skeletal muscle.  In contrast, gamma motor neurons are smaller, and therefore conduct slower, and innervate muscle spindles.  Loss of upper motor neurons leads to: o Paralysis. o Exaggerate spinal reflexes (spasticity).  o Loss o o o No muscle atrophy. of lower motor neurons leads to: Paralysis. Loss of spinal reflexes. Denervation and atrophy of muscle. Myotatic (stretch) reflexes  Muscle stretch activates specialised sensory endings in the muscle, the muscle fibres (stretch receptors), which cause a reflex activation of spinal (alpha) motor neurons, thereby preventing further stretch.  Allows the system to compensate a sudden increase or decrease in load. o Unloading of a muscle unloads muscle spindles, causing a short-latency inhibition of alpha motor neurons.   Afferent impulses from the muscle spindles in the extensor muscle evokes a monosynaptic excitation of the extensor (agonist) motor neurons. o Release of glutamate at the synapse with the extensor motor neurons produces excitatory post-synaptic potentials (EPSPs) due to the influx of Na+ in the extensor motor neurons. o Synapse is weak, but summation may result in the generation of an action potential in the extensor motor neurons. However, the same muscle spindles have a disynaptic inhibition, via an interneuron, of the flexor (antagonist) motor neurons. o Release of glutamate at the synapse with the interneuron evokes strong EPSPs that trigger action potentials and release of glycine at the synapse with the flexor motor neurons. o Release of GABA/glycine produces inhibitory post-synaptic potentials (IPSPs) due to the influx of Cl- in the flexor motor neurons. Muscle spindles  Highly sensitive stretch receptors located in parallel to the extrafusal (skeletal) muscle fibres.  As extrafusal muscle fibres contract (and muscle shortens), the spindle becomes slack.  This unloading causes a reduction in the spontaneous firing of muscle spindle sensory endings, deceasing tone.  However, due to innervation from gamma motor neurons, muscle spindles remain contracted with the extrafusal muscle. Golgi tendon organs  Located in the musculotendinous junctions.  They are simple stretch receptors (no innervation), similar to Ruffini endings in the skin.  Axons are rapidly-conducting 1b afferents.  Functions as force detectors, rather than length detectors like muscle spindles.  They are silent when the muscle is relaxed and do not respond when the muscle is stretched passively.  They are activated during muscle contraction and can sense the forces generated by individual motor units.  They exert negative feedback onto the alpha motor neurons via the interneuron to ensure that there is no excess force produced.