Transmission of Somatosensory Information PDF

Transmission of Somatosensory Information PH5208 Dr. Moira Jenkins Somatosensory information is transmitted into the brain via a “multi-synaptic relay” − somatosensory neurotransmission starts with signals generated from peripheral receptors … peripheral receptive fields “trigger zone” primary...

Transmission of Somatosensory Information PH5208 Dr. Moira Jenkins Somatosensory information is transmitted into the brain via a “multi-synaptic relay” − somatosensory neurotransmission starts with signals generated from peripheral receptors … peripheral receptive fields “trigger zone” primary afferent fibers cell bodies (dorsal root ganglia) synapse to 2nd order relay neuron (dorsal horn) receptive field: • nerve endings contain stimulus-gated ion channels  generator (receptor) potentials trigger zone: • high density of voltage-gated Na+ channels  threshold to trigger action potentials primary afferent fibers: • action potentials propagate along  transmission to first synapse in the dorsal horn Sensory System • Receptors have 3 main functions 1. To transduce the appropriate signals into electrical signals (sensations) 2. Aid in the control and coordination of movements (feed forward, feedback) 3. Help maintain arousal (reticular activating system) Somatosensory Afferent PathwaysSomatotopic, paired, decussate, chain of neurons • Somatosensory pathways consist of the chain of neurons, from receptor organ to the cerebral cortex, that are responsible for the perception of sensations. • Somatosensory pathways travel along different routes depending on the information carried. DCML Spinothalamic Ascending pathways Descending pathways Dorsal/posterior column Lateral Motor Systems lateral corticospinal Modern Discriminatory touch Vibration Proprioreception Joint position sense Line and Limb somatotopy Primitive Pain Temperature Crude touch Itch Tickling Anterolateral Sexual system/Spinothalamic vestibulospinal anterior corticospinal Medial Motor Systems Somatotopic organization Dorsal column Somatotopy-localize a stimulus to a specific area on the body 2-point discriminationhelps distinguish between stimuli fine touch, vibration, and proprioception (position sense) Fasciculus gracilis Fasciculus cuneatus Dorsal column – medial lemniscal fine touch, vibration, and proprioception (position sense) 1. 2. 3. 4. 1° afferent cell body in DRG; dorsal horn  fasiculus gracilis Ascend cord, synapse on gracile nucleus Crosses, synapse in thalamus VPL Axons travel through internal capsule to primary somatosensory cortex Large fibers, faster than Spinothalamic tract, mostly mechanoreceptors Spinothalamic (pain, temperature, and crude touch) 1. cell body in DRG 2. enters into dorsal root 3. SYNAPSES; 2nd order neurons cross in spinal cord & terminate in thalamus VPL 4. 3rd order neuron projects to somatosensory cortex Afferent neurons in the spinothalamic pathway activated by pricking the left foot with a pin Sensory Trigeminal Pathways Dorsal column medial lemniscal pathway (MLP) for the body and the main sensory trigeminal pathway (MSTP) for the face. Spinothalamic pathway (NSTP) for the body and the spinal trigeminal pathway (STP) for the face. touch nervous encoding of an external stimulus perception [CNS] Instead, somatosensory information is transmitted from the periphery to the primary sensory cortices via a multi-synaptic relay peripheral mechanoreceptor → neural encoding 2nd order projection (relay) neuron 3rd order projection (relay) neuron → perception primary afferent signaling dorsal horn thalamus cerebral cortex Why this extra complexity? “Opportunity” to integrate, process, and distribute sensory information as it is “decoded” and “recoded” at each synaptic relay decoding → presynaptic release of neurotransmitter in response to inputs recoding → postsynaptic response to the released neurotransmitter Physiology: An Illustrated Review (TannerThies) Fig. 5.2 − each synaptic relay along the pathway starts with “faithful transmission” of the encoded sensory information, but then … Central Processing of Sensory Information • All sensory information is processed in a series of hierarchical relay stations and each station has a site for integration (convergence or divergence) • Most afferent information is modified by excitatory or inhibitory interneurons at each synapse • The 3 principle ways of MODIFYING afferent input are • 1. Feed-back inhibition • 2. Feed-forward inhibition • 3. Distal inhibition (descending analgesic tract) Integration The postsynaptic response to just one single primary afferent input to a dorsal horn relay neuron will not be strong enough to drive that relay neuron to fire at the same frequency as the input conveyed by that one afferent fiber integration: the response that is “sent forward” from each relay neuron reflects the average strength of multiple inputs that all converge upon that relay neuron Integration − through the process of summation the relay neuron determines the average strength of its synaptic inputs by an array of receptors that comprise the receptive field for the 2nd neuron the receptive field for the relay neuron encompasses the contiguous array (touching) of receptive fields for each of the primary afferent fibers that converge upon that relay neuron receptive field for the relay neuron 1st order receptive fields primary afferents 2nd order synaptic relay ascending tract − the extent of convergence (how many primary afferent fibers) contributes towards determining the size of the receptive field for the relay neuron − the brain’s perception of the specific location of the stimulus upon the body surface is determined by the size of the receptive field for the signals that reach the brain − e.g. “two-point” discrimination testing processing: the strength of signaling from the relay neuron in response to its afferent inputs can be modulated by other signals that either strengthen or lessen neurotransmission from the afferent inputs dorsal horn synaptic relay of peripheral sensory input ascending sensory signaling dorsal horn interneuron Local interneurons in the dorsal horn modulate second order neuron response Interneuron releases its neurotransmitter to vary the strength of synaptic transmission from the primary afferent fiber to the second order relay neuron excitatory (+) amplifies synaptic transmission inhibitory (-) diminishes synaptic transmission ascending transmission inhibitory interneuron diminished transmission via this interneuronal circuit, transmission from relay neurons that receive input from receptive fields immediately surrounding the central point upon which the stimulus energy is focused, will be inhibited (lessened) focused attention at the central point at which the stimulus is applied greater specificity in identifying the somatotopic location of the stimulus Processing Somatosensory input flowing into the dorsal horn will excite inhibitory interneurons, as well as its own relay neuron ascending sensory signaling inhibitory interneuron diminished transmission of afferent signaling along this “line” due to input from the inhibitory interneuron The responsiveness of dorsal horn relay neurons to afferent sensory signaling is therefore restrained through both feedforward and feedback inhibitory pathways forward (“ascending” towards the brain) 1 2 brain backward (away from the brain) feedforward inhibition feedback inhibition Filter out weaker signals → the brain more accurately focuses attention upon the specific location and nature of the strongest stimulus see also: Kandel & Schwartz Fig. 21-11 Processing-Lateral Inhibition