Neuroscience 5a – Touch and Proprioception Anil Chopra 1. List the major somatosensory modalities 2. Define Proprioception and list the main types of receptor 3. Explain the terms: receptor, stimulus threshold and intensity, adaptation, receptive field and lateral inhibition as applied to somatosensory systems. 4. Demonstrate on a diagram of model of the brain the 1° and 2° sensory cortex and the parietal association cortex 5. Describe the pathway of touch/proprioception for information from the body and face 6. Explain the somatotopic organisation of the touch pathway and sensory cortex 7. Outline the main circumstances in which somatosensory deficits may occur 8. Explain the clinical use of 2-point discrimination and why sensitivity caries between regions of the body. Somatosensory System: involves the information coming from the skin, muscles joints and ligaments. Touch: includes fine touch (light discriminative), vibration and pressure. Proprioception – Joint position, muscle length, muscle tension – acts to provide position sense (Mechanoreceptors, carried via the dorsal columns pathway to the somatosensory cortex) » Proprioception provides a position sense to the body by measuring such things as joint position, muscle length and muscle tension » All receptors for touch and proprioception are mechanoreceptors Receptors The receptors for touch are found as peripheral nerve terminals of axons of dorsal root ganglion cells. They are all mechanoreceptors and they fire action potentials when either nerve endings or connective tissue structures on the nerve endings become deformed. As the endings become more and more deformed, depolarisation occurs until the threshold level is reached. At this point the action potential is fired. Different nerve endings and the structures that relate to them have different threshold levels. There are 2 main types of receptors for touch and pressure: Fast adapting receptors: these are mainly receptors associated with touch, movement and vibration and fire action potentials only when stimuli change. They include: Pacinian Corpuscle – pressure and vibration. Meissner’s Corpuscle – light touch Some free nerve endings Slow adapting receptors: these are generally receptors that are associated with pressure. They constantly fire action potentials and change in frequency depending on the strength of the stimulus. These include: Merkle’s Corpuscle – touch Ruffini Corpuscle – pressure Receptor – Modified terminals of the peripheral axons of the primary sensory organs which transduce a mechanical signal (deformation) into electrical signals. All receptors for touch and proprioception are mechanreceptors The function of mechanoreceptors depends on: The type of modified terminal is present. See above for different types. Degree of specialisation – from free nerve endings to elaborate accessory structures Location – e.g. various layers of the skin, around hair shaft etc. Muscle spindle detects changes in muscle length, Golgi organ tendons detect change in muscle tension Physiological properties – activation threshold determines sensitivity (all touch and proprioception receptors have a low threshold). May be slow or fast Generally the more complicated the receptor the more specific the stimulus Stimulus Threshold and Intensity – The stimulus threshold dictates at what level of stimulation the receptor fires, the lower the threshold the more sensitive the receptor. The intensity of the signal is coded by the frequency of firing, i.e. the faster firing the bigger the signal. The amplitude of the signal does not change. Usually firing frequency is related to Log(stimulus intensity) i.e. if stimulus increases 10 fold, firing frequency doubles Adaptation – Dictates whether the receptor is constantly firing or only fires in bursts; slow adapting or fast adapting. Mechanoreceptors used for touch and proprioception are a mixture of both. The decrease in receptor sensitivity that occurs in the presence of a maintained stimulus, getting rid of the noise. Receptive Field – Is the number of receptors innervated by one sensory nerve. The larger the receptive field size, i.e. more receptors on each nerve, the lower the acuity. The density of receptive fields varies all over the body. The size of the field also dictates the size of region around the neuron that will respond to a stimuli Somatosensory System Generally the somatosensory system is made up of 3 neurones. (1) The first starts from the skin/muscles/joint and has its cell body in the dorsal root ganglion. It then goes from the dorsal root ganglion, up the dorsal column (gracile and cuneate faciculus) of the spinal cord and synapses in the brainstem (specifically the gracile and cuneate nuclei in the medulla). The inputs from the face come from the trigeminal nerve (V). (2) The second projects through the brain stem anterolaterally (to the opposite side, passing up the medial leminiscus) to synapse in the thalamus. (3) The third goes from the thalamus to the appropriate part of the somatosensory cortex. NB: the pathway is somatotopic so the inputs are arranged in a way that resembles the body map from dermatome to cortex with information added laterally as you go up. Each limb has its own termination pathway in the somatosensory cortex; here the information from sensory stimuli becomes conscious sensation: » Somatosensory I (SI) – is the primary somatosensory area. The body map is distorted according to the relative density of input from different parts of the body, i.e. the face and hands have the greatest area of processing. The response of neurons in SI varies, and most only respond to a particular modality such as pressure, vibration etc. » Others respond to abstract properties such as movement or shape. SI has projections to SII and the posterior parietal cortex » Pain is processed in the SI » Somatosensory II (SII) – receives intracortical projections from SI » Posterior Parietal Cortex – combines different types of somatic sensation with other modalities, this is necessary for interpretation of spatial relationships Damage of the somatosensory system can lead to anaesthesia and parasthesia, but few neurological diseases affect the somatosensory system specifically. Lateral inhibition - enhances differences between adjacent inputs and so sharpens the resolution of a signal. Collateral branches of neurons make inhibitory synapses with surrounding relay neurons. E.g. if six receptor fields are stimulated, the middle one will carry the highest discharge frequency due to lateral inhibition, will have a higher chance of inhibiting its neighbours and so activating a relay neuron. This increase in intensity of signal allows the cortex t more accurately pinpoint the location of stimulation. Somatotopic organisation is maintained through the entire pathway from dermatomes to cortex.