Introductory Sensory Function PDF

Introductory Sensory Function PL1400/PL1001 1 Learning Outcomes List the various modalities of sensation List the various sensory receptors Define the term receptive field Explain the significance of the Two-Point Discriminitive Test Outline how generator...

Introductory Sensory Function PL1400/PL1001 1 Learning Outcomes List the various modalities of sensation List the various sensory receptors Define the term receptive field Explain the significance of the Two-Point Discriminitive Test Outline how generator potentials are set up Outline how action potentials are set up in sensory neurones Describe how sensory information reaches the Sensory Cortex 2 Overview of Sensory Physiology Function of sensory physiology is to allow us to interact with the world around us and transduce/convert different types of energy into electrical signals called action potentials (AP) Skin as a sensory organ First, the different types of energy are detected by sensory receptors > leads to generation of AP This sensory information (AP) is usually transmitted to an area of the brain called the sensory cortex for interpretation/perception > see diagram Perception starts with stimulation of sensory receptors, each of which responds to a certain stimulus 3 Types of Sensory Modality (Type of Sensation) Receptor Chemoreceptors Chemical changes Thermoreceptors Warm & Cold Photoreceptors Light Mechanoreceptors Pressure Hair cells in ear Nociceptors Noxious stimuli (generate pain) Proprioreceptors Information about body and limb position > i.e. important for control of movement Sensory receptors can be rapidly adapting or slowing adapting. Slowly adapting receptors continue to discharge as long as the receptor is being stimulated, whereas rapidly adapting receptors don’t…discharge is reduced or may cease 4  Skin Receptors Sensory Receptors transduce different forms of energy into action potentials in sensory nerves. To do this they exhibit threshold properties 5 Sensory Neurones Note the direction of the arrows! 6 Receptive Field Receptive Field: of a sensory neuron is the area of skin that, once stimulated produces a response in that neuron> learn this definition! Receptive fields are of variable size, and varies inversely with numbers of receptors in the area Receptive fields are smaller in areas requiring greater sensitivity 7 Skin is a very important sensory organ: Two-point Discrimination Test > is a measure of tactile acuity > small receptive fields in areas of skin with large numbers of cutaneous receptors 8 How do sensory receptors transmit sensory information? Generator (Receptor) Potentials: Local graded changes in membrane potential. Action potentials are generated when generator potentials reach threshold for depolarization - The bigger the stimulus the greater number of action potentials generated - 9  Sensory Pathways to the Brain How does sensory information reach our sensory cortex? so that we have conscious perception of a sensation. Thalamus is an important relay centre for sensory Information A Medial Lemniscal Tract and B Spinothalaminc Tract Note: Sensory innervation of the face is not shown on this slide but is mainly via sensory fibers of the Trigeminal Nerve (Cranial Nerve V). Sensory neurons synapse in brainstem nuclei, including the main sensory trigeminal nucleus and the mesencephalic nucleus of the trigeminal nerve and on to the thalamus via trigeminothalamic tract, and finally to the somatosensory cortex A B 10  A. Medial Lemniscal Tract (Dorsal Columns) Dorsal columns are first order neurons and composed of large myelinated axons which enter the spinal cord at different levels, via the dorsal route. They then travel up the cord in a highly organized pathway > Somatotopic Representation They then synapse in the medulla on either of the dorsal column nuclei i.e. nucleus gracilis (lower body & legs) or nucleus cuneatus (upper body & arms) to become second order neurons Second order neurons known as the Medial Lemniscus synapse in thalamus on third order neurons which travel on up to somatosensory cortex The Dorsal columns carry sensory information from skin, muscles, tendons and joints regarding ….. Proprioception - Position of body parts and awareness of movement Discriminative touch - Light touch, Precise location (two-point discrimination) and stereognosis 11 Pressure and Vibration sense  B. Spinothalamic Tract/Pathway Anterior and Lateral - Transmit sensory information regarding pain, temperature, crude touch, itch and tickle Also involves a 3 neuron relay system between the receptor and the sensory cortex The third order axons eventually terminate in precise areas of the somatosensory cortex somatosensory cortex is mapped out with different areas of body having defined locations >Sensory homunculus - 12 Sensory Homunculus: Right half of this slide The area of Sensory Cortex given over to an area of the body is not determined by the size of the body part – Rather by the number of sensory receptors in that area of the body 13 Final look at the SENSORY CORTEX SC located in the parietal lobe All incoming sensory information must reach here for conscious perception of the sensation 14