Sensory System - Lecture Notes PDF

## Sensory System Dr O.M. Olumide Department of Physiology Lagos State University College of Medicine ### Lecture Topics - Introduction to sensory receptors - Types, Properties, receptor generator potential - Skin Sensation - Touch - Pressure - Pain - Temperature receptors -...

## Sensory System Dr O.M. Olumide Department of Physiology Lagos State University College of Medicine ### Lecture Topics - Introduction to sensory receptors - Types, Properties, receptor generator potential - Skin Sensation - Touch - Pressure - Pain - Temperature receptors - Proprioception - Muscle proprioceptors - Joint receptors - Vestibular Apparatus - Thalamus; Thalamocortical System - Thalamus Syndrome - Somatosensory Cortex ### Introduction - Nerve cells are classified into: - **Motor neurons:** carry motor impulse from CNS to the peripheral (effector) organs like muscles, glands, blood vessels hence are known as efferent nerve cells - **Sensory neurons:** sensory impulse from periphery to the CNS (afferent nerve cells) ### Introduction contd - One of the characteristics of a living organism is in its ability to respond to stimuli. - Importantly, the sensory system is highly evolved and processes thousands of incoming messages simultaneously. - This complexity is what allow you to be aware of your surroundings and take appropriate actions - Sensation is the ability to transduce, encode and ultimately perceive information provided by a stimuli from external or internal environment. ### Introduction continued - Fundamental rules govern the way the nervous system carries out sensation - **Receptors** (converts energy associated with mechanical forces, sound waves, odorant molecules, or chemicals into neural signals) called afferent sensory signals (ASS) - The ASS convey information about the stimulus to the brain. - ASS activates central neurons that represents qualitative and quantitative stimulus, and in some cases its location. ### Why is this important? - Assessment of sensory systems during clinical evaluation of patients to infer **nature** and **location** of neurological problems. ### SENSORY SYSTEM #### Sensory Receptors - Information about the internal and external environment reaches the Central Nervous System via a variety of sensory receptors. - These receptors are biological transducers that convert stimuli in the environment to electrical signals or action potentials in neurons. - Sensory transduction converts chemical, light, thermal, mechanical stimuli into graded potentials. - If the graded potentials are above the threshold, action potentials pass from the receptor along an afferent sensory neuron to the CNS where the signals are integrated. - Some stimuli pass upward to the cerebral cortex where they reach conscious perception but others are acted upon without our awareness. - **Modality of sensation:** sensory receptors are highly sensitive to one particular type of stimulus. nerve fibers transmit only impulses despite the different ways of sensation. - **labeled line** - **Note:** Each of the different types of sensation that we experience i.e. pain, touch, sight, sound etc. is called modality of sensation. Therefore, the specificity of nerve fiber for transmitting only one modality of sensation is called “**the labeled line principle**”. - At each synapse along the way, the nervous system can modulate and shape information. - Sensory stimuli that reach the conscious level include the special senses of vision, hearing, taste, smell and equilibrium. - Others include somatic senses of touch, pressure, temperature, pain and proprioception. - Somatic senses are the nervous mechanisms that collect sensory information from all over the body. ### Classification - **Exteroceptors** occurs at or near the surface of the skin and are sensitive to stimuli outside or on the surface of the body e.g. cutaneaous receptors, chemoreceptors, telerecptors. - **Interoceptors:** They respond to stimuli that arise from within the body. These include visceroceptors and propioceptors - **Interoceptor** (visceroceptor) respond to stimuli occurring in the body from visceral organs and blood vessels e.g. stretch receptors, baroreceptors, chemoreceptors, osmoreceptors - **Proprioceptor** respond to stimuli occurring in the skeletal muscles, tendons, ligaments and joints e.g. muscle spindle, Golgi tendon organ, pacinian corpuscle, free nerve ending. - Receptors can also be classified based on the kinds of stimuli or energies they respond to. - These include the following: - Mechanoreceptors - Thermoreceptors - Nociceptors - Photoreceptors - Chemoreceptors ### 1. Mechanorececeptors - a) Skin tactile sensibilities (epidermis and dermis) - i) Free nerve endings - ii) Expanded tip endings e.g. merkel' disc and ruffini endings - iii) Encapsulated endings e.g. Pacinians, Meissner’ and krauses' corpuscles - iv) Hair end-organs - b) Deep tissue sensibilities - i) Free nerve endings - ii) Expanded tip endings - iii) Encapsulated endings - c) Muscle endings - i) Muscle spindles - ii) Golgi tendon receptor - d) Hearing - i) Sound receptors of cochlear - ii) Equilibrium e.g. vestibular receptors - e) Arterial blood pressure - i) Baroreceptors of the carotid sinuses and aorta ### 2. Thermoreceptors - i) Cold e.g. cold receptors - li) Warmth e.g. warm receptors ### 3. Nociceptors - e.g. free nerve or naked nerve endings ### 4. Electromagnetic receptors or Photoreceptors - e.g. rods and cones. ### 5. Chemoreceptors - i) Taste: receptors of taste buds - ii) Smell: receptors of olfactory epithelium - iii) Arterial oxygen e.g. receptors of aortic and carotid bodies - iv) Osmolality e.g. neurons in or near supraoptic nuclei. - v) Blood carbon dioxide e.g. receptors in or on the surface of medulla and aortic and carotid bodies. - vi) Blood glucose, amino acid, fatty acid receptors in hypothalamus. ### Properties of Sensory Receptors ### 1. Specificity of Response – Muller's Law - Each receptor type has an **adequate stimulus**, a particular form of energy which it is most sensitive to e.g. an adequate stimulus for the rods and cones in the eyes is light. (Law of specific nerve energies). - The specificity of response is also known as the Law of specific nerve energies. - Although, receptors are specific for one form of energy, they can respond to most forms if the intensity is high enough. ### 2. Adaptation - When a maintained stimulus of constant strength is applied to a receptor, the frequency of the action potential in its sensory nerve declines over time. This phenomenon is known as **adaptation**. - The receptors that adapt rapidly to stimuli are called **phasic receptors** e.g touch receptors such as pacinian corpuscles, meissner's corpuscles. - Those receptors that adapt slowly to stimuli are known as **tonic receptors** e.g merkel's disc, ruffini nerve endings. The receptors in Carotid sinus, muscles spindles and those for cold and pain adapt also slowly. ### 3. Response to the increase in the strength of stimulus - Receptors respond more to the increase in the strength of the stimulus. - Weber - Frechner’ law states that the change in response of a receptor to a stimulus is directly proportional to the logarithmic increase in stimulus intensity. ### 4. Sensory transduction - It is the process by which the receptors convert energy (stimulus) in the environment to electrical impulses (action potentials). - When a receptor is stimulated, it responds by sending information to central nervous system (CNS). Series of events occur to achieve this function such as development of receptor potential in the receptor cell and action potential in the sensory nerve. - Sensory transduction varies, depending on the types of receptors. ### 5. Receptor Potential - This is a non-propagatory transmembrane potential. - The minimum stimulus required to activate a receptor is known as the **threshold**. - Subthreshold stimulus when applied to a receptor gives rise to **receptor potential**. Receptor potential is also called **generator potential**. It is short lived and does not obey all or none law. - When the magnitude of the receptor potential is above 10mV, an **action potential** is generated in the sensory nerve. ### Mechanisms of Receptor Potential. - Different receptors can be elicited in one of these ways to Cause receptor potential: - 1.By mechanical deformation of the receptor which stretches the receptor membrane and opens ion channels. - 2.By application of a chemical to the membrane which opens ion channels. - 3.By change in temperature of the membrane which alters the permeability of the membrane. - 4.By the effect of electromagnetic radiation on a retinal visual receptor which either directly or indirectly changes the receptor membrane potential. ### Using the pacinian corpuscle to site receptor potential - **Stimulus** -pressure - It compresses when pressure is applied. This causes elongation or change in shape of the corpuscle. - The change in shape leads to deformation of the center core fiber of the corpuscle - There is opening of the sodium gated channel which allows the positively charged sodium ion to enter the interior of the core fiber - A mild depolarization is produced - There increased positivity inside the fiber, which is the “receptor potential.” - The receptor potential in turn induces a local circuit of current flow, that spreads along the nerve fiber - Local circuit reaches the first node of ranvier within the corpuscle and causes opening of voltage gated sodium channel with sodium ion into the nerve fiber leading to development of action potential ### Receptor potential-Action potential - A diagram of the receptor potential-action potential is shown. ### 1. Mechanorececeptors - a) Skin tactile sensibilities(epidermis and dermis) - i) Free nerve endings - ii) Expanded tip endings e.g. merkel’disc and ruffini endings - iii) Encapsulated endings e.g. Pacinians, Meissner’ and krauses' corpuscles - iv) Hair end-organs ### The Somatic Sensory System - The somatic sensory system has two major subsystems - Detection of Mechanical Stimuli (touch, vibration, pressure, and cutaneous tension - Mechanoreceptors and Proprioceptors - Detection of Painful Stimuli and temperature ### Mechanoreceptors and Proprioceptors - Mechanosensory processing of external stimuli is initiated by cutaneous and subcutaneous mecahnoreceptors - Other receptors are located in muscles, joints and deep structures and are called proprioceptors ### SKIN OR CUTANEOUS SENSATIONS - There are four cutaneous senses. - i) Touch - ii) Pressure - iii) Temperature (cold, warmth) - iv) Pain - Note: Sustained touch gives rise to pressure. ### Touch-pressure receptors - They are one of the most common receptors in the body. - They are found both in superficial layers of the skin and in deeper regions of the body such as the viscera. Some of these receptors are free nerve endings such as those that wrap around the base of hairs, others are complex receptors. - Most of them are difficult to study due to their small in size. However, the pacinian corpusle is one of the largest receptors in the body. - They compose of nerve endings encapsulated in layers of connective tissue. They are found in the subcutaneuos layers of skin, muscles, joints and internal organs. The concentric layers of connective tissue in the corpuscle create large receptive fields. - They respond best to high frequency vibration whose energy is transferred through the connective tissue capsule to the nerve ending where it opens mechanically gated ion channels. - Pacinian corpuscles are rapidly adapting **phasic receptors** that turn off even if the stimulus continues. Other encapsulated endings include Meissner's corpuscles and Krause's end bulb are also rapidly adapting receptors. - The expanded endings such as Merkel' disc and Ruffini endings have varied distributions in different parts of the body. - The expanded or encapsulated endings appear to function as mechanoreceptors that respond to tactile stimuli. - Merkel'disk and Ruffini endings are slowly adapting **tonic receptors**. - Touch receptors are most numerous in the skin of the fingers and lips and relatively scarce in the skin of the trunk ### Mechanoreceptors of tactile information - These mechanoreceptors are highly sensitive i.e. they have low-threshold - Weak stimulation on the skin induces them to produce action potentials - They are innervated by large myelinated axons. - This ensures rapid central transmission of tactile information. ### They include: - Meissner's corpuscles - Pacinian Corpuscles - Merkel's Corpuscles - Ruffinni's Corpuscles ### Meissner's corpuscles - Lie beneath the dermal papillae beneath the epidermis of fingers, palms, soles - Are elongated receptors - Comprise several lamellae of Schwann cells - Contains one or more afferent nerve fibre that generate rapidly adapting action potential - Receptors respond maximally but briefly - Receptors respond to minimal skin depression - Most common "**glabrous**" skin mechanoreceptors - Afferent fibres account for 40% of the sensory innervation of the human hand - Are efficient transducers of relatively low frequency vibrations (30-50Hz) ### Pacinian Corpuscles - Large encapsulated endings located in the subcutaneous tissue, receptor for pressure and vibration - Found in **interosseous membranes** and mesenteries of the gut - Possess an onion-like capsule - One or more afferent axons lie at the centre of the structure - The capsule acts as a filter allowing stimulation at high frequencies (250-350Hz) - Adapt more rapidly than Meissner's corpuscle - Lower response threshold - Stimulation induces a sensation of vibration or tickle - 10-15% of the cutaneous receptors in the hand - Like Meissner's provide information about the dynamic qualities of mechanical stimuli ### Merkel's Corpuscles - Located in the epidermis - Aligned with papillae lie beneath the dermal ridges - 25% of the mechanoreceptors of the hand - Dense in the finger tips, lips and external genitalia - Play a role in the static discrimination of shapes, edges, rough textures ### Ruffinni's Corpuscles - Are elongated, spindle-shaped, capsule-like specializations located deep in the skin - Also present in tendons and ligaments - Sensitive to cutaneous stretching produced by digit or limb movements. - 20% of receptors in the human hand ### A Diagram of Sensory Receptors - A diagram of different types of sensory receptors is shown. ### A Diagram of Receptors for Special and Somatic Senses - A diagram of receptors for Special and Somatic Senses is shown. ### Types of Somatosensory Nerve fibres - A table showing the type of nerve fibre, information carried, myelin sheath, diameter and conduction speed is shown. ### Proprioceptors - They provide information about mechanical activities arising from the body's musculoskeletal system. - Proprioceptors means **receptors for self**. - They give detailed and continuous information about the position of limbs and other parts of the body in space. ### Types of Proprioceptors - Low-threshold mechanoreceptors - Muscle spindles - Golgi Tendon Organs - Joint receptors ### Propioception - This is mediated by sensory receptors in the muscles and joints of the body that make us aware of our body position in space and of the relative location of various body parts to each other. - If you close your eyes and raise your arm above your head you are aware of its position in space because of the activation of various proprioceptors in the muscles and joints. - Proprioceptors are slowly adapting and probably pacinian corpuscles in the synovia and ligaments. Impulses from slowly adapting spray endings, pacinian corpuscles, touch receptors in the skin & other tissues and muscle spindles are synthesized in the cortex into a conscious picture of the body in space. - Proprioceptive information is transmitted up the spinal cord in the dorsal column. A good deal of this proprioceptive input goes to the cerebellum but some pass via the medial lemnisci and thalamic radiation to the cortex. Disease of the dorsal columns produce ataxia because of the interruption of the proprioceptive input to the cerebellum. ### Muscle Spindles - Found in skeletal muscle - Has four to eight intrafusal muscle fibres. - The largest intrafusal fibres have nuclei collected in an expanded region (**nuclear bag fibres**). - Nuclei of the remaining smaller intrafusal fibres are in a single line (**nuclear chain fibres**). ### Afferent Axons of Muscle Spindles - Myelinated sensory axons Group la (**Primary sensory ending**) innervate the middle portion of both types of intrafusal fibres. - Group II axons provide the secondary innervation to the nuclear chain fibres with minor branch to nuclear bag fibres. - γ motor neurons from the spinal cord can contract the intrafusal muscle fibres. ### Function of Muscle Spindles - They provide information for the degree of stretch of fibres / length of muscle. ### Golgi Tendon Organs - Are low threshold mechanoreceptors. - Are innervated by branches of Group 1b afferents. - Afferents are distributed among the collagen fibres that form the tendons. - They inform the CNS about changes in muscle **tension**. ### Joint Receptors - Are rapidly adapting mecahnoreceptors - Found around joints - They gather information about limb position and joint movement. ### Diagram of Skin Receptors - A diagram of skin receptors is shown, including Meissner corpuscle, Pacinian corpuscle, Ruffini's corpuscles, Merkel's disks, and free nerve endings. ### Diagram of Muscle Spindles - A diagram of a muscle spindle is shown, including axon of a motor neuron, extrafusal muscle fibers, axons of g motor neurons, Group I and II afferent axons, intrafusal muscle fibers, nuclear chain fiber, subcapsular space, nuclear bag fiber, and capsule surrounding spindle. ### Temperature (Cold Or Warmth) - There are two types of temperature sense organs or receptors. - i) Those responding maximally to temperature slightly above the body temperature are known as **warmth receptors**. - ii) The ones that respond maximally to temperature slightly below body temperature are called **cold receptors**. - There are discrete cold-sensitive and warth sensitive spots in the skin. There are 4-10 times as many cold spots as warm. - The temperature receptors are naked nerve endings that terminate in the subcutaneous layers of the skin. - Cold receptors respond from 10ºC - 38ºC and warm receptors from 30ºC - 45ºC. - The afferents for cold are A delta and C nerve fibres. - The afferents for warmth are C nerve fibres. - These afferents relay information to the postcentrai gyrus via the **lateral spinothalamic tract** and the thalamic radiations. - Below a skin temperature of 20°C and above 40°C, there is no adaptation but there is adaptation between 20°C and 40°C. - Above 45°C, tissue damage begins to occur and pain sensation is perceived. - Temperature receptors play an important role in thermoregulation. ### PAIN AND NOCICEPTORS - The sense organs for pain are free nerve endings found in almost every tissue of the body. Nociceptors or pain receptors are activated by a variety of strong noxious stimuli that cause or have potential to cause tissue damage. - Pain is a perceived sensation rather than a stimulus. Pain is an adaptive, protective response to environmental stress. - For example, if we did not feel joint discomfort with over use, we would damaged our joint more quickly. - Pain can be elicited by multiple types of stimuli, which are classified as mechanical, thermal, and chemical pain stimuli. - Some of the chemicals that excite the chemical type of pain are bradykinin, serotonin, histamine, potassium ions, acids, acetylcholine, and proteolytic enzymes. - In addition, prostaglandins and substance P enhance the sensitivity of pain endings but do not directly excite them. ### Fast and Slow Pain - Pain impulses are transmitted by 2 nerve fibre systems. One nociceptor system is made up of small myelinated A delta nerve fibres and the other consists of unmyelinated C nerve fibres. - These latter nerve fibres are found in the lateral division of the dorsal roots and are often called dorsal root C fibres. Both nerve fibre groups end in the dorsal horn. A delta nerve fibres terminate primarily on neurons in laminal I while C nerve fibres terminate in laminal II and III. - The synaptic transmitters secrected are glutamate and substance P, for fast and slow pain respectively. Some of the axons of the dorsal horn neurons end in the spinal cord and brain stem. Others enter the anterolateral system. A few ascend in the posteriolateral portion of the cord. Some of the ascending nerve fibres project to the specific sensory relay nuclei of the thalamus and from there to the cerebral cortex. - Pain activates 3 cortical areas: SI,SII and cingulate gyrus on the side opposite the stimulus. - The cingulate gyrus is involved in emotion and cingulate gyrectomy has been reported to lessen the distress associated with chronic pain. - Fast pain: it is sharp and localized and often followed by slow Pain. It is due to quicker transmission of impulse by the myelinated A delta nerve fibres. - Slow pain: It is dull, intense, diffuse with unpleasant feeling. Slow pain is due to activity of Unmyelinated C nerve fibre. - For instance, when you stab your toe, you first experience a quick stabbing sensation (fast pain) followed shortly by a dull throbbing (slow pain). ### Referral of Pain - Pain in the heart and other internal organs (viscera) is often poorly localized and may be felt in areas far remote from the sight of the stimulus. - This is called referral of pain. Eg pain of cardiac ischaemia is often felt in the neck, left shoulder and arm. ### Mechanism of referred pain - Branches of visceral pain fibers synapse in the spinal cord on the same second-order neurons (1 and 2) that receive pain signals from the skin. - When the visceral pain fibers are stimulated, pain signals from the viscera are conducted through at least some of the same neurons that conduct pain signals from the skin, and the person has the feeling that the sensations originate in the skin.