Sensing and Seeing: Somatosensory 1-3 Notes PDF

Sensing and Seeing: Somatosensory 1-3 The function of a sensory system is to detect changes in the environment (external or internal) and relay that information to the central nervous system to elicit an appropriate response. Sometimes the response is a stereotypic reflex, which may or may not invol...

Sensing and Seeing: Somatosensory 1-3 The function of a sensory system is to detect changes in the environment (external or internal) and relay that information to the central nervous system to elicit an appropriate response. Sometimes the response is a stereotypic reflex, which may or may not involve the cerebral cortex and sometimes the response is a conscious decision based on previous experience and knowledge. The sensory systems that we will discuss in this course include somatosensory (touch, temperature, pain, itch, pressure, position), olfactory (odors), gustatory (taste), visual (light), and vestibular (movement). In all the sensory systems, the basic steps of sensory perception are the same. A receptor detects a stimulus and translates it into an electrical signal. A sensory neuron then transmits the electrical signal to the central nervous system. This information is modulated based on inputs from other sensory receptors. Synapses on neurons in the primary sensory cortex of the brain are associated with conscious perception of the stimulus; however, no all sensory information reaches the level of the cortex. Some sensory information is processed by the spinal cord or brainstem. Sensory receptors are classified according to the type of stimulus they detect. Mechanoreceptors are activated by physical force, such as touch, stretch, and vibration. Photoreceptors are activated by light. Nociceptors, or pain receptors, respond to tissue damage or stimuli that are associated with tissue damage. Chemoreceptors are activated by the binding of specific chemicals, and thermoreceptor respond to heat or cold temperature. Sensory receptor can also be classified based on whether they are part of the sensory neuron or are a - - - - - - - N - separate cell. Primary receptors are specialized terminals of sensory neurons that detect touch, pain, and temperature. Secondary receptors, which are specialized cells, separate from the sensory neuron, include rods and cones of the retina and hair cells in the inner ear. For both primary and sensory receptors, the stimulus is converted to an electrical signal through the opening or closing of ion channels. A group of channels that is important in sensory reception are the transient receptor potential (TRP) channels. These channels are permeable to sodium and calcium, and therefore cause a depolarization in response to the stimulus. TRP channels are important in sensing pain, temperature, taste, pressure and light. Receptor adaptation is an important concept in sensory systems. Adaptation refers to a decrease in sensitivity to a prolonged stimulus. Slow adapting receptor (also called tonic receptors) show a decreased rate of action potential production in response to a prolonged stimulus. Rapidly adapting receptors (also called phasic receptors) only produce action potential when the stimulus changes (beginning and end of a prolonged stimulus). The somatosensory system is the sensory system devoted to detection of tactile, proprioceptive, thermal, and pain stimuli. The receptors are located in the skin, muscles, tendons, joints, teeth, mucous membranes, and corneas. The somatosensory pathways consist of three neurons: the sensory neuron in the peripheral nervous system and two interneurons in the central nervous system. The sensory neuron’s axon is found in a spinal or cranial nerve and the cell body is located in the dorsal root ganglion near the spinal cord. The sensory neuron synapses on the second-order neuron in the spinal cord, and its axon travels cranially in the spinal cord to the thalamus, where it synapses with the third-order neuron. The third-order neuron relays information from the thalamus to the primary somatosensory cortex. The skin receptors that detect non-painful touch (light touch, - - - - - - - - - vibration, stretch, pressure) are located in the superficial or deep dermis and are divided into 4 types based on their structure and function. They are all specialized nerve endings (primary receptors), often surrounded by layers of connective tissue that help detect the stimulus. This group includes both rapid and slow adapting receptors. You do not need to memorize the names and differences between these receptors; just know that they exist and occasionally are clinically important (for example, Merkel cells can undergo malignant transformation to form Merkel cell carcinoma). Nociceptors detect actual or potential damage to tissues. Many of these receptors use transient receptor potential channels to transform the stimulus into an electrical signal. There are different receptors that respond to mechanical pain, cold pain, hot pain, and pinprick pain. Blocking nociceptors and the neurons that transmit the signals from nociceptors to the sensory cortex is an area of interest for development of analgesics (drugs for the relief of pain). Thermoreceptors that detect non-painful heat and cold also use TRP channels to generate an electrical signal. The low threshold receptors, which detect cold, are also activated by some chemicals, such as methanol, causing a cold sensation. Similarly, high threshold receptors, which detect heat, can be activated by the chemical capsaicin in spicy food. We have already talked about muscle spindles and Golgi tendon organs, which are proprioceptors that detect movement and position of the limbs. Muscle spindles are located inside skeletal muscles and detect stretch of the muscle. Golgi tendon organs are located in tendons connecting muscle to bone and provide information about the degree of muscle contraction. There are also receptors in joints (joint kinesthetic receptors) that detect extreme flexion and extension, in order to prevent damage to the joint or surrounding tissues. For all of these sensory modalities, the first order somatosensory neuron is a pseudounipolar neuron, with its cell body located in a dorsal root ganglion of the spinal cord or a cranial nerve ganglion. The peripheral process extends from the site of stimulation to the cell body and the central process extends from the cell body to the central nervous system, where it synapses with a second order sensory neuron. The peripheral process of the sensory neuron is the equivalent of its axon, and the speed of conduction depends on the diameter of the axon and whether or not it is myelinated. The fastest somatosensory nerve fibers are for proprioception (action potentials travel at up to 280 mph). Pain fibers are slightly slower (80 mph), and information about chronic pain and nonpainful temperature is transmitted at an even slower speed (4 mph). The peripheral processes of sensory neurons in the body travel in spinal nerves, along with motor information travelling from the central nervous system to the skeletal muscles. A spinal nerve may contain sensory information that eventually enters the central nervous system through a dorsal root, which corresponds to one of the segments of the spinal cord (C2-C8, T1-T13, L1L7, and S1 in dogs). Each of those dorsal roots receives sensory information from a strip of skin called a dermatome, as well as from muscles, tendons, and connective tissue in that area of the body. The cell bodies of the sensory neurons are located in dorsal root ganglia, near the spinal cord. In addition to the cell bodies of the somatosensory neurons, the dorsal root ganglia contain a population of support cells called satellite cells, and the axons carrying signals to and from the cell bodies. (Inflammation of the dorsal root ganglia is called ganglioneuritis and inflammation of the nerve roots is called radiculoneuritis.) The central processes of the somatosensory neurons enter the spinal cord and usually synapse in the dorsal horn of the grey matter. Remember that the spinal cord is organized with the grey matter (neuron cell bodies) in a butterfly shape at the center of the cord, with the white matter on the periphery, organized into ventral, lateral, and dorsal funiculi. With a few exceptions, the dorsal half of the spinal cord contains sensory neurons and axons and the ventral half contains motor neurons and axons. The grey matter of the spinal cord is organized, based on function, into layers called Rexed’s laminae. There are two major somatosensory pathways that transmit information about touch, pain, temperature, and body position to the cerebral cortex. The first is the dorsal somatosensory pathway, also called the dorsal column pathway. This pathway carries information about touch, position, and vibration from the hindlimb and forelimb. This pathway involves three neurons: the first order neuron is the sensory neuron we have already discussed. The central process of this sensory neuron does not synapse in the spinal cord, but rather ascends in the dorsal funiculus (in the medial fasciculus gracilis for axons carrying information from the hindlimb and in the more laterally located fasciculus cuneatus for axons carrying information from the forelimbs). These axons synapse on second-order neurons located in nuclei in the brainstem (nucleus gracilis for the hindlimb and nucleus cuneatus for the forelimb). The axons of these second-order neurons cross midline and ascend to the thalamus, where they synapse of third-order neurons. The third order neurons transmit information to the primary somatosensory cortex. The other somatosensory pathways are the spinothalamic tracts. These pathways carry information about pain, temperature and light touch. The sensory neuron synapses on a second-order neuron in the dorsal horn, soon after entering the spinal cord. The axon of the second order neuron crosses midline and ascends in the lateral or ventral funiculus to the thalamus, where it synapses on the third-order neuron. Similar to the dorsal somatosensory pathway, the third-order neuron synapses in the primary somatosensory cortex. First order sensory neuron axons typically branch, so in addition to synapsing on second-order neurons, they also synapse on other interneurons that mediate local effects, such as spinal reflexes. There are also other ascending pathways that transmit information to parts of the brain other than the cerebral cortex: the spinoreticular tract projects to the reticular formation and helps orient attention to a painful stimulus; the spinovestibular tract projects to the vestibular nuclei and carries information about posture and balance; and the spinocerebellar tract projects to the cerebellum and carries information needed for coordination of motor activity. The thalamus is part of the diencephalon and acts as a relay station for almost all sensory information going to the cerebral cortex. Remember that the thalamus consists of two halves that are fused along midline at the interthalamic adhesion. The thalamus is divided into functional nuclei, which are groups of neurons with a common purpose. The nuclei that contain the cell bodies of third-order neurons of the somatosensory system are located in the ventral part of the thalamus. The primary somatosensory cortex is located just caudal to the primary motor cortex. The neurons in the cortex are organized in a somatotopic map, meaning that each part of the cortex receives information from a particular part of the body. The amount of the primary somatosensory cortex devoted to a particular body part reflects how sensitive that area is to stimuli, and how important sensation in that area is to the animal’s survival (finding food, sensing predators, communicating with others). In humans, the largest areas of the somatosensory cortex are devoted to the tongue and hands, while in cats the face (particularly the vibrissae or whiskers) are overrepresented. The somatosensory cortex is also organized by sensory modality, so that information about light touch to the left foot synapses on neurons located just rostral to the neurons that receive information about the position of the foot. Sensory processing involves connections within the primary somatosensory cortex, between the different modalities, and connections from the primary somatosensory cortex to other areas of the cortex, including the premotor cortex, for planning a motor response to the stimuli. Although itch (pruritis) is not classically included with the somatosensory system, it probably should be. Itch is an unpleasant sensation that evokes a motor response (scratching). Itch is usually caused by chemicals (called pruritogens) such as histamine. The chemicals bind to TRP type receptors called pruriceptors, and the information is transmitted to the spinal cord by slow C type axon fibers. The signal then ascends to the brain in the spinothalamic tract. Somatosensory information from the head of the head travels from the receptors to the central nervous system via three cranial nerves (V, VII, IX) and one spinal nerve (C2). The trigeminal nerve (CN V) provides most of the somatosensory innervation, with VII, IX and C1 each innervating part of the pinna. The trigeminal nerve has three divisions: the ophthalmic nerve innervates the skin around the eye; the maxillary nerve innervates the dorsal part of the head; and the mandibular nerve innervates the ventral part of the head. The peripheral processes of neurons in these three divisions relay information about touch, pain, temperature, itch, jaw position, and muscle stretch, to the central nervous system. The cell bodies of these neurons are found in the trigeminal ganglia, which are located on the floor of the skull, lateral to the pituitary gland. The central processes of these neurons synapse on second order neurons in the trigeminal sensory nuclei, located in the brainstem and midbrain. Axons carrying information about pain and temperature synapse in the spinal nucleus (located in the medulla), axons carrying information about light touch and proprioception synapse in the pontine nucleus, and axons carrying information from muscle spindles synapse in the mesencephalic nucleus. Second-order neurons with their cell bodies in these nuclei send axons via the trigeminothalamic tract to the thalamus, where they synapse on third-order neurons. The third order neurons project to the somatosensory cortex. The sensory pathways for the facial and glossopharyngeal nerves are very similar, except that the axons of the first order neurons travel in the facial and glossopharyngeal nerves, instead of the trigeminal nerve. The first-order neurons synapse on second order neurons in the trigeminal sensory nuclei, the second-order neurons synapse on third-order neurons in the thalamus, and the third-order neurons project to the somatosensory cortex. Sensation from the internal organs (visceral sensation) is not as well understood as sensation from the skin, muscles, tendons, and mucous membranes (somatic sensation). The sensory receptors on the viscera are called enteroreceptors (or interoreceptors) as opposed to exteroreceptors. These receptors include mechanoreceptors that detect stretch and distention, chemoreceptors that detect pH, oxygen and carbon dioxide levels, and nociceptors that detect tissue damage, inflammation and ischemia. In humans, visceral pain is described as dull or aching and is often poorly localized or referred to a part of the body distant from the site of the stimulus (ischemia in the myocardium can be perceived as pain in the arm or jaw. In veterinary species, visceral pain can be caused by gastrointestinal or urinary tract obstruction or inflammation in the viscera (such as pancreatitis). The signs of colic in horses include stretching, rolling, pawing, or kicking/biting at the flanks. In dogs with pancreatitis, a posture with the thorax lower than the rump (“downward dog”) is characteristic. Visceral pain receptors are similar to somatic pain receptors, and are usually free nerve endings with specialized TRP channels. A painful stimulus causes the channels to open and, if enough channels open, an action potential is generated. Most viscera are innervated by both spinal nerves and the nerves of the autonomic nervous system (vagus and sacral nerves for the parasympathetic system and splanchnic nerves for the sympathetic system). Pain signals often travel in the dorsal somatosensory and spinothalamic tracts, while the vagus nerve transmits information about non-painful stimuli, such as hunger, fullness, and nausea. Both visceral and somatic pain are classified as nociceptive or physiologic pain, and are due to tissue damage or stimuli that can signal potential tissue damage. Nociceptive pain is contrasted with neuropathic pain, which is due to damage to the nervous system. In humans, this type of pain is described as burning, prickling, or tingling. Neuropathic pain can be caused by trauma, metabolic disease, neurotoxic chemicals, infectious agents, or neoplastic disease, and can affect the peripheral nervous system (sensory receptors, spinal or cranial nerves) or the central nervous system (ascending tracts in the spinal cord, the thalamus, or the somatosensory cortex). Damage to the nervous system can cause pain signals by opening channels that result in action potentials, decreasing the threshold for firing action potential, inhibiting interneurons that modulate pain signal, or creating abnormal neural pathways. In animals, signs of neuropathic pain include chewing, biting, scratching at a particular site, crying out with no evidence of external stimuli, and hypersensitivity to painful or non-painful stimuli. Neuropathic pain is also suspected if other causes of pain have been excluded. -