Sensory System Overview

**Sensory System Overview** - **Sensory receptors**: Can either be: - **Specialized endings of afferent neurons**. - **Separate cells** that signal the afferent neuron. - **Transduction**: Involves the opening or closing of ion channels in response to a stimulus. This convert...

**Sensory System Overview** - **Sensory receptors**: Can either be: - **Specialized endings of afferent neurons**. - **Separate cells** that signal the afferent neuron. - **Transduction**: Involves the opening or closing of ion channels in response to a stimulus. This converts external stimuli into electrical signals that the nervous system can understand. - Variable stimulus intensity: The strength of the stimulus can vary. - *Produces variable receptor potentials: When a stimulus is applied, it generates receptor potentials in the receptor membrane.* - Amplitude: The amplitude of these receptor potentials depends on the stimulus intensity. - Insulates the axon: Myelin acts as an insulator, increasing the speed of signal transmission***.*** - Threshold: The first node of Ranvier is where the action potential is generated if the receptor potential reaches the threshold. - ***Action potentials:*** If the threshold is reached, action potentials are produced at the first node of Ranvier. - ***Variable frequencies:*** The frequency of action potentials depends on the stimulus intensity. Higher intensity stimuli produce higher frequencies of action potenti***als.*** - Action potentials down the axon: Action potentials travel down the axon to the axon terminal. - Neurotransmitter: The axon terminal releases neurotransmitters, which transmit the signal to the next neuron in the central nervous system. **Principles of Sensory System Organization.** 1**. Specific sensory receptor types are sensitive to certain modalities and submodalities**. - Different types of sensory receptors are specialized to detect specific types of stimuli, such as touch, temperature, pain, vision, hearing, taste, and smell. **2. A specific sensory pathway codes for a particular modality or submodality.** - Each sensory modality has its own dedicated pathway in the nervous system. This ensures that the brain can accurately interpret and process different types of sensory information. 3\. **The ascending pathways are crossed so that sensory information is generally processed by the side of the brain opposite the side of the body that was stimulated.  ** - Most sensory pathways cross over to the opposite side of the brain. This allows for a more integrated and coordinated processing of sensory information from both sides of the body. 4\. **In addition to other synaptic relay points, all ascending pathways, except for those involved in smell, synapse in the thalamus on their way to the cortex**.   - The thalamus is a relay station in the brain that receives and processes sensory information before it is sent to the cerebral cortex for further processing. 5**. Information is organized such that initial cortical processing of the various modalities occurs in different parts of the brain.**   - Different areas of the cerebral cortex are responsible for processing different types of sensory information. For example, the visual cortex processes visual information, while the auditory cortex processes auditory information. 6\. **Ascending pathways are subject to descending controls.** - The brain can influence the processing of sensory information through descending pathways. This allows the brain to selectively attend to certain **Stimulus and CNS Activity** - **Intensity of Stimuli**: Changes in stimulus intensity produce variable receptor potentials, which then result in different frequencies of action potentials in the CNS. **Sensory Pathways** - **General Sensory Pathway Layout**: Three neurons in series (primary, secondary, and tertiary) form the sensory pathway from peripheral receptors to the brain. - **Somatotopic Representation**: Sensory pathways are organized topographically in the brain, meaning different body areas are represented in specific brain regions. **Two Key Sensory Pathways:** 1. **Anterolateral Pathway (Spinothalamic)**: 1. **Carries information on pain and temperature:** The first-order neurons of the spinothalamic tract carry sensory information related to pain and temperature from the periphery (e.g., skin) to the spinal cord. 2. **Enters the spinal cord and synapses with a secondary neuron in gray matter at that level:** The first-order neurons enter the spinal cord and synapse with second-order neurons in the gray matter of the spinal cord. 3. **Secondary neuron then crosses over (decussates) in the spinal cord then ascends to the thalamus:** The second-order neurons cross over to the opposite side of the spinal cord (decussation) and ascend to the thalamus. 4. **A third neuron in the thalamus then directs the information to the somatosensory cortex:** In the thalamus, the third-order neurons receive the sensory information and relay it to the somatosensory cortex, where it is interpreted as pain or temperature sensation. **Dorsal Column Pathway**: 1. **Carries general somatic information (proprioception, fine touch & vibration):** The first-order neurons of the dorsal column-medial lemniscus pathway carry sensory information related to proprioception, fine touch, and vibration from the periphery (e.g., muscles, joints, skin) to the spinal cord. 2. **First neuron enters the spinal column then begins to ascend on the dorsal column:** The first-order neurons enter the spinal cord and ascend on the dorsal column. 3. **1st order neuron synapses with a secondary neuron in the brainstem:** In the brainstem, the first-order neurons synapse with second-order neurons. 4. **2nd order neuron decussates in the brainstem and then travels to the thalamus where 3rd neuron projects to the somatosensory cortex:** The second-order neurons cross over to the opposite side of the brainstem (decussation) and ascend to the thalamus. In the thalamus, the third-order neurons receive the sensory information and relay it to the somatosensory cortex, where it is interpreted as proprioception, fine touch, or vibration sensation. **Brown-Séquard syndrome** is a neurological condition that results from damage to one side of the spinal cord. This damage can disrupt the dorsal column-medial lemniscus pathway, leading to a **dissociation of sensory loss**. Patients with Brown-Séquard syndrome may experience loss of proprioception, fine touch, and vibration on the same side as the spinal cord injury, while pain and temperature sensation may be lost on the opposite side. **Nonspecific pathways** are sensory pathways that provide **background information** about touch and temperature from the periphery. They are not as precise as the specific pathways (like the spinothalamic tract and dorsal column-medial lemniscus pathway) but contribute to our overall sense of touch and temperature. **Polymodal neurons** are neurons that can carry **more than one type of sensory information**. These neurons may respond to multiple stimuli, such as touch, temperature, and pain. They are important for integrating different sensory inputs and providing a more comprehensive understanding of our environment. - **Located on postcentral gyrus:** The primary somatosensory cortex is situated on the postcentral gyrus, a prominent ridge in the parietal lobe of the brain. - **Neurons in this area receive sensory info from entire body WRT somatic sensations**: Neurons in the primary somatosensory cortex receive sensory information from the entire body regarding somatic sensations, which include touch, pressure, temperature, and pain. - **Contralateral input:** The sensory information received by the primary somatosensory cortex is contralateral, meaning that it comes from the opposite side of the body. For example, sensory information from the right side of the body is processed by the left primary somatosensory cortex. - **Is there a resemblance to motor somatotopic map?** Yes, there is a striking resemblance between the primary somatosensory cortex and the primary motor cortex. Both areas have a somatotopic map, which means that different areas of the cortex represent different parts of the body. In the primary somatosensory cortex, the representation of body parts is distorted, with larger areas devoted to more sensitive regions, such as the hands and face. This is known as the homunculus. **Receptors and Stimulus Properties** - **Four Basic Receptor Types**: 1. **Mechanoreceptors**: Respond to physical deformation. 2. **Thermoreceptors**: Detect temperature changes. 3. **Nociceptors**: Respond to painful stimuli. 4. **Proprioceptors**: Sense body position and movement. - **Four Key Stimulus Properties(Sensory representation)**: 5. **Modality**: Refers to the type of stimulus (e.g., touch, temperature). - The brain associates impulses from specific receptors with specific modalities (e.g., phantom limb). 6. **Location**: Defined by the **receptive field** size and overlap of receptors. - **Lateral Inhibition**: Sharpens the contrast between active and inactive neurons, improving stimulus location precision. - **Each receptor has a receptive field:** Each sensory receptor has a specific area of the body or sensory organ that it is responsible for detecting stimuli. This area is called the receptive field. - **They are of varying size + overlap:** Receptive fields can vary in size and often overlap with each other. This overlap helps to ensure that sensory information is processed efficiently and accurately. - **Convergence leads to ↑ sensitivity but ↓ resolution (acuity):** When multiple sensory receptors converge onto a single neuron, it leads to increased sensitivity. This means that the neuron can detect weaker stimuli. However, this also results in decreased resolution or acuity, as it becomes more difficult to precisely localize the source of the stimulus. 7. **Intensity**: Proportional to the number of action potentials generated. - Stronger stimuli recruit more receptors. (**Recruitment / population coding)**. 8. **Duration**: Depends on receptor type (**tonic vs. phasic).** **Slowly adapting receptors** (Figure a) exhibit a gradual decrease in their firing rate over time when exposed to a constant stimulus. This type of receptor is well-suited for detecting sustained stimuli, such as pressure or joint position. Examples of slowly adapting receptors include Merkel\'s disks and Ruffini corpuscles. **Rapidly adapting receptors** (Figure b), on the other hand, show a rapid decrease in their firing rate and then quickly stop firing when exposed to a constant stimulus. These receptors are sensitive to changes in stimuli, such as the onset or offset of a touch or vibration. Examples of rapidly adapting receptors include Meissner\'s corpuscles and Pacinian corpuscles. The **off response** shown in Figure b is a brief increase in firing rate that occurs when the stimulus is removed. This off response is characteristic of rapidly adapting receptors and is important for detecting changes in stimuli. **Pain Mechanism** - **Pain Definition**: A complex sensation perceived as pricking, burning, etc. - **Nociceptors**: Pain receptors that respond to various stimuli like **prostaglandins**, **serotonin**, **bradykinin**, **Histamine and K+** and more. **Types of Pain Fibers:** 1. **Type Aδ fibers**: Myelinated, conduct sharp pain signals quickly. 2. **Type C fibers(polymodal)** : Unmyelinated, slower, and carry dull, aching pain. - **Pain Pathways**: Project through the lateral spinothalamic tract to the thalamus and then to different brain areas (e.g., reticular formation, limbic system). - Perception of pain is associated with our autonomic responses due to interconnections between cortex, thalamus, brainstem & hypothalamus & ANS - - **Neurotransmitters**: Involved in pain signaling include **substance P**, **glutamate**, and **CGRP (Calcitonin Gene-Related Peptide)**. **Gate Control Theory of Pain(created by Melzack and Wall)** **Original theory revised \~ 1993** - Balance of activity in dorsal horn (substantia gelatinosa) between nociceptive **(Aδ,C)** & non nociceptive **(Aβ**) afferents that converge at a particuler level of spinal cord - **Non-nociceptive fibers (Aβ)** activate inhibitory interneurons that release **enkephalins**, reducing pain signals. - Techniques like **TENS (Transcutaneous Electrical Nerve Stimulation)** stimulate these fibers to reduce pain perception. - **Explains how a rub + electroanalgesia (TENS\*) provide relief from pain as they stimulate large type Aβ fibres which effectively ↓ impulses from pain afferents to brain. Topographically specific** - Perception of pain can also be altered by input from central areas as well viz conscious desire to fight pain + placebo - Pain can be controlled by higher brain centers, including **periaqueductal gray (PAG)**, ( 3^rd^ and 4^th^ ventricle) which modulates pain via descending pathways. - PAG neurons inhibit pain by releasing **enkephalins**, which block pain neurotransmitters like **substance P**. - Enkephalins bind with mu receptors of interneurons prevent release of substance p from incoming pain afferents - Afferents from nociceptors are "blocked"/inhibited - This explains maternal behavior + heroic deeds - Periaqueductal grey matter (PAG) in midbrain - PAG also receives input from hypothalamus, cortical areas + presence of opioid receptors - Interneurons contain endogenous opioid peptides: - enkephalin, dynorphin & - β endorphins (blocked by naloxone = opiate antagonist) - Encaphalin -supresses transmition of pain informationbetween 1^st^ and 2^nd^ order neurons **Referred Pain** - **Referred Pain**: Pain felt in an area far from the source (e.g., heart attack causing pain in the arm). - Occurs because visceral and somatic sensory fibers converge at the same point in the spinal cord, leading the brain to misinterpret the pain source. - **Theories:** - 1**)Due to convergence ↓ acuity**. Somatic sensation more common so brain "misinterprets" origins of sensory impulse - 2)**Afferents from viscera** → ↓ threshold of 2nd order neurons ie closer to firing point! Thereafter any minor stimulus from somatic area depolarizes nerve & pain appears to come from somatic area - Spinothalamic tract (2nd order) project to thalamus (3rd order) then to cortex (cingulate gyrus +insular cortex - An explanation of referred pain? - **Referred pain an example of how convergence leads to a ↓ in acuity.** - **Referred pain → visceral + somatic afferents converge on the same 2° neurons in the spinal cord** **Neuropathic vs. Nociceptive Pain** - **Neuropathic Pain**: Caused by nerve damage, often persistent and described as burning or tingling. It is more resistant to treatment. - Patients also often present with allodynia - Due to damage/ lesions to the NS itself (CNS or PNS) eg diabetes, multiple sclerosis, entrapment, infection (*herpes zoster*) or surgery. - It's a case of "sending the wrong signal to the brain" - Narcotics + electro-analgesia ? - Management sympathetic blockade, tricyclics, etc - Neuropathic pain network (NPN) - **Nociceptive Pain**: Caused by tissue injury or inflammation, usually resolves once the injury heals.