Chapter 6 Gathering information; the sensory systems
Gathering information the sensory systems The parts of the nervous system that receive and process information are termed sensory systems. There are five senses common to most mammals; vision, smell, hearing, taste, and touch. - However, some mammals have developed other special systems that aid in their survival; the best examples are echolocation (in bats and dolphins) and electroreception (in the platypus). - Some mammals, such as the mole, have lost almost all their visual sense, and rely almost entirely on smell and touch.
Gathering information the sensory systems The sensory systems have two basic roles detection and understanding. Detection refers to the recognition of information, while understanding refers to using this information appropriately. Detection is the role of specialized receptor cells; the highest level of understanding occurs in the cerebral cortex.
Chapter 6. Gathering information the sensory systems When we speak of the senses, we are generally referring to conscious perception, -but this is not the only way that sensory information is used by the central nervous system. Conscious perception is just one aspect of sensory function, since the information collected by sensory receptors is registered at many levels in the nervous system. In many cases, the conscious experience of sensation may occur well after other control centers have already acted on the information. This highlights the distinction between receptor activity, indicating that a receptor has been stimulated, and perception, which is the conscious experience that results from the cortex receiving information from the receptor activity.
Receptors Receptors convert energy of a particular type (such as light, vibration, or pressure) into a graded electrical potential, which is transmitted to the central nervous system by a sensory axon. - The process of converting a physical stimulus into a graded electrical potential is called transduction. - Receptors use changes in the permeability of ion channels to alter their membrane potential. If the stimulus is strong enough, it will generate an action potential in the sensory axon. - Each action potential in a sensory axon is identical, but stimuli of different intensity or duration can alter the rate of firing of the sensory axon, and so give more information about the stimulus. Each sensory modality employs an exclusive pathway to the cortex, making specific and ordered connections, so the brain can make sense of the information it receives.
Gathering information the sensory systems; Receptors Most vertebrates have similar types of receptors, but individual groups of mammals have specializations in terms of the exact type and number of receptors. Primates are largely visual animals and have many more visual receptors, including some specialized to detect color. Rats and mice, which usually live in dark and confined environments, have fewer visual receptors, but have many tactile receptors linked to whiskers, which sweep in front of them as they move around.
Gathering information the sensory systems; Receptors The electrical signal generated by the receptor is passed on to a ganglion cell, whose job is to carry the signal to the brain or spinal cord. In the case of fine touch sensation from a fingertip, this is the dorsal root ganglion neuron has a long axon that travels from the fingertip, past its ganglion cell body, and into the spinal cord. - In the central nervous system, this same axon can travel rostrally and pass the signal on to a neuron in the hindbrain. - The axon arising from this hindbrain neuron crosses to the opposite side and travels to the thalamus. - Thus, the left thalamus receives touch information from the right side of the body. The final link consists of a thalamic cell axon which sends the information to the cerebral cortex.
Receptors This sequence of information trans - mission, from receptor, ganglion cell, relay neuron, thalamic neuron, and finally to cortex, is present in almost all sensory systems. The basic plan does have some important variations; not all the information is sent to the opposite side of the nervous system. In the case of the auditory system there are both crossed and uncrossed pathways, which are interconnected in an elaborate system for detecting the exact direction from which the sound came.
Sensory Maps Keeping sensory maps intact A common property of sensory projections is that they retain some kind of spatial arrangement for their entire passage through the nervous system. The maintenance of spatial (topographical) arrangement results in what is called somatotopy for the touch system, which means that the map of the skin of the body in proper spatial order is transmitted to the cortex. In the case of the visual system, the retinal map is maintained (retinotopic organization) and in the case of the auditory system, the tonal map is transmitted intact (tonotopic organization).
Interpretation and understanding Understanding involves the interpretation of sensory information, so that the brain can take appropriate action. - At the simplest level, sensory stimuli may provoke an immediate reflex response. - At the next level, sensory interpretation might include making use of sensory feedback to modify a pattern of activity, such as altering walking behavior on the basis of feedback from the soles of the feet and the angles of leg joints. At a more conscious level, understanding might include using the cerebral cortex to decode the sounds of speech, and using the speech content as part of a decision making process for action. The essence of understanding is the interpretation of sensory information on the basis of prior learning, in order to select the most useful responses.
Gathering information the sensory systems Sensory areas in the cerebral cortex Skin sensation is received by the primary somatosensory cortex (S1), which lies immediately behind the motor cortex. The primary visual cortex (V1) is located more caudally in the occipital pole of the cortex. - In the human brain, the Sensory V1 cortex areas is in the cerebral cortex mostly located on the medial side of the occipital lobe. The primary auditory cortex (A1) is located in the temporal region. - In the human brain, the A1 cortex is mostly hidden in the lower bank of the lateral fissure. Visceral sensation (including taste and sensation from internal organs) is received by the insula (taste), deep in the lateral fissure.
Sensory areas in the cerebral cortex The S1 cortex contains a map of the whole skin surface of the opposite side of the body, with the parts of the body properly arranged. In the V1 cortex, there is a precise point-to-point map of the visual field that is seen by the eye. Although these cortical maps are spatially coherent, they are distorted in an interesting way. - Some parts are represented by a much bigger cortical area than others. - In humans the areas receiving touch information from the lips and fingertips are huge compared with the area devoted to the rest of the body.
Sensory areas in the cerebral cortex Each area of primary sensory cortex is surrounded by secondary areas that progressively analyze and interpret sensory information. - For example, the image received by the visual cortex may be recognized in a related cortical area as the face of someone we know. Finally, different kinds of sensory information (touch, vision, hearing) are pooled in association cortical areas in the parietal and temporal lobes. - This enables the brain to correlate what we see with what we hear and feel.
Gathering information the sensory systems Sensory areas in the cerebral cortex The olfactory pathway has a completely different organization. - The olfactory pathway does not cross to the opposite side of the brain as the touch pathway does, - The thalamus is not involved in the chain from periphery to cerebral cortex. - Instead, the olfactory pathways go directly to the olfactory cerebral cortex, an area near the amygdala called the piriform cortex.
Gathering information the sensory systems; Somatosensory system The somatosensory system carries information about touch, deep pressure, pain, and temperature from the skin, joints and muscles. There are two major somatosensory pathways, 1) the dorsal columnmedial lemniscus pathway and 2) the spinothalamic pathway.
Gathering information the sensory systems; Somatosensory system Each consists of three neurons forming a chain from the receptors to the cerebral cortex. 1) The primary sensory neuron has its cell body in a spinal ganglion or cranial nerve ganglion, 2) The secondary sensory neuron has its cell body in the gray matter of the spinal cord or in the hindbrain, 3) The tertiary sensory neuron has its cell body in the thalamus.
Somatosensory system These pathways cross the midline so that the sensory information from one side of the body is brought to the opposite cerebral hemisphere. A major difference between the two pathways is the levels at which they cross the midline; - Temperature and pain-related fibers (spinothalamic tract) cross almost immediately where they enter the spinal cord. - Fine touch and proproception (dorsal column-medial lemniscus pathway) does not cross until it reaches the hindbrain.
Dorsal column-medial lemniscus pathway The main touch pathway. It transmits signals from low-threshold mechanoreceptors in the skin, muscles, and joints. This enables tactile discrimination, vibration detection, form recognition, and proprioception (sense of position from joint and muscle receptors). 1. First-order (primary) neurons are located in the dorsal root ganglia. - In the lower extremities, the axons of these ganglion cells form the gracile fasciculus, - while in the upper extremities they form the cuneate fasciculus. - These fasciculi are collectively known as the dorsal columns.
Dorsal column-medial lemniscus pathway The dorsal columns are somatotopically organized, with the gracile axons placed medially, and the cuneate axons lateral to them. 2. The axons ascend in the dorsal columns and terminate in the gracile and cuneate nuclei of the hindbrain. - This is where the second-order neurons are located. - The axons of these neurons cross the midline and form a compact bundle called the medial lemniscus. 3. It ascends through the brainstem to synapse with third order neurons in the ventroposterior nucleus of the thalamus. - The thalamic neurons send their axons to the somatosensory cortex in the postcentral gyrus.
Spinothalamic tracts There are two spinothalamic tracts, the lateral tract and the ventral tract. Information about pain and temperature is conveyed by the lateral spinothalamic tract while touch sensation is conveyed by the ventral spinothalamic tract. Axons connect the skin with the dorsal horn of the spinal cord, where they synapse almost immediately with second order neurons in the substantia gelatinosa, a loose aggregate of neurons at the head of the dorsal horn. The cell bodies of neurons that make up the spinothalamic tract are located principally within the dorsal horn of the spinal cord, and their axons cross the midline via the anterior white commissure of the spinal cord. The axons then continue along the length of the spinal cord toward the thalamus, where they ultimately synapse with thirdorder neurons, which in turn transmit signals to the somatosensory cortex.
Pain and nociception It is important to distinguish between pain, a state of distress usually related to injury or damage, and nociception, the detection of actual or likely tissue damage. Pain and nociception are closely related, but can occur independently. - A person under general anesthetic has working nociceptors that can signal tissue damage and trigger reflexes, but does not experience pain because there is no conscious awareness of distress to motivate behavior. - Conversely, in some cases of chronic pain, the nervous system can end up abnormally sensitized so that the pain state persists even when the nociceptors are no longer active, a condition called neuropathic pain.
Pain and nociception Pain is a signal that the body may be damaged, and the injury must be avoided as a high priority. Because pain is such an important signal to the nervous system, it is typically not subject to the same kinds of filtering that occur with other senses. - For example, most senses will adapt to a stimulus that persists for a long time, and the system will respond to it less and less over time. - However, if nociception persists for long periods, chemical feedback mechanisms make nociceptors become more sensitive and can cause previously unresponsive receptors to become activated (called peripheral sensitization).
Pain and nociception In CNS pathways carrying nociceptive information, chronic stimulation causes other interactions, which make the pathways more responsive to incoming stimuli (called central sensitization). Sensitized second-order neurons become hypersensitive to nociception, and may also signal pain when triggered by innocous stimuli, such as touch. Clinically this painful response to innocuous stimuli is called allodynia.
Nociceptors Nociceptors are triggered by actual or potential tissue damage. Generally, nociceptors are free nerve endings in tissue and respond to mechanical, thermal, or chemical stimuli. The sensory fibers that carry nociception are of two types ; fast and slow. 1. The fast nociceptor pathway is associated with sharp, well-localized pricking or stinging pain. - These sensations travel via thin myelinated fibers (A delta fibers) to the superficial dorsal horn of spinal cord or, in the case of the head, to the caudal spinal trigeminal nucleus. - The transmitter substance used is glutamate, which gives a rapid, brief transmission. - The information travels in the spinothalamic tract to reach the ventroposterior thalamic nuclei, and is passed on to the somatosensory cortex.
Nociceptors 2. The slow nociceptor pathway is associated with dull, burning, or aching sensations. - These sensations travel via slower, unmyelinated fibers (C fibers) to deep layers of dorsal horn of the spinal cord or the caudal spinal trigeminal nucleus. - The transmitter substance used by this pathway is substance P. - The pathway projects via the spinothalamic trac to intralaminar nuclei of the thalamus, and is associated with poorly localized pain that is unusually distressing. - As well as sending information to the cerebral cortex, - Nociceptor systems have a vital role in triggering reflexes that protect the body from the source of the pain, such as the withdrawal reflex in the limbs. - While the main touch pathways cross the midline in the hindbrain, pain pathways cross in the spinal cord just above the level where the nerves enter.
Visceral pain Visceral nociception is triggered by smooth muscle spasm, organ distension, inflammation, or ischemia (loss of blood supply). We are NOT usually conscious of visceral sensation, the brain finds it hard to localize the source of visceral pain; in some cases, the pain may seem to come from somewhere on the body surface. This phenomenon is called referred pain. HEADACHE The brain itself has no nociceptors, but the blood vessels and meninges are very sensitive, and can cause pain in the head. Headache is at times caused by pain in a distant area, such as the neck or the teeth; In these cases, the pain is said to
Visceral pain For an example, the gall bladder inflammation triggers visceral nociceptors, but instead of localizing the pain in the abdomen, the brain perceives the source to be a patch of skin on the shoulder. - The reason for percieving pain in the shoulder is that both the shoulder and the coating of the diaphragm (near the gall bladder) are supplied by the C4 nerve.
The endogenous opiate system The brain produces a number of chemicals that act like the opium derivatives, heroin and morphine. These chemicals (enkephalins and endorphins) are therefore called endogenous opiates. Endogenous opiates are used by the nervous system to control the transmission of nociception by reducing neurotransmission at synapses. - They thus can also alter mood, which affects the perception of nociception as pain. - These control mechanisms are essential at times when it is necessary to temporarily disregard nociception, such as in a life-threatening situation. Experiencing pain under those circumstances would distract the brain from organizing an escape behavior, so nociception is temporarily blocked off.