The Visual System PDF

Lesson Proper for Week 7 **THE VISUAL SYSTEM** - The eye is the major sensory organ involved in **vision ** - Light waves are transmitted across the cornea and enter the eye through the pupil.  - **Cornea **is the transparent covering over the eye. It serves as a barrier be...

Lesson Proper for Week 7 **THE VISUAL SYSTEM** - The eye is the major sensory organ involved in **vision ** - Light waves are transmitted across the cornea and enter the eye through the pupil.  - **Cornea **is the transparent covering over the eye. It serves as a barrier between the inner eye and the outside world, and it is involved in focusing light waves that enter the eye.  - **Pupil **is the small opening in the eye through which light passes, and the size of the pupil can change as a function of light levels as well as emotional arousal. When light levels are low, the pupil will become dilated, or expanded, to allow more light to enter the eye. When light levels are high, the pupil will constrict, or become smaller, to reduce the amount of light that enters the eye.  - The pupil's size is controlled by muscles that are connected to **Iris**, which is the colored portion of the eye. **Anatomy of the Visual System** download (1).png - After passing through the pupil, light crosses the **lens** , a curved, transparent structure that serves to provide additional focus. - The lens is attached to muscles that can change its shape to aid in focusing light that is reflected from near or far objects.  - In a normal-sighted individual, the lens will focus images perfectly on a small indentation in the back of the eye known as the **fovea**, which is part of the **retina**, the light-sensitive lining of the eye.  - The fovea contains densely packed specialized photoreceptor cells.  - These **photoreceptor **cells, known as **cones**, are light-detecting cells.  - The cones are specialized types of photoreceptors that work best in bright light conditions.  - Cones are very sensitive to acute detail and provide tremendous spatial resolution.  - They also are directly involved in our ability to perceive color.  - While cones are concentrated in the fovea, where images tend to be focused, rods, another type of photoreceptor, are located throughout the remainder of the retina. - Rods are specialized photoreceptors that work well in low light conditions, and while they lack the spatial resolution and color function of the cones, they are involved in our vision in dimly lit environments as well as in our perception of movement on the periphery of our visual field.  - Rods and cones are connected (via several interneurons) to retinal ganglion cells. Axons from the retinal ganglion cells converge and exit through the back of the eye to form the optic nerve.  - The optic nerve carries visual information from the retina to the brain  - There is a point in the visual field called the blind spot:  - Even when light from a small object is focused on the blind spot, we do not see it. We are not consciously aware of our blind spots for two reasons  - First, each eye gets a slightly different view of the visual field; therefore, the blind spots do not overlap.  - Second, our visual system fills in the blind spot so that although we cannot respond to visual information that occurs in that portion of the visual field, we are also not aware that information is missing.  - The Optic nerve from each eye merges just below the brain at a point called the optic chiasm  - Once inside the brain, visual information is sent via a number of structures to the occipital lobe at the back of the brain for processing.  - Visual information might be processed in parallel pathways which can generally be described as the "what pathway" (the ventral pathway) and the "where/how" pathway (the dorsal pathway). - As mentioned above, light enters your eyes as a wave. It is important to understand some basic properties of waves to see how they impact what we see.  - Two physical characteristics of a wave are: **Amplitude **& **Wavelength ** - The amplitude of a wave is the height of a wave as measured from the highest point on the **wave **(**peak **or **crest**) to the lowest point on the wave (trough).  - **Wavelength **refers to the length of a wave from one peak to the next  - **Wavelength** is directly related to the frequency of a given wave form.  - **Frequency **refers to the number of waves that pass a given point in a given time period and is often expressed in terms of **Hertz (Hz)**, or cycles per second.  - Longer wavelengths will have lower frequencies, and shorter wavelengths will have higher frequencies. **Amplitude and Wavelength** download (13).png - In humans, light wavelength is associated with perception of color. Within the visible spectrum, our experience of red is associated with longer wavelengths, greens are intermediate, and blues and violets are shorter in wavelength. (An easy way to remember this is the mnemonic.  ROYGBIV: red, orange, yellow, green, blue, indigo, violet.) The amplitude of light waves is associated with our experience of brightness or intensity of color, with larger amplitudes appearing brighter. - We do not see the world in black and white; neither do we see it as two dimensional (2-D) or flat (just height and width, no depth). Let's look at how color vision works and how we perceive three dimensions (height, width, and depth). - Normal-sighted individuals have three different types of cones that mediate color vision.  - Each of these cone types is maximally sensitive to a slightly different wavelength of light.  - According to the Young-Helmholtz trichromatic theory of color vision, shown in Figure 9, all colors in the spectrum can be produced by combining: Red, Green, Blue  - The three types of cones are each receptive to one of the colors. **Color Vision** download (15).png - The trichromatic theory of color vision is not the only theory---another major theory of color vision is known as the **opponent-process theory. ** - According to this theory, color is coded in opponent pairs: black-white, yellowblue, and green-red.  - The basic idea is that some cells of the visual system are excited by one of the opponent colors and inhibited by the other. So, a cell that was excited by wavelengths associated with green would be inhibited by wavelengths associated with red, and vice versa.  - One of the implications of opponent processing is that we do not experience greenish-reds or yellowish-blues as colors. Another implication is that this leads to the experience of negative afterimages.  - An **afterimage **describes the continuation of a visual sensation after removal of the stimulus.  Stare at the white dot for 30--60 seconds and then move your eyes to a blank piece of white paper. What do you see? This is known as a negative afterimage, and it provides empirical support for the opponent-process theory of color vision. - But these two theories---the trichromatic theory of color vision and the opponent process theory---are not mutually exclusive. Research has shown that they just apply to different levels of the nervous system. For visual processing on the retina, trichromatic theory applies: the cones are responsive to three different wavelengths that represent the cells respond in a way consistent with opponent-process theory. - Our ability to perceive spatial relationships in three-dimensional (3-D) space is known as **depth perception**.  - With depth perception, we can describe things as being in front, behind, above, below, or to the side of other things.  - Our world is three-dimensional, so it makes sense that our mental representation properties. We use a variety of cues in a visual scene to establish our sense of depth.  - Some of these are **Binocular cues**, which means that they rely on the use of both eyes.  - One example of a binocular depth cue is** binocular disparity**, the slightly different view of the world that each of our eyes receives.  - To experience this slightly different view, do this simple exercise: extend your arm fully and extend one of your fingers and focus on that finger. **Depth Perception** - A 3-D movie works on the same principle: the special glasses you wear allow the two slightly different images projected onto the screen to be seen separately by your left and your right eye. As your brain processes these images, you have the illusion that the leaping animal or running person is coming right toward you  - Although we rely on binocular cues to experience depth in our 3- D world, we can also perceive depth in 2-Darrays.Think about all the paintings and photographs you have seen.  - Generally, you pick up on depth in these images even though the visual stimulus is 2-D. When we do this, we are relying on a number of **monocular cues**, or cues that require only one eye.  - If you think you can't see depth with one eye, note that you don't bump into things when using only one eye while walking--- and, in fact, we have more monocular cues than binocular cues - Vision is not an encapsulated system. It interacts with and depends on other sensory modalities.  - For example, when you move your head in one direction, your eyes reflexively move in the opposite direction to compensate, allowing you to maintain your gaze on the object that you are looking at.  - This reflex is called the **vestibulo-ocular reflex**. It is achieved by integrating information from both the visual and the vestibular system (which knows about body motion and position).  - You can experience this compensation quite simply. First, while you keep your head still and your gaze looking straight ahead, wave your finger in front of you from side to side. **Integration with Other Modalities ** - Notice how the image of the finger appears blurry. Now, keep your finger steady and look at it while you move your head from side to side.  - Notice how your eyes reflexively move to compensate the movement of your head and how the image of the finger stays sharp and stable.  - Vision also interacts with your proprioceptive system, to help you find where all your body parts are, and with your auditory system, to help you understand the sounds people make when they speak. You can learn more about this in the multimodal module.  - Finally, vision is also often implicated in a blending-of-sensations phenomenon known as **synesthesia**.  - Synesthesia occurs when one sensory signal gives rise to two or more sensations. The most common type is grapheme-color synesthesia.  - About 1 in 200 individuals experience a sensation of color associated with specific letters, numbers, or words: the number 1 might always be seen as red, the number 2 as orange, etc. But the more fascinating forms of synesthesia blend sensations from entirely different sensory might elicit a sensation of green, for example, and the timbre of violin a deep purple. MECHANISMS OF PERCEPTION: HEARING, TOUCH, SMELL, TASTE, AND ATTENTION TYPES OF SENSORY AREAS OF CORTEX The sensory areas of the cortex are, by convention, considered to be of three fundamentally different types: primary, secondary, and association.  The primary sensory cortex of a system is the area of sensory cortex that receives most of its input directly from the thalamic relay nuclei of that system.  The secondary sensory cortex of a system comprises the areas of the sensory cortex that receive most of their input from the primary sensory cortex of that system or from other areas of secondary sensory cortex of the same system.  Association cortex is any area of cortex that receives input from more than one sensory system. Most input to areas of association cortex comes via areas of secondary sensory cortex. In recognition of the hierarchical organization of sensory systems, psychologists sometimes divide the general process of perceiving into two general phases: sensation and perception.  Sensation is the process of detecting the presence of stimuli, and perception is the higher order process of integrating, recognizing, and interpreting complete patterns of sensations.    AUDITORY SYSTEM  The function of the auditory system is the perception of sound. Sounds are vibrations of air molecules that stimulate the auditory system; humans hear only those molecular vibrations between about 20 and 20,000 hertz (cycles per second).  Ear is the primary receptor of auditory stimuli. While its well-known function is hearing, it also helps us in maintaining our body balance. The structure of an ear is divided into three segments, called the external ear, the middle ear, and the inner ear.   External Ear : It contains two main structures, namely Pinna and Auditory meatus. Pinna is a cartilaginous funnel-shaped structure that collects sound waves from the surroundings.  Auditory meatus is a canal protected by hair and wax that carries sound waves from pinna to the tympanum or ear drum   Middle Ear : The middle ear starts with tympanum, a thin membrane highly sensitive to sound vibrations. This is followed by the tympanic cavity.  It is connected to the pharynx with the help of Eustachian tube, which maintains the air pressure in tympanic cavity.  From the cavity the vibrations pass to three ossicles known as malleus (hammer), incus (anvil), and stapes (stirrup). They increase the intensity of sound vibrations about 10 times, and send them to the inner ear.  Inner Ear : The inner ear has a complicated structure known as membranous labyrinth, which is encapsulated in a bony shell called bony labyrinth.  A lymph-like fluid is found in the space between bony labyrinth and membranous labyrinth. This is called perilymph.  The bony labyrinth has three semicircular canals at right angle to each other, a cavity,  called vestibule, and a coiled structure  called cochlea. The semicircular canals have fine hair cells, which are highly sensitive to postural changes as well as changes in the body orientation.  Inside the bony cochlea, there is a membranous cochlea, which is also known as scala media.  It is filled with endolymph, and has a spirally coiled membrane, called basilar membrane.  It has got fine hair cells arranged in a series to form the organ of corti. STRUCTURE OF THE HUMAN EAR IMG\_256 WORKING OF THE EAR  Pinna collects the sound vibrations and serves them to the tympanum through the auditory meatus.  From the tympanic cavity the vibrations are transferred to the three ossicles, which increase their strength and transmit them to the inner ear. In the inner ear the cochlea receives the sound waves.  Through vibrations the endolymph is set in motion, which also vibrates the organ of corti.  Finally, the impulses are sent to the auditory nerve, which emerges at the base of cochlea and reaches the auditory cortex where the impulse is interpreted. SOUND AS A STIMULUS  We all know that sound is the stimulus for ears. It results from pressure variations in the external environment. Any physical movement disturbs the surrounding medium (i.e. air), and pushes the air molecules back and forth.  This results in changes in pressure that spread outward in the form of sound waves, travelling at a rate of about 1,100 ft/sec.   These changes travel in waves much like the ripples set up by a stone thrown into a pond. When these sound waves strike our ears, they initiate a set of mechanical pressure changes that ultimately trigger the auditory receptors.  The simplest kind of sound wave is one that causes successive pressure changes over time in the form of a single repeating sine wave (Fig.5.3).  Sound waves vary in amplitude as well as in wavelength. Amplitude is a general measure of stimulus magnitude. It is the amount of change in pressure, i.e. the extent of displacement of the molecules from the position of rest.  Wavelength is the distance between the two crests. Sound waves are basically formed due to alternate compression and decompression (rarefaction) of air molecules.  A complete change in pressure from compression to rarefaction and again to compression makes a cycle of the wave.  Sound waves are described in terms of their frequency, which is measured in terms of cycles per second. Its unit is called Hertz (Hz)  Frequency and wavelength have an inverse relationship.  When the wavelength increases, the frequency decreases, and when the wavelength decreases, the frequency increases.   Amplitude and frequency both are physical dimensions. Besides these, there are three psychological dimensions of sound, namely  Loudness of the sound is determined by its amplitude. Sound waves with large amplitude are perceived as loud; those with small amplitude are perceived as soft. Loudness is measured in decibels (db).  Pitch refers to highness or lowness of a sound. The higher the frequency, the higher will be the pitch. The range of hearing is generally 20 Hz-20,000 Hz.  Timbre refers to the nature or quality of a sound. The timbre of a sound reflects the complexity of its sound waves. Most of the sounds found in natural environments are complex. SOMATOSENSORY SYSTEM  Somatosensations: sensations from your body. It has three separate but interacting systems  1\. Exteroceptive system - Senses external stimuli interacting with the skin  2\. Proprioceptive system - Monitors body position; Receptors in the muscles, joints & organs of balance 3\. Interoceptive system - General information on the internal body conditions; Ex: Temperature/BP EXTEROCEPTIVE SYSTEM  It has three distinct divisions for perceiving different types of stimuli  Mechanical (touch)  Thermal (temperature)  Nociceptive (pain) CUTANEOUS RECEPTORS  Receptors in the skin; it has many types;  a\. Free nerve endings - Simplest; neuron endings with no specialized structures; Sensitive to temperature change & pain b\. Pacinian corpuscles - Largest & deepest; Adapt rapidly; Respond to sudden displacements of skin, not constant pressure  c\. Merkel's disks - Adapt slowly; Respond to gradual skin indentation d\. Ruffini endings - Adapt slowly; Respond to gradual skin stretch  When constant pressure is applied to the skin, there is a burst of firing in all of the receptors, corresponding to the sensation of touch. But after a bit, only the slowly adapting receptors stay active & the sensation changes (often becoming unnoticeable) so to maintain constant input, you move & manipulate objects in your hands. Stereognosis: identification of objects by touch. Each type has its own unique structure, but they all basically work the same way  Stimuli to the skin changes the chemistry of the receptor, which changes the permeability of the receptor cell membrane to ions, which sends a neural signal  Dermatomes - Nerve fibers from cutaneous receptors come together and enter the spinal cord at the doral root. The area of the body innervated by the left & right dorsal root at a given spinal segment is a dermatome. TWO MAJOR SOMATOSENSORY PATHWAYS  1\. Dorsal-column medial-lemniscus system -- Sends information about touch & proprioception  Ipsilateral, decussates at the dorsal column nuclei, contralateral  Neurons of this path that start in the toes are the longest neurons in the human body!  2\. Anterolateral system -- sends information about pain & temperature  Spinothalamic tract  Neurons decussate immediately upon entering the spinal cord & travel up contralaterally  However, there is overlap in the type of info each pathway carries  If both paths are cut by a spinal cord injury, there will be no sensation from below that point. CORTICAL AREAS OF SOMATOSENSATION  The primary somatosensory cortex is located on the postcentral gyrus  Most input is contralateral  It is organized somatotopically; according to a map of the body surface  Referred to as the homunculus ("little man")  Secondary somatosensory cortex is just ventral to the primary  Association cortex is in the posterior parietal lobe SOMATOSENSORY AGNOSIAS  Astereognosia - Inability to recognize objects by touch; Rare  Asomatognosia - Inability to recognize parts of your own body; Usually only affects the left side of the body after damage to the right posterior parietal lobe. PERCEPTION OF PAIN  Pain is the response to any kind of harmful stimulation. It serves as a warning system  There is no clear cortical area involved in pain. Although the anterior cingulate cortex is activated during the emotional reaction to physical pain  Amazingly, we can exhibit a lot of control over our perception of pain  Gate-control theory: descending cognitive signals from the brain can activate neural gate circuits in the spinal cord to block incoming pain signals DESCENDING PAIN-CONTROL CIRCUIT  Activity in the periaqueductal gray has analgesic (pain blocking) effects  Also has specialized receptors for opioids, including endorphins; Potentially involves stimulation of serotonergic neurons   Neuropathic Pain - Severe chronic pain in the absence of a recognizable pain stimulus; Often after an injury has healed & there should be no more reason for pain. THE CHEMICAL SENSES: SMELL & TASTE  These senses respond to chemicals in our environment  Smell for airborne chemicals  Taste for those that dissolve in our oral cavity  Smell & taste are highly integrated  Together they produce what we know as flavor  We use these senses primarily to recognize flavor, but many other species use it for communication, via pheromones OLFACTORY SYSTEM: SMELL  Receptor cells are in the upper part of your nose, within the olfactory mucosa  The axons of these neurons actually project through the cribriform plate in your skull & enter the olfactory bulbs (It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OFC) and the hippocampus where it plays a role in emotion, memory and learning), which go via the olfactory tracts to the brain.  Your olfactory receptor neurons can be regenerated throughout your life  Primary olfactory cortex: piriform cortex; Medial temporal cortex next to the amygdala  Only sensory system that does not first go through the thalamus!! GUSTATORY SYSTEM: TASTE  Taste receptors are on the tongue & elsewhere in the oral cavity  Occur in clusters of 50 called taste buds  So each taste bud sends out many axons and many individual neural signals  The 5 traditional tastes  1.Sweet   2\. Salty  3\. Sour 4\. Bitter   5.Umami  But not every taste we experience can be made from any combo of those 5... GUSTATORY PATHWAY  Afferent neurons leave the mouth as the facial, glossopharyngeal & vagus cranial nerves; which terminate in the solitary nucleus of the medulla, to the ventral posterior nucleus of the thalamus, to the primary gustatory cortex  Primary cortex: near the face area of the somatosensory homunculus DAMAGE TO THE CHEMICAL SENSES  Anosmia: inability to smell  Caused by blows to the head that rip the olfactory nerves as they pass through the cribriform plate  Symptom along with several other neurological disorders  Ageusia: inability to taste; Rare ATTENTIONAL PROCESSES  In the previous section we have discussed some sensory modalities that help us in collecting information from the external world and also from our internal system.  A large number of stimuli impinge upon our sense organs simultaneously, but we do not notice all of them at the same time. Only a selected few of them are noticed.  For example, when you enter your classroom you encounter several things in it, such as doors, walls, windows, paintings on walls, tables, chairs, students, schoolbags, water bottles, and so on, but you selectively focus only on one or two of them at one time. The process through which certain stimuli are selected from a group of others is generally referred to as attention.  At this point it may be noted that besides selection, attention also refers to several other properties like alertness, concentration, and search.  Alertness refers to an individual's readiness to deal with stimuli that appear before her/him. While participating in a race in your school, you might have seen the participants on the starting line in an alert state waiting for the whistle to blow in order to run. Concentration refers to focusing of awareness on certain specific objects while excluding others for the moment.  When the field of awareness is centered on a particular object or event, it is called focus or the focal point of attention.  On the contrary, when the objects or events are away from the center of awareness and one is only vaguely aware of them, they are said to be at the fringe of attention.  Attention has been classified in a number of ways. A process-oriented view divides it into two types, namely Selective and Sustained  Sometimes we can also attend to two different things at the same time. When this happens, it is called divided attention. Automatic processing has three main characteristics;  \(i) It occurs without intention,  \(ii) It takes place unconsciously, and  \(iii) It involves very little (or no) thought processes SELECTIVE ATTENTION  Selective attention is concerned mainly with the selection of a limited number of stimuli or objects from a large number of stimuli. We have already indicated that our perceptual system has a limited capacity to receive and process information.  This means that it can deal only with a few stimuli at a given moment of time. The question is, which of those stimuli will get selected and processed?  Psychologists have identified a number of factors that determine the selection of stimuli. THEORIES OF SELECTIVE ATTENTION  Filter theory was developed by Broadbent (1956).  According to this theory, many stimuli simultaneously enter our receptors creating a kind of "bottleneck" situation. Moving through the short-term memory system, they enter the selective filter, which allows only one stimulus to pass through for higher levels of processing. Other stimuli are screened out at that moment. Filter-attenuation theory was developed by Triesman (1962) by modifying Broadbent's theory.  This theory proposes that the stimuli not getting access to the selective filter at a given moment of time are not completely blocked. The filter only attenuates (weakens) their strength. Thus some stimuli manage to escape through the selective filter to reach higher levels of processing. Multimode theory was developed by Johnston and Heinz (1978). This theory believes that attention is a flexible system that allows selection of a stimulus over others at three stages.  At stage one the sensory representations (e.g., visual images) of stimuli are constructed;  At stage two the semantic representations (e.g., names of objects) are constructed; and  At stage three the sensory and semantic representations enter the consciousness SUSTAINED ATTENTION  While selective attention is mainly concerned with the selection of stimuli, sustained attention is concerned with concentration. It refers to our ability to maintain attention on an object or event for longer durations.  It is also known as "vigilance". Sometimes people have to concentrate on a particular task for many hours. Air traffic controllers and radar readers provide us with good examples of this phenomenon.  They have to constantly watch and monitor signals on screens. The occurrence of signals in such situations is usually unpredictable, and errors in detecting signals may be fatal. Hence, a great deal of vigilance is required in those situations FACTORS INFLUENCING SUSTAINED ATTENTION  Several factors can facilitate or inhibit an individual's performance on tasks of sustained attention.  Sensory modality is one of them. Performance is found to be superior when the stimuli (called signals) are auditory than when they are visual.  Clarity of stimuli is another factor. Intense and long lasting stimuli facilitate sustained attention and result in better performance.  Temporal uncertainty is a third factor. When stimuli appear at regular intervals of time they are attended better than when they appear at irregular intervals. Spatial uncertainty is a fourth factor. Stimuli that appear at a fixed place are readily attended, whereas those that appear at random locations are difficult to attend. IMG\_258 Lesson Proper for Week 9 THE SENSORIMOTOR SYSTEM Hierarchical Organization The sensorimotor system is organized like a large effective company; the "president" (associated cortex) issues general commands and lower levels (motor neurons and muscles) take care of details; the advantage of this hierarchical arrangement is that higher levels are left free to focus on the complex functions. Motor Output is Guided by Sensory Input Like a large company, the sensorimotor system carefully monitors the external world and the consequences of its own actions, and it acts accordingly; only ballistic movements (brief, all-or-none, high speed movements) are not guided by sensory feedback. Learning Changes the Locus of Sensorimotor Control As a new company develops, more and more tasks become part of the routine and are taken over by lower levels of the organization; the same thing happens in the sensorimotor system; after much practice lower levels perform well-learned tasks with little higher involvement. Posterior Parietal Association Cortex Before an effective response can be initiated, the sensorimotor system must know the positions of various parts of the body and of objects in he external world; current thinking is that the posterior parietal cortex performs this function.  The posterior parietal cortex receives input from visual, auditory, and somatosensory systems ( that is why it is considered to be associated cortex) most of its output goes to secondary motor cortices.  In addition to disrupting the accuracy of movements, large lesions of posterior cortex can produce apraxia and contralateral neglect . Apraxia Apraxia is the inability to perform movements when requested to do so ( in the absence of simple sensory or motor deficits, motivational deficits, or intellectual deficits).  for example an apraxic patient may have difficulty demonstrating hammering movements when asked to do so but be perfectly capable of spontaneously hammering a nail.  Apraxia is almost always associated with left hemisphere damage, but its symptoms are always bilateral.  Right parietal damage often produces deficits on the WAIS block-design subtest; this is referred to as constructional apraxia. Other type of apraxia called object apraxia - patient uses wrong object for certain programmed movements (like brushing teeth with a comb instead of toothbrush). Contralateral Neglect Patients with contralateral neglect fail to respond to visual, auditory, and somatosensory stimuli from the contralateral half of the body.  Contralateral neglect is usually produced by very large right parietal lesions.  Patients with contralateral neglect may shave only the right half of their face, eat food from only the right half of their plate, put only their right leg in their pants. Dorsolateral Prefrontal Association Cortex Projections to this area are from the posterior parietal cortex; this area in turn projects to parts of the  Secondary Motor Cortex  Primary Cortex  Frontal Eye Field  Research on nonhuman primates has suggested that the prefrontal association cortex is involved in assessment of external stimuli and the initiation of responses to them; neurons here may be activated by characteristics of an object, its location, or by the response that the object elicits.  Further research shows that the motor neurons firing the earliest (prior to a motor task) are located in the dorsolateral prefrontal cortex, indicating that this area may be key in decisions regarding voluntary response initiation. Secondary Motor Cortex There are three areas of secondary motor cortex:  the premotor cortex  the supplementary motor area  cingulate motor areas.  They all send information to primary motor cortex; all receive input from primary motor cortex; all are interconnected with one another; and all send axons to the motor circuits of brainstem. Functionally, each of these areas produces complex movements when stimulated; are activated both before and during voluntary movements; and are active when either side of the body is involved in a movement.  Premotor cortex neurons often respond to both visual and touch stimuli; it appears to encode spatial relations of external cues and program movements guided by these cues.  Much of the supplementary motor area (SMA) is in the longitudinal fissure.  The cingulate motor cortex lies on the cingulate gyrus, just below the SMA. Secondary motor cortex is involved in the planning, programming and generation of complex motor sequences. Motor Homunculus  Primary motor cortex  Primary motor cortex is in the precentral gyrus of the frontal lobe; it is somatotopically organized  The motor homunculus has a disproportionate representation of hands and mouth; in fact, two different areas of each primary cortex control the contralateral hand. Neurons in primary motor cortex seem to code for a preferred direction of movement; they fire most just before and during the movement; they fire most when the movement is in the preferred direction and less as the direction deviates from the preferred one. Lesions of primary motor cortex produce contralateral asterognosia; they reduce the speed and force of contralateral movements, and they make it difficult to move one body part independently of others (They do not produce paralysis). Cerebellum and Basal Ganglia Both are important subcortical sensorimotor structures, but neither participates directly in the transmission of signals to the spinal cord.  Their role seems to be to integrate and coordinate the activity of structures at various levels of the sensorimotor system. Cerebellum The cerebellum constitutes 10% of the brain's mass, but it contains over half the brain's neurons; it is organized systematically in lobes  It receives inputs from  primary  secondary motor cortex  from brainstem motor nuclei  from somatosensory  vestibular systems It is thought to correct deviations from intended movements. effects of diffuse cerebellar damage include loss of the ability to precisely control movement, to adjust motor output to changing conditions, to maintain steady postures, exhibit good locomotion, to maintain balance, to speak clearly, and to control eye movements. Long-recognized role in motor learning, more recently appreciated for a role in the fine-tuning and learning of nonmotor cognitive responses. Basal Ganglia The basal ganglia are part of a loop that receives information from various parts of the cortex and transmits it back to motor cortices via the thalamus  Basal ganglia are involved in sequencing of movements, like the cerebellum, its role has recently been expanded to include a variety of nonmotor cognitive tasks.  Basal ganglia function compromised in patients with  Parkinson's Disease (due to loss of dopamine from substantia nigra)  Huntington's Disease (due to loss of cells in basal ganglia) Descending Motor Pathways Neural signals are conducted from the primary motor cortex to the motor neurons of the spinal cord over four different pathways. Two pathways descend in the dorsolateral region of the spinal cord---collectively known as the dorsolateral motor pathways, and two descend in the ventromedial region of the spinal cord---collectively known as the ventromedial motor pathways. Dorsolateral Corticospinal Tract and Dorsolateral Corticorubrospinal Tract One group of axons that descends from the primary motor cortex does so through the medullary pyramids---two bulges on the ventral surface of the medulla---then decussates and continues to descend in the contralateral dorsolateral spinal white matter. This group of axons constitutes the dorsolateral corticospinal tract. Most notable among its neurons are the Betz cells--- extremely large pyramidal neurons of the primary motor cortex. A second group of axons that descends from the primary motor cortex synapses in the red nucleus of the midbrain. The axons of neurons in the red nucleus then decussate and descend through the medulla, where some of them terminate in the nuclei of the cranial nerves that control the muscles of the face. The rest continue to descend in the dorsolateral portion of the spinal cord. This pathway is called the dorsolateral corticorubrospinal tract (rubro refers to the red nucleus). Ventromedial Corticospinal Tract and Ventromedial Corticobrainstemspinal Tract The direct ventromedial pathway is the ventromedial corticospinal tract, and the indirect one---as you might infer from its cumbersome but descriptive name---is the ventromedial cortico-brainstemspinal tract. The long axons of the ventromedial corticospinal tract descend ipsilaterally from the primary motor cortex directly into the ventromedial areas of the spinal white matter. As each axon of the ventromedial corticospinal tract descends, it branches diffusely and innervates the interneuron circuits in several different spina segments on both sides of the spinal gray matter. Sensorimotor Spinal Circuits Muscles  Motor units are the smallest units of motor activity. Each motor unit comprises a single motor neuron and all of the individual skeletal muscle fibers that it innervates. Skeletal muscle comprises hundreds of thousands of threadlike muscle fibers bound together in a tough membrane and attached to a bone by a tendon. Acetylcholine, which is released by motor neurons at neuromuscular junctions, activates the motor end-plate on each muscle fiber and causes the fiber to contract. Contraction is the only method that muscles have for generating force, thus any muscle can generate force in only one direction. All of the motor neurons that innervate the fibers of a single muscle are called its motor pool. Skeletal muscle fibers are often considered to be of two basic types: fast and slow. Fast muscle fibers, as you might guess, are those that contract and relax quickly. Although they are capable of generating great force, they fatigue quickly because they are poorly vascularized (have few blood vessels, which gives them a pale color). In contrast, slow muscle fibers, although slower and weaker, are capable of more sustained contraction because they are more richly vascularized (and hence much redder). Many skeletal muscles belong unambiguously to one of two categories: flexors or extensors. Flexors act to bend or flex a joint, and extensors act to straighten or extend it. Any two muscles whose contraction produces the same movement, be it flexion or extension, are said to be synergistic muscles; those that act in opposition, like the biceps and the triceps, are said to be antagonistic muscles. Activation of a muscle can increase the tension that it exerts on two bones without shortening and pulling them together; this is termed isometric contraction. Or it can shorten and pull them together; this is termed dynamic contraction. Stretch Reflex and Withdrawal Reflex When the word reflex is mentioned, many people think of themselves sitting on the edge of their doctor's examination table having their knees rapped with a little rubberheaded hammer. The resulting leg extension is called the patellar tendon reflex (patella means "knee"). This reflex is a stretch reflex---a reflex elicited by a sudden external stretching force on a muscle. We are sure that, at one time or another, you have touched something painful---a hot pot, for example---and suddenly pulled back your hand. This is a withdrawal reflex. Unlike the stretch reflex, the withdrawal reflex is not monosynaptic. When a painful stimulus is applied to the hand, the first responses are recorded in the motor neurons of the arm flexor muscles about 1.6 milliseconds later, about the time it takes a neural signal to cross two synapses. Reciprocal Innervation Reciprocal innervation is an important principle of spinal cord circuitry. It refers to the fact that antagonistic muscles are innervated in a way that permits a smooth, unimpeded motor response: When one is contracted, the other relaxes. Central Sensorimotor Programs and Learning Characteristics of Central Sensorimotor Programs  Central sensorimotor programs are CAPABLE of Motor equivalence  Sensory information that controls central  Sensorimotor programs Is not necessarily conscious.  Central sensorimotor programs can develop without practice.  Practice can create central sensorimotor programs #### Lesson Proper for Week 10 **DEVELOPMENT OF THE NERVOUS SYSTEM** **NEURODEVELOPMENT** - - **Phase of Development** - - - - - - - IMG\_256 **Introduction of the Neural Plate** - - - - - - - - - - - - **Stem Cells** - - - **Neural Proliferation (birth of new neurons)** - - - - - - **Migration** - - - - - - - - - -  **Neural Crest** - **Aggregation** - - - - - **Axon growth and Synapse formation** - - - **Axons growth** - - - - - - - IMG\_258 **Synaptogenesis** - - - - - **Neurons death and Synapse rearrangement** - - - - - **Life preserving Chemicals** - - - - - **Postnatal Brain Cerebral**  **Development in Human Infants** - - - - - **Development of the Prefrontal Cortex** - - - - **Effects of experience on Neural Circuits** - - - -  **Early of experience and development** - - - - - - - - - **Competitive nature or experience and neurodevelopment** - - - - **How experience might influence Neurodevelopment** - - - - - **Neuroplasticity in Adults** - - - **Effects of experience on adult Cortex reorganization** - - - - - **Autism** - - - - - - - - IMG\_260 - - - - - - - - - - - - - - - **William\'s Syndrome** - - -  ***[Lesson Proper for Week 11]*** Brain Damage and Neuroplasticity Brain Tumors  IMG\_256 A tumor (neoplasm) is a mass of cells that grows independently of the rest of the body; a cancer  Affecting whichever part of the brain it is growing in  Can be benign or malignant (worse than benign: can spread)  20% of brain tumor are meningiomas (encased in meanings)  Encapsulated, growing within their own membranes  Usually benign, surgically removable  Most brain tumors are infiltrating  Grows usually through surroundings tissue  Malignant, difficult to remove or destroy  About 10% of brain tumors are metastatic they originate elsewhere, usually the lungs Cerebrovascular disorders  Stroke -- a sudden onset cerebrovascular event that causes brain damage Cerebral hemorrhage -- bleeding in the brain  Cerebral ischemia -- disruption of blood supply  Cerebral hemorrhage -- blood vessel ruptures  Aneurysm -- a weakened point in a blood vessel that makes a stroke more likely; may be congenital (present at birth) or due to poison or infection. Cerebral ischemia - disruption of blood supply  Thrombosis -- a plug (made up of tissue) forms in the brain  Embolism -- a plug forms elsewhere and moves to the brain  Arteriosclerosis -- wall of blood vessels thicken, usually due to fat deposits  Source:[https://www.cdc.gov/stroke/types\_of\_stroke.html ](https://www.cdc.gov/stroke/types_of_stroke.html%C2%A0); Damage due to cerebral ischemia Does not develop immediately  Most damage is a consequence of excess neurotransmitter release -- especially glutamate  Blood deprived neurons become overactive and release glutamate  Glutamate over activates its receptors, especially NMDA receptors leading to an influx of Nati and Ca2 Influx of Nati and Ca2titriggers The release of still more glutamate  A sequence of internal reactions that ultimately kill the neuron Cause excitotoxic cell death (because glutamate is an excitatory neuron) Ischemia induced brain damage Takes time  Does not occur equally in all parts of the brain  Mechanism of damage vary with the brain structure affected CA1 in the hippocampus us very prone to damage NMDA is an ionotropic glutamate receptor Allows Nati and Ca2ti to flow through  Too much Ca2ti causes apoptosis How do we prevent excitotoxicity (the toxic cascade) from spelling? Cool down the brain & the body Closed head injuries  Brain injuries due to blows that do not penetrate the skull -- the brain collides with the skull  Contrecoup injuries -- contusions are often on the side of the brain opposite to the blow  Coup injuries - are often on the side of the brain where the blow occurs Contusions -- closed head injuries that involve damage to the cerebral circulatory system hematoma (bruise) forms  Concussions when there is disturbance of consciousness following a blow to the head and no evidence of structural damage.  While there is no apparent brain damage with a single concussion, multiple concussions may result in a dementia referred to as "punch drunk syndrome". If they don't remember what happened blackout IMG\_258 Brain Infections  Invasion of the brain by microorganisms  Encephalitis -- the resulting inflammation  Bacterial infections  Often lead to abscesses pockets of pus  May inflame meninges, creating meningitis  Treat with penicillin and other antibiotics  Viral infections  Some preferentially attack neural tissues Some causes  Bacterial  Syphilis -- may produce a syndrome of insanity and dementia known as general paresis. Syphilis bacteria are passed to the non infected and enter a dormant stage for many years. Viral  Rabies -- high affinity for the nervous system  Mumps and herpes -- typically tissues other than the brain  Viruses may lie dormant for years  Nuerotoxins  May enter general circulation from the GI tract or lungs, or through the skin   Toxic psychosis -- chronic insanity produced by a neurotoxin  Ex. The Mad Hatter -- hat makers often had toxic psychosis due to mercury exposure  Ex. 'crackpot' comes from people who would drink tea out of lead teapots   Bisphenol (BPA was used in plastics)  Endocrine disruptor weak hormone like properties  Obesity neurological issues, cancer, thyroid problems  Linked to anxiety, depression, hyperactivity, and inattention in children Genetic Factors  Most neuropsychological diseases of genetic origin are associated with recessive genes why?  Overall health, reproductive fitness  Not likely these people would reproduce  Down syndrome  0.15% of births, probability increase with advancing maternal age  Extra chromosome 21 created during ovulation  Characteristic disfigurement, mental retardation, other health problems Nueropsychological Disease  Epilepsy  Primary symptoms is seizures, but not all who have seizures have epilepsy (alcoholics seizure threshold is very high)  Epileptics have seizures generated by their own brain dysfunction  Affecting about 1% of the population  Difficult to diagnose due to the diversity and complexity of epileptic seizures  Types of seizures  Convulsions -- motor seizures  Some are merely subtle change of thought, mood, or behavior  Blank out for awhile could think someone is daydreaming  Cause (don't know exactly)  Brain damage  Genes -- over 70 known so far  Diagnosis  EEG -- electroencephalogram  Seizures associated with high amplitude spikes during the seizure  Seizures often preceded by an aura, such as a smell, hallucination, or feeling  Aura's nature suggests the epileptic focus (where the seizure is starting. If it's associated with a smell it could be generated from the olfactory area, etc.)  Warns epileptic of an impending seizure  Partial epilepsy -- does not involve the whole brain  Localized to specific areas  Generalized epilepsy -- involves the entire brain  Whole brain becomes overactive to a certain extent Partial seizures -- Simple  \- Symptoms are primarily sensory or motor or both (Jacksonian seizures)  Ex. Twitching  \- Symptoms spread as epileptic discharge spreads -- Complex  \- Often restricted to the temporal lobes (temporal lobe epilepsy)  \- Patient engages in compulsive and repetitive simple behaviours (automatisms)  Someone will be doing something that looks normal. Could be buttoning and unbuttoning  \- More complex behaviours seem normal Generalized seizures  Grand mal (Tonicclonic seizure) worst seizure to have  \- Loss consciousness and equilibrium  When someone falls to the ground and just shakes  \- Tonicclonic convulsions  Rigidity (tonus)  Tremors (clonus)  \- Resulting hypoxia may cause brain damage  Lack of oxygen to the brain  \- Petit mal (absence seizure)  Not associated with convulsions  A disruption of consciousness associated with a cessation of ongoing behavior (look like someone daydreaming intensely)  Most common in children before puberty. Goes away after IMG\_260 Parkinson's disease  A movement disorder of middle and old age affecting about.5% of the population  Pain and depression commonly seen before the full disorder develops  Tremor at rest is the most common symptom of the full blown disorder  Dementia is not typically seen  Initiating physical and mental activities is difficult  No single cause  Associated with degeneration of the substantia nigra, whose neurons release dopamine  Almost no dopamine in the substantia nigra of Parkinson's patients  Treated temporally with Lidopa and Cardidopa (DOPA decarboxlyyase imhibitor)  Lidopa can cross blood brain barrier; dopamine cannot Though movements is easier, involuntary movement can still occur (ex. Bobbing head while walking)  Loses effectiveness over time  Linked to about ten die rent gene mutations  Deep brain stimulation of subthalamic nucleus reduces symptoms  Subthalamic nucleus inhibits the thalamus, therefore the cortex is no excited and there's no movement.  Need to block the subthalamic nucleus so the cortex can be stimulated. Huntington's disease  A rare. Progressive motor disorder of middle and old age with a strong genetic basis  Begins with fidgetiness and progressive to jerky movements of entire limbs and severe dementia  Death usually occurs within 15 years o Caused by single dominant gene (huntingtin)  First symptoms usually not seen until age 40  50% chance to get it if one parent has it Multiple sclerosis  Canada has highest rates of MS  \- 300 out of 100,000 have it  Higher in females than males (2fl1)  Higher in Caucasians  Increased in people who have lived in a colder climate (THEORY)  MS can show up in people between the ages 14 to 25  A progressive disease that attacks CNS myelin, leaving areas of hard scar tissue (sclerosis)  Nature and severity of deficits vary with the nature, size, and position of sclerotic lesions  Periods of remission are common  Attacks myelin  Symptoms include visual disturbances, muscle weakness, numbness, tremor, and loss of motor coordination (ataxia)   Epidemiological studies find that incidence of MS is increased in those who spend childhood in a cool climate. MS is rare amongst Africans and Asians  Strong genetic predisposition and many genes involved  An autoimmune disorder -- immune system attacks myelin  Drugs may retard progression or block some symptoms  IMG\_262 Alzheimer's disease  Most common cause of dementia -- likelihood of developing it increase with age  Progressive, with early stages characterized by confusion and a selective decline in memory  Early selective decline in memory  Intermediate difficulty swallowing and controlling the bladder  Definitive diagnosis only at autopsy -- must observe nuerofibrillary tangles (found in neurons) and amyloid plagues (damage to dendrites & axons, loss of synapses), and neuronal loss  Several genes associated with early onset AD synthesize amyloid or tau, a protein found in the tangles  Genes increase risk, but don't necessarily cause it  Which come first, amyloid plaques or neurobrillary tangles? Genetic research on early onset AD support amyloid hypothesis (amyloid first)  Primary disease symptom amyloid hypothesis  Decline in acetylcholine levels is one of the earliest signs of AD Nothing to prevent/slow down Alzheimer's  Neuroplastic responses to nervous system damage  Degeneration -- deterioration  Cutting axons (axotomy) is a common way to study responses to neuronal damage  Anterograde degeneration of the distal segment -- between the cut and synaptic terminals (from the point of the cut and forwards towards synaptic terminal)  \- Cut from cells' metabolic center -- swells and breaks within a few days  \- Retrograde degeneration of the proximal segment -- between the cut and cell body (from the point of the cut and backwards towards dendrite)  Progresses slowly -- if regenerating axon makes a new synaptic contact, the neuron may survive  Regeneration -- growth of damaged neurons  Neural regeneration Does not proceed successfully in mammals and other higher vertebrates -- capacity for accurate axonal growth is lost in maturity  Regeneration is virtually nonexistent in the CNS of adult mammals and unlikely, but possible, in the PNS Neural regeneration in the PNS  Minimal damage if the original Schwann cell myelin sheath is intact, regenerating axons may grow through them to their original targets  Medium damage if the nerve is served and the ends are separated, they may grow into incorrect sheaths  Maximal damage if ends are widely separated, no meaningful regeneration will occur Mammal PNS neurons regenerate, CNS don't why?  CNS neurons can regenerate if transplanted into the PNS, while PNS neurons won't regenerate in the CNS  Schwann cells promote regeneration (they are only in the PNS) \- Neurotrophic factors stimulate growth  \- CAMs provide a pathway Oligodendroglia actively block regeneration (in the CNS) Recognization  Existing connections can strengthen  Collaterals (like terminal buttons) can sprout and compensate for the loss of the damaged area. Recovery  Difficult to conduct controlled experiments on populations of brain damaged patients  Can't distinguish between true recovery and compensatory changes  Cognitive reserve -- education and intelligence -- thought to play an important role in recovery of function -- may permit cognitive tasks to be accomplished in new ways \- People with higher IQ/intelligence tend to recover quicker Adult neurogenesis may play a role in recovery \- We know it does happen, but don't know why Treating nervous system damage  Reducing brain damage by blocking neurodegeneration  \- Various neurochemicals can clock or limit neurodegeneration Apoptosis inhibitor protein -- introduced in rats via a virus  Nerve growth factor -- blocks degeneration of damaged neurons  Estrogens -- limits or delay neuron death Neuroprotective molecules tend to also promote regeneration \- While regeneration does not normally occur in the CNS, experimentally it can be induced directing growth of axons by transplanting. Schwann cells  Olfactory unsheathing cells Promoting recovery by promoting regeneration  Promoting recovery by neurotransplantation of stem cells \- Fetal tissue  Fetal substantia nigra cells used to treat MPTP treated monkeys (PD model)  Treatment was successful Limited success with humans \- Caused bizarre side effects after awhile  \- Placebo effect occurred Stem Cells  \- Rats with spinal damage "cured", but much more research is needed Promoting recovery by rehabilitative training  Monkeys recovered hand function from induced strokes following rehab training  Constraint induced therapy in stroke patients -- tie down functioning limb while training the impaired one -- creates a competitive situation to foster recovery. \- Make the damaged side of the brain to do the work helps decrease recovery time Facilitated walking as an approach to treating spinal injury \- Getting into/out of the machine takes a very long time, not used very often. Benefits of cognitive and physical exercise  \- Correlations in human studies between physical/cognitive activity and resistance or recovery from neurological injury and disease  People who are more active mentally/physically are less likely to get these things  \- Rodents raised in enriched environments are resistant to induced neurological conditions (epilepsy, model of Alzheimer's, Huntington's, etc.) Physical activity promotes adult neurogenesis in rodent hippocampus