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Module 6: The Peripheral Nervous System: Afferent Division CHAPTER 4.1: INTRODUCTION  PNS = link between the periphery and CNS  The afferent division senses and sends information about both the internal and external environment to the CNS  CNS receives input, interprets it, and initiates response  The efferent division, under direction of the CNS, serves to consciously control various tissues and organs CHAPTER 4.2: PNS: AFFERENT DIVISION  Afferent neurons = sensory or receptor neurons that carry nerve impulses (AP) from receptors or sense organs to the CNS o The stimulus is sensed in the brain and perceived based on the location of the brain that it is stimulated o Afferent neurons have a single long dendrite, short axon, and smooth rounded cell body  Fight-or-flight response o Stress can be physical, psychological or emotional o Afferent division responds to stress for arousal, perception, and determination of efferent output o Example of seeing a bear in the woods:  Afferent division provides info to the CNS about external environment  arousal occurs  Hearing the noise behind you, you turn your head towards the sounds  To focus on the bear, your eyes use accommodation  pupils dilate to allow in more light  The decision to flee or fight is most likely based on your ability to judge distance from the bear  Somatosensation o The modality of a stimulus is determined by which sensory neuron is activated and where the stimulus terminates in the brain o Primary somatosensory area is located in the parietal lobes o Somatosensory reacts to stimuli by 4 primary receptors  Thermoreceptors  Mechanoreceptors  Photoreceptors  Chemoreceptors o Sensory info is transmitted through an AP from receptors via an afferent nerve through the spinal cord to the brain  Sensation: Internal and external o Afferent info about the internal environment never reaches conscious awareness  Ex: blood pressure, heart rate etc.

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Visceral afferent  incoming pathway for information derived from the internal viscera (body organs) o Afferent info derived from receptors located at the body surface or in the muscles/joints typically reaches conscious awareness  Sensory afferent  incoming pathway for info derived from the body surface or muscles/joints  Sensory info is either categorized as:  Somatic sensation: Body sense arising from body surface including somaesthetic sensation from the skin and proprioception from the muscles, joints, inner ear, skin  Special senses: including vision, hearing, taste, smell, and equilibrium Perception: Conscious awareness of surroundings o Perception is the conscious interpretation of the external world created by the brain by a pattern of nerve impulses delivered from sensory receptors o Our response range is limited; therefore, what we perceive isn’t reality o Information channels to our brains are not high-fidelity recorders  Some stimuli are accentuated while others are supressed and ignored o Cerebral cortex further manipulates data combining it with other info in our memory o Our cortex often completes the picture for us  Ex: optical illusions demonstrate this o Our perceptions do not replicate our reality

CHAPTER 4.3: RECEPTOR PHYSIOLOGY  Afferent neurons have receptors at its peripheral ending that responds to stimuli in both the external and internal environment  Somatosensory receptors: Consist of a neuron with an exposed receptor  Special senses receptors: Turns a mechanical stimulation (non-neural) into a neural signal by synapsing it onto a sensory neuron o Ex: sounds are converted this way  Transduction: The process by which receptors convert other forms of energy into electrical signals  CNS is able to distinguish stimuli from the PNS by: o Modality o Intensity o Location o Duration  Adequate stimuli and threshold o Each type of receptor is specialized to respond best to one type of energy  The specific energy is called an adequate stimulus  Ex: receptors in the eye are most sensitive to light

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The perception of a stimulus depends on the modality of receptor rather than the type of stimulus o Threshold = minimum stimulus required to activate the receptor o Types of receptors according to their adequate stimulus  Photoreceptors:  Responsive to visible wavelengths of light  Mechanoreceptors:  Sensitive to mechanical energy  Ex: skeletal muscle receptors sensitive to stretch  Thermoreceptors  Sensitive to varying amounts of heat  Chemoreceptors  Sensitive to specific chemicals  Include receptors to smell and taste, chemical content of digestive tract, and oxygen and carbon dioxide in the blood o Uses of information detected by receptors  Essential for the control of efferent output, both for regulating motor behaviour and maintain homeostasis  Afferent input provides information for the CNS to use in directing activities necessary for survival  Processing of sensory info in the RAS is critical for cortical arousal and consciousness  Gives rise to our perceptions of the world around us  May be stored in CNS for future reference  Can have a profound effect on our emotion Stimuli and receptor permeability o A receptor is either:  A specialized ending of the afferent neuron  A separate receptor cell closely associated with the peripheral ending of the neuron o Stimulation of receptor alters membrane permeability  opens ion channels o Effect is an inward flux of Na o Generator potential: Local depolarizing change of potential in a receptor IF the receptor is specialized ending of an afferent neuron o Receptor potential: Local depolarizing change of potential in a receptor IF the receptor is a separate cell o Receptor and generator potentials duration and amplitude vary  Stronger the stimulus = greater change in ion permeability = larger the receptor potential o Action potentials do NOT take place in the receptor itself Receptor potentials and action potentials: FIGURE 4-2 o If a receptor or generator potential has enough magnitude it may issue an action potential  triggers the open of Na+ channels in its region

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Separate receptor cell  Receptor potential triggers the release of a chemical messenger that diffuses into a space between the receptor cell and the afferent neuron (similar to a synapse)  Chemical messenger binds with a specific protein receptor site on the afferent neuron and opens the Na+ channels  Specialized afferent ending  Local current flow between the activated receptor and the cell membrane adjacent to the receptor brings about opening of Na+ channels  In either case:  If the ionic flux is big enough to bring adjacent neuron to threshold, an action potential is initiated and travels along afferent nerve to the CNS o The intensity of a stimulus is reflected by the magnitude of the receptor potential  Larger receptor potential = greater frequency of AP generated in the afferent neuron  Stronger stimuli usually affect larger areas  Stimulus intensity is distinguished by both frequency and by the number of receptors activated Adaptation to sustained stimulation o Adaptation: Process by which some receptors diminish the extent of their depolarization despite sustained stimulus strength  Receptor “adapts” to the stimulus by no longer responding to it o Types of receptors according to their speed of adaptation  Two types of receptors:  Tonic receptors: o Do not adapt at all, or slowly adapt o Important in situations where it is valuable to maintain the information about a stimulus o Ex: muscle stretch receptors, joint proprioceptors  Phasic receptors: o Rapidly adapting receptors o No longer responds to the stimulus, but when the stimulus is removed, the receptor slightly depolarizes which is called off response o Useful in situations where it is important to change in stimulus intensity rather than relay the same info o Ex: tactile (touch) receptors so you are not continually conscious about pressure while sitting on a chair for example

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Labelling somatosensory pathways

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o On reaching the spinal cord, the afferent info can either:  Come part of a reflex arc  May be relayed upward toward to the brain for further processing  Pathways for this are called somatosensory pathways containing discrete chain of neurons or labelled lines o Labelled lines and location of stimulus  First-order sensory neuron: Afferent neuron with its peripheral receptor first detecting the stimulus which synapses on a   Second-order sensory neuron: Either the spinal cord or medulla which synapses on a   Third-order sensory neuron: In the thalamus  By having these separate locations different types of incoming info are kept separate within the labelled lines b/t the periphery and the cortex o Phantom pain  Pain perceived as happening in the foot on a person whose foot has been amputated Acuity, receptive field size, and lateral inhibition o Receptive field: Restricted region of the skin surface surrounding the neuron in which that particular neuron will respond to  The smaller the receptive field, the greater its acuity or discriminative ability  Ex: feeling something with your fingertips vs. your elbow  You feel more info with your fingers because their receptive fields are small  The elbow has large receptive fields in which subtle differences cannot be detected o Lateral inhibition: Blockage of further transmission of stimulus into weaker inputs increases the contrast between unwanted and wanted info so that the pencil can be precisely located  Think about when you press a pencil against your skin, you can only feel the pencil where it is pressing, not in the whole receptive field Mechanoreceptors o Sensitive to pressure, stretch, vibration, acceleration and sound o 5 types:  Pacinian corpuscles  Located in the skin and respond to touch and deep pressure  Useful in detecting moderately rough to rougher surfaces  Oval shaped, wrapped by a layer of connective tissue  Classified as a phasic tactile receptor  Rapidly adapting and respond to transient touches  Adaptation involves both mechanical and electrochemical components  Meissner’s corpuscles

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Phasic, tactile, myelinated Responsible for sensing light touch Encapsulated nerve ending surrounded by connective tissue capsule  Concentrated in areas sensitive to touch such as the fingers, lips, and nipples Merkel’s discs  Most sensitive to low frequency virbations  Slow adapting  Found in superficial layers of the skin and mucosa  Clustered beneath ridges of the fingertips that make up fingerprints  Extremely sensitive to tissue replacement Ruffini corpuscles  Slow adapting, myelinated nerve endings found in the deep layers of skin  Encapsulated and tied to a local collagen matrix  Respond to stretch and torque  Take information about deformation within joints and continuous pressure Free nerve endings  Found in the skin, around hair roots, in eyes and many other tissues  Can be myelinated or unmyelinated  Most abundant of the skin receptors  Specialize in detecting touch and pressure but are also important for temperature and sensing pain  Sensations of ticking and itching detected by free nerve endings

CHAPTER 4.4: PAIN  An unpleasant sensation associated with nociceptor stimulation (perception of physiological pain)  Stimulation of nociceptors o Categories of pain receptors  Mechanical nociceptors: Respond to mechanical damage such as cutting, crushing or pinching  Thermal nociceptors: Respond to temperature extremes, especially heat  Chemical nociceptors: Respond to equally many kinds of irritating chemicals released from damaged tissue o Fast and slow afferent pain fibres  Pain can either be fast or slow  Pain impulses from nociceptors are transmitted to the CNS via one of the two types of afferent fibres  A- delta fibres: Largest and fastest

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o Signals resulting from cold, warmth, and mechanical stimuli are transmitted through A fibres at rates of 630m/s  C-fibres: Smallest and slowest o Slow pain, heat, cold, and mechanical stimuli at a rate of 1-2 m/sec  Fast pain is the initial brief, sharp, prickling sensation that is easily localized  Slow pain is the dull, aching, poorly localized sensation that is longer and more unpleasant  Slow pain pathway  activated by chemicals o Higher-level processing of pain input  Primary afferent pain fibres synapse with second-order interneurons at the dorsal horn of the spinal cord  Then, afferent pain fibres release neurotransmitters to those in the next line  2 best known pain neurotransmitters  Substance P o Activates ascending pathways that transmit nociceptive signals to higher levels for further processing o Ascending pathways have different destinations in the cortex, thalamus and reticular formation o Cortical somatosensory processing areas localize pain whereas other areas make the pain conscious o Reticular formation increases alertness associated with an encounter  Glutamate o Acts on two different types of plasma membrane receptors o 1) Glutamate + AMPA  Leads to permeability changes that ultimately result in the generation of APs in the dorsal horn cell  Aps transmit pain message to higher centres o 2) Glutamate + NMDA  Leads to calcium entry into the dorsal horn cell  Ca acts as a second-messenger system and makes the dorsal horn neuron more excitable than usual  Hyperexcitability contributes in part to the exaggerated sensitivity of an injured area  this helps us avoid activities that may cause further injury Brain’s built-in analgesic system

o CNS contains a built-in pain suppressing analgesic system that suppresses transmission in the pain pathways as they enter the spinal cord o Descending analgesic pathway  Electrical stimulation of periaqueductal grey matter results in profound analgesia  Same for stimulation of the reticular formation  Suppresses pain by blocking the release of substance P from afferent pain-fibre terminals  Depends on the presence of opiate receptors  Endorphins, enkephalins and dynorphins are all endogenous  released from the descending pathway and bind with opiate receptors on afferent nerve  binding suppresses release of substance P  Morphine acts in a similar way CHAPTER 4.5: EYE: VISION  Vision is where the eye captures patterns of illumination in the environment as an optical picture on a layer of light sensitive cells called the retina  Protective mechanisms o Eyelids act as a shutter to protect the front of the eye from things in the environment o Tears act as an eye washing fluid that flows across the eye and drains into each corner o Eyelashes trap fine airborne debris such as dust before it falls into the eye  A fluid-filled sphere o Each eye is spherical, fluid filled structure composed of 3 layers  Sclera/cornea  Sclera  a tough outer layer of connective tissue. Visible white part of the eye  Cornea  transparent layer where light passes through to the interior of the eye  Choroid/ciliary body/iris  Choroid  contains many blood vessels that nourish the retina  Choroid layer forms the ciliary body and iris  Retina  Consists of an outer layer and an inner nervous tissue layer  Latter contains rods and cones  photoreceptors that convert light energy into nerve impulses o Interior of the eye  Consists of 2 fluid filled cavities separated by the lens  Vitreous humour  jelly like substance between the lens and the retina  Important in maintaining the shape of the eye  Aqueous humour  watery fluid between the cornea and lens  Important for carrying nutrients for the cornea and lens

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Produced at a rate of 5 mL/day by a capillary network within the ciliary body

The iris o A thin, pigmented smooth muscle that forms a visible ring-like structure with the aqueous humour o Pupil  round opening in the iris  Size of this can be adjusted by variable contractions of the iris muscles to admit more or less light o Contains 2 sets of smooth muscle  Circular and radial o Pupil gets smaller when the circular (constrictor) muscle contracts and forms a smaller ring  Decrease amount of light getting into the eye  Controlled by parasympathetic fibres o Pupil gets larger when the radial (dilator) muscle shortens  Increase the amount of light getting into the eye  Controlled by sympathetic fibres o Iris muscles are controlled by the ANS Properties of light waves o Light: Form of electromagnetic radiation composed of particle like individual packets of energy called photons that travel in a wavelike fashion o Distance between 2 peaks = wavelength  Visible light range is between 400 and 700 nm  Shorter wavelengths are sensed as violet and blue  Longer wavelengths are sensed as orange and red o Intensity: Amplitude or height of the wave o Light waves diverge (radiate outward) in all directions from every point of a light source o Forward movement of a light wave in particular direction is known as a light ray o Divergent light rays must be bent inward to be focused back into a focal point onto the retina o Refraction  bending of a light ray  Convex  surface curves outward. Bring light waves close together  Concave  surface curves inward. Spread light waves closer apart The eye’s refractive structures o The curved corneal surface contributes to the eye’s total refractive ability o The refractive ability of the lens can be adjusted by changing its curvature as needed for near or far vision Accommodation o Accommodation is the ability to adjust the strength of the lens o The strength of the lens depends on its shape which is controlled by the ciliary muscle o Ciliary body has 2 main components

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Ciliary muscle  circular ring of smooth muscle attached to the lens by suspensory ligaments  Capillary network that produces aqueous humour o When the ciliary muscle is relaxed, the suspensory ligaments are stiff and they pull the lens into a flattened, weakly refractive shape o As the ciliary muscle contracts, the circumference decreases, slackening the tension in the suspensory ligaments; therefore the lens its more spherical o Greater the curvature = more refractive abilities o Ciliary muscle relaxed = lens is for far vision o Ciliary muscle contracts = lens is more convex and for near vision o Ciliary muscle is controlled by the ANS, with sympathetic stimulation causing relaxation and parasympathetic causing contraction o A mature lens cannot repair itself o Presbyopia: Occurs with aging, the muscles lose their elasticity which causes people to need corrective lenses for near vision o Cataract: When the elastic fibres in the lens become opaque so that light rays cannot pass through them o Nearsighted  Myopia  The eyeball is too long or lens is too strong, a near light source is brought into focus on the retina without accommodation  A far light source is focused in front of the retina is blurry  Has better near vision than far vision o Farsighted  Hyperopia  The eyeball is too short or the lens is too weak  Far objects are focused with only accommodation  Near objects are focused behind the retina and is blurry  Has better far vision than near vision The retinal layers o Responsibility of the eye is focus light rays onto rods and cons which transform light enery into electrical signals for the CNS o Neural portion of the retina has 3 layers  Outermost layer which contains rods and cones  Middle layer which contains bipolar cells  Inner layer which contains ganglion cells who join to form the optic nerve o Point in which blood vessels pass through the optic nerve I called the optic disc  This region is often called a blind spot because no image can be detected due to lack of rods and cones o Light must pass through ganglion and bipolar layers before reaching photoreceptors in all areas EXCEPT the fovea  In the fovea (located in the centre of the retina), bipolar and ganglion cells are pulled aside so that light strikes the photoreceptors directly

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The fovea only contains cones which makes...