Chapter 16 Notes PDF

Chapter 16 – Notes 2025-01-04 5:16 PM Sensory, Motor & Integrative Systems Synapses: The point of connection between a nerve cell and another cell Specifically, a synapse is a specialized junction at which a nerve cell (a ne...

Chapter 16 – Notes 2025-01-04 5:16 PM Sensory, Motor & Integrative Systems Synapses: The point of connection between a nerve cell and another cell Specifically, a synapse is a specialized junction at which a nerve cell (a neuron) communicates with a target cell Target cells include: Nerves Muscles Glands Types: Electrical Synapses Relatively rare in vertebrates. The membranes of the two cells are in tight contact producing electrical coupling, which enables a nerve impulse (or action potential) arriving at the presynaptic nerve ending to pass swiftly and reliably to the next cell Two cells are touching each other Works even faster than chemical synapses More synchronized Chemical Synapses More complex, the presynaptic and postsynaptic cells are physically separated by a gap roughly 10 – 15 times larger than an electrical synapse (the synaptic cleft). This prevents simple electrical transmission of the action potential to the postsynaptic cell Transmission is accomplished by the release of a chemical neurotransmitter from the presynaptic cell Ability to control how much chemical is release Neurotransmitters: Maybe delete this slide EPSP In the case of neurotransmitters that excite the postsynaptic membrane, the pore permits positively-charged sodium ions to move into the cell, making the potential across its membrane less negative This local depolarization is known as an excitatory post-synaptic potential (EPSP), and its amplitude is determined (roughly) by the number of vesicles released from the presynaptic cell If it is sufficiently large, the synaptic potential reaches threshold and initiates an action potential in the cell If the target cell is a neuron the action potential sweeps along its fibre If it is a muscle, it also propagates over the surface of the muscle cell and causes it to contract Inhibitory Transmitters Not all synaptic transmission is excitatory Inhibitory transmitters also exist which render the post-synaptic cell less excitable and thus less likely to generate an action potential They often act on receptors that act as channels for chloride (M/C) or potassium ions, and generally make the interior of the postsynaptic cell even more negative (hyperpolarization) Acetylcholine is the excitatory transmitter at nerve-skeletal muscle synapses Glutamate is the main excitatory transmitter in the CNS Examples of excitatory neurotransmitters: Acetylcholine Glutamate Examples of inhibitory neurotransmitters: Glycine Gamma aminobutyric acid (GABA) Serotonin: 1. The presynaptic cell makes serotonin (5-hydroxytryptamine, 5HT) from the amino acid tryptophan and packages it in vesicles in the end terminals Vitamin B6 helps this process Most serotonin is in your gut 2. An action potential passes down the presynaptic cell into its end terminals 3. The action potential stimulates the vesicles containing serotonin to fuse with the cell membrane and exocytose serotonin into the synaptic cleft 4. Serotonin passes across the synaptic cleft, binds with specialized proteins receptors on the membrane of the postsynaptic cell and depolarizes the postsynaptic cell If the depolarization reaches the threshold level, a new action potential will be propagated in that cell 5. The remaining serotonin molecules in the cleft and those release by the receptors after use get degraded by the enzyme monoamine oxidase (MAO) 6. In the presynaptic cell MAO destroys the reabsorbed serotonin molecules This enables the nerve signal to be turned "off" and readies the synapse to receive another action potential 7. Catechol-o-methyl transferase (COMT) is a similar enzyme that targets catecholamines (dopamine, epinephrine, norepinephrine), but not serotonin. Whereas MAO will target catecholamines MAOI (monoamine oxidase inhibitor) = Antidepressant Serotonin can act as a modulator Modulation: Longer lasting, slower acting The action of 'fast' neurotransmitters is brief, because they unbind quickly from their receptors and are then rapidly cleared from the synaptic cleft, usually by breakdown into inactive substances or reuptake into the cell Because the receptor channels remain open only as long as a neurotransmitter is bound, and because binding is only transient, the synaptic potential is also brief and the membrane potential returns rapidly to its resting level Many other transmitters, sometimes called modulators (including serotonin, dopamine, noradrenaline, and many small peptide molecules), act more slowly and for much longer period of time These neurotransmitters do not always act as modulators Much slower acting but much longer lasting In general, their receptors do not act as channels but instead activate messenger molecules inside the cell, which can initiate a variety of responses, even including the switching-on/off of genes It used to be thought that each nerve fibre releases only one neurotransmitter ('Dale's principle'), but it is now known that two or more transmitters and/or modulators can be produced by individual nerve terminals Neurotransmitters: Enkephalin (Opiate) Aka. Endorphins Type Neuropeptide Postsynaptic Effect Inhibitory Locations Brain, Spinal Cord Functions Reduce stress, promote calm, natural painkiller Cold and exercise increases endorphins Synapse Abuse: Most drugs that work on the brain, as well as drugs of abuse, act on synapses Nicotine Activates nicotinic acetylcholine receptors (especially in the brain) causing dopamine release (euphoria) Curare Traditionally used by South American Natives as an arrow poison paralyzes prey because it acts antagonistically on the acetylcholine receptor - Blocks ACh in the neuromuscular junction and therefore blocks neuromuscular transmission (flaccid paralysis) Morphine & Heroin Act on opiate receptors Cannabis Act on cannabinoid receptors Cocaine Works differently, it blocks the reuptake system that clears dopamine from the synaptic cleft Consequently, dopamine stays within the synaptic cleft for longer, which explains why cocaine acts as a stimulant Nerve gas (Sarin) More in a similar fashion as cocaine, by blocking the removal of the transmitter acetylcholine at nerve-muscle synapses Constant muscle contraction Spastic muscle paralysis Disorders of Synaptic Function: Myasthenia Gravis Inherited neuromuscular disorder Occurs when the body produces antibodies which act antagonistically to the acetylcholine receptors on muscle fibers This causes them to be bound to the cell, and the reduced number of receptors at the cell surface means that neurotransmission is compromised Consequently the patient is easily fatigued Graves Disease Auto-antibodies are agonistically bound to the thyroid gland, causing an increase in thyroid hormone Epilepsy Sometimes linked to a decrease in the efficiency of inhibitory transmission (GABA) in the brain, leading to over-excitability of networks of neurons Too much excitation Too little of GABA Depression & Schizophrenia There is some evidence that the major psychiatric conditions, depression and schizophrenia, involve disorders of synapses in which serotonin and dopamine, respectively, act as neurotransmitters Schizophrenia – too much dopamine Depression – linked with serotonin (too little) Neural Circuits: Neurons never function in isolation; they are organized into ensembles or circuits that process specific kinds of information The arrangement of neural circuits varies greatly according to the intended function, some features are characteristic of all such ensembles The synaptic connections that define a circuit are typically made in a dense tangle of dendrites, axon terminals, and glial cell processes that together constitute a neuropil Few cell bodies are found here Thus, the neuropil between nerve cell bodies is the region where most synaptic connectivity occurs The direction of information flow in any particular circuit is essential to understanding its function Afferent neurons, efferent neurons, and interneurons are the basic constituents of all neural circuits Afferent neurons Nerves that carry information toward the central nervous system (or more centrally within the spinal cord/brain) Efferent neurons Nerves that carry information away from the brain or spinal cord Interneurons Nerve cells that only participate in the local aspects of a circuit are called interneurons or local circuit neurons Neural circuits are both anatomical and functional entities. A simple example is the circuit that controls the myotatic (or "knee-jerk") spinal reflex The afferent limb of the reflex is sensory neurons of the dorsal root ganglion in the periphery These afferents target neurons in the spinal cord The efferent limb comprises motor neurons in the ventral horn of the spinal cord with different peripheral targets One efferent group projects to flexor muscles in the limb, and the other to extensor muscles The third element is interneurons in the ventral horn of the spinal cord The interneurons receive synaptic contracts from the sensory afferent neurons and make synapses on the efferent motor neurons that project to the flexor muscles The synaptic connections b/w the sensory afferents and the extensor efferents are excitatory, causing the extensor muscles to contract. Conversely, the interneurons activated by the afferents are inhibitory, and their activation by the afferents diminishes electrical activity in motor neurons and causes the flexor muscles to become less active The result is a complementary activation and inactivation of the synergist and antagonist muscles that control the position of the leg More complex circuits involve processes such as addiction Neuronal Pools: The human body has about 10 million sensory neurons, 20 billion interneurons, and one-half million motor neurons. The interneurons are organized into a smaller number of neuronal pools Neuronal pools are only interneurons A neuronal pool Is a group of interconnected neurons with specific functions Neuronal pools are defined on the basis of function A pool may be diffuse, involving neurons in several regions of the brain, or localized, with neurons restricted to one specific location in the brain or spinal cord Estimates of the actual number of pools range between a few hundred and a few thousand Each pool has a limited number of input sources and output destinations, and each may contain both excitatory and inhibitory neurons. The output of the entire pool may stimulate or depress the activity of other pools, or it may exert direct control over motor neurons or peripheral effectors The pattern of interaction among neurons provides clues to the functional characteristics of a neuronal pool It is customary to refer to such a "wiring diagram" as a neural circuit, just as we refer to electrical circuits in the wiring of a house We can distinguish 5 circuit patterns 5 Circuit Patterns: Divergence Divergence is the spread of information from one pool to multiple pools Permits the broad distribution of a specific input Considerable divergence occurs when sensory neurons bring information into the CNS, for the information is distributed to neuronal pools throughout the spinal cord and brain For example, visual info arriving from the eyes reaches your conscious awareness at the same time it is distributed to areas of the brain that control posture and balance at the subconscious level Convergence Convergence, several neurons synapse on the same postsynaptic neuron Several patterns of activity in the presynaptic neurons can therefore have the same effect on the postsynaptic neuron Through convergence, the same motor neurons can be subject to both conscious and subconscious control For example, the movements of your diaphragm and ribs are now being controlled by your brain at the subconscious level, but the same motor neurons can also be controlled consciously, as when you take a deep breath and hold it Two neuronal pools are involved, both synapsing on the same motor neuron Multiple inputs with a few amount of outputs Serial Processing Information may be relayed in a stepwise fashion, from one neuronal pool to the next This pattern is called serial processing Serial processing occurs as sensory info is relayed from one part of the brain to another For example, when your brain wants to send info from one hemisphere to the other, it does so serially Not common in the brain – only see it when crossing hemispheres Parallel Processing Occurs when several neurons or neuronal pools process the same information at one time Divergence must take place before parallel processing can occur Thanks to parallel processing, many responses can occur simultaneously For example, when you step on a sharp object, sensory neurons that distribute the info to a # of neuronal pools are stimulated. As a result of parallel processing, you might withdraw your foot, shift your weight, move your arms, feel the pain, and shout "ouch!" All at the same time Reverberation Some neural circuits utilize feedback to produce reverberation – this can be positive OR negative In this arrangement, collateral branches of axons somewhere along the sequence extend back toward the source of an impulse and further affect the presynaptic neuron Reverberation is like a feedback loop involving neurons; once the circuit is activated, it may continue to stimulate itself or shut itself off Very complicated examples of reverberation among neuronal pools in the brain may help maintain consciousness, muscular coordination, and normal breathing Consciousness – RAS Positive feedback Breathing Negative feedback Actual start of chapter 16: Sensation: Sensation – is a conscious or unconscious awareness of external or internal stimuli The nature of the sensation and the type of reaction generated vary according to the destination of the impulses in the CNS and the types of receptors stimulated Sensory impulses relayed to the spinal cord may serve as input for spinal reflexes, such as the stretch reflex When sensory impulses reach the lower brain stem It can elicit more complex reflexes such as changes in heart rate and breathing rate When sensory impulses reach the cerebral cortex – most complex The brain can precisely locate and identify specific sensations such as touch, pain, hearing or taste Sensation Vs. Perception: Sensation Any stimulus the body is aware of If we have the receptor, our brain is aware of the sensation What are we aware of? Heat, pain, touch, smells We are aware of these b/c we have receptors that pick up these type of stimuli What are we not aware of? X – rays, ultra-high frequency sound waves, UV light We have no sensory receptors for those stimuli You don’t feel the x-ray but it doesn’t mean it doesn’t have an effect Perception The conscious awareness & interpretation of a sensation Localization and identification Memories of perception are stored in the cortex Can we sense something without perceiving it? Can you feel your blood sugar right now? - no Your just aware of the effects Sensory Modality: Sensory modality is the property by which one sensation is distinguished from another (each of your different senses) Ie. The different senses Different types of sensations include Touch, pain, temperature, vibration, hearing, vision Often a single receptor can only detect one of these types of stimuli, but there are exceptions Two classes of sensory modalities 1. General Senses 2. Special Senses 1. General Senses Include both somatic and visceral senses, the latter of which provide information about conditions within internal organs Somatic senses Tactile sensations (touch, pressure, vibration, tickle and itch) Thermal sensations (warm and cold) Pain sensations Proprioceptive sensations Proprioceptive sensations can be both conscious and unconscious They allow perception of both the static positions of limbs and body parts (joint and muscle position sense) and movements of the limbs and head Visceral senses Provide information about conditions within internal organs 2. Special Senses Include the modalities of smell, taste, vision, hearing, and equilibrium (all in the face and all have to do w/ cranial nerves) The Process of Sensation: Begins in a sensory receptor, which can be either a specialized cell or the dendrites of a sensory neuron For sensations to arise, 4 events typically occur – if any of these 4 steps are missing, no sensation occurs Event 1 Stimulation of the sensory receptor An appropriate receptor must be present in the area of the stimulus Event 2 Transduction of the stimulus A sensory receptor transduces (converts) energy from a stimulus into a graded potential Eg. Odorant molecules in the air stimulate olfactory receptors in the nose, which transduce the molecules chemical energy in the form of a graded potential Vary in amplitude and are not propagated Converting energy into electricity Event 3 Generation of impulses when graded potentials reach threshold Impulses then propagated towards the CNS Sensory neurons that conduct impulses from the PNS into the CNS are called first order neurons (1˚) Event 4 Intergration of sensory input by the CNS A particular region of the CNS receives and integrates the sensory nerve impulses Conscious sensations or perceptions are integrated in the cerebral cortex Sensory Receptors: Receptor structure may be simple or complex General Sensory Receptors Somatic Receptors No structural specializations in free nerve ending that provide us with pain, tickle, itch, temperatures Some structural specializations in receptors for touch, pressure & vibration Special Sensory Receptors Special Sense Receptors Very complex structures – vision, hearing, taste & equilibrium Classifying Sensory Receptors: 1. Structural classification 2. Type of stimuli they detect 3. Type of response to a stimulus 4. Location of receptors & origin of stimuli Structural Classification 1. Free nerve endings Bare dendrites – the ends of neurons Lack any structural specialization Pain, temp, tickle, itch & light touch The terminal branches of the neuron are unmyelinated and spread throughout the dermis and epidermis Most basic of all sensory receptors 2. Encapsulated nerve endings Dendrites enclosed in connective tissue capsule Pressure, vibration & deep touch 3. Separate sensory cells Sensory receptors for most special senses Specialized/separate cells that respond to stimuli by synapsing with the first order neurons Vision, taste, hearing, balance - special senses (except smell) Completely separate cell – isnt a neuron – synapses onto a neuron Stimuli Detected 1. Mechanoreceptors Detect physical or mechanical stress Touch, pressure, vibration, hearing, proprioception, equilibrium Most broad category – all of these sensation involve physically moving the receptor 2. Thermoreceptors Are of two types one that responds to an increase in temperature other that responds to a decrease in temperature 3. Nociceptors Detect potential damage to tissues Respond to intense mechanical deformation, excessive heat or chemical signals Tissue damage releases chemicals which stimulate nociceptors Signal is sent prior to the potential damage occurring 4. Chemoreceptors Detect chemicals Chemicals that we taste and smell (arterial oxygen), blood osmolality, (blood CO2), (Blood glucose), amino acids, fatty acids, pH Chemoreceptors in aorta and carotid 5. Photoreceptors Detect visible light Rods – very sensitive, but doesn’t distinguish colour Cones – less sensitive, 3 kinds of cones containing red, green, or blue sensitive pigment 6. Other receptors Ampullae of Lorenzini - respond to electric fields, salinity, and to temperature, but function primarily as electroreceptors Baroreceptors - specialized mechanoreceptors that respond to pressure Electromagnetic receptors - respond to electromagnetic waves Hyperalgesia If the initial stimulus of pain leads to an increased sensitivity to subsequent painful stimuli Stimulation-produced analgesia If descending pathways inhibit the transmission of pain stimuli, it leads to a suppression of pain Opioids are involved in this mechanism Referred pain If both visceral and somatic afferent converge on the same interneuron, excitation of one can lead to excitation of the other, leading to the pain being felt at a site different from the actual injured part We arn't sure how exactly this works Pain-gate theory Stimulating non-pain afferent fibers can inhibit neurons in the pain pathway This concept is used in transcutaneous electric nerve stimulation (TENS) Response to Stimuli Amplitude of potentials vary with stimulus intensity 1. Generator potential A type of graded potential Free nerve endings, encapsulated nerve endings & olfactory receptors produce generator potentials When large enough, it generates a nerve impulse in a first-order neuron Standard action potential 2. Receptor Potential A type of graded potential Vision, hearing, equilibrium, and taste receptors produce receptor potentials Receptor cells release neurotransmitter molecules on first-order neurons producing postsynaptic potentials PSP may trigger a nerve impulse If it’s a separate cell – it’s a receptor potential Location of Receptor 1. Exteroceptors Near surface of body Receive external stimuli hearing, vision, smell, taste, touch, pressure, pain, vibration & temperature 2. Interoceptors Monitors internal environment (BV or viscera) Receive internal stimuli concentration in blood, blood pressure etc. not conscious except for pain or pressure 3. Proprioceptors In muscles, tendons, and joint capsules Adaptation: Most sensory receptors exhibit adaptation – the tendency for the generator or receptor potential to decrease in amplitude during a maintained constant stimulus Receptors may adapt rapidly or slowly Decrease in sensitivity to long-lasting stimuli Leads to a decrease in responsiveness of a receptor - bad smells disappear - very hot water starts to feel only warm - this is why you can smell your own home Tonic receptor Is a sensory receptor that adapts slowly to a stimulus and continues to produce action potentials over the duration of the stimulus In this way it conveys information about the duration of the stimulus Some tonic receptors are permanently active and indicate a background level stimulus Examples: nociceptors (don’t adapt) and proprioceptors (don’t adapt) Phasic receptor Is a sensory receptor that adapts rapidly to a stimulus – changes in the stimulus The response of the cell diminishes very quickly and then stops It does not provide information on the duration of the stimulus; instead some of them convey information on rapid changes in stimulus intensity and rate An example of a phasic receptor is the Pacinian corpuscle Variability in Tendency to Adapt Rapidly adapting receptors (smell, pressure, touch) Specialized for detecting changes Slowly adapting receptors (pain, body position) Specialized for detecting duration Quiz 2: Tactile Sensations: Tactile sensations are 1. Touch 2. Pressure 3. Vibration 4. Itch 5. Tickle All sensations we feel with our fingertips Perceive differences among these sensations, but arise by activation of some of the same types of receptors Also, some receptors sense multiple stimuli Touch: Crude touch Ability to perceive something has touched the skin, even though its exact location, shape, size or texture cannot be determined "Something touched you somewhere" Discriminative touch Provides specific information about a touch sensation, such as exactly what point on the body is touched plus the shape, size and texture of the source of stimulation More discriminative touch in fingers rather than forearm The two are carried on separate spinal pathways Two Types RAPIDLY adapting touch receptors (Phasic receptors) 1. Corpuscles of Touch encapsulated (Meissner corpuscles) Receptors for touch located in the dermal papillae of glabrous skin aka. Hairless skin (right at the top of the dermis) Abundant in the fingertips, hands, eyelids, tip of the tongue, lips, nipples, soles, clitoris, and tip of the penis (hairless skin) Each corpuscle is an egg-shaped mass of dendrites enclosed by a capsule of CT Corpuscle = encapsulated – every corpuscle is encapsulated 2. Hair root plexuses Found in hairy skin Consist of free nerve endings(dendrites) wrapped around hair follicles Detect movements on the skin surface that disturb hairs Eg. An insect landing on a hair causes movement of the hair shaft that stimulates the free nerve ending Two Types: SLOWLY adapting touch receptors (Tonic receptors) 1. Type I Cutaneous Mechanoreceptors Saucer-shaped, flattened free nerve endings that make contact with merkel cells of the stratum basale (Merkel Discs) Function in touch and pressure IMPORTANT CHART: STUDY Most abundant in fingertips, hands, lips, and external genitalia - hairless skin 2. Type II Cutaneous Mechanoreceptors Elongated, encapsulated receptors located deep in the dermis, and in ligaments and tendons – also used as proprioreceptors (Ruffini Corpuscles) Abundant in hands (especially fingers) and the soles, but found all over the body Sensitive to stretching of skin that occurs as digits or limbs are moved, also helps detect pressure When a drink is slipping from your hand, these receptors detect it Pressure: Pressure is sustained sensation over a large area - "broad touch" Receptors that contribute to sensation of pressure include: Type I and Type II cutaneous mechanoreceptors (merkel discs and ruffini corpuscles) Vibration: Sensations of vibration result from rapidly repetitive sensory signals from tactile receptors Receptors for vibration sensation are: Meissner Corpuscles (encapsulated) Pacinian Corpuscles (encapsulated) Meissner Corpuscles Detect lower-frequency vibrations Pacinian/Lamellar Corpuscles Detect higher-frequency vibrations Adapt rapidly Distributed throughout the body In deep dermis, and subQ; submucosal tissues that underlie mucous and serous membranes; around joints, tendons, and muscles; in periosteum; in mammary glands, external genitalia; certain viscera such as a pancreas and urinary bladder Itch: Itch is a bit of a mystery Triggers include mechanical stimulus, electricity, temperature and certain chemicals Free nerve endings Bradykinin and histamine are well known itch stimulators There is no agreed upon theory as to what the functionality of itch is in humans Tickle: Tickle is also a bit of a mystery Tickle is stimulation of free nerve endings only by someone else, we arn't sure why we can't tickle ourselves, but many ideas have been presented Tickle can be divided into 2 categories One of these evokes laughter and the other is considered annoying Like itch, we aren't certain why the sensation of tickle exists Pacinian corpuscles also thought to mediate tickle response Phantom Limb Sensation: Patients with a limb amputated may still experience sensations such as itching, pressure, tingling, or pain as if the limb were still there 75% of amputees experience this, the majority of these experience painful sensations Very distressing to an amputee – most report that the pain is severe, and that it often does not respond to traditional pain medication therapy Alternative treatments may include electrical nerve stimulation, acupuncture, and biofeedback, mirror therapy (most effective) One older explanation Cerebral cortex interprets impulses arising from the distal portions of damaged sensory neurons that previously carried impulses from the limb as coming from the non-existent (phantom) limb One newer explanation The brain contains networks of neurons that generate sensations of body awareness Neurons in the brain that previously received sensory impulses from the missing limb are reorganized (plasticity), and now sensations from other areas of the body create a phantom sensation from the former limb Thermal Sensations: Free nerve endings with 1mm diameter receptive fields on the skin surface Cold Receptors Located in the stratum basale of the epidermis – more abundant than hot receptors Attached to medium-diameter, myelinated A fibers Few connect to small-diameter, unmyelinated C fibers Temperatures between 10˚ and 35˚ activate cold receptors Warm Receptors Not as abundant as cold receptors Located in the dermis Attached to small-diameter, unmyelinated C fibers Activated by temperatures between 30˚ and 45˚ Responds to chemicals – eg. Hot/cold gel Warm Vs Cold Both adapt rapidly at first, but continue to generate impulses at low frequency Pain is produced below 10 degrees and over 45 degrees 5 degree window where cold and hot receptors are both activated Skin degrees (shell temperature) = 32 degrees if its colder than your skin – its is going to activate your cold receptors If its warmer than your skin – it is going to activate your warm receptors Pain Sensations: Nociceptors Free nerve endings found in almost every tissue of the body – not found in the brain, hair, nails Activated by intense thermal, mechanical or chemical stimuli Tissue irritation or injury releases chemicals such as prostaglandins, substance P, kinins, and potassium ions (K+) that stimulate nociceptors May persist even after a pain-producing stimulus is removed b/c pain-mediating chemicals linger Exhibit very little (if any) adaptation (tonic) Conditions that can elicit pain include: Excessive distention (stretching) of a structure Prolonged muscular contractions Muscle spasms Ischemia You can perceive the difference in onset of fast pain and slow pain best when you injure a body part that is far from the brain because the conduction distance is long When you stub your toe, you feel the sharp sensation of fast pain and then feel the slower, aching sensation of slow pain Fast Pain (Acute) Occurs rapidly after stimuli (0.1 second) Pain is often felt as acute, sharp, or pricking pain Pain felt from a needle puncture or knife cut to the skin Not felt in deeper tissues Nerve impulses propagate along medium-diameter, myelinated A fibers Superficial tissues Slow Pain (Chronic) Begins more slowly & increases in intensity May be excruciating – felt as chronic, burning, aching, or throbbing pain Pain associated with a toothache Can occur both in the skin and in deeper tissues or internal organs Pain conducted along small-diameter, unmyelinated C fibers Deeper tissue or superficial tissue Superficial Somatic Pain Somatic pain that arises from the stimulation of receptors in the skin Deep Somatic Pain Somatic pain that arises from skeletal muscle, joints and tendons Visceral Pain Unlike somatic pain, is usually felt in or just under the skin that overlies the stimulated organ Localized damage (cutting) intestines may cause no pain, but diffuse visceral stimulation can be severe - distension of a bile duct from a gallstone – cholic pain - distension of the ureter from a kidney stone – cholic pain Pain may also be felt in a surface area far from the stimulated organ in a phenomenon known as referred pain Referred Pain Visceral pain that is felt just deep to the skin overlying the stimulated organ or in a surface area far from the organ Skin area & organ are served by the same segment of the spinal cord Heart attack is felt in skin along left arm since both are supplied by spinal cord segment T1 – T5 Pain Threshold vs Tolerance: Pain Threshold How strong does the stimulation have to be before it elicits a pain response? Ie. Before the stimulus reaches receptor threshold Involves only the PNS Constant among population Pain Tolerance How much pain an individual can take? Involves the CNS Varies widely among population - not well understood Drugs for Pain Analgesic Drugs Aspirin, ibuprofen and naproxen block/inhibits formation of prostaglandin and thromboxane, which stimulate nociceptors Inhibit the cyclooxygenase enzyme Non-steroidal anti-inflammatories Local Anesthetics Novocaine or Lidocaine, are sodium channel blockers They provide short-term pain relief by inhibiting the response of voltage-gated sodium channels along the axons of first order pain neurons Morphine & Opiates Alter the quality of pain perception in the brain; pain is still sensed in the PNS, but the signal is affected in the CNS to either be blocked or perceived differently Causes euphoria Many pain clinics use anticonvulsant and antidepressant medications to treat those suffering from chronic pain or nerve pain Gabapentin Proprioception: Proprioceptive Sensations Allow us to know where our head and limbs are located and how they are moving even if we are not looking at them, so that we can walk, type, or dress without using our eyes Kinesthesia The perception of body movements Proprioceptors Embedded in muscles (especially postural muscles) tendons and joints, inform us of the degree to which muscles are contracted, the amount of tension on tendons, and the positions of joints Proprioceptive or Kinesthetic sense This awareness of the activities of muscles, tendons, and joints and of balance or equilibrium Proprioceptors adapt slowly and only slightly (tonic) Brain continually receives nerve impulses related to the position of different body parts and makes adjustments to ensure coordination Proprioceptors allow weight discrimination, the ability to assess the weight of an object Helps to determine the muscular effort necessary to perform a task Proprioceptive information is sent to both the cerebellum & cerebral cortex Cortical information is conscious Cerebellar information is unconscious 3 Types of Proprioceptors 1. Muscle Spindles withing skeletal muscles 2. Tendon Organs within tendons 3. Joint kinesthetic receptors in or around joint capsules Muscle Spindle: Proprioceptors in skeletal muscles that monitor changes in length of skeletal muscles & participate in the stretch reflex Each muscle spindle consists of several slowly adapting sensory nerve endings that wrap around 3 – 10 specialized muscle fibers called intrafusal muscle fibers Function Main function of muscle spindles is to measure muscle length – how much a muscle is being stretched Either sudden or prolonged stretching of the central areas of the intrafusal muscle fibers stimulates the sensory nerve endings The resulting nerve impulses propagate into the CNS – to the somatic sensory areas of the cerebral cortex, which allows conscious perception of limb positions and movement Impulses from muscle spindles also pass to the cerebellum, where the input is used to coordinate muscle contractions Gamma Motor Neurons In addition to their sensory nerve endings near the middle of intrafusa fibers, muscle spindles contain motor neurons called gamma motor neurons These terminate near both ends of the intrafusal fibers and adjust the tension in a muscle spindle to variations in the length of the whole muscle Eg. When a muscle shortens, gamma motor neurons stimulate the ends of the intrafusal fibers to contract slightly Keeps the intrafusal fibers taut and maintains the sensitivity of the muscle spindle to stretching of the muscle As the frequency of impulses in its gamma motor neuron increases, a muscle spindle becomes more sensitive to stretching of its mid-region Alpha Motor Neurons Surrounding the muscle spindle are muscle fibers called extrafusal muscle fibers Supplied by a large – diameter A fibers called alpha motor neurons Cell bodies of both Y and A motor neurons are located in the anterior gray horn of the spinal cord (or in the brain stem for muscles in the head) Stretch Reflex During the stretch reflex, impulses in muscle spindle sensory axons propagate into the spinal cord and brain stem and activate alpha motor neurons that connect to extrafusal muscle fibers in the same muscle Activation of a skeletal muscle, which relieves the stretching Summary Specialized intrafusal muscle fibers enclosed in a CT capsule and innervated by gamma motor neurons Stretching of the muscle stretches the muscle spindles sending sensory information back to the CNS Spindle sensory fibers monitor changes in muscle length Brain regulates muscle tone by controlling alpha motor neurons Golgi Tendon Organ: Protects tendons and their associated muscles from damage due to excessive tension/load Penetrating the capsules one sensory nerve ending that entwines among and around the collagen fibers of the tendon When tension is applied to a muscle, the GTO generates nerve impulses that propagate into the CNS, providing information about changes in muscle tension Tendon reflexes decrease muscle tension causing inhibition of a muscle Summary Found at junction of tendon & muscle Consist of an encapsulated bundle of collagen fibers laced with sensory fibers When the tendon is overly stretched, sensory signals propagate toward the CNS & result in muscle relaxation Joint Kinesthetic Receptors: Present within and around the articular capsules of synovial joints Free nerve endings and Ruffini corpuscles in the capsules of joints respond to pressure Small pacinian corpuscles in the CT outside articular capsules respond to changes of speed of joints during movement Somatic Sensory Pathways: Somatic sensory pathways relay information from somatic receptors to the primary somatosensory area in the cerebral cortex and to the cerebellum The pathways consist (usually) of three neurons: 1. First–order 2. Second–order 3. Third–order Axon collaterals of somatic sensory neurons simultaneously carry signals into the cerebellum and the reticular formation of the brain stem 1. First-order neurons Conduct impulses from the somatic receptors into the CNS (brainstem or spinal cord) - Peripheral nerve --> CNS From the face, mouth, teeth and eyes, somatic sensory impulses propagate along cranial nerves to the brain stem From the body, neck and posterior head, somatic sensory impulses propagate along spinal nerves into the spinal cord 2. Second-order neurons Conducts impulses from CNS to the thalamus (major relay station) Cross over to opposite side of body in the brainstem or spinal cord before ascending to the thalamus Sensory info from one side of the body reaches the contralateral ½ of the thalamus This is where crossing occur It will cross and then ascend to the thalamus 3. Third-order neurons Conduct impulses from the thalamus to the primary somatosensory area - postcentral gyrus of the parietal lobe Somatic Sensory Pathways: Somatic sensory impulses entering the spinal cord ascend to the cerebral cortex via two general pathways: 1. The posterior column pathway 2. Spinothalamic pathways Also have the pathways to reach the cerebellum via the spinocerebellar tracts 1. Posterior Column Posterior Column - Medial Lemniscus Pathway (PCML) - Sensory Nerve impulses for touch, pressure, vibration and conscious proprioception from the limbs, trunk, neck, and posterior head Ascend to the cerebral cortex via the PCML Posterior column – only sensory Touch (discriminative touch), pressure, vibration, and proprioception Everything but the face Name of the pathway comes from the names of two white-matter tracts that convey the impulses: 1. The posterior column of the spinal cord 2. The medial lemniscus of the brain stem Impulses Conducted: Discriminative Touch Ability to recognize specific info about a touch sensation, such as point of touch, shape, size and texture of source aka stereognosis Stereognosis – the ability to perceive the form of an object by using sense Proprioception Awareness of the precise position of body parts Kinesthesia Awareness of directions of movement Weight Discrimination Ability to asses weight of an object Vibration & Pressure Specific touch with vibrations and pressure First-order neurons Extend from sensory receptors in the limbs, trunk, neck, and posterior head into the spinal cord and ascend to the ipsilateral medulla oblongata The cell bodies of these first-order neurons are in the posterior (dorsal) root ganglia of spinal nerves In the spinal cord, their axons form the posterior (dorsal) columns, which consist of two parts: 1. The gracile fasciculus – closer to midline, lower half of body 2. The cuneate fasciculus – further from midline, upper half of body Second-order neurons First order neurons synapse with second-order neurons whose cell bodies are located in the gracile nucleus or cuneate nucleus of the medulla Second order neuron – is the lemniscus Nerve impulses for touch, pressure, vibration and conscious proprioception from the upper limbs, upper trunk, neck and posterior head Propagate along axons in the cuneate fasciculus and arrive at the cuneate nucleus Nerve impulses for touch, pressure, vibration and conscious proprioception from the lower limbs and lower trunk Propagate along axons in the gracile fasciculus and arrive at the gracile nucelus The axons of the second-order neurons cross to the opposite side of the medulla and enter the medial lemniscus Thin ribbon-like tract that extends from the medulla to the ventral posterior nucleus (VPN) of the thalamus Third-order neurons In the thalamus the axon terminals of second-order neurons synapse with third-order neurons, which project their axons to the primary somatosensory area of the cerebral cortex Spinothalamic Pathways: Sensory 3 neuron pathway IMPORTANT CHART: STUDY The anterolateral or spinothalamic pathways carry mainly pain and temperature impulses, but they also carry... Tickle and itch sensations Name Sensation Location Synapse Other Crude poorly localized touch, pressure and vibration (touch, pressure and vibrations are all poorly localized on these tracts) PCML Discriminatory Touch Everywhere – not face Medulla Gracille – lower extremity Everything but the face Pressure Cunate – upper extremity Vibration Contralateral Proprioception 3 neurons Cortex VPN Spinothalamic Pain Everywhere – not face Spinal cord Anterior – itch, tickle crude ect. Temperature Lateral – pain and temperature Itch Contralateral Tickle 3 neurons Crude touch, pressure, vibration Cortex VPN First-order neurons Travel through the posterior (dorsal) roots of spinal nerves to connect to second order nerves at the level of the spine in which they entered Trigeminothalmic All sensations Face Pons or medulla Contralateral 3 neurons Synapse with second-order neurons in the posterior gray horn of the spinal cord – immediately synapses in the spine Cortex VPN Second-order neurons Cross to the opposite side of spinal cord – continue upward via either the lateral spinothalamic tract or the anterior spinothalamic tract Spinocerebellar Proprioception Everywhere – not face Spinal cord Ipsilateral 2 neurons Lateral spinothalamic tract – carries pain & temperature Cerebellum – no cortex – not conscious Anterior – anatomical division Anterior spinothalamic tract – carries tickle, itch, crude touch, pressure and vibrations Posterior – anatomical division Axons of second order neurons synapse with third-order neurons in the thalamus (VPN) Third-order neurons Project into the primary somatosensory area of cerebral cortex Trigeminothalamic Pathway: Sensory Trigemino – trigeminal neve Thalamic – thalamus Nerve impulses for most somatic sensation (tactile, thermal, pain, proprioception) from the face, nasal cavity, oral cavity ascend to the cerebral cortex along the trigeminothalamic pathway Consists of a 3-neuron set Sensory tract – face sensation First-order neurons Extend from somatic sensory receptors in the face, nasal cavity, oral cavity and teeth into the brain stem through the trigeminal nerve Goes to one or two areas – either pons or medulla Second-order neurons Cross over and go to the thalamus VPN Third-order neurons From the thalamus to the primary somatosensory area of cortex Spinocerebellar Pathways: Proprioceptive tract – unconscious proprioception Spino – spine Cerebellar – cerebellum Doesn’t go to the cerebral cortex Two tracts in spinal cord – posterior spinocerebellar tract and anterior spinocerebellar tract Major proprioceptive impulses from trunk and limbs of one side of the body to the ipsilateral cerebellum These pathways are critical for maintenance of posture and balance and to coordinate, smooth and refine skilled movements Doesn’t follow some of the rules Only 2 neuron pathway – b/c we don’t go from thalamus to cortex Ipsilateral tract Primary Somatosensory Area: Occupies the postcentral gyrus of the parietal lobes Somatic sensory signals from the left side of the body to the right cerebral hemisphere and vice versa The lips, face, tongue and thumb provide input to large regions in the somatosensory area Relative sizes of cortical areas are presented as the homunculus Proportional to number or sensory receptors Proportional to the senitivity of each part of the body Can be modified with learning – plasticity (ability to change) Syphilis: Bacterial infection caused by treponema pallidum Late stage syphilis can show neurological signs and symptoms at which point it is known as neurosyphilis One such sign is progressive degeneration of the posterior portion of the spinal cord – posterior column As the posterior column takes up most of the posterior spinal cord, symptoms will mainly include a loss of the sensory information carried on this tract The most noticeable of these symptoms is the loss of conscious proprioception Sufferers walk with a noticeable gait impairment **** Somatic Motor Pathways: Lower motor neurons extend from the CNS to skeletal muscles – PNS Lower motor neurons are peripheral nerves These lower motor neurons are also called the final common pathway because many regulatory mechanisms summate before a single signal is sent on these peripheral neurons The final messenger that tells the muscle to contract or not LMNs extend as cranial nerves to skeletal muscles of the face and head, and as spinal nerves to skeletal of the limbs and trunk – only LMNs output from CNS to skeletal mm Control of Body Movement Motor portions of cerebral cortex – frontal lobe – precentral gyrus (primary motor area) Initiate & control precise movements Basal ganglia help: Establish muscle tone Integrate semi-voluntary automatic movements Inhibit excess movement Cerebellum Coordinates smooth movement Helps maintain posture and balance Local Circuit Neurons Aka Interneurons Location Close to lower motor neuron cell bodies in the brain stem and spinal cord Receive input from somatic sensory receptors (eg. Nociceptors & muscle spindles) and higher centers in brain Help coordinate rhythmic activity in specific muscle groups (eg. Alternating flexion & extension of lower limbs during walking) Local circuit neurons & lower motor neurons receive input from upper motor neurons Upper Motor Neurons UMNs from the cerebral cortex are essential for planning, initiating and directing sequences of voluntary movements CNS motor neurons Other UMNs originate in motor centers of the brain stem Regulate mm tone Control postural muscles Help maintain balance and orientation of head and body (influenced by basal ganglia and cerebellum) Basal Ganglia Neuron of the basal ganglia provide input to UMNs Assist movement by initiating and terminating movements, suppress unwanted movements and establish normal level of mm tone Cerebellar Neurons Also control activity of UMNs Interconnect the cerebellum with motor areas of the cerebral cortex (via the thalamus) and the brain stem Prime f(x) is to monitor differences b/w intended movements and movements actually performed Then issues commands to UMN to reduce errors in movement This coordinates body movements and helps maintain normal posture and balance Descending Somatic Motor Pathways: Axons of UMN extend from brain to LMNs via two types of descending somatic motor pathways: 1. Direct pathways 2. Indirect pathways Direct pathways From cerebral cortex to spinal cord & out to muscles via LMN Cortex ---> downward Indirect pathways Includes synapses in motor centres in brain stem from basal ganglia, thalamus, reticular formation & cerebellum – then to the LMNs Modify the signal from the direct pathway ---> makes the movement nice and smooth (refine it) Paralysis Damage of LMNs produces flaccid paralysis Damage to UMNs causes spastic paralysis Direct Motor Pathways: Aka. Pyramidal Pathways Provide input to LMNs via axons that extend directly from cerebral cortex The direct (pyramidal) pathways include: 1. Lateral & anterior corticospinal tracts 2. Corticobulbar tracts 1 million upper motor neurons in cerebral cortex Nerve impulses for voluntary movements propagate from UMN in primary motor area and premotor area of cerebral cortex to LMNs Axons descend from the cerebral cortex to the medulla oblongata where axon bundles form the ventral bulges known as pyramids 90% of UMN fibres decussate (cross over) in the medulla Right side of brain controls left side muscles 10% remain on same side, they will eventually decussate at the SC where they synapse with interneurons or LMNs in either: Nuclei of cranial nerves Anterior horns of spinal cord Types: 1. Lateral corticospinal tract 2. Anterior corticospinal tract 3. Corticobulbar tract 1. Lateral Corticospinal Tract The 90% of the fibers that decussate in the medulla make up this tract Control muscles in limbs for precise, agile, and highly skilled movements – hands and feet Eg. Button a shirt or play piano More muscles allows for finer movements, but also requires more nervous control to maintain dexterity 2. Anterior Corticospinal Tract The 10% of axons that do not cross in the medulla Decussate in the Spinal Cord and then synapse with interneurons of LMN in anterior gray horn Axons of LMNs exit cervical and upper thoracic segments of SC in the anterior roots of spinal nerves Controls movements of neck, trunk, and girdle muscles 3. Corticobulbar Tracts Control skeletal muscles of head Cortico – cortex & bulbar – brainstem (cortex --> brainstem) Direct motor pathway for cranial nerve activity Form left and right cerebral peduncles – descend from cerebral cortex to brain stem Some decussate, some do not UMNs synapse with LMNs of Cranial Nerves exiting brain stem Cortex to nuclei of cranial nerves III, IV, V, VI, IX, X, XI & XII Control precise and voluntary movements of eyes, tongue, chewing, facial expressions & speech Primary Motor Cortex: The primary motor area is located in the precentral gyrus of the frontal lobe Where the sensation comes from Upper motor neurons plan and initiate voluntary movements Different mm's are represented unequally in the primary motor area The cortical area devoted to a muscle is proportional to the number of motor units More cortical area is needed if the number of motor units in a muscle is high - vocal cords, tongue, lips, fingers & thumb Paralysis: LMN Damage Flaccid paralysis No voluntary movement on same side as damage No reflex actions Muscle limp & flaccid Decreased muscle tone UMN Damage Variable, but often spastic paralysis Paralysis on opposite side from injury – not always on the opposite side Increased muscle tone Exaggerated reflexes If it happens up in the brain – contralateral If it happens in the spinal cord – variations Indirect Motor Pathways: Indirect or extrapyramidal pathways include all somatic motor tracts other than the corticospinal and corticobulbar tracts Involve the motor cortex, basal ganglia, thalamus, cerebellum, reticular formation, and nuclei in the brain stem Indirect tracts: 1. Rubrospinal Tracts 2. Tectospinal Tracts 3. Vestibulospinal Tracts 4. Lateral Reticulospinal Tracts 5. Medial Reticulospinal Tracts Complex polysynaptic circuits Include basal ganglia, thalamus, cerebellum, reticular formation Descend in spinal cord as 5 major tracts All 5 tracts end upon interneurons or lower motor neurons Types: 1. Rubrospinal Rubro - red & spinal – spine Conveys nerve impulses from the red nucleus to contralateral skeletal muscles for gross and precise movements of the upper limbs 2. Tectospinal Tecto – tectum & spinal – spine Conveys nerve impulses from the superior colliculus to contralateral skeletal muscles that move the head and eyes in response to visual stimuli 3. Vestibulospinal Vestibulo – vestibular & spinal – spine Conveys nerve impulses from the vestibular nucleus in the pons and medulla to regulate ipsilateral muscle tone for maintaining balance in response to head movement When you turn your head, your body adjusts weight distribution and this is due to this pathway 4. Lateral Reticulospinal Reticulo – reticular formation & spinal – spine Conveys nerve impulses from the reticular formation to facilitate flexor reflexes inhibit extensor reflexes decrease muscle tone in muscles 5. Medial Reticulospinal Reticulo – reticular formation & spinal – spine Conveys nerve impulses from the reticular formation to facilitate extensor reflexes inhibit flexor reflexes increase muscle tone in muscles Final Common Pathway Lower motor neurons receive signals from both direct & indirect upper motor neurons Sum total of all inhibitory & excitatory signs determines the final response of the lower motor neuron & the skeletal muscles Integrative Functions of the Cerebrum: The integrative functions include: Sleep Wakefulness Memory Sleep & Wakefulness Role of the Reticular Activating System (RAS) Sleep and wakefulness are integrative functions that are controlled by the reticular activating system Arousal, or awakening from a sleep, involves increased activity of the RAS - when the RAS is activated, the cerebral cortex is also activated and arousal occurs - the result is a state of wakefulness called consciousness RAS has connections to cortex & spinal cord Many types of inputs can activate the RAS Pain, light, noise, muscle activity, touch Coma is sleep-like state Defined in a number of different ways, but the most common objective indicator used is the Glasgow Coma Scale (GCS) Circadian rhythm Any biological process that cycles on a daily schedule ~ 24 hour biological clock – some evidence suggests its longer than 24 hours Established by the hypothalamus EEG recordings show large amount of activity in cerebral cortex when awake EEG is out best tool for defining sleep/wake cycles Sleep A state of altered consciousness or partial unconsciousness from which an individual can be aroused by different stimuli During sleep activity in the RAS is very low There are two types of sleep: 1. Non-rapid eye movement sleep – NREM 2. Rapid eye movement sleep – REM 1. Non-Rapid Eye Movement (RNEM) Consists of four stages – each defined by the activity on an EEG A regular night consists of phasing from one stage to the next in a typical sleep cycle Stage 1 Person drifting off with eyes closed – first few minutes Hypnic jerk occurs here – when you jerk "falling from a building" Stage 2 Fragments of dreams – boring dreams Eyes may roll side to side (slow roll) Stage 3 Very relaxed, moderately deep 20 minutes after stage 1 body temperature & BP drop, breathing rate drops Marks the change from light sleep to deep sleep Stage 4 Deep sleep or slow-wave sleep Parasomnia occurs here Bed-wetting, sleep walking, sleep talking, night terrors etc. Delta waves 2. Rapid Eye Movement (REM) Most dreams occur during REM sleep Sleep cycles occur over ~90 minutes – people become paralyzed in this stage Go from stage 1 – 4 of NREM Go up to stage 2 of NREM To REM sleep The first REM of the night is very short – every time REM sleep is entered, it lasts longer Cycles repeat until REM sleep totals 90 – 120 minutes Neuronal activity & oxygen use is highest in REM sleep Total sleeping & dreaming time decreases with age Sleep Problems Parasomnia – stage 4 Delayed sleep phase disorder REM sleep behaviour disorder Narcolepsy – excessive day time sleepiness (get strong urges to fall asleep that they cant control) Learning & Memory Learning The ability to acquire new knowledge or skills through instruction or experience Memory The process by which that knowledge is retained over time For an experience to become part of memory, It must produce persistent functional changes that represent the experience in the brain The capability for change with learning is called plasticity