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Sensory Pathways & Somatic Nervous System

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Sensory Pathways & Somatic Nervous System Sensory Pathways & Somatic Nervous System Chapter 15 How Does Brain Differentiate Sensations? Pain impulses make brain aware of injuries and infections. Impulses from eye, ear, nose and tongue make brain aware of what you saw, heard, smelled or tasted. Impulses from joints and ear make brain aware of body position and balance. How does brain differentiate between different impulses/sensations? Depends on which part of the brain these impulses reach. Examples: Occipital lobe translates impulses as light. Temporal lobe translates impulses as sound. Cerebellum translates impulses as equilibrium and balance. General Senses vs. Special Senses There are sensory receptors present all over the body to make brain aware of its environment…sensation! All body senses can be divided into two categories: General senses Special senses General Senses vs. Special Senses Epidermal cells of skin Epidermal cells of skin Dendrites Free nerve endings Sensory nerve fiber Primary sensory Tactile receptors on skin cortex General senses: Monitor simpler, general outside/inside environment. Examples: sense of touch, pressure, temperature, pain and body position. Receptors for general senses: Widely distributed throughout the body… simple receptors. Are dendrites of sensory neurons….which could be naked (free nerve endings) or encapsulated (Meissner’s corpuscle/ Tactile corpuscle). Integrated by primary sensory cortex in the parietal lobe of cerebrum. General Senses vs. Special Senses Special senses: Monitor more complex senses. Examples: sense of vision (sight), auditory (hearing), olfaction (smell), gustation (taste) and equilibrium (balance). Receptors for special senses: Are specialized cells (neurons or epithelial cells). Are found in clusters and localized only in certain parts of the body… complex receptors. Integrated by specific lobes of cerebrum and cerebellum. Characteristics of Receptors 1. When receptors are stimulated they generate impulses impulses are sent to the brain via sensory neurons. 2. Receptors can be nerve endings or specialized cells (neurons or epithelial cells). 3. Each receptor monitors a certain radial area called its receptive field. Larger the receptor field less sensitive (poor ability to localize a stimulus), e.g. on arm Smaller the receptor field more sensitive (precise localization of a stimulus), e.g. on fingertips, nose Receptors – More Characteristics 4. Each receptor specializes in one type of stimulus….receptor specificity. But it can be stimulated weakly by other types of stimuli. Classification of receptors based on the type of stimuli: 1) Mechanoreceptor – are stimulated by mechanical stimulus such as touch, pressure, vibrations: a) Tactile receptors – are stimulated by touch, pressure and vibrations as in skin, inner ear (cochlea). b) Baroreceptors – detect pressure changes in organs such as blood vessels, urinary, digestive and respiratory organs. c) Proprioceptors – sense changes in body positions as in skeletal muscle, joints, tendons, ligaments, ear. 2) Thermoreceptors – are stimulated by temperature changes as in skin and internal organs. 3) Chemoreceptors – detect chemical changes as in the nose, tongue and the wall of blood vessels (monitor pH, O2 and CO2 level). 4) Nociceptors – are pain receptors that are stimulated by injuries and infections…present everywhere in the body. Receptors – More Characteristics 5. Receptors can temporarily change their threshold value……sensory adaptation….to accommodate to a long-term stimulus. Adaptation: reduction in the sensitivity of the receptors in response to constant stimulus. Examples: Jump into the swimming pool initially feels cold after a few minutes, water no longer feels as cold! Cook something on the stove initially strong odor after a few minutes, not as strong anymore! Rapidly adapting/Phasic receptors: Normally inactive, but become active for a short time in response to a change in the conditions they monitor. Whose threshold changes very quickly. Examples: receptors for smell, taste, pressure and touch. Slowly adapting/Tonic receptors: Are always active. Whose threshold does not change easily. Examples: receptors for pain, body position, chemicals in blood. Receptors – More Characteristics 6. Receptors can be classified according to their location. Exteroceptors: That are present on the surface of the body. Detect changes in the external environment. Examples: receptors in the skin for temperature, pressure and touch. Interoceptors/Visceroceptors: That are present in the wall of internal organs. Detect changes in the internal environment.. Examples: receptors in the wall of blood vessels (chemoreceptor & baroreceptor) and other internal organs. Proprioceptors: Are present in the joints, tendons, ligaments, muscles and ear. For maintaining body positions and balance. Pain Sensation Pain: refers to the discomfort or an alarm system to inform the brain of an injury or infection or changes that may endanger the body. Detected by receptor called nociceptors….which are nerve endings. Present in just about any tissue and organ of the body. Can be stimulated by any type of stimulus. Integrated by thalamus and limbic system of cerebrum. Acute pain: occurs rapidly after a stimulus is applied….as in injuries. Chronic pain: begins gradually and increases in intensity over time, e.g. osteoarthritis, migraine headaches, cancer etc. Somatic pain: arises from receptors present in the skin, skeletal muscles or joints, e.g. finger cut, overstretched muscle. Visceral pain: arises from receptors present in the visceral organs, e.g. renal colic, labor pain. Phantom pain: pain interpreted by the brain as coming from the nonexistent limb….as after amputations. Referred pain: a sense of pain in a region other than the site of origin, e.g. pain in hip/thigh because of herniated disc. Sensory and Motor Pathways When a change takes place in the external environment Receptors are stimulated Impulses are generated Impulses travel to the spinal cord Then to the brain….afferent sensory pathway Brain integrates the information Motor impulses travel to the spinal cord Impulses exit to go to the effector….efferent motor pathway Response takes place. How do afferent sensory and efferent motor pathways run???? Afferent Sensory Pathways Dorsal gray horn Cerebrum Going from the receptor to the brain….at least 3 Dorsal root neurons are involved….first order neuron, second Dorsal root ganglion order neuron, third order neuron. When a change takes place in the external environment Sensory receptors are stimulated. Impulses are generated. Impulses travel through the sensory neuron or the first order neuron that enters the spinal cord through the posterior gray horn. First order neuron synapses with interneuron or the second order neuron that originates in the spinal cord. Second order neuron cross to the opposite side of the spinal cord and pass upward through the brain stem to the thalamus. It then synapses with third order neuron that takes impulses to the somatosensory cortex of cerebrum for perception of sensation. Efferent Motor Pathways Dorsal gray horn Dorsal root Going from the brain to the voluntary effector Dorsal root ganglion (somatic)….at least 2 neurons are involved….upper motor neuron and lower motor neuron. Once the brain has integrated the sensory information Upper motor neurons from primary motor cortex of cerebrum take impulses down through descending tracts to brain stem or the spinal cord Upper motor neurons intercept with lower motor neurons in the spinal cord Lower motor neurons exit spinal cord through anterior gray horn Take motor impulses to skeletal muscles for the somatic (decussation of pyramids) response. Upper motor neuron lesions Plantar extension Babinski reflex (normal response in infants) Plantar flexion (normal response in adults) Positive Babinski sign in adults (abnormal response) Upper motor neuron (descending spinal tract) lesion occurs due to stroke, traumatic brain injury or multiple sclerosis…results in:- a) Muscle weakness. b) Decreased muscle tone. c) May exhibit positive Babinski reflex- extensor plantar reflex (big toe extends upward and the remaining toes fan out-normal in infants less than 2 years old due to incomplete myelination of the corticospinal tract). Positive Babinski's sign is always pathological in adults but normal in children less than 2 years. Disorders ALS / Amyotrophic Lateral Sclerosis / Lou Gehrig’s Disease Progressive degeneration of upper and lower motor neurons paralysis of muscles muscles atrophy. Later stages- affects respiratory muscles-leads to respiratory failure and death. Cerebral Palsy- Affects voluntary motor performance… not progressive. Appears during infancy or childhood and persist throughout life. Difficult birthing process, maternal exposure to alcohol and drugs, congenital defects CNS functions are permanently impaired…motor skill, posture and balance, memory, speech and learning..
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