A sensation is an awareness of external or internal changes whether we are aware of them or not. General senses have receptors that are widely spread throughout the body and include the skin, various organs and joints. Special senses are specialized receptors confined to structures in the head.
Sensory receptors are going to be either complex or simple. General sensory receptors are rather simple. They have no structireal specializations in free nerve endingst that provide us with pain, tickle, itch, and temperature. Our special sensory receptors are very complex structures and include vision, taste, hearing and smell.
There are several types of receptors. They include mechanoreceptors, thermoreceptors, photoreceptors, chemoreceptors and pain receptors (aka nociceptors). Mechanoreceptors are for touch, pressure, hearing and equilibrium. Thermoreceptors are for temperature. Photoreceptors are for light intensity. Chemoreceptors are for chemical changes. Pain receptors are for physical or chemical changes to our tissues.
The receptive field is the area of the body that when stimulated will cause a response from an afferent neuron. Basically it is the area around a receptor. There are different sizes of receptive fields.
Adaptation is a change in sensitivity to long lasting stimuli or the ability to ignore unimportant stimuli. A decreased response from receptors occurs from a particular stimulus. The sensory impulses become less frequent and can even stop. We have two types of adapting receptors. They are either fast or slow. The rapidly adapting receptors include smell, pressure and touch and are specialized for detecting pain. Slowly adapting receptors are for pain, body position, and chemical composition of the blood. Their nerve impulses continue as long as the stimulus persists.
The rest of this entry is about general senses. Specialized senses are special and have their own.
Receptors for the general senses exist all over our bodies. We have exteroceptive senses that are associated with the body surface for touch, pressure, temperature and pain. We have visceroceptive senses that recognize changes in the viscera includeing blood pressure stretching blood vessels and ingesting a meal. And lastly we have proprioceptive senses that are associated with changes in muscles and tendons.
Meissner corpuscles or corpuscles of touch are egg-shaped masses of dendrites enclosed by a capsule of connective tissue. These are rapidly adapting receptors. They are found in the dermal papillae of hairless skin (fingertips, hands, eyelids, tip of tounge, lips, nipples, clitoris and tip of penis). They generate impulses mainly at the onset of a touch.
Hair root plexuses are also rapidly adapting touch receptors. They are found in our hairy skin. Their free nerve ending wrap around the hair follicles and deduct movements on the skin that disturb the hair.
Merkel discs are also called tactile discs or type I cutaneous mechanoreceptors. These are slowly adapting touch receptors. They are saucer-shaped flat free nerve endings. They are found in our fingertips, hands, lips and external genitalia.
Ruffini corpuscles are also called type II cutaneous mechanoreceptors. They are elongated, encapsulated receptors that are located deep in the dermis and ligaments and tendons. They are only found in the hands and soles.
Pacinian or lamellated corpuscles are large, oval structures that are composed of a multilayered connective tissue capsule that enclose a dendrite. They are found in joints, tendons and muscles and in the periosteum., our mammary glands, external genitalia, pancreas and urinary bladder.
Somatic tactile sensations summary:
Touch is either crude or discriminate. Crude touch is the ability to perceive that something has touched the skin. Discriminate touch provides our brains with location, size and texture of source. Pressure is always sustained over a large area. A vibration is a set of rapidly repetitive sensory signals. An itch is from a chemical stimulation of free nerve endings and a tickle is the stimulation of free nerve endings only by someone else. Some types of somatic tactile sensations are more rapidly adapting than others.
Thermoreceptors respond to changes in temperature. They are located in our skin and our hypothalamus. Our thermoreceptors are free nerve endings that are approximately 1 millimeter thick. They are rapidly adapting receptors. Our warm receptors are in the dermis and respond to temperatures at 25 degrees Celsius, but are unresponsive at temperatures greater than 45 degrees. Cold receptors are in the stratum basale and are sensitive to temperatures between 10 and 20 degrees. Pain receptors respond to the extremes.
Pain nerve pathways also have two parts. There are acute and chronic pain pathways. Acute pain fibers are also known as A-delta fibers. They are thin and myelinated and conduct rapid impulses. They are associated with sharp pain and are very localized. Chronic pain fibers are exactly the opposite. They are C fibers, they are thin, but unmyelinated. They conduct slow impulses over a large pain.
Pain is regulated through the thalamus, which allows us to be aware of the pain. It is then cent to the cerebral cortex which judges the intensity of the pain, locates the source and produces the emotional and motor responses necessary. Pain inhibiting substances include ekephalins, serotonin and endorphins. Pain relief comes from multiple sites of analgesic actions. Aspirin and ibuprofen block the formation of prostaglandins that stimulate nociceptors in the hypothalamus. Novacain blocks the conduction of nerve impulses along pain fibers (aka no more sodium channels). Morphine, as well and percocet and vicoden lessen the perception of pain in the brain.
Proprioceptors send information to the spinal cord and centeral nervous system about body position and length and tention of muscles. The main kinds of proprioreceptors are pacinian corpuscles (speed of joint movement), muscle spindles (are in the skeletal muscles) and golgi tendon organs (which are in tendons and protect from over stretching).
Baroreceptors and viscreal function provide information on the pressure or volume of organs.
Chemoreceptors are a major part of homeostasis.
Somatic sensory pathways relay information from somatic receptors to the cerebral cortex. A first order neuron conducts the impulse to the central nervous system. Then, the second order neurons conduct impulses from the brain or cord to the thalamus. When the impulse crosses over to the other side of the body, this is called decussate. The third order neurons conduct impilses from the thalamus to primary somatosensory cortex (aka the postcentral gyrus of the parietal lobe). An axon collateral of the somatic sensory neurons simultaneously carry signals into the cerebellum and the reticular formation of the brain.
Spinothalamic pathway of the central nervous system has both lateral and anterior tracts. The lateral spinothalamic tract carries impulses for pain, cold and warmth. The anterior spinothalamic tract carries ticke, itch, crude touch and pressure. The first cell body is in the dorsal root ganglion. The second cell body is in the cray matter of the cord and sends fibers to the other side of the cord and up through white matter to synapse with the thalamus. The third order cell body is in the thalamus and projects to the cerebral cortex.
Motor information goes away from the brain in a similar pattern to somatic sensory pathways. The upper motor neurons extend from the cerebral cortex to either the brain stem or the cord. The neural circuits involving basal ganglia and cerebellum regulate the activity of upper motor neurons. Lower motor neurons extend from the brain stem (via cranial nerves) and the spinal cord (via spinal nerves) to skeletal muscles. If you have spastic paralysis, there is damage to the upper neurons. If you have flaccid paralysis, you have damage to lower motor neurons.
A direct pathway carries voluntary control to skeletal muscles. It foes from the motor area of the cerebral cortex to the spinal cord and out to the muscles. An indirect pathway carries involuntary control for subconscious movements. They include synapses in basal ganglia, thalamus and reticular formation.
The amount of the cortex devoted to a muscle is proportional to the number of motor units in that muscle. Muscles that produce precise movements have gross motor units. Muscles of the legs have few motor units.
The cerebral cortex initiates and controls precise movements. Basal ganglia help establish muscle tone and integrates semi-voluntary movements. The cerebellum helps make movements smooth and maintains posture and balance. Descussation occurs in the medulla oblongata such that one side of the brain controls the other side of the body.
Integrative functions include sleep, wakefulness, memory and emotions. The hypothalamus establishes a circadian rhythm. A portion of the reticular formation increases activity of the cerebral cortex. Many sensory stimuli can activate RAS - pain, movement, bright lights, and alarm clocks. RAS consists of neurons whose axons projkect from the reticular formation through the thalamus to the cerebral cortex. We have increased activity of the RAS which causes awakening from sleep.
Learning is the ability to acquire new knowledge or skills. Memory is the structural and functional changes that represent the experience in the brain. Neurons make new proteins and sprout new dendrites to the new information. Immediate memory lasts only for a few seconds. Short term memory lasts for hours related to electrical and chemical events. Long term memory lasts for days to years and it is said to be related to anatomical and biochemical changes at synapses.