6 Lecture - Somatosensory Receptors Cutaneous Receptors (Fig 11.13) 1. Merkel's discs, hair follicles, Meissner and Pacinian Corpuscles and Ruffini's endings are all sensitive to touch. They contain mechanoreceptors that activate ion channels when their shape is changed. 2. Some of these receptors, such as hair follicles, are nerves that are wrapped around the root of a hair. Others, such as the corpuscles, are nerves contained within capsules. 3. These nerves run from the skin all the way to the spinal cord. 4. Nerve endings are located in several places. a. Superficial dermis Meissner's Corpuscle and Merkel's discs. Both are very sensitive to fine mechanical stimuli and also have a very small receptive field to allow for acuity. Between these two, Meissner's are phasic. This means that they adapt very slowly and respond to the duration of the stimulus. On the other hand, Merkel's are tonic. They adapt rapidly, respond to the onset and offset of a stimulus. They let the individual know about fluctuations in stimuli. b. Dermis Pacinian Corpuscles and Ruffini's endings. Both are sensitve to deep pressure and have a very large receptive field to receive more information. Between these two, Pacinian are phasic like Meissner's. On the other hand, Ruffini's are tonic like Merkel's discs. c. Hair follicles phasic with very small receptive fields. d. Free Nerve Endings very tiny. They run into the dermal layer, close to the epidermis. They have a very large receptive field because they are highly branched. These are sensitive to temperature (hot or cold) and pain, not pressure like the others. Some nerves are so tiny they are not myelinated because this promotes action potentials. Encoding Stimulus Modality (Online Slide 1) 1. Law of Specific Nerve Energies Each nerve is specialized (due to ion channels, etc.) to receive and transmit a specific stimulus. For example, mechanoreceptors deliver information about pressure, olfactory receptors about scent, thermoreceptors about temperature, etc. 2. Principle of Labeled Lines - The route of delivery of the stimulus has no role in what the stimulus ends up being. For example, you can excite a mechanoreceptor in any way, but it will only deliver the signal it was built to deliver. It will not translate any information about light or temperature, just pressure. Therefore, the sensation is determined by the receptor pathway, not the stimulus itself. Spinal Cord And Brain (Fig 10.6) 1. The Central Nervous System (CNS) is made up of the brain and spinal cord while the Peripheral Nervous System (PNS) contains all other nerves branching off from the brain and spinal cord. 2. Spinal nerves contain both sensory input neurons and motor output neurons. Cross Section Of A Spinal Cord (Fig 10.8) 1. Sensory afferent neurons run from sensory receptors in the skin toward the spinal cord. They run through a clump of cell bodies called the dorsal root ganglion. This is where all the cell bodies of those neurons are located, far away from the actual stimulus site. 2. After passing through the root ganglion, the axon reaches its terminal in the gray matter of the spinal cord. This area is concentrated with cell bodies and dendrites. It is surrounded by white matter, which consists of myelinated axons. 3. Once information has been processed, the resulting action that is generated is run out of the spinal cord through a motor efferent neuron. This leaves from the ventral root and forms a large synapse with the muscle that will carry out the response. Cross Section Of A Spinal Cord (Fig 10.9) 1. The white matter containing myelinated axons has many tracts that run up and down connecting the brain and spinal cord. The ascending tracts transmit sensory information to the brain while the descending tracts send information back about actions to take. There are also tracts that interconnect the spinal cord with other parts of itself. The Brain (Fig 10.11c) 1. All tracts from the spinal cord end up in the midbrain. The midbrain is part of the brain stem that also
contains the medulla oblongata and pons. Parts of the midbrain release amino neurotransmitters that control brain activity, such as consciousness and awareness, from the brainstem. 2. There are many other control centers such as the hypothalamus, the thalamus, etc. 3. The thalamus is special because it is the gateway between the midbrain and the cerebral cortex. Almost all sensory information that ends up in the midbrain has to go through the thalamus before being processed in the cerebral cortex. Only olfactory sensation does not have to. Lobes Of The Cerebral Cortex (Fig 10.13) 1. Major lobes of the brain are separated by sulci. The two are called the central sulcus, separating the frontal from the parietal lobe, and the lateral sulcus, separating those lobes from the temporal lobe. 2. The function of the occipital lobe is only primary visual sensation. 3. The function of the temporal lobe is auditory as well as higher-level visual sensation. 4. The function of the parietal lobe is sensation as well as higher-level visual sensation. 5. The function of the frontal lobe is anything with planning and decision-making, such as movement. General Pathway For Transmitting Somatosensory Information To the Cerebral Cortex (Fig 11.6) 1. A stimulus is received by nerves in the PNS. They are transmitted along a first-order afferent neuron. 2. It reaches the spinal cord or brain stem, where it transferred is to a second-order neuron in the CNS. 3. From there, it reaches the thalamus, where it is transferred to a third-order neuron and finally into the cerebral cortex. 4. This is only a general pathway for free nerve endings. Larger neurons go directly to the medulla instead of the spinal column. The Two Somatosensory Pathways (Fig 11.14) 1. The first is called the Dorsal Column-Medial Lemniscal Pathway. It transmits information from mechanoreceptors and propioceptors to the thalamus. a. First-order neurons originate in the periphery and enter the dorsal horn of the spinal cord. b. The main branch of the axon ascends to the ipsilateral (same side as stimulus) brainstem. c. The dorsal columns are tracts of white matter that run to the brain. The first-order neurons terminate in the medulla, where the second-order neuron picks up. d. The second-order neurons cross over to the contralateral (opposite side as stimulus) side of the medulla and continue up to the brain. e. The second- and third-order neurons synapse in the thalamus, which transmits to the cortex. 2. The second is called the Spinothalamic Tract. It transmits information from thermoreceptors and nociceptors to the thalamus. a. First-order neurons originate in the periphery and enter the dorsal horn of the spinal cord. b. They synapse here with second-order neurons, which cross over to the contralatl side of the spine. c. They ascend into the thalamus, where they synapse with third-order neuron d. The transmit information to the cerebral cortex. Primary Somatosensory Areas Of The Brain (Fig 10.15) 1. Specific parts of the brain respond to stimuli from specific parts of the body. The more concentrated regions of the body have larger areas of response in the cerebral cortex. 2. Adjacent neurons have adjacent response areas in the cerebral cortex as well. The Eye (Fig. 11.18a) 1. The outermost layer of the eye is the cornea. 2. The next layer is the iris, which has a hole in it called the pupil. 3. The next layer contains the lens, which can adjust its shape via ciliary muscles. 4. There is vitreous humor filling the cavity of the eye behind the lens and aqueous humor in front of it. 5. An image is formed on the retina, the back portion of the inner layer in the eye. Control Of The Pupil (Fig 11.27) 1. Dilation occurs only when the sympathetic nervous system is activated. When this occurs, the pupil dilates. When it is inhibited, the pupil remains its normal size. This only happen during periods of intense activity, excitement, fright and stress, not during normal parasympathetic responses.
2. Constriction occurs when the parasympathetic nervous system is activated. When this occurs, the pupil constricts. When it is inhibited, the pupil remains its normal size. Refraction By A Convex Lens System (Fig 11.23) 1. A given point in the visual field comes to focus on a single point in the retina. 2. Refraction of light waves as they pass through the convex cornea and lens of the eye cause the image to be inverted and reversed on the retina. 3. The brain can understand this and interprets images to be right-side up. Mechanism Of Accomodation (Fig 11.25) 1. The pupil changes its size by changing the length of the muscles that stem from it to the iris. When those muscles are relaxed, it puts tension on zonular fibers running to the lens. This causes the lens to flatten out, which helps focus distant objects. 2. In order to focus on near objects, the opposite happens. The ciliary muscles contract, reducing the tension on the zonular fibers running to the lens. This causes the lens to become rounder, which helps focus on near objects. Focusing On Distant And Near Objects (Fig 11.24) 1. Distant objects have nearly parallel light rays reflecting off of them. In order to focus these, the lens needs to be flattened so that it weakly refracts these rays. 2. Near objects have diverging light rays reflecting off of them. In order to focus these, the lens needs to be rounded so that it can strongly refract these rays. Normal And Near-Sighted Vision (Fig 11.26 a,b) 1. For a normal-sighted person, distant objects do not require accommodation while near objects due because of the refractive power that is necessary to focus. Therefore, for distant objects, the lens remains flat while for near objects, the lens becomes rounder. These people can shift their lens shape enough to focus on both types of objects easily. 2. For a near-sighted person, the lens of the eye is too strong. That means its refractive power is so high that distant objects do not come into sharp focus. In order to balance this, a concave lens can be used to diverge light rays from distant objects. This allows the eye's lens to refract them, since it has a high refractive power, and that corrects this person's vision. a. This person's eyeball is a little too long, so that the image focuses in front of the retina instead of on it. They are said to have myopia. Far-Sighted Vision (Fig 11.26c) 1. For a far-sighted person, the lens of the eye is too weak. That means its refractive power is not high enough to focus on near objects. In order to balance this, a convex lens can be used to increase the total power of the two lenses (the eye's and the artificial). This allows the person to see near objects sharply. a. This person's eyeball is a little to short, so that the image focuses behind the retain instead of on it. They are said to have hyperopia.