PNS 1.docx

Afferent division of the PNS Brain and the spinal cord receive information about: Internal environment= visceral stimuli. External environment= sensory stimuli. 2 types of afferent divisions in the PNS Sensory afferent e.g., vision, hearing, taste, and smell= conscious input Visceral afferent e.g.,...

Afferent division of the PNS Brain and the spinal cord receive information about: Internal environment= visceral stimuli. External environment= sensory stimuli. 2 types of afferent divisions in the PNS Sensory afferent e.g., vision, hearing, taste, and smell= conscious input Visceral afferent e.g., blood pressure, heart rate= subconscious input. Efferent division of the PNS Somatic nervous systemcontrols skeletal muscles (voluntary control) Autonomic nervous systemcontrols sympathetic and parasympathetic nervous system such as smooth muscle, cardiac muscle, and glands. Enteric nervous systemcontrols digestive organs only! Network of fibres that are localised in digestive track. Vision as an example The part of the brain that controls the vision is in the cortex and localised in the occipital region. Features of the eyes Iris= gives the eyes the colour Pupil= round opening through which the light enters the eye. Sclera= made of up of connective tissue. Tough outer layer of the eye Structures that protect the eye. Eye lids= protects from mechanical issue Eyelashes= traps fine particles Tears= lubricant and germ-killing fluid Tears are produced by lacrimal gland and drains into tiny canals and reaches lacrimal sacs. The eye is a fluid filled structure. Has 3 different layers External sclera (outermost layer) Choroid (middle layer which is highly pigmented) Retina (innermost layer) that contains photoreceptors. The eye contains 2 separate cavities that are separated by analytical lens. (Lens must be transparent to let the light in) The region between the lens and the retina is filled with vitreous humor (also transparent) which helps to maintain the spherical shape of the eyeball. Between the lens and the cornea and this is filled with aqueous humor. Iris muscle is controlled by automimic system to dilate and contract. Process of refraction The eye refracts entering light to focus the image on the retina. 2 main refractive structures are cornea and retina. Cornea never changes its refractory ability whilst the lens can change its refractory abilities in a process called accommodation which is mediated by the ciliary muscle. Light travels faster through the air than any other mediums The course of the direction of the light changes if it strikes the surface at any other angle than perpendicular. When light ray hits the surface of the cornea, slows down and changes direction and gets is focused on the retina. If a light ray strikes the cornea perpendicularly and not any other angle, it doesn’t change the direction and is focused on the light-sensitive retina. The main function of the eye is to focus the light from the environment on the photoreceptor cells of the retina which are rods and cones. Structure of retina Photoreceptors consists of rods and cones Cones can be the colour green, blue and red. Rods are grey colour which enable to us to see in grey light. Bipolar cells Can interact with photoreceptor cells on one side and ganglion cells on the other side. Horizontal and amacrine cells Retinal ganglion cells Axons of the retinal ganglion cells forms optic nerve for transmission into the CNS. Direction of light travels from optic fibres to the photoreceptors whereas direction of retinal visual processing starts with the photoreceptors and goes to optic nerve. The light finds first the ganglion cells, amacrine cells, bipolar cells, horizontal cells and then photoreceptors. This happens everywhere aside from one region called fovea. Fovea is in exact centre of retina and contains no ganglion cells, amacrine cells, bipolar cells, and horizontal cells. It only contains photoreceptors as they are localised in the fovea and the light immediately strikes the photoreceptors only. Structure of rods and cones Both rods and cones contain 3 segments Outer segment- houses discs that contain photopigment called rhodopsin. Rods contains more discs than cones. Inner segment- contains metabolic machinery e.g., nucleus, Golgi, and mitochondria. Synaptic terminal- faces bipolar cells. Stores and releases neurotransmitter Rods (for night vision) Cones (day vision) More localised in the periphery of retina Localised in exact centre of retina Only grey colour Blue, green, and red Very light sensitive due to more discs therefore more photopigment Output from lots of rods converge via bipolar cells on a single ganglion cell: low acuity (poor resolution) Each cone is connected to a particular ganglion cell: high acuity (good resolution) The photopigment called rhodopsin is made up of 2 components. Plasma membrane protein called opsin. Vitamin A derivative that’s called retinal When light hits, the retinal change sits confirmation from cis to trans (retinal is light sensitive part) an opsin stays exactly same. This causes a process called phototransduction which is the process of converting light stimuli into electrical signals. In dark Depolarisation of photoreceptors Opens Ca2+ Releases neurotransmitters Further retinal processing in bipolar and ganglion cells Reaches ganglion cells and forms action potential which travels along the optic nerve. In light Hyperpolarisation of photoreceptors Less Ca2+ released. Less release of neurotransmitter Less signal relayed to ganglion cells. Visual information processing The optic nerve carries information from lateral and medial part of each eye. Reaches optic chiasm. Localised underneath the hypothalamus. Light enters the eye Light sensors convert light into electrical signals The electrical signals pass through the optic nerve to lateral geniculate nucleus which relays the information to the visual cortex. Example of pain Pain is triggered on stimulation of nociceptors. Function is to bring conscious perception tissue damage that is occurring or about to occur. It is a multidimensional experience. 3 types of pain receptors Mechanical nociceptors- reacts to pinching and cutting. Thermal nociceptors- reacts to heat. Polymodal nociceptors- reacts to mechanical, thermal and chemical triggers. There are 2 different pain pathways. Fast pain Slow pain Stimulation of machinal and thermal nociceptors Stimulation of polymodal nociceptors Small, myelinated A-delta fibres Small Non-myelinated C fibres Easily localised Poorly localised Immediate pain Slow persistent pain Processing of pain input If there is a stimulus, the nociceptor releases neurotransmitter. 2 main neurotransmitters used in pain pathway are glutamate and substance P (substance P is specific to only pain pathway) Nociceptors are synapses with dorsal horn in spinal cord (excitatory neurones)- activates higher centre in the brain. Somatosensory cortex is responsible for localisation of the pain. Thalamus is responsible for perception of the pain. Reticular formation is responsible for alertness. Hypothalamus is responsible for behavioural and emotional response to pain. The brain contains and built-in analgesic system which suppresses transmission in the pain pathways as they enter spinal cord. Part of CNS stimulate inhibitory interneurons in dorsal horn- they produce endogenous opioid (decreases level of pain) which blocks the release of glutamate and substance P.