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The Function of the Nervous System: To allow an immediate response to stimuli in the environment The Cells of the Nervous System: A. Neurons (aka “Nerve cells”) -specially designed to transmit an electrical signal -Consist of 3 parts: 1. The Cell Body (which contains the nucleus) 2. The Dendrites (branches which carry the signal TOWARDS the cell body) 3. The Axons (branches which carry the signal AWAY from the cell body) -A “nerve” consists of one or more neurons -two neurons in a row, are separated by a “synapse” (a gap) -when the nerve signal gets to the end of the axon, it releases chemicals called “neurotransmitters” which cross the synapse, and initiate the signal in the dendrites of the next neuron -Often, axons will be wrapped in a “Myelin sheath” (fat) -Gaps in this sheath are known as “nodes of Ranvier” B. Neuroglia -“supporting” cells; support, insulate and protect neurons -“Schwann cells” = neuroglia which produce the myelin sheath The Impulse of a Neuron Neurons are said to carry an electrical impulse, which is unlike a wire carrying an electrical current To understand this impulse, we must focus on a small section of the neuron’s dendrite or axon: When this small section is at rest (not carrying an impulse), we find there is a charge difference inside vs. outside the membrane -this charge is slightly negative on the inside of the membrane, slightly positive outside of the membrane -this charge is called the “resting membrane potential” -it is caused by an uneven distribution of ions: -Outside the membrane there is a high concentration of Sodium (Na+) ions, and a low conc. of Potassium (K+) ions -Inside the membrane, there is a low conc. of Sodium (Na+) ions, and a high conc. of Potassium (K+) ions -Other ions are involved as well -Diffusion of these ions does not occur in or out of the membrane, as the membrane is impermeable to these ions The Neuron’s Action Potential As a result of a stimulus, the membrane of the section of the neuron becomes alternatively permeable to these ions -First, Sodium “gates” in the membrane open, allowing Na+ ions to enter the cell -As the Na+ ions enter, the section “depolarizes” -The Sodium gates then close -Next, Potassium gates open, allowing K+ ions to leave the cell -As the K+ ions exit, the section “repolarizes” -The Potassium gates then close -Finally, sodium-potassium pumps in the membrane pump sodium ions out, and potassium ions in, restoring concentrations This change in the membrane potential of this section of the neuron is known as an “Action Potential” -An action potential then occurs in the next section of the neuron -The sequence of action potentials appearing to “travel” down a neuron is known as the neuron’s “Impulse” The Effect of Myelination As a result of the myelin sheath around the neuron, the movement of ions (and therefore, action potentials) can NOT occur in these areas -The only places they can occur is in the Nodes of Ranvier -therefore, the impulse seems to “jump” from node to node -As a result, the impulse travels approx 10 times faster -This is known as “Saltatory” conduction of an impulse The Organization of the Nervous System 1. The Central Nervous System (CNS) A. The Brain B. The Spinal Cord 2. The Peripheral Nervous Sytem (PNS) A. The Sensory or Afferent Division -impulses lead from sense organs towards the CNS B. The Motor or Efferent Division -impulses lead away from the CNS towards “effectors” (muscles and/or glands) 1. Somatic Nervous System (voluntary) -controls the skeletal muscles 2. Autonomic Nervous System (involuntary) -controls the cardiac and smooth muscles a. Sympathetic system -typically, “speeds up” effectors -the “fight or flight” response b. Parasympathetic system -typically, “slows down” effectors The Reflexes Reflexes are rapid, predictable, involuntary responses to stimuli Reflexes may be somatic (deal with skeletal muscles) or autonomic (deal with smooth muscles, glands, or the heart) Reflexes are a result of the “reflex arc” 1. The sensory receptor 2. The sensory neurons 3. The integration center (either the brain OR the spinal cord) -determines what the stimulus “means” and what the response should be 4. The motor neurons 5. The effector organs (muscle or gland) The Central Nervous System Protection: 1. Bone 2. The Meninges (3 layers of connective tissue) -Cerebrospinal fluid acts as an intermediate between blood and the CNS (the blood-brain barrier) Structures of the Central Nervous System: The Brain The origin of 12 pairs of nerves, which are called the “cranial nerves”, as well as the spinal cord Consists of 4 major sections: The cerebrum, the diencephalon, the brain stem, and the cerebellum The Cerebrum: -Contains many gyri (outfoldings) and sulci (grooves) -Is the largest portion of the human brain -Is responsible for most memory, logic, emotional response, interpretation of sensation, and voluntary movement -Is divided into left and right cerebral hemispheres by the longitudinal fissure -Each cerebral hemisphere is divided into lobes: The Frontal lobe and the Parietal lobe (both deal with speech, memory, sensory, and motor functions), the Temporal lobe (hearing and olfaction functions) and the Occipital lobe (visual functions) -The frontal lobe contains a major portion of the “limbic system” which -The two cerebral hemispheres are connected (and communicate) as a result of the corpus callosum The Diencephalon: -Sits atop of the brain stem -allows the brain stem to communicate with the cerebrum -Consists of the thalamus, the hypothalamus, and the pituitary gland, and the pineal gland -The the hypothalamus helps to regulate autonomic functions -The thalamus screens out “unimportant information” to the cerebrum The Cerebellum: -Coordinates the contractions of skeletal muscles’ movement which was initiated by the cerebrum The Brain Stem: -Consists of the midbrain, the pons, and the medulla oblongata -The pons controls breathing -The medulla oblongata controls heart rate, blood pressure, breathing rate, swallowing, and other involuntary functions The Spinal Cord -Acts as the connection between the brain and most of the PNS -Stretches from the brain down to the L1 vertebra -Inferior to the L1 vertebra, it degrades into a group of individual nerves called the “cauda equina” -The origin of 31 pairs of nerves, called “spinal nerves”, which exit between the vertebrae -Consists of sensory (afferent) nerve tracts, motor (efferent) nerve tracts, as well as integration sites for spinal reflexes The Special Senses Vision: The result of light causing photoreceptors to produce action potentials These action potentials are then sent to the brain via the Optic nerve Structures associated with Vision: The photoreceptors: two types – Rods which detect low intensity light in degrees of “black and white”, and Cones which detect color, but only in high intensity light The Retina: The back wall of the eyeball, which contains all the photoreceptors The fovea centralis: The portion of the retina that contains primarily cones; outside the fovea, there are mainly rods The optic disc: the portion of the retina where the optic nerve attaches; there are no rods or cones located here To get a focused image of the proper light intensity to fall upon the retina, the following structures of the eyeball are required: The cornea: Bends (refracts) light The pupil: Allows light into the eyeball The Iris: The colored portion that regulates the size of the pupil The lens: refracts light, but can muscles can change its shape to focus the image (accommodation) Hearing The result of liquid pushing upon mechanoreceptors, producing action potentials that are sent to the brain via the Vestibulocochlear nerve Structures associated with Hearing: The Pinna: collects the sound The Tympanic membrane: vibrates in response to sound The Pharyngotympanic (auditory) tube: opens up into the throat, allowing equal air pressure on both sides of the tympanic membrane The Ossicles (the malleus, the incus, and the stapes): vibrate in response to the tympanic membrane vibrating The stapes pushes on the oval window of the cochlea, which causes waves of the liquid “endolymph” within the cochlea These waves of endolymph push upon a membrane (the tectorial membrane) that pushes down on the mechanoreceptors Chemical Senses: Taste and Smell The result of chemicals binding to chemoreceptors, producing action potentials that are sent to the brain Olfaction (smell): Caused by olfactory receptors in the nasal cavity The impulses travel to the brain via the olfactory nerve In the brain, the olfactory pathways are integrated with the limbic system, thus explaining the strong emotional response to smell Gustation (taste): Caused by groups of chemoreceptors called taste buds These taste buds are actually located within little hills (papillae) located upon the tongue When stimulated, the impulses are sent to the brain via the 3 cranial nerves Taste, is a function of 5 different chemoreceptors, which detect the following taste sensations: sweet, sour, bitter, salty, and “Umami” (delicious – as in steak)