Nervous System Introduction center consciousness self controls skeletal muscle emotions form images storing memories problem solving creative

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1 Nervous System Introduction 1. The Nervous System is highly complex. It is the seat of our conscious experience, personality, and behavior. Since the brain is the center of consciousness it gives us our sense of self. 2. brain controls skeletal muscle contractions, emotions, the ability to form images, etc. 3. the brain is capable of storing memories and of complex thought to include problem solving and creative thinking 4. The brain helps to maintain homeostasis (i.e., maintaining a stable internal environment). The cells of the body and the organs they make up must work together in a coordinated fashion. The two organ systems dedicated to homeostasis: endocrine and nervous system. 5. The nervous system uses electrical (nerve impulses) and chemical means (neurotransmitters) to allow cells to rapidly send messages to each other. Organization of the Nervous System 1. Only one NS that is divided into logical parts with the major divisions: CNS and PNS 2. Central Nervous System (CNS) a. brain and spinal cord b. command center of the NS 3. Peripheral Nervous System (PNS) a. found outside of the CNS

2 b. consists of cranial and spinal nerves, and all of their branches, ganglia, motor end plates, and sensory receptors c. 2 Divisions 1. sensory (afferent) division of the PNS - neurons that carry impulses from sensory receptors in the periphery to the CNS (e.g., pain, touch, taste) a. somatic sensory impulses from receptors in the skin, skeletal muscles, bones, and joints b. visceral sensory impulses from internal organs mainly in the thoracic and abdominal cavities (e.g., heart, lungs, kidneys, blood vessels, intestine) 2. motor (efferent) division of PNS a. somatic motor (voluntary) somatic motor neurons that conduct impulses from CNS to skeletal muscles, and allow conscious control of skeletal muscle b. autonomic (involuntary or visceral) motor visceral motor neurons that conduct impulses from CNS to smooth muscle, cardiac muscle, and glands Histology of Nervous System 1. Two Types of NS cells: neurons and neuroglial cells 2. neurons (structure of typical neuron) a. basic characteristics 1. ability to conduct nerve impulses 2. longevity a. function from birth until death (neurons can function optimally for over 100 yrs) 2

3 b. neurons in brain continue to form throughout early childhood and they may continue to form in adults from stem cells 3. largely amitotic a. most mature neurons are non-mitotic b. most neurons lack the centrosome complex which is needed for the formation of the mitotic spindle. Since neurons usually lose their centrosome complex during differentiation, they cannot divide. If neurons are lost during disease or injury, they cannot be replaced. c. brain damage is essentially irreversible (permanent loss of function since repair is minimal). Deficits: cognitive, motor, and sensory d. pockets of stem cells in the NS that can form new neurons 1. stem cells within the brain and heart can regenerate new brain and heart cells under proper conditions; even in old age there are unspecialized stem cells in the CNS that can divide and develop into new neurons 2. olfactory neurons and some hippocampal regions contain stem cells that can produce new neurons throughout life by neurogenesis. 3. new neurons that form in the adult come from stem cells and not from the mitotic divisions of mature neurons. 4. In humans, new neurons continually form throughout adulthood in the dentate gyrus of the hippocampus and in the striatum. Adult-born neurons are interneurons and do not project to other areas of the brain. About 700 new neurons form in the hippocampus each day. 4. high metabolic rate (lot of chemical activity in cells) a. require a lot of oxygen and glucose to make ATP (neurons die within a few minutes from lack of oxygen) b. many ATP-dependent reactions occurring in neurons c. large number of mitochondria (100 s per neuron) b. cell body (soma) of multipolar neuron (typical neuron) 1. site where nucleus is located; contains neuroplasm 2. axon hillock portion of cell body of a multipolar neuron from which the axon arises a. the axon hillock is always the point at which the axon attaches to the cell body b. summation of IPSPs and EPSPs occurs at the axon hillock to generate the first action potential which self-propagates as a nerve impulse down the axon. c. the first action potentials associated with a nerve impulse occur at the axon hillock since it contains a rather dense concentration of ion channels with voltage-sensitive gates d. although the nerve impulse begins at the axon hillock of the cell body where the first action potentials occur, only the axon can generate a nerve impulse c. neuron processes (extend from cell body) 1. dendrites ( little branches ) some neurons have up to 400,000 dendrites a. short, branched (lacey-like), and numerous (1000 s); unmyelinated b. make up the dendritic zone; numerous dendrites increase the total surface area for receiving impulses from other neurons c. part of neuron that is stimulated by other neurons d. receptive end of neuron, because contains receptors on dendrites and cell body for neurotransmitters from other neurons 2. axon (nerve fiber) a. arises from the axon hillock of the cell body 3

4 b. only one axon per neuron although it may contain branches called collaterols (most axons don t have collaterols) and terminal branches (may be numerous; 10,000 or more per axon) at their ends; terminal branches synapse (form a junction) with other neurons and effector cells; terminal branches are also called telodendria c. long process that conducts impulse towards the axon terminals (bouton or synaptic knob) at the end of each terminal branch d. nerve impulse originates on the axon hillock, then self-propagates down the axon e. axoplasm cytoplasm inside axon f. axolemma plasma membrane g. axon terminal end of axon where neurotransmitter-filled vesicles are found f. myelin sheath (fatty covering over most axons) made by Schwann cells (PNS) and oligodendrocytes (CNS) 1. Function a. protect and insulate the axon b. increase the speed of conduction 1. unmyelinated 5 mph 2. myelinated mph (100m/sec) 50x faster than unmyelinated 2. Neurolemmocyte or Schwann cells (type of neuroglial cell) a. form the myelin sheaths of axons found in the PNS b. about 1mm in length c. several 100 Schwann cells are needed to myelinate an axon d. neurilemma peripheral or outermost part of a Schwann cell; the neurilemma is a sheath that envelopes axons; it is discontinuous as a result of the nodes of Ranvier (the outer part of an oligodendrocyte is not called the neurilemma; this term is only applied to Schwann cells) 3. nodes of Ranvier (myelin sheath gaps) a. gaps between adjacent Schwann cells; voltage-gated ion channels (Na+, K+) concentrated at nodes b. also between oligodendrocytes, but they are fewer in number c. nodes are about 1 mm apart 4

5 4. Even unmyelinated fibers have a thin Schwann cell wrapped loosely around them, but it is only a single wrapping that overlaps rather than multiple layers. The single layer of Schwann cell on an unmyelinated axon is called the neurilemma. Thus, unmyelinated fibers have a neurilemma but do not possess a myelin sheath. 5. oligodendrocytes (type of neuroglial cell) a. surround the axons of CNS neurons to form their myelin sheaths b. one oligodendrocyte may form portions of the myelin sheaths of several CNS axons (in fact, it can be up to 60 axons at the same time) c. nodes of Ranvier occur but they are more widely spaced than in the PNS 6. Schwann cells and oligodendrocytes as is the case with neuroglial cells can divide by mitosis 3. Neuroglia, or glial cells, are closely associated with neurons, providing a protective and supportive network; there are tens of billions of neurons in the nervous system and they are outnumbered 10:1 by neuroglial cells. There are 6 types of glial cells (4 in CNS and 2 in PNS). Glial cells make up about 50% of brain s total mass. a. CNS glial cells (only found in the CNS): astrocytes, microglia, ependymal, and oligodendrocytes b. Astrocytes ( star cells) 1. most abundant glial cells of the CNS; they form a supportive framework for neurons 2. pass nutrients (e.g., glucose) from the blood to neurons 3. help to take up free K + ions in the fluids around neurons so they do not accumulate and interfere with nerve conduction (i.e., help maintain extracellular ion balance) 5

6 4. important to the formation of the blood-brain barrier (BBB) a. formed by endothelial cells of brain capillaries that are joined by tight junctions and the cells are surrounded by astrocytes. b. Brain capillaries are less permeable than capillary beds elsewhere and this creates a barrier across which certain pathogens like bacteria and chemicals such as neurotoxins (e.g., botulin) cannot cross. c. The blood-brain barrier makes it difficult to deliver drugs to the brain. 5. Although the feet of the astrocytes contribute to the blood-brain barrier, their major role is to stimulate capillary endothelial cells to form and maintain tight junctions with one another c. Microglia (20% of all glial cells) 1. small macrophages that develop from white blood cells that leave blood vessels and wander within the CNS. 2. phagocytize bacteria, dead neurons and glial cells d. Ependymal cells 1. are glial cells of the CNS that line the cavities of the brain and spinal cord 2. they wiggle their cilia to help circulate cerebrospinal fluid. 3. fluids leaking through ependymal cells become cerebrospinal fluid; ependymal cells cover the capillaries of the choroid plexi e. Oligodendrocytes are glial cells of the CNS that wrap around neuron fibers, forming myelin sheaths. f. Peripheral NS glial cells 1. Schwann cells, or neurolemmocytes, are glial cells of the PNS that surround axons, forming the myelin sheath. 2. Satellite cells surround neuronal cell bodies in PNS ganglia. Little is known about their function. g. Brain tumors: most adult brain tumors are composed of cancerous glial cells which are mitotically active throughout life. These tumors are called gliomas. Classification of Neurons 1. Structure (number of processes extending from cell body) a. multipolar (most common, 99% of all neurons) 1. many dendrites; one axon 2. account for most neurons in the brain and spinal cord and PNS motor neurons (somatic and autonomic) that travel out to muscles and glands b. bipolar (only sensory) 1. two short processes out of cell body, an axon and a dendrite 2. found only in special sense organs: cochlea of inner ear (hearing), retina of the eye (vision), and olfactory cells in nasal mucosa (smell) c. unipolar (only sensory) most sensory neurons 1. one short process comes off cell body and branches to form a long axon (peripheral process goes to receptor and central process goes to CNS) 2. dendrite is where the impulse begins at one end of axon (some authors call this dendrite a receptive ending rather than a dendrite) 2. Function (based on direction nerve impulse travels with respect to the CNS) a. sensory (afferent) neurons 1. conduct sensory impulses from PNS receptors to CNS (brain and spinal cord) 2. carry information concerning the sense of touch, pain, blood pressure, and temperature 6

7 3. most are unipolar, although some are bipolar b. interneurons 1. neurons found in the CNS; make up most of the brain and spinal cord 2. all parts of the neuron are inside the CNS 3. interneurons are associated with brain functions like thoughts, problem-solving, creativity, control of the heartbeat, emotions, and the stimulation of skeletal muscle (control and integration of body functions) 4. all interneurons are multipolar c. Motor (efferent) neurons 1. conduct impulses from the CNS to effector cells 2. stimulate muscles (skeletal, smooth, cardiac) and glandular cells that secrete chemicals (e.g., salivary gland) 3. motor neurons are multipolar Structure of Nerves and Tracts 1. Nerve a. found in PNS b. bundle of axons in the PNS (similar to electrical wires) c. surrounded by a CT sheath and contain blood vessels 7

8 d. CT covering (structure similar to skeletal muscle) 1. epineurium covers the whole nerve; dense regular CT 2. perineurium covers axons bundled into fascicles; dense irregular CT 3. endoneurium covers individual axons; loose CT 2. Tract a. found in CNS b. bundle of axons in the CNS c. surrounded by a CT sheath (similar to nerve) Sensory Receptors 1. sensory receptors a. structures in periphery that respond to 1. sensations (stimuli that we perceive) like touch, pain, temperature change 2. internal changes in the body like blood pressure, CO2 levels, ph b. sensory receptors convert the energy of a stimulus (e.g., touch, blood pressure change, heat, pain) to a nerve impulse. When one form of energy is converted to another this is called transduction c. always associated with the dendrites of sensory neurons d. sometimes the dendrites are associated with other cells such as connective tissue 2. most sensory neurons are unipolar, although some are bipolar 3. receptors respond to changes in the body to provide information about the change to the brain (e.g., temperature change on the skin, pain as a result of tissue damage) 4. Overview of Pathway a. dendrite of a unipolar sensory neuron responds to a stimulus 8

9 b. stimulation of the dendrite triggers a nerve impulse that spreads along the neuron s axon and into spinal cord to stimulate an interneuron c. the interneuron then conducts the impulse along its axon to the brain for processing 5. Modality refers to the type of stimulus or the sensation it produces. Vision, pain, and taste are examples of sensory modalities. A modality is a type of physical phenomenon that one can sense such as sight, hearing, pain, touch, vibrations, heat and cold. Sensory Receptors: General Types 1. Exteroceptors on the body surface 2. Interoceptors within the body 9

10 Specific Types of Sensory Receptors 1. Nociceptors (pain; noci = pain) unencapsulated (not covered by CT) a. found within the dermis and epidermis of the skin and within internal organs (brain tissue lacks pain receptors, but they are found in the walls of blood vessels within the brain) b. respond to potentially damaging stimuli that might result in tissue damage 1. thermal nociceptors: burning heat and extreme cold 2. mechanical nociceptors: excessive pressure 3. chemical nociceptors a. inflammatory chemicals (histamine, bradykinin, various acids) released by dead and dying cells at the site of an injury b. also respond to strong chemicals like capsaicin giving a burning sensation (found in hot chili peppers and mace or pepper spray) b. respond mostly to tissue damage 1. dead and dying cells release chemicals (e.g., bradykinin) that stimulate nociceptors 2. also respond to most vigorous stimuli to include intense cold and heat and strong pressure against the skin 3. the sense of pain is important so that we are aware of tissue damage. In the absence of pain people are not aware of minor injuries to the point that they don t take care of them. This can lead to infection or a worsening of the injury (e.g., athletes and others will continue to work through the pain and if not for the pain would stress their bodies even more) 10

11 c. structure: free dendrite of a unipolar sensory neuron; it is unencapsulated by connective tissue d. Bradykinin 1. chemical released by cells as they die that stimulate nociceptors and trigger pain 2. potent stimulator of pain endings; if inject a small quantity under the skin it will induce intense pain in that area 3. other pain-stimulating chemicals: acids that release excess H +, histamines, prostaglandins 4. bradykinin is also involved in BP regulation as a vasodilator e. Pain Pathway (3-neuron pathway to pain centers in the brain) 1. strike thumb with a hammer 2. tissue damage results in dead and dying cells that release pain-stimulating chemicals (e.g., bradykinin) that bind to receptors on the dendritic zone of the unipolar sensory neuron to trigger an impluse 3. dendrite at end of unipolar sensory neuron associated with the modality of pain sends impulses along its axon; this neuron is referred to as a 1 st order neuron 4. the axon enters a peripheral nerve 5. impulse travels along axon into the spinal cord 6. within the spinal cord the impulse stimulates an interneuron (2 nd order neuron) whose axon decussates or crosses over to the other side of the CNS 7. the axon of the interneuron travels up the spinal cord in the spinothalamic tract (two other tracts carry pain, but this is the most important one) 8. the interneuron stimulates another interneuron (3 rd order neuron) in the thalamus of the brain (within the diencephalon) 9. the 3 rd order neuron takes the info to two areas of the brain found in the cerebral cortex a. pain interpretation center in the somatosensory association area 1. part of the somatosensory association area of the parietal lobe of the cerebrum 2. provides the sensation of pain ( ouch ); one feels pain in the brain, but not the body part b. primary somatosensory cortex 1. on the post-central gyrus of the parietal lobe 2. contains a map of the body that allows the cerebrum to localize the source of the pain by projection (neurons that make up this area form a map of the body) 10. Effects of Advil (ibuprofen) and Aspirin a. tissue injury damages cell membranes. b. A fatty acid named arachidonic acid escapes from injured cells. It is acted on by an enzyme called cyclo-oxygenase and converted to prostaglandins which stimulate nociceptors. c. Ibuprofin (Advil, Motrin) and aspirin decrease the activity of the enzyme, thus decrease the production of prostaglandins and decrease the pain associated with tissue damage. f. Phantom Pain 1. feelings of pain from an amputated body part (also itching and tingling sensations) 2. after amputation, the severed axons of sensory neurons associated with pain continue to fire for a day or so until the wound heals over 3. Result: brain projects source of pain to body part that no longer exists 4. Projection: sensory areas of the brain determine the source of the stimulus 5. patient with an amputated foot feels pain in toes that are missing g. Referred Pain 1. pain from an injured organ causes the feeling of pain in uninjured organs 11

12 2. angina pectoris a. pain from a damaged heart is referred to left side of chest, left shoulder, and inside of left arm b. may be an indicator of atherosclerosis (blockage) in arteries that serve the heart c. blocked arteries can result in tissue damage downstream which results in the release of pain-stimulating chemicals 3. referred pain is due to convergence of neural pathways a. axons (unmyelinated, thus not insulated) carrying pain from the heart travel with axons that carry pain from the skin of the upper left chest, left shoulder, and the inner part of the left arm b. impulses within axons carrying pain from the heart and the skin converge on the same 2 nd -order interneurons in the spinal cord. The brain can t distinguish the exact source of the pain when it comes from the heart because it feels like it is coming from the heart and the skin areas. Since there are more pain receptors in the skin as compared to the heart, the greatest sense of pain from heart damage can come from skin areas on the upper left side of the body even though it may be completely uninjured. Pain and Temperature Pathways 12

13 2. Touch and Pressure Receptors (types of mechanoceptor) a. Tactile corpsucles (Meissner s Corpuscles) touch receptors 1. thin CT capsule around the spiraled dendritic end of a unipolar sensory neuron (encapsulated) 2. found in the papillary layer of the dermis of hairless skin (palms, soles, and lips) 3. Meissner s corpuscles are limited to hairless regions of the body and are common on the lips, palms, fingertips, nipples, glans of the penis, and soles of feet 4. sensitive to very light touch; help to determine textures (smooth like silk versus grainy like sandpaper) b. Lamellar corpuscles (Pacinian corpuscles) pressure receptors 1. thick, multilayered CT capsule around the dendrite of a sensory neuron (encapsulated) 2. primarily located deep into the reticular layer of the dermis 3 stimulated by heavy pressure to the skin 3. Chemoceptors (unencaspulated) a. respond to certain chemical changes in the body b. smell, taste, ph changes, CO 2 levels in the blood 4. Photoceptors found in the retina of eye and respond to light; unencapsulated 5. Thermoceptors respond to temperature changes (hot or cold); unencapsulated 6. Proprioceptors (special type of stretch receptor)(encapsulated) a. found in tendons, joint or articular capsules, and skeletal muscles b. detect the contraction of muscles c. function to supply motor centers in the cerebrum of the brain with information about the 1. strength of contraction which generates tension in muscles and tendons 2. direction that a body part is moving (determine which muscles are contracting) d. Golgi Tendon Organs stimulated by the stretching of a tendon when its muscle contracts 1. found inside tendons 2. respond when a tendon stretches as its muscle contracts e. muscle spindle stimulated by the stretching of a muscle when an antagonistic muscle contracts 1. found inside skeletal muscles 2. respond to the stretching of a muscle as the antagonistic muscle contracts; slight stretching of a muscle stimulates sensory receptors in muscle called muscle spindles; the muscle spindles monitor changes in the length of the muscle; during forearm flexion, muscle spindles in the tricep are activated as the biceps contracts. 3. e.g., muscle spindles in the bicep fire as the bicep stretches when the tricep contracts 4. dendrites spiral around intrafusal muscle fibers within the belly of the muscle (contractile muscle fibers are called extrafusal fibers); intrafusal fibers are enclosed within a CT capsule 7. Merkel (tactile) disks (unencapsulated) a. free nerve endings associated with disk-shaped epidermal cells (Merkel cells) b. function as light touch receptors 8. Hair follicle receptors (unencapsulated) a. free nerve endings that wrap around hair follicles b. light touch receptors that are stimulated when hairs move 13

14 Muscle spindle and Golgi Tendon Organ 7. Itch receptors a. stimulated by chemicals released at the site of tissue damage (e.g., histamines) b. hydrocortisone cream can reduce the itch Sensory Adaptation 1. repeated stimulation to most sensory receptors causes them to decrease their firing rate which decrease the strength of the sensation 2. receptors that adapt include smell (perfume fades over time), touch (don t feel a shirt against the skin after a while), and temperature (body adapts to a hot tub) 3. pain receptors do not adapt over time, thus one feels pain until the tissue damage is repaired. Since nociceptors don t adapt, one needs to take pain killers to relieve the pain 14