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2401 : Anatomy/Physiology Dr. Chris Doumen Week 5 Neural Tissue Introduction TextBook Readings ♦ Pages 388 through 397. ♦ Make use of the figures in your textbook ; a picture is worth a thousand words ! ♦ Work the Problems and Questions at the end of the Chapter The process of homeostasis makes sure that the processes that occur in the body are maintained within normal physiological limits. In addition, our body constantly reacts to a multitude of signals, be it external or internal signals. Two body systems are responsible for dealing with these signals and controlling the state of homeostasis • • Endocrine system : releases hormones into the blood stream, with a slow action response Nervous system : communicates by means of electrical signals, resulting in an immediate response The nervous functions: ♦ ♦ ♦ system has 3 overlapping Sensory : uses sensory receptors to detect internal and external changes (stimuli) Integrative: analyzes the sensory information, stores it and makes decision Motor : responds to stimuli by initiating muscular contractions or glandular secretions The branch of Neurology is the study of the normal and disordered nervous system Over view of the N.S. The nervous system can be divided into 2 principal parts : 1. ♦ ♦ The Central Nervous system (CNS) consists out of the Brain and Spinal Cord (integrating and command center ) it is connected to the sensory receptors, muscles and glands in the Peripheral Nervous system ♦ • Collin County Community College District 2. The Peripheral Nervous system (PNS) ♦ consists out of cranial ( arise from the brain) and spinal ( from the spinal cord) nerves ♦ they carry impulses in and out of the CNS the input component of the PNS ( from sensory receptors to the CNS) = sensory division (afferent nerves) • somatic afferent part : input from skin, skeletal muscle and joints • visceral afferent part: input from the organs and glands the output component ( from CNS to the muscles, glands) = motor division (efferent nerves) 2401 : Anatomy/Physiology Page 2 of 6 This motor division of the PNS has an additional two functional subdivisions A. The Somatic Nervous system • also called voluntary nervous system • composed of somatic motor nerve fibers that conduct impulses from CNS to skeletal muscles B. The Autonomic Nervous System • called involuntary nervous system • consist of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles and glands. Two branches exist in the ANS • Sympathetic : usually involves expenditure of body energy, stimulates • Parasympathetic : restores and conserves body energy, contra acts on the activities of the Sympathetic Nervous system . Histology of the Nervous System The tissue making up the nervous system contains highly packed cells. But notwithstanding the complexity of the nervous system, nervous tissue is made up of two principle types of cells • • Neurons : the excitable nerve cells that conduct the electrical signals Supporting cells or neuro-glial cells : support, nurture and protect the neurons A. Neurons • • • • • they relay the electrical signals can be short or extremely long functional contact between 2 neurons = synapse in mature individuals, neuron do not grow or repair themselves ( can't divide anymore) they have a long longevity and have an extremely high metabolic rate Page 3 of 6 1. Anat om y • Cell B ody or Soma • contains the nucleus, mitochondria, ER • Rough ER plus ribosomes = Nis sl bodies) ; the provide a gray texture to neuron bodies • Lacks centrioles ; thus neurons can not divide Dendrite s • short highly branched processes, always unmyelinated Ax on • long cylindrical projection with terminal bulbs (synap tic t e rminals ) containing the neurotransmitters • surrounded by an ax olem ma, contains ax oplasm a and neurofibrils, mitochondria, lysosomes • Area where axon connects with cell body is called the axon hilloc k • • Dendrites specialize in receiving the stimulus from other neurons or receptors to the cell body. Axons conduct the nerve impulse from the cell body to the cell body or dendrites of other neurons. Important Terminology • • • • A ne rve fiber is the terminology for a single axon. A ne rve is a bundle of nerve fibers running in the same path in the PNS, surrounded by connective tissue. Ne rve c ell bodi es clustered together in the PNS are called Ganglia Bundl e of nerve fi be rs in the CNS are termed Tracts. 2401 : Anatomy/Physiology 2401 : Anatomy/Physiology Page 4 of 6 The Synapse This is the special site where two neurons communicate with each other. The axon will in most cases ‘meet’ up with a dendrite from another neuron and a s ynp as e is formed. ♦ The neuron with the axon ( passing on the electrical information) is called the presynap tic neu ron • The neuron with the dendrite (receiving the electrical information) is referred to as the post- s ynaptic ne uron. At a synpase, no direct cell to cell contact occurs. Instead, a small space exists between the axon and the dendrite. This is called the synap tic cle ft. Axonal Transport • • • a mechanism that transports material up and down the axon, using microtubules, neurofillamants and actin fillaments the axon contains all cell organelles but no nucleus or Nissl bodies. Therefore, transport is needed for new protein and other material to be delivered to remote areas away from the cell body. 2 types of transport • "slow" axonal transport: goes only one way ( body to axon) • "fast" axonal transport : goes both ways and uses "molecular" motors ( ATP dependent "motor" proteins called kine sin ) Since there is no direct contact, the electrical information cannot be passed on directly. Instead, the information is now passed on via specific molecules called N eu rot ransmit te rs . Neurotransmitters are packed in s ynap tic ve sicles in the synaptic terminals of an axon. Specific receptors on the post synaptic membrane ( the membrane of the dendrite) bind these neurotansmitters and turn that signal back into an electrical signal. The details of these processes will become clear at the end of these discussions. Keep in mind that the presynaptic neuron is always a neuron. The postsynaptic entity receiving the information can be another neuron, or it can be a muscle ( the synapse area is then called a neu ro-m uscul ar ju nc tion) or it can be a gland ( with a neuro-glandular junction) 2401 : Anatomy/Physiology Page 5 of 6 Classification Struct ural • • • of Neurons Classific ation Multipolar neurons • Typical neuron displays numerous branching dendrites and a single axon. • Most common in human and major type in CNS Bipolar neurons • Have typically one dendrite and one axon • Rare in adult human, only found in special sense organs Unipolar neurons • Have a single process that emerges form the cell body, and divides T-like into a proximal and distal branch. • Collective length of both branches are considered to be an axon Func ti onal • • • • Afferent or sensory neurons: transmit nerve impulses from receptors to the brain, spinal cord. There are somatic sensory neurons and visceral sensory neurons Efferent or motor neurons: convey motor nerve impulses from CNS to effectors (muscles, glands). Somatic motor neurons activate your skeletal muscles and visceral motor neurons activate effectors other than the skeletal muscles. Association or inter-neurons : carry signals from one neuron to another Pre-synaptic and post-synaptic neurons. B. Neuroglial C ells There are 4 types of neuroglial cells that are only found i n th e CNS ! Epe ndymal Cells These are specialized epithelial cells that line the ventricles of the brain and the central canal within the spanl cord. In the ventricles these are ciliated. They are instrumental in the production of the cerebrospinal fluid and in circulating this fluid around. Olig ode ndroc yt es • • These cells are only found in the CNS They wrap themselves around axons and insulate them with the many wrappings of their cell membrane This is the myelin sh eath Ast rocyte s • These cells are only found in the CNS and have a variety of functions. Microglia • • • Maintain blood-brain barrier by wrapping around CNS capillaries. Repair damaged neural tissues Control the interstitial fluid environment (control electrolyte balance, absorb/recycle neurotransmitters,… ) • • • These cells are only found in the CNS They function like macrophages They are tiny and move along neural tissue removing debris and pathogens membrane 2401 : Anatomy/Physiology The other neuroglia are found Pe riphe ral Ne rvous S ys tem only in Page 6 of 6 the Schwann Cell or N eu rilemm oc yt es • These are similar to oligodendrocytes in that they wrap themselves around axons and form myelin sheats Amphic yt es • or Satellit e c ells These cells are similar in function as Astrocytes. They are specifically found in the Ganglia of the PNS.