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Unit IV: Coordination Reflex Arc Chapter 11 – pp 363-371 Chapter 12 – pp 413-421 Internal Coordination • Endocrine and nervous system maintain internal coordination – endocrine = – nervous = • Reflex Arc: 1. sense organs receive information 2. brain and spinal cord determine responses 3. brain and spinal cord issue commands to glands and muscles Subdivisions of Nervous System Central nervous system (CNS) Peripheral nervous system (PNS) Brain Two major anatomical subdivisions • Central nervous system (CNS) Spinal cord Nerves • Peripheral nervous system (PNS) – consists of nerves and ganglia Ganglia Neural Tissue A nerve is a bundle of nerve fibers (axons) wrapped in fibrous connective tissue. • emerge from the CNS to carry signals between organs • Neurons and neuroglial cells Functions of Neurons 1. Excitability (irritability) – stimuli 2. Conductivity – produce traveling electrical signals 3. Secretion – at end of nerve fiber, a neurotransmitter is secreted Structure of a Neuron • Soma (cell body) • Dendrites • Axon (nerve fiber) − trigger zone • Myelin Sheath – some nerve fibers – insulating layer (mostly lipid) – formed by neuroglia A Representative Neuron Functional Classes of Neurons Dendrites Structural Classes of Neurons Dendrites Dendritic process • Multipolar neuron Cell body Axon • Bipolar neuron • Unipolar neuron Synaptic terminals Dendrites Initial segment Axon Axon Synaptic terminals – Peripheral fiber carries impulses from source of sensation – Central fiber carriers impulses to spinal cord • Anaxonic neuron Axon Synaptic terminals Neuroglial Cells • Also known as Glial cells • Outnumber neurons • General functions: − Protect − Support, maintain physical structure of neural tissue − Repair − Maintain nutrient supply to neurons Types of Neuroglial Cells Occur in the Central Nervous System • Oligodendrocytes – form myelin sheaths in CNS • Ependymal cells – line cavities of brain and spinal cord – produce and circulate CSF • Microglia – macrophages (WBC) – Phagocytosis – in areas of infection, trauma or stroke Types of Neuroglial Cells • Astrocytes – most abundant glial cells – cover brain surfaces – formation of blood-brain barrier – regulate composition of cerebrospinal fluid – remove neurotransmitters and potassium ions from ICF – repairing damaged tissue with scar tissue Occur in the Peripheral Nervous System • Schwann cells – form myelin sheath in PNS • Satellite cells – surround somas of neurons in ganglia Neuroglial Cells of CNS Astrocytes Section of spinal cord Capillary Oligodendrocyte Ependymal cell Unmyelinated axon Myelin (cut) Microglial cell Neurons Gray matter Myelinated axons White matter Nodes Myelination Demyelination Hallmark of some neurodegenerative autoimmune diseases: • Multiple Sclerosis – Demyelination in CNS – Own immune system attacks and damages myelin – Scars form in white matter of CNS – Cause unknown, no cure • Cerebral Palsy – Damage to developing oligodendrocytes usually during infancy – Mutations, lack of oxygen, interruption of blood flow – Treatment of symptoms, no cure • Leukodystrophies – Results from defect in the gene that controls the production of only one component molecule of myelin – Affects growth and maintenance of white matter – Inherited, no cure Nerve Signal Depends on two factors: • Presence/absence of myelin • Diameter of fiber - large/thicker fibers have more surface area for signals • Functions – fast signals employed when speedy responses are needed – slow signals used when quick responses are not important Regeneration of Nerve Fiber in PNS Site of injury Step 1: the axon and myelin degenerate and fragment. Step 2: The Schwann cells proliferate and macrophages remove the debris distal to the injury site. Step 3: The axon grows along the path created by the Schwann cells. Step 4: As the axon elongates, the Schwann cells wrap around it. Axon Myelin Macrophage Regeneration Tube Reflexes Properties: – Require stimulation – Quick – Involuntary – Stereotyped Reflex Arc: Somatic receptors → afferent nerves → integrating center → efferent nerves → skeletal muscles The Stretch Reflex • When a muscle is stretched, it contracts and maintains increased muscle tone (stretch reflex) – helps maintain equilibrium and posture – balances tension in extensors and flexors – mediated by the brain • Reciprocal inhibition prevents muscles from working against each other • Very sudden muscle stretch causes tendon reflex – knee-jerk (patellar) reflex is monosynaptic reflex The Tendon Reflex To brain 6 Primary afferent neuron stimulates inhibitory interneuron 4 Primary afferent neuron stimulates alpha motor neuron to extensor muscle 7 Interneuron inhibits alpha motor neuron to flexor muscle 5 3 Primary afferent neuron excited Alpha motor neuron stimulates extensor muscle to contract 2 Muscle spindle stimulated 1 Extensor muscle stretched 8 Flexor muscle (antagonist) relaxes Flexor (Withdrawal) Reflexes • Occurs during withdrawal of a limb from pain • Contract flexor muscles • Polysynaptic reflex arc • Not just one sudden response as in stretch reflex • Ipsilateral reflex arc Crossed Extensor Reflexes • Maintains balance by extending other limb • Contract extensor muscles • Contralateral reflex arcs explained by pain at one foot causes muscle contraction in other leg Golgi Tendon Reflex • Golgi tendon organs in a tendon near its junction with a muscle • Excessive tension on tendon inhibits motor neuron – muscle contraction decreased • Also functions when some parts of muscle contracts more than others