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Nervous System – Ch 7 Introduction Neurons – nerve cells Transmit information in the form of electrochemical changes (nerve impulses) Neuron structure Cell body Dendrites – numerous; receive messages Axons – send information; usually one per neuron Nerves – bundles of axons Neuroglial cells – supporting cells that provide neurons with physiological requirements and function Two groups of nervous system: Central nervous system (CNS) – brain and spinal cord Peripheral Nervous System (PNS) – nerves and other body parts General Functions of Nervous System Sensory Function Sensory receptors gather information by detecting changes inside and outside the body (light, sound, temp, O2 concentration) • Convert information into nerve impulses that are integrated, added to memory, or translate to sensations Integrative Function Conscious or subconscious decisions Motor Functions Use peripheral neurons to carry impulses to effectors (responsive structure) Two categories • Somatic nervous system – consciously controlled; skeletal muscle • Autonomic nervous System – involuntary; heart muscle Neuroglial Cells Neuroglia Functions: Fill spaces provide structural framework produce myelin carry on phagocytosis Types of Neuroglial Cells: Microglial cells: scattered through CNS; support neurons and phagocytize bacterial cells and cell debris Oligodendrocytes: occur in nerve fibers; provide myelin around axons in brain and spinal cord Astrocytes: found between neurons and blood vessels; provide structural support, join parts, regulate nutrient and ion concentration in tissues, form scar tissue Ependymal cells: form epithelial-like membrane to cover specialized brain parts (choroid plexuses) and form inner linings that enclose brain spaces (ventricles) and spinal cord (central canal) The PNS contains neuroglial called Schwann cells that form the myelin sheath around axons. Neuron Structure Vary in size and shape Structure: Neuron cell body Consist of cytoplasm, cell membrane, and organelles Chromatophilic substance (Nissl bodies) similar to rough ER also contain ribosomes Contain a nucleus and nucleolus Do not divide Dendrites Short, highly branched Main receptive surface Axons Arise from elevation of cell body (axonal hillock) to conduct nerve impulses away from the cell body Many mitochondria, microtubules, neurofibrils Single structure that may have many branches Large ones are covered by sheaths of Schwann cells (membrane covering). Membrane is composed of myelin (lipoprotein) Myelin sheath - higher proportion of lipids in cell membrane Neurilemmal sheath – surrounds myelin sheath; contains cytoplasm and nuclei Nodes of Ranvier – gaps between Schwann cells With myelin sheath – myelinated Also found in CNS Appear white – white matter Unmyelinated axons and neurons form the gray matter When damaged, the PNS regenerate due to neurilemma; the CNS do not because of lack of neurilemma Classifying Neurons Differ in size, structure, shape of cell bodies Vary in length and size of axons and dendrites Vary in function Some carry impulses to brain or spinal cord Some transmit impulses out of brain or spinal cord Some conduct impulses from neuron to neuron Bipolar Neurons Cell body has two processes, one from each end One is axon, one is dendrite Eyes, nose, ear neurons Unipolar Neurons Single process from body that branches into two that function as a single axon In Peripheral process – dendrite association near peripheral body parts Central process – enters the brain or spinal cord specialized masses called ganglia Multipolar Neurons Many processes from cell body One is an axon All the others are dendrites Most neurons in brain and spinal cord are multipolar Sensory Neurons Also called afferent neurons Carry nerve impulses from peripheral body to brain or spinal cord Have specialized receptor ends on dendrites OR dendrites are closely related with receptor cells in skin and sensory organs Changes stimulate receptors, triggering a nerve impulse Most are unipolar, some are bipolar Interneurons Also called association or internuncial neurons Lie within brain or spinal cord Multipolar and link other neurons Transmit impulses from one part of brain or spinal cord to another Direct incoming sensory impulses Motor Neurons Efferent Neurons Multipolar Carry nerve impulses out of the brain or spinal cord to effectors Stimulate muscles to contract and glands to release secretions Synapse Nerve impulses travel from neuron to neuron along pathways. The junction between communicating neurons is a synapse. Gap between neurons is synaptic cleft. Neurotransmitters Chemical that reacts with specific receptors to create a nerve impulse Acetylcholine – controls skeletal muscle actions Norepinephrine – “good” feeling, low levels may lead to depression Dopamine – “good” feeling Serotonin – sleep Histamine – alertness Endorphins – reduce pain Nitric oxide – vasodilation, memory pg. 218 chart of neurotransmitters Types of Nerves Nerves are cordlike bundles of nerve fibers held together by connective tissue layers that conduct impulses Sensory nerves – impulses into the brain or spinal cord Motor nerves – carry impulses to muscles or glands Mixed nerves – include both of the above Nerve Pathways Routes that nerve impulses follow in the nervous system Reflex arcs are the simplest pathways that constitute reflexes. Reflexes are subconscious responses to stimuli within or outside the body. Help maintain involuntary actions such as heart rate, breathing rate, blood pressure and digestion Knee-jerk reflex – simple, only using two neurons; helps maintain upright posture Withdrawal reflex – unexpected touch to something painful; protective by limiting tissue damage Meninges Membranes beneath the bony coverings of the skull and vertebral column for protection Three layers Dura mater – outermost layer of connective tissue, blood vessels, and nerves; interior periosteum of skull bones and between lobes of brain; surround spinal cord for protection. Arachnoid mater – thin membrane lacking blood vessels in the middle Pia Mater – thin layer of nerves and blood vessels that nourish the cells of the brain and spinal cord; hugs surfaces of organs Cerebrospinal fluid (CSF) – in between arachnoid and pia maters. Spinal Cord Nerve column passing from brain into vertebral canal Begins at the foramen magnum and terminates at the first and second lumbar vertebrae. Structure of Spinal Cord Consists of 31 segments that give rise to spinal nerves which branch to various body parts Cervical enlargement is the thickening of the spinal cord in the neck region that supplies nerves to upper limbs The lumbar enlargement is in the lower back giving nerves to lower limbs. Divided into right and left halves by the anterior median fissure and posterior median sulcus grooves. Functions of Spinal Cord Two major functions: conducting nerve impulses and center for spinal reflexes Axons of the spinal cord provide two way communication between the brain and the body parts Ascending tracts carry sensory information to the brain Descending tracts conduct motor impulses from the brain to the muscles Knee-jerk and withdrawal reflexes are spinal reflexes because they pass through the spinal cord