The Nervous System Chapter 9 Nervous System • The master controlling and communicating system of the body • Functions: – Sensory input – monitoring stimuli – Integration – interpretation of sensory input – Motor output – response to stimuli Organization of the Nervous System • Central nervous system (CNS) – Brain & spinal cord – Integration and command center • Peripheral nervous system (PNS) – Paired spinal & cranial nerves – Carries messages to and from the spinal cord and brain Peripheral nervous system (PNS) • Two functional divisions: – Sensory (afferent) division • Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain • Visceral afferent fibers – transmit impulses from visceral organs to the brain; Ex) feeling full – Motor (efferent) division • Transmits impulses from the CNS to effector organs Motor Division: Two Main Parts • Somatic nervous system – Conscious control of skeletal muscles • Autonomic nervous system (ANS) – Regulates smooth muscle, cardiac muscle, and glands (unconscious) – Divisions: • Sympathetic – fight or flight system; increase in heart rate, blood pressure, blood sugar • Parasympathetic – housekeeping systems (resting/digestive); keeps functions going under normal conditions Histology of the Nerve Tissue • The two principle cell types of the nervous system are: – Neurons excitable cells that transmit electrical signals – Supporting cells cells that surround and wrap neurons (neuroglial or glial cells) • • • • Provide a supportive scaffolding for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth Neurons (Nerve Cells) • Lose their ability to divide • Very long lived • High metabolic rate – need astrocytes to get nutrients from capillaries • NEED OXYGEN • No centrioles • Dendrites and Cell bodies receive graded potentials Neuron Structure • Neurons vary considerably in size & shape, but they have certain features in common: – Cell body – Nerve fibers – Axon – Dendrites Nerve Cell Body • Contains the nucleus and nucleolus • The major biosynthetic center • Is the focal point for the outgrowth of neuronal processes • Has well-developed Nissl bodies (rough ER) • Contains an axon hillock – cone-shaped area from which axons arise Dendrites of Motor Neurons • Short, tapering, and diffusely branched processes • They are the receptive, or input, regions of the neuron • Electrical signals are conveyed as graded potentials (not action potentials) Axons: Structure • Slender processes of uniform diameter arising from the axonal hillock of the cell body • Long axons are called nerve fibers • Usually there is only one unbranched axon per neuron • Axonal terminal – branched terminus of an axon Axons: Function • Generate and transmit action potentials • Secrete neurotransmitters from the axonal terminals • Movement along axons occurs in two ways – Anterograde – toward the axonal terminal – Retrograde – away from the axonal terminal Myelin Sheath • Whitish, fatty (protein-lipoid), segmented sheath around most long axons • It functions to: – Protect the axon – Electrically insulate fibers from one another – Increase the speed of nerve impulse transmission Myelin Sheath & Neurilemma: Formation • Formed by Schwann cells in the PNS • A Schwann cell: – Envelopes an axon in a trough – Encloses the axon with its plasma membrane – Has concentric layers of membrane that make up the myelin sheath • Neurilemma – remaining nucleus and cytoplasm of a Schwann cell Nodes of Ranvier • Gaps in the myelin sheath between adjacent Schwann cells • They are the sites where axon collaterals can emerge Types of Neurons and Neuroglial Cells • Neuron Classification (structural): – Multipolar – three or more processes – Bipolar – two processes (axon & dendrite) – Unipolar – single, short process Types of Neurons & Neuroglial Cells • Neuron Classification (function): – Sensory (afferent) – transmit impulses toward the CNS (stimulus) – Motor (efferent) – carry impulses away from the CNS (reaction) – Interneurons (association neurons) – shuttle signals through CNS pathways Types of Neurons & Neuroglial Cells • Classification of Neuroglial Cells – Microglial cells – Oligodendrocytes – Astrocytes: – Ependymal cells Microglial Cells • Small, oval cells w/ spiny processes • Scattered throughout the CNS • Support neurons & phagocytize bacterial cells & cellular debris Oligodendrocytes • Occur in rows along nerve fibers • Form myelin within the brain & spinal cord • Insulation Astrocytes • Most abundant, versatile, & highly branched glial cells • Cling to neurons at the synaptic endings & cover capillaries • Functions: – Support & brace neurons – Anchor neurons to their nutrient supplies – Guide migration of younger neurons Ependymal Cells • Range in shape from squamous to columnar • Line the central cavities of the brain and spinal column • Make a barrier for spinal fluid Spinal Cord (structure) • Consists of 31 segments – each gives rise to a pair of spinal nerves • Cervical enlargement – in the neck region, supplies nerves to the upper limbs • Lumbar enlargement – in the lower back, supplies nerves to the lower limbs Spinal Cord (function) • Has 2 main functions: – Conducting nerve impulses – Serving as a center for spinal reflexes • Nerve tracts provide a two-way communication system between the brain and body parts outside the nervous system Spinal Cord (function) • Ascending tracts – carry sensory information to the brain • Descending tracts – conduct motor impulses from the brain to muscles and glands • Nerve fibers within ascending and descending tracts are axons