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Nervous System. Master control and communication system. Nervous System: Functions. Three overlapping functions Sensory receptors monitor changes inside and outside the body Change – a stimulus Gathered information – sensory input CNS Processes and interprets sensory input
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Nervous System • Master control and communication system
Nervous System: Functions • Three overlapping functions • Sensory receptors monitor changes inside and outside the body • Change – a stimulus • Gathered information – sensory input • CNS Processes and interprets sensory input • Makes decisions – integration • Dictates a response by activating effector organs • Response – motor output
Basic Divisions of the Nervous System: CNS • Central nervous system (CNS) • Brain and spinal cord • Integrating and command center
Basic Divisions of the Nervous System: PNS • Peripheral nervous system (PNS) • Outside the CNS • Nerves extending from brain and spinal cord • Cranial nerves • Spinal nerves • Link all regions of the body to the CNS
Sensory Input and Motor Output • Sensory signals picked up by sensory receptors • Carried by afferent nerve fibers of PNS to the CNS • Motor signals are carried away from the CNS • Carried by efferent nerve fibers of PNS to effectors • Innervate muscles and glands
Sensory Input and Motor Output • Divided according to region they serve • Somatic body region • Visceral body region • Results in four main subdivisions • Somatic sensory • Visceral sensory • Somatic motor • Visceral motor
Somatic Sensory • Somatic sensory • General somatic senses – receptors are widely spread • Touch, pain, vibration, pressure, and temperature • Proprioceptive senses – detect stretch in tendons and muscle • Body sense – position and movement of body in space • Special somatic senses • Hearing, balance, vision, and smell
Visceral Sensory • Visceral sensory • General visceral senses – stretch, pain, temperature, nausea, and hunger • Widely felt in digestive and urinary tracts, reproductive organs • Special visceral senses – taste
Somatic Motor • Somatic motor • General somatic motor – signals contraction of skeletal muscles • Under voluntary control • Often called “voluntary nervous system”
Visceral Motor • Visceral motor • Regulates the contraction of smooth and cardiac muscle and gland secretion • Makes up autonomic nervous system • Controls function of visceral organs • Often called “involuntary nervous system”
Peripheral Nervous System Summary Figure 12.3
Types of Sensory and Motor Information Figure 12.3
Types of Sensory and Motor Information Figure 12.3
Nervous Tissue • Cells are densely packed and intertwined • Two main cell types • Neurons – transmit electrical signals • Support cells (neuroglial cells) – nonexcitable • Surround and wrap neurons
The Neuron • The human body contains billions of neurons • Basic structural unit of the nervous system • Specialized cells conduct electrical impulses along the plasma membrane • Graded potentials • Action potentials
The Neuron: Special Characteristics • Longevity – can live and function for a lifetime • Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception • High metabolic rate – require abundant oxygen and glucose
The Cell Body or Soma (also called Perikaryon) • Size varies from 5–140µm • Contains nucleus, organelles plus other structures • Chromatophilic bodies (Nissl bodies) • Clusters of rough ER and free ribosomes • Stain darkly and renew membranes of the cell • Neurofibrils – bundles of intermediate filaments • Form a network between chromatophilic bodies
The Cell Body • Most neuronal cell bodies • Located within the CNS (clustered in nuclei) • Protected by bones of the skull and vertebral column • Ganglia – clusters of cell bodies in PNS
Cell Body Structure Figure 12.4
Neuron Processes: Dendrites • Dendrites • Extensively branching from the cell body • Transmit electrical signals (graded potentials) toward the cell body • Chromatophilic bodies – only extend into the basal part of dendrites • Function as receptive sites
Neuron Processes: Axons • Axons (nerve fibers) • Neuron has only one, but it can branch • Impulse generator and conductor • Transmits action potentials away from the cell body • Chromatophilic bodies absent • No protein synthesis in axon
Neuron Processes: Axons • Axons • Neurofilaments, actin microfilaments, and microtubules • Provide strength along length of axon • Aid in the transport of substances to and from the cell body • Axonal transport
Neuron Processes • Axons • Branches along length are infrequent • Axon collaterals • Multiple branches at end of axon • Terminal branches (telodendria) • End in knobs called axon terminals (also called end bulbs or boutons) Neuron Structure
Neuron Processes: Action Potentials • Nerve impulse (action potential) • Generated at the initial segment of the axon • Conducted along the axon • Releases neurotransmitters at axon terminals • Neurotransmitters – excite or inhibit neurons • Neuron receives and sends signals
Synapses • Site at which neurons communicate • Signals pass across synapse in one direction • Presynaptic neuron • Conducts signal toward a synapse • Postsynaptic neuron • Transmits electrical activity away from a synapse
Two Neurons Communicating at a Synapse Figure 12.6
Structure of a Synapses PLAY Synapse Figure 12.8a, b
Signals Carried by Neurons: Resting Membrane Potential • Plasma membranes of neurons conduct electrical signals • Resting neuron – membrane is polarized • Inner, cytoplasmic side is negatively charged
Changes in Membrane Potential • Signals occur as changes in membrane potential
Directional Signals • Stimulation of the neuron depolarization • Inhibition of the neuron hyperpolarization
Action Potentials Figure 12.9a, b
Action Potentials on Axons • Strong depolarizing stimulus applied to the axon hillock triggers • Action potential • Membrane becomes positive internally • Action potential travels the length of the axon • Membrane repolarizes itself
Action Potentials on Axons Figure 12.9c–e
Graded Potentials on Dendrites and the Cell Body • Natural stimuli applied to dendrites and the cell body • Receptive zone of the neuron • Membrane stimulation causes local depolarization • A graded potential – inner surface becomes less negative • Depolarization spreads from receptive zone to the axon hillock • Acts as the trigger that initiates an action potential in the axon
Synaptic Potentials • Excitatory synapses • Neurotransmitters alter the permeability of the postsynaptic membrane • Leads to an inflow of positive ions • Depolarizes the postsynaptic membrane • Drives the postsynaptic neuron toward impulse generation
Synaptic Potentials • Inhibitory synapses • The external surface of the postsynaptic membrane becomes more positive • Reduces the ability of the postsynaptic neuron to generate an action potential
Classification of Neurons • Structural Classification • Functional Classification
Structural Classification of Neurons Classification based on number of processes • Multipolar • Bipolar • Unipolar (pseudounipolar)
Multipolar Neurons • Possess more than two processes • Numerous dendrites and one axon Figure 12.10a–c
Bipolar Neurons • Possess two processes • Rare neurons – found in some special sensory organs Figure 12.10a–c
Unipolar (Pseudounipolar) Neurons • Possess one single process • Start as bipolar neurons during development Figure 12.10a–c
Afferent neurons • Afferent (sensory) neurons – transmit impulses toward the CNS • Virtually all are pseudounipolar neurons (some true bipolar) • Cell bodies in ganglia outside the CNS • Short, single process divides into • The central process – runs centrally into the CNS • The peripheral process – extends peripherally to the receptors
Afferent Neurons Periphery CNS Axon terminals Sensory receptors
Efferent Neurons • Efferent (motor) neurons • Carry impulses away from the CNS to effector organs • Most efferent neurons are multipolar • Cell bodies are within the CNS • Form junctions with effector cells
Interneurons • Interneurons (association neurons) – most are multipolar • Lie between afferent and efferent neurons • Confined to the CNS
Neurons Classified by Function Figure 12.11
Variety of Interneurons • Purkinje cell, stellate cell, granule cell, and basket cell • Located in the cerebellum • Pyramidal cell – located in the cerebral cortex