CHP 9 NERVOUS TISSUE CHP 11 AUTONOMIC NERVOUS

CHP 9: NERVOUS TISSUE CHP 11: AUTONOMIC NERVOUS SYSTEM CHP 12: SOMATIC SENSES AND SPECIAL SENSES

Chapter 9: Nervous Tissue Learning Objectives � � � Describe the organization of the nervous system. Explain the three basic functions of the nervous system. Contrast the histological characteristics and the functions of neuroglia and neurons. Distinguish between gray matter and white matter. Describe how a nerve impulse is generated and conducted. Explain the events of synaptic transmission and the type of neurotransmitters used.

Structures of the Nervous System

Organization of the Nervous System

SUBDIVISIONS OF PNS Somatic Nervous System (SNS) � Sensory neurons conveys information from somatic receptors in head, body wall, and limbs, and from receptors for special senses of vision, hearing, taste, and smell to CNS � Motor neurons conduct impulses from CNS to skeletal muscles only; can be consciously controlled, voluntary actions Autonomic nervous system (ANS) � Sensory neurons convey information from autonomic sensory receptors, located in visceral organs (stomach, lungs) to CNS � Motor neurons conduct nerve impulses from CNS to smooth muscles, cardiac muscles and glands; cannot be consciously controlled, involuntary actions � Two divisions of ANS are sympathetic division and parasympathetic division; these divisions usually perform opposite functions � “Fight-or-flight” responses emergency actions (sympathetic) � “Rest-and-digest” activities GI tract activities (parasympathetic)

Enteric Nervous System (ENS) � � nn. Get a copy of checkpoint questions 1 -3, complete them and hand them in… b Get a copy of checkpoint questions 1 -3, complete them and hand them in…

Functions of the Nervous System � Sensory functions _ (AFFECTORS) Sensory receptors_ detect stimuli inside and outside the body. - Sensory or afferent neurons carry information from cranial to spinal nerves into brain and spinal cord or visa versa � Integrative functions _(LIKE A CONTROL CENTER) Process sensory information by analyzing and storing some of it and by making decisions for appropriate responses - Interneurons; have short axons that connect with neurons in brain, spinal cord, and ganglion; are majority neurons in the body

Functions…continued � Motor functions _(EFFECTORS) Respond the integrative decisions - Motor of efferent neurons carry information from brain toward spinal cord or out of brain to spinal cord into cranial or spinal nerves

HISTOLOGY OF NERVOUS TISSUE � Two types of cells � Neurons unique functions of the nervous system; sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions � Neuroglia support, nourish, and protect neurons and maintain homeostasis in the intestinal fluid that baths neurons

Neurons � Three parts: � Cell Body nucleus surrounded by cytoplasm; includes RER, lysosomes, mitochondria, Golgi synthesizes cellular molecules needed for a neuron’s operation Processes or extensions: � Dendrites (“little trees”) multiple per single axon combined with cell body receiving and input parts of a neuron �short, tapering, and highly branched, tree-branch array emerging from cell body � Axons_ conducts nerve impulses toward another neuron, muscle fiber, or gland cell �long, thin, cylindrical projection that joins cell body at a cone-shaped elevation

Classification of Neurons � STRUCTURAL CLASSIFICATION � Multipolar neurons: have several dendrites and one axon; most in brain and spinal cord � Bipolar neurons: have one main dendrite and one axon; retina of the eye, inner ear, olfactory area of brain � Unipolar neurons: dendrites and one axon fused together forming a continuous process that emerges from cell body; begin in embryo as bipolar neurons; most function as sensory receptors for touch, pressure, pain, or thermal stimuli. Cell bodies of most of this type located in ganglia of spinal and cranial nerves.

Classification of Neurons � FUNCTIONAL CLASSIFICATION � Sensory or afferent neurons: once sensory receptor activated, these form an action potential in their axon that is conveyed into the CNS through spinal and cranial nerves contain sensory receptors at their distal ends or are located just after sensory receptors that are separate cells; most unipolar in structure � Motor or efferent neurons: convey action potential away from CNS to effectors (muscles and glands) in PNS through cranial and spinal nerves Most are multipolar in structure � Interneurons or association neurons: integrate incoming sensory information from sensory neurons and then elicit a motor response by activating appropriate motor neurons Located within CNS between sensory and motor neurons; most

Neuroglia � � Smaller than neurons 5 -50 times more numerous � ”glue” that holds nervous tissue together � do not generate or conduct nerve impulses � can multiply and divide in mature nervous system � in case of injury or disease multiply to fill in spaces formerly occupied by neurons � Gliomas: brain tumors derived from glia called gliomas; very malignant and grow rapidly

Myelination Myelin sheath, many-layered covering composed of lipids and protein, surround the axons of most of our neurons. � Two Functions: (1) insulates the axon (2) increases the speed of nerve impulse conduction The amount of myelin increases from birth to maturity �

Collections of Nervous Tissue Clusters of Neuronal Cell Bodies � Ganglion: cluster of neuronal cell bodies located in PNS � Nucleus: cluster of neuronal cell bodies in CNS Bundles of Axons � Nerve: bundle of axons located in PNS; cranial nerves connect brain to periphery and spinal nerves connect spinal cord to periphery � Tract: bundle of axons located in CNS; tracts interconnect neurons in spinal cord and brain

Gray and White Matter � � White matter: myelinated and unmyelinated axons of many neurons which is white in color; also has blood vessels Gray matter: neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia white is grayish pink in color; also has blood vessels

ACTION POTENTIALS � � AKA nerve impulses Two features of plasma membrane needed for action potentials in muscle fibers and in neurons � � � Membrane potential difference in the amount of electrical charge inside and outside plasma membrane. � � existence of resting membrane potential presence of specific types of ion channels membrane that has potential is polarized Resting membrane potential voltage difference between the inside and outside of a plasma membrane when not responding to a stimulus, in muscle fibers and neurons � voltage created by flow of ions

Ion Channels � Two types of ion channels: � Leakage channels allow small but steady stream of ions to leak across the membrane � Gated channels open and close on command Voltage-gated channels are used to generate and conduct action potentials; open in response to a change in membrane potential

Resting Membrane Potential

Generation of Action Potential � � Action potential (AP) or impulse generates rapidly occurring events that decrease and increase the membrane potential and eventually restore it to its resting state Ability of muscle fibers and neurons to convert stimuli into action potential is called electrical excitability. Stimulus in cell’s environment changes resting membrane potential; if stimulus causes cell to depolarize to a critical level; called a threshold (about -55 m. V) then an action potential arises Two main phases: � Depolarizing phase- rapidly occurring events that decrease and eventually reverse polarization of membrane, makes inside more positive than outside; Na+ ions move into cell � Repolarizing phase- membrane polarization is restored to resting state; Na+ ions move back out cell restoring charge to original state

All-or-None Principle

Depolarizing phase Reversal of polarization Threshold Stimulus 6 After-hyperpolarizing phase 7. Resting membrane potential

Conduction of Nerve Impulses � Also called propagation � Way cells communicate information from one part of body to another � Nerve impulses travel from where they arise, usually axon hillock, along axon to axon terminal � Positive feedback process

Continuous & Saltatory Conduction � Continuous conduction � Step-by-step process; impulses travel a short distance in 10 milliseconds � Occurs in unmyelinated axons (muscle fibers) Have smallest diameter � Saltatory conduction � Impulses leap from one node of Ranvier to the next � Occurs in myelinated axons Have largest diameter

Synaptic Transmission � How synapses neurons communicate with other neurons or with effectors through a series of events � neuron sending the signal is called the presynaptic neuron � neuron receiving the signal called the postsynaptic neuron A synaptic cleft separates the presynaptic and postsynaptic neurons Neurotransmitters Different neurotransmitters are found in synaptic vesicles. These different neurotransmitters have different effects.

Postsynaptic: Deactivating Neurotransmitters � � Essential neurotransmitters are removed in order to restore normal synaptic function Three ways: � Diffuse away from synaptic cleft (out of reach of receptors) � Destroyed by enzymes � Actively transported back into neuron (reuptake)