Anatomy

64

Nervous Tissue and Neurophysiology

Objectives • Describe the two major divisions of the nervous system and their characteristics. • Identify the structures/functions of a typical neuron. • Describe the location and function of neuroglia. • Explain how resting potential is created and maintained. • Describe the events in the generation and propagation of an action potential. • Define the structure/function of a synapse. • List the major types of neurotransmitters and neuromodulators. • Explain the processing of information in neural tissue.

Nervous system overview • Nervous system

• Provides swift, brief responses to stimuli to control homeostasis (hand off a hot stove)

• Communicates via action potentials along neurons • Endocrine system

• Adjusts metabolic operations and directs long-term changes • Communicates via hormones in the bloodstream. Growth and development

• Nervous system includes • All the neural tissue of the body • Basic unit = neuron

Divisions of the nervous system • CNS (Central Nervous system)

• Brain and spinal cord • PNS (Peripheral Nervous system)

• Neural tissue outside CNS • Afferent division brings sensory information from receptors to CNS • Efferent division carries motor commands to effectors

• Efferent division includes somatic nervous system and autonomic nervous system

65

Organization

Nervous System

Central Nervous System Peripheral Nervous System

Brain Spinal Cord Afferent Efferent Periphery to CNS CNS to Periph.

Autonomic Somatic CNS to Skeletal Glands, Smooth

& CardiacMuscle

Sympathetic Parasympath. During Stress During Normal

Functioning

Neuron structure • Perikaryon – cytoplasm surrounding the nucleus

• Neurofilaments, neurotubules, neurofibrils • Soma • Dendrites • Axon • Axon hillock • Collaterals with telodendria • Synaptic terminals (knob)

The Anatomy of a Multipolar Neuron

Synapse • Site of intercellular communication • Neurotransmitters released from synaptic knob of presynaptic neuron

Neuron classification • Anatomical

• Anaxonic • Unipolar • Bipolar • Multipolar

66

Functional • Sensory neurons (afferent)

• deliver information from exteroceptors, interoceptors, or proprioceptors about external and internal environmental changes

• Motor neurons (efferent) • Responds by initiating muscular contractions or glandular secretions

• Interneurons (association neurons) • Located entirely within the CNS • Distributes and interprets sensory input and coordinate motor output

Neuroglia of the central nervous system • Ependymal cells

• Related to cerebrospinal fluid • Astrocytes

• Largest and most numerous • Oligodendrocytes

• Myelination of CNS axons – White matter of CNS • Microglia

• Phagocytic cells

Neuroglia of the peripheral nervous system • Satellite cells

• Surround neuron cell bodies within ganglia • Schwann cells

• Ensheaths axons in the PNS. Forms myelin sheath

Neurophysiology: Ions and Electrical Signals

The transmembrane potential • Electrochemical gradient

• Sum of all chemical and electrical forces acting across the cell membrane • Sodium-potassium exchange pump stabilizes resting potential at ~70 mV

Changes in the transmembrane potential • Membrane contains

• Passive (leak) channels that are always open • Active (gated) channels that open and close in response to stimuli

Three types of active channels • Chemically regulated channels (binding to a neurotransmitter) • Voltage-regulated channels (change in transmembrane potential as in action potentials) • Mechanically regulated channels – respond to physical distortion (touch, pressure)

67

Graded potential • A change in potential that decreases with distance

• Localized depolarization or hyperpolarization

Action Potential • Appears when region of excitable membrane depolarizes to threshold (-60mV) • Voltage-regulated Na+ channels open – Na+ ions flood into cell • Transmembrane potential rises to +30mV via positive feedback • Voltage-gated K+ channels open, and K+ ions exit cell • Votage-gated Na+ channels close at +30mV • Repolarization K+ positive charges exit cell and Na+ positive cease entering cell • K+ channels start to close a –70mV, but are slow • Repolarization continues until –90mV causing hyperpolarization

• Steps involved • Membrane depolarization and sodium channel activation • Sodium channel inactivation • Potassium channel activation • Return to normal permeability

Characteristics of action potentials • Generation of action potential follows all-or-none principle • Refractory period lasts from time action potential begins until normal resting potential

returns • Continuous propagation

• Spread of action potential across entire membrane in series of small steps – Relatively slow and characteristic of unmyelinated neuron.

• Salutatory propagation • Action potential spreads from node to node, skipping internodal membrane – Much

faster and a characteristic of myelinated neuron.

Axon classification • Type A fibers, Type B fibers, Type C fibers

• Based on diameter, myelination, and propagation speed

General properties of synapses • Chemical synapses

• Most common • Excitatory neurotransmitters cause depolarization and promote action potentials • Inhibitory neurotransmitters cause hyperpolarization and suppress action potentials

68

Cholinergic synapses • Release acetylcholine (ACh), the most common and best understood neurotransmitter

Other neurotransmitters • Adrenergic synapses release norepinephrine (NE) • Other important neurotransmitters include

• Dopamine • Serotonin • GABA (gamma aminobutyric acid)

Alteration of Synaptic Conduction

Botulism (Clostridium botulinum)

Food poisioning from improperly canned or cooked foods

The toxin from this organism inhibits release of acetylcholine (ACh)

Inhibits muscle contraction=weak, pupils dysfunction, dysphagia

Victims suffocate from no contraction of diaphragm

Crack Cocaine

Affects nuerotransmitters: dopamine, norepinephrine, serotonin these regulate mood & motor functions

Crack interferes w/ inactivation & return of neurotransmitters to presynaptic neuron

Resulting buildup results in feelings of euphoria

Can also result in high HR, high BP, convulsions, insomnia

Neurotransmitter shortages may result in depression

Information processing • Simplest level of information processing occurs at the cellular level

• Excitatory and inhibitory potentials are integrated through interactions between postsynaptic potentials

Postsynaptic potentials • EPSP (excitatory postsynaptic potential) = depolarization

• EPSP can combine through summation • Temporal summation or Spatial summation

• IPSP (inhibitory postsynaptic potential) = hyperpolarization • Most important determinants of neural activity are EPSP/IPSP interactions

Presynaptic inhibition • GABA release at axoaxonal synapse inhibits opening calcium channels in synaptic

knob • Reduces amount of neurotransmitter released when action potential arrives