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Neural tissue

 


An Overview of the Nervous System

Nervous system overview

      Nervous system

    Provides swift, brief responses to stimuli

      Endocrine system

    Adjusts metabolic operations and directs long-term changes

      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

    Efferent division carries motor commands to effectors

   Efferent division includes somatic nervous system and autonomic nervous system

 


Neurons

Neuron structure

      Perikaryon

    Neurofilaments, neurotubules, neurofibrils

      Axon hillock

      Soma

      Axon

      Collaterals with telodendria

 

Synapse

      Site of intercellular communication

      Neurotransmitters released from synaptic knob of presynaptic neuron

 

Neuron classification

      Anatomical

    Anaxonic

    Unipolar

    Bipolar

    Multipolar

 

Functional

    Sensory neurons

   deliver information from exteroceptors, interoceptors, or proprioceptors

    Motor neurons

   Form the efferent division of the PNS

    Interneurons (association neurons)

   Located entirely within the CNS

   Distribute sensory input and coordinate motor output


Neuroglia

Neuroglia of the Central Nervous System

      Four types of neuroglia in the CNS

    Ependymal cells

   Related to cerebrospinal fluid

    Astrocytes

   Largest and most numerous

    Oligodendrocytes

   Myelination of CNS axons

    Microglia

   Phagocytic cells

 

Neuroglia of the Peripheral Nervous System

      Two types of neuroglia in the PNS

    Satellite cells

   Surround neuron cell bodies within ganglia

    Schwann cells

   Ensheath axons in the PNS

 


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

      Voltage-regulated channels

      Mechanically regulated channels

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

      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

      salutatory propagation

    action potential spreads from node to node, skipping internodal membrane

 

Axon classification

      Type A fibers

      Type B fibers

      Type C fibers

    Based on diameter, myelination and propagation speed

 

Muscle action potential versus neural action potential

      Muscle tissue has higher resting potential

      Muscle tissue action potentials are longer lasting

      Muscle tissue has slower propagation of action potentials

 


Synaptic Activity

Nerve impulse

      Action potential travels along an axon

      Information passes from presynaptic neuron to postsynaptic cell

 

General properties of synapses

      Electrical

    Rare

    Pre- and postsynaptic cells are bound by interlocking membrane proteins

 

General properties of synapses

      Chemical synapses

    More common

    Excitatory neurotransmitters cause depolarization and promote action potential generation

    Inhibitory neurotransmitters cause hyperpolarization and suppress action potentials

Cholinergic synapses

      Release acetylcholine (ACh)

      Information flows across synaptic cleft

      Synaptic delay occurs as calcium influx and neurotransmitter release take appreciable amounts of time

      ACh broken down

    Choline reabsorbed by presynaptic neurons and recycled

      Synaptic fatigue occurs when stores of ACh are exhausted

 

Other neurotransmitters

      Adrenergic synapses release norepinephrine (NE)

      Other important neurotransmitters include

    Dopamine

    Serotonin

    GABA (gamma aminobutyric acid)

 

Neuromodulators

      Influence post-synaptic cells response to neurotransmitter

      Neurotransmitters can have direct or indirect effect on membrane potential

    Can exert influence via lipid-soluble gases


Information Processing

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

   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

 

Presynaptic facilitation

      Activity at axoaxonal synapse increases amount of neurotransmitter released when action potential arrives

    Enhances and prolongs the effect of the neurotransmitter

 

Rate of generation of action potentials

      Neurotransmitters are either excitatory or inhibitory

    Effect on initial membrane segment reflects an integration of all activity at that time

      Neuromodulators alter the rate of release of neurotransmitters

 

Rate of generation of action potentials

      Can be facilitated or inhibited by other extracellular chemicals

      Effect of presynaptic neuron may be altered by other neurons

      Degree of depolarization determines frequency of action

      potential generation