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