introduction to sensory

Neurons and Synaptic Transmission

Learning Outcomes

  • Define Neurone

  • Classify neurones based on:

    • Structure

    • Function

    • Axon length and diameter

    • Speed of action potential conduction

  • Differentiate between:

    • Leak channels

    • Voltage-gated channels

    • Chemically-gated channels

    • Ion pumps

  • Differentiate between:

    • Resting potential

    • Graded potential

    • Action potential

    • Discuss structures of the neuron as well as ion movements and membrane potentials

  • Compute equilibrium potentials for Na and K using the Nernst equation

  • Draw the action potential curve and distinguish between the transmission of an action potential in myelinated vs nonmyelinated neurons

    • Emphasize the importance of sodium and potassium ions in impulse transmission

  • Describe the sequence of events in synaptic transmission

  • List major neurotransmitters and describe their functions

  • Differentiate between excitatory and inhibitory post-synaptic potentials in terms of their effects on membrane potential and action potential generation likelihood

  • Predict effects of changes in:

    • Extracellular fluid (ECF) or intracellular fluid (ICF) ion concentrations

    • Channel function

    • Neurotransmitter abundance

    • Other relevant parameters on resting membrane potential, action potentials, synaptic transmission, and post-synaptic potentials

Nervous System Overview

  • Central Nervous System (CNS):

    • Brain: Receives and processes sensory information, initiates responses, stores memories, generates thoughts and emotions

    • Spinal Cord: Conducts signals to and from the brain, controls reflex activities

  • Peripheral Nervous System (PNS):

    • Motor Neurons: CNS to muscles and glands

    • Sensory Neurons: Sensory organs to CNS

    • Somatic Nervous System: Controls voluntary movements

    • Autonomic Nervous System: Controls involuntary responses

    • Sympathetic Division: "Fight or Flight"

    • Parasympathetic Division: "Rest or Digest"

Structural Components of Neuron

  • Neuron: Basic functional unit of the nervous system

  • Components of Neurons:

    • Cell Body (Soma): Contains large round nucleus, prominent nucleolus, surrounded by cytoplasm, mitochondria, ribosomes, rough endoplasmic reticulum (RER)

    • Dendrites: Sensitive processes; the receiving/input portion of the neuron forming synaptic connections with other neurons

    • Axon: Long cytoplasmic process capable of propagating an action potential

Fundamental Properties of Neurons

  1. Excitability: Ability to respond to stimuli (changes in body and external environment)

  2. Conductivity: Producing traveling electrical signals

  3. Secretion: When electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted

Classification of Neurons

According to Number of Processes:

  • Unipolar Neurons: Most sensory neurons of the peripheral nervous system (PNS)

  • Bipolar Neurons: Occur in special senses (e.g., retina)

  • Multipolar Neurons: Most common in the CNS, with one axon and multiple dendrites

  • Multipolar Interneuron (Anaxonic Neuron): Occurs mostly in the brain, has more than two processes without distinguishable axons

According to Functions:

  • Sensory (Afferent) Neurons: Carry impulses from receptors to the spinal cord

  • Relay Neurons: Interneurons that lie between sensory and motor pathways in CNS; process, store, and retrieve information

  • Motor (Efferent) Neurons: Carry impulses from the brain to effectors (glands and muscles)

According to Length:

  • Golgi Type I Neuron: Long axon

  • Golgi Type II Neuron: Short or no axon (Anaxonic neuron)

Impulse Conduction and Axonal Transport

  • Axonal Transport: Required for proteins made in the soma to be delivered to the axon and axon terminal

    • Anterograde Transport: Speeds of up to 400 mm/day for organelles, enzymes, vesicles, small molecules

    • Retrograde Transport: For recycled materials and pathogens, speeds up to 10 mm/day

Electrical Potentials & Currents

  • Neuron Doctrine: Nerve pathway is not a continuous "wire"; a series of separate cells

    • Electrical Potential: Difference in concentration of charged particles within the cell

    • Electrical Current: Flow of charged particles from one point to another within the cell

  • Nerve cells are polarized with a resting membrane potential of -70 mV (relatively negative charge inside neurons)

Resting Membrane Potential

  • Resting Membrane Potential: Approximately -70 mV; determined by selective permeability of the plasma membrane

    • Membrane is very permeable to K⁺, leading to diffusion out until an electrical gradient attracts K⁺ back

    • Membrane is much less permeable to Na⁺; Na⁺/K⁺ pump expels 3 Na⁺ for every 2 K⁺ it brings in

    • Requires substantial ATP to maintain this gradient

Ionic Basis of Resting Membrane Potential,m

  • ECF Concentrations:

    • Na⁺: 145 mEq/L

    • K⁺: 4 mEq/L

  • ICF Concentrations:

    • Na⁺: 12 mEq/L

    • K⁺: 155 mEq/L

    • Large anions that cannot escape the cell

Equilibrium Potential

  • Equilibrium Potential: The potential at which there is no net movement of the ion across a membrane

    • Calculated using the Nernst Equation:

    • E<em>Na=−61/zimesextlograc[extNa]</em>i[extNa]<em>oE<em>{Na} = -61/z imes ext{log} rac{[ ext{Na}]</em>i}{[ ext{Na}]<em>o} where $E{Na} = +52 mV$, $[ ext{Na}]i = 20 mM$, and $[ ext{Na}]o = 145 mM$ (z=1, since Na⁺ is monovalent)

    • E<em>K=−61/zimesextlograc[extK]</em>i[extK]<em>oE<em>{K} = -61/z imes ext{log} rac{[ ext{K}]</em>i}{[ ext{K}]<em>o} where $E{K} = -96 mV$, $[ ext{K}]i = 150 mM$, and $[ ext{K}]o = 4 mM$ (z=1, since K⁺ is monovalent)

Graded / Local Potentials

  • Local disturbances in membrane potential occur when stimulated

    • Depolarization: Decreases the potential across the cell membrane due to opening of gated Na⁺ channels

    • Na⁺ rushes in down concentration and electrical gradients

    • Produces a change in voltage called local potential

    • Characteristics of Local Potentials:

    • Graded: Vary in magnitude with stimulus strength

    • Decremental: Get weaker as they spread

    • Reversible: As K⁺ diffuses out, pumps restore balance

    • Can be either excitatory or inhibitory (hyperpolarizing effect)

Action Potentials

  • More dramatic changes in membrane produced where high density of voltage-gated channels occur

    • If threshold potential (-55 mV) is reached, voltage-gated Na⁺ channels open leading to depolarization

    • Peaks at +35 mV, Na⁺ channels close, K⁺ channels open for repolarization

    • Hyperpolarization can occur due to negative overshoot

Characteristics of Action Potentials:

  • Follows an all-or-none law; the voltage gates either open or don’t

  • Nondecremental: Do not decrease in strength over distance

  • Irreversible: Once initiated, the process cannot be stopped

The Refractory Period

  • Absolute Refractory Period: No additional action potential can be triggered while Na⁺ gates are open

  • Relative Refractory Period: A strong stimulus may trigger a new action potential while K⁺ gates are open

  • Refractory period occurs at a small patch of membrane, which quickly recovers

Impulse Conduction

In Unmyelinated Fibers:

  • Threshold voltage in trigger zone begins the impulse

  • The nerve signal is a chain reaction of sequential opening of voltage-gated Na⁺ channels down the entire axon

    • Travels at approximately 2 m/sec

Saltatory Conduction in Myelinated Fibers:

  • Action potentials jump from node of Ranvier to node of Ranvier

    • Myelin sheath increases signal speed due to fewer voltage-gated channels in myelin-covered regions compared to nodes of Ranvier

  • Voltage-gated channels are densely packed at nodes (up to 12,000 per µm²)

Synapses Between Two Neurons

  • Presynaptic Neuron: Releases neurotransmitter onto the second neuron (postsynaptic neuron) through synaptic cleft

    • The number of synapses on a postsynaptic cell can vary significantly (e.g., 8000 on spinal motor neuron, 100,000 on cerebellar neurons)

Postsynaptic Potentials

Excitatory Postsynaptic Potentials (EPSP):

  • Positive voltage change, making the postsynaptic cell more likely to fire

    • Caused by Na⁺ flowing into the cell

    • Example neurotransmitters: Glutamate, Aspartate

Inhibitory Postsynaptic Potentials (IPSP):

  • Negative voltage change, making the postsynaptic cell less likely to fire (hyperpolarization)

    • Caused by Cl⁻ flowing into the cell or K⁺ leaving the cell

    • Example neurotransmitters: Glycine, GABA

Table of Neurotransmitters and Neuropeptides

  • Acetylcholine (ACh):

    • Found in neuromuscular junctions, most synapses in autonomic nervous system;

    • Excites skeletal muscles, inhibits cardiac muscle, varying effects on smooth muscle and glands

  • Excitatory Amino Acids:

    • Glutamate: Major excitatory neurotransmitter in the brain (~75% of excitatory synaptic transmission)

    • Aspartate: Similar effects to glutamate

  • Inhibitory Amino Acids:

    • Glycine: Most common inhibitory neurotransmitter in the spinal cord

    • GABA (γ-aminobutyric acid): Most common inhibitory neurotransmitter in the brain

  • Monoamines:

    • Norepinephrine, Epinephrine, Dopamine, Serotonin, Histamine

  • Neuropeptides:

    • Substance P, Enkephalins, B-endorphin, Cholecystokinin (CCK)

Excitatory Cholinergic Synapse

  • Nerve signal opens voltage-gated calcium channels, triggering the release of ACh across the synapse

  • ACh receptors open Na⁺ channels, producing local potential (postsynaptic potential)

  • When threshold (-55 mV) is reached, triggers action potential

Cessation & Modification of the Signal

  • Mechanisms for turning off stimulation:

    • Diffusion of neurotransmitter away from synapse into extracellular fluid (ECF)

    • Synaptic knob reabsorbs amino acids and monoamines by endocytosis and breaks them down with monoamine oxidase

    • Acetylcholinesterase degrades ACh in the synaptic cleft (choline is reabsorbed and recycled)

    • Neuromodulators modify synaptic transmission by affecting receptor numbers, neurotransmitter release, synthesis or breakdown

Summation of Postsynaptic Potentials

  • The likelihood of neuron firing is determined by the net input of other cells.

  • Temporal Summation: Single synapse receives multiple EPSPs in a short time period.

  • Spatial Summation: Single synapse receives multiple EPSPs from different presynaptic cells.

Neuroglia

  • Supporting cells of the nervous system, constituting about half the volume of CNS

  • Do not generate action potentials

  • Example: Schwann cells produce myelin sheath, which electrically insulates axons and increases speed of nerve impulse propagation.k