Neuron Structure and Classification

Neuron Structure

• Myelinated axons in the CNS lack neurilemmae, while unmyelinated axons are encased by Schwann cell cytoplasm.
• Gray matter is composed of unmyelinated axons.

Neuron Classification (by structure)

• Bipolar neurons: Have two processes and are mainly sensory neurons found in the eyes, ears, and nose.
• Unipolar neurons: Have one process and are commonly sensory neurons that transmit impulses to the CNS via afferent pathways. Their cell bodies are located in ganglia.
• Multipolar neurons: Represent 99% of neurons and have many processes. Most are found in the CNS.
  • Most multipolar neurons are interneurons, responsible for integrating sensory input or motor output within the CNS.
  • Some multipolar neurons are motor neurons, transmitting impulses from the CNS to effectors (muscles or glands) via efferent pathways.

Neuron Classification (by function)

• Sensory neurons: Afferent neurons that carry impulses to the CNS. Most are unipolar, with some being bipolar.
• Interneurons: Association neurons that link other neurons. They are multipolar and located in the CNS.
• Motor neurons: Efferent, multipolar neurons that carry impulses away from the CNS to effectors.

Neuroglia in the CNS

• Astrocytes: Connect neurons to blood vessels, exchange nutrients and growth factors, form scar tissue in response to brain injury, aid in metabolism, regulate ion concentrations, and form part of the blood-brain barrier (BBB).
• Oligodendrocytes: Myelinate CNS axons and provide structural support.
• Microglia: Phagocytic cells that also provide structural support.
• Ependymal cells: Line the central canal of the spinal cord and ventricles of the brain. They cover the choroid plexuses and help regulate the composition of cerebrospinal fluid (CSF). These cells are cuboidal or columnar and often ciliated.

Neuroglia of the PNS

• Schwann cells (neurolemmocytes): Produce myelin sheath around some peripheral axons, crucial for regeneration of damaged peripheral nerve fibers, and increase the speed of nerve impulse transmission.
• Satellite cells: Support clusters of neuron cell bodies (ganglia) and function similarly to astrocytes in the CNS.

Axonal Regeneration

• Mature neurons generally do not divide. If the cell body is injured, the neuron usually dies.
• Neuron regeneration in the PNS: If a peripheral axon is injured, it may regenerate. The axon segment separated from the cell body and its myelin sheath will break down, but the Schwann cells and neurilemma remain. These Schwann cells act as a guiding sheath for the growing axon, thanks to neuroglial cells secreting growth hormones. If the growing axon re-establishes its former connection, function may return; otherwise, function could be lost.
• Neuron regeneration in the CNS: CNS axons lack the neurilemma, which would act as a guiding sheath for regeneration. Oligodendrocytes do not proliferate after injury, making regeneration unlikely.

Synapses

• Neurons communicate at synapses, sites where one neuron transmits a nerve impulse to another neuron.
• The presynaptic neuron sends the impulse, while the postsynaptic neuron receives it.
• The synaptic cleft separates the two neurons.

Synaptic Transmission

• Synaptic transmission is a one-way transfer of information.
• The impulse travels down the axon of the presynaptic neuron, reaching the axon terminal.
• When the impulse reaches the synaptic knob, it triggers an influx of calcium ions (Ca2+).
• This influx leads to the release of neurotransmitters from synaptic vesicles through exocytosis.
• Neurotransmitters exert either excitatory or inhibitory effects on the postsynaptic neuron.

Cell Membrane Potential

• A cell membrane is usually electrically charged, or polarized, with the inside being negatively charged relative to the outside. This is due to the unequal distribution of ions inside and outside the membrane.
• This polarization is crucial for the conduction of impulses in neurons and muscle fibers.

Resting Membrane Potential (RMP)

• Requires differences in potassium (K+) and sodium (Na+) concentrations inside and outside the cells, as well as differences in the permeability of the plasma membrane to these ions.
• The resting neuron is not being stimulated.
• Na+ and K+ ions move according to diffusion rules, moving from areas of high concentration to low concentration.
• There is a 70 millivolt (mV) potential difference between the inside and outside of the cell.
• The neuron's membrane is polarized, with more K+ ions inside the cell, more Na+ ions outside, and more negatively charged proteins inside.
• The resting cell membrane is more permeable to K+ ions than Na+ ions.
• The inside of the cell is negative relative to the outside.
• The RMP for a typical neuron is -70 mV, due to this unequal charge distribution.
• If the resting potential changes, the sodium-potassium pump restores it.

Local Potential Changes

• Neurons are excitable cells that detect stimuli and respond by changing their resting potential.
• A common response is to open a gated ion channel.
• If the membrane potential becomes more negative (further away from zero), the membrane is hyperpolarized.
• If the membrane potential becomes less negative (closer to zero), the membrane is depolarized.
• This change in a positive direction is called depolarization. If the depolarization is subthreshold it does not generate an action potential.
• When enough sodium ions enter the cell and the membrane potential depolarizes to threshold (-55 mV), another type of sodium channel opens, triggering an action potential. These channels are found along the axon, particularly near the origin in an area called the "trigger zone."

Synaptic Transmission (Review)

• Synaptic transmission involves the transfer of a nerve impulse from one neuron to another.
• Released neurotransmitters cross the synaptic cleft and interact with specific receptors on the membrane of the postsynaptic neuron.
• Neurotransmitters can have varying effects: some open ion channels, while others close them.
• Chemically gated ion channels respond to neurotransmitters.
• Local potentials resulting from changes in chemically gated ion channels are called synaptic potentials.
• Excitatory neurotransmitters increase the permeability to Na+ ions, bringing the membrane closer to threshold, which increases the likelihood of generating impulses.
• Inhibitory neurotransmitters move the membrane further from threshold, decreasing the likelihood of generating impulses.

Summation of EPSPs and IPSPs

• Excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) are summed together in a process called summation.
• A net excitatory effect increases the probability of an action potential.
• A net inhibitory effect does not generate action potentials.
• Summation of all inputs typically occurs at the trigger zone (axon hillock).