Nerve 1: Understanding Neurons

Nerve 1

• Nerve 1 is a topic presented by Dr. Neven Makram Aziz.

Neuron

• A neuron is the fundamental unit of the nervous system.
• Billions of nerve cells (neurons) make up the nervous system.
• Neurons are excitable cells, capable of generating and transmitting action potentials.
• Neurons resemble other body cells in their basic structure, containing cytoplasm, a nucleus, mitochondria, and other organelles enclosed by a cell membrane.
• Neurons also perform cellular functions like protein synthesis (by Nissl granules) and energy production (by mitochondria).
• Neuron cell bodies form the gray matter of the brain and spinal cord, grouped together as nuclei in the central nervous system (CNS).
• Neuron extensions (dendrites and axons) are usually referred to as nerve fibers.
• Nerve fibers form the white matter of the brain and spinal cord.
• Neurons possess specialized structures (synapses) and chemicals (neurotransmitters) for communication with each other.

Neuron Structure

• (Diagram on page 4 shows)
  • Dendrites - branch-like structures that receive signals from other neurons.
  • Nucleus - the control center of the cell.
  • Cell body - contains the nucleus and other cellular components.
  • Axon - a long, slender projection that transmits signals away from the cell body.
  • Myelin sheath - insulating layer around some axons, increasing transmission speed.
  • Axon ending - the terminal part of the axon where signals are passed to other cells.

Synapse

• A synapse is a contact point (without continuity) between presynaptic and postsynaptic neurons.
• Information transmission occurs across the synapse.
• A space (synaptic cleft) separates pre- and postsynaptic neurons at the synapse.
• Neurotransmitters are released into the synaptic cleft to transmit signals across the synapse.

Types of Synapses

• I. According to site:
  • Between two neurons.
  • Between a neuron and a muscle.
  • Between a neuron and a gland.
• II. According to site of contact:
  • Axodendritic (most common, least excitable).
  • Axosomatic.
  • Axonaxonic (least common, most excitable).

Electrical vs. Chemical Synapses

• Electrical synapses are rare.
• Electrical synapses utilize gap junctions for signal transmission (bi-directional).
• Chemical synapses are the most prevalent in the CNS.
• Chemical synapses use neurotransmitters for signal transmission (one-directional).

Properties of Neurons

• Excitability: the ability to respond to a stimulus.
• Conductivity: the ability to transmit a nerve impulse (active process).
• Neurons do not fatigue.

Stimulus

• A stimulus is any change in the environment that triggers a response in living tissue.
• Stimulus types include: electrical, chemical, mechanical, and thermal.

Factors Affecting Stimulus Response

• Rate of application: A sudden change elicits a stronger response than a slow change of the same intensity.
• Strength and duration of stimulus: strength-duration curve illustrates this relationship. A minimum stimulus strength (rheobase) is needed for a response; a minimum duration is also needed.
• Stronger stimuli require less time to elicit a response, and visa versa (within limits).
• There's a minimal stimulus intensity (rheobase) needed to elicit a response; lowering intensity below this doesn't elicit a response even with longer durations.

Threshold (Rheobase) Stimulus

• Threshold stimulus is the minimum intensity of current needed to stimulate a nerve, often of a very long duration (like galvanic current).
• Stimuli with lower intensities are called subthreshold.
• Two types of stimulation exist:
  • Galvanic current: low intensity, long duration.
  • Faradic current: high intensity, short duration.

Utilization Time (UT)

• UT is the longest time needed for a rheobase (threshold) stimulus to evoke a response.

Minimal Duration (t)

• There's a minimal duration (t) for a stimulus to be effective; reducing duration below t prevents stimulation regardless of intensity.

Chronaxia

• Chronaxia is the time required for a stimulus of double the rheobase intensity to evoke a response.
• Chronaxia is useful to compare the excitability of different tissues.
• Tissues with shorter chronaxia are more excitable.

Resting Membrane Potential (RMP)

• RMP is the potential difference between the inner and outer surfaces of the nerve membrane under normal conditions.
• RMP is approximately -70 mV, indicating a negative charge inside the nerve cell.
• RMP is measured using microelectrodes and an oscilloscope.

Causes of RMP

• Active Na+/K+ pump: The pump actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell, maintaining a charge difference.
• Selective membrane permeability: The nerve cell membrane is significantly more permeable to potassium ions (K+) than to sodium ions(Na+). Because they open (or close) at different times, this establishes a membrane potential.
• Na+ channels are closed in resting conditions, while K+ channels are open. Na+ and K+ have different hydration energies, affecting their movement across the membrane.
• Non-diffusible ions (proteins, sulfate, phosphate) inside the cell contribute to the negative charge inside, along with the pump and selective permeability, contributing to overall negative potential.

This quiz explores the fundamental unit of the nervous system, the neuron, as presented by Dr. Neven Makram Aziz. It covers the structure, function, and communication mechanisms of neurons, including their role in forming gray and white matter in the brain and spinal cord.