Nervous System Overview and Neuron Structure

Nervous System Overview

• The nervous system is a complex network of specialized cells, neurons, that transmit information throughout the body.
• It comprises the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), which includes nerves extending from the CNS to the rest of the body.
• The nervous system's primary function is to receive, process, and respond to stimuli, enabling communication and coordination within the organism.

Neuron Structure

• Neurons are fundamental units of the nervous system, characterized by their unique structure.
• The cell body (soma) contains the nucleus and other organelles.
• Dendrites are branched extensions that receive signals from other neurons or sensory receptors.
• An axon is a long, slender projection that transmits signals away from the cell body to other neurons, muscles, or glands.
• The axon terminal is the end of the axon where neurotransmitters are released to communicate with the next cell.
• Myelin sheath, a fatty insulating layer, surrounds the axon in many neurons, significantly increasing the speed of signal transmission.
• Nodes of Ranvier are gaps in the myelin sheath along the axon that facilitate saltatory conduction.

Action Potentials

• Action potentials are rapid, transient electrical signals that travel along the axon of a neuron.
• They are initiated by a stimulus that depolarizes the neuron's membrane potential to a threshold level.
• Depolarization opens voltage-gated sodium channels, allowing sodium ions to rush into the axon.
• This rapid influx of positive charge further depolarizes the membrane, leading to a positive feedback loop.
• Repolarization occurs as voltage-gated potassium channels open, allowing potassium ions to flow out of the axon, restoring the resting membrane potential. This is followed by a refractory period during which the neuron cannot fire another action potential.
• The action potential is an "all-or-none" phenomenon, meaning its amplitude is constant regardless of the stimulus strength above the threshold.

Synaptic Transmission

• Synapses are specialized junctions where neurons communicate with other neurons, muscle cells, or gland cells.
• Presynaptic neurons release neurotransmitters into the synaptic cleft.
• Neurotransmitters bind to receptors on the postsynaptic cell, triggering a response.
• Neurotransmitters can be excitatory, increasing the likelihood of an action potential in the postsynaptic neuron, or inhibitory, decreasing the likelihood.
• Synaptic transmission is crucial for information processing and integration within the nervous system. The precise timing and location of neurotransmitter release influences the overall outcome of neural signaling.

Conduction Mechanisms

• Continuous Conduction: In unmyelinated axons, action potentials propagate along the entire length of the axon, sequentially depolarizing each adjacent section of the membrane.
• Saltatory Conduction: In myelinated axons, action potentials "jump" between the nodes of Ranvier, significantly increasing the speed of signal transmission. Myelin insulates the axon, preventing ion leakage. The signal regenerates at each node, minimizing signal degradation.
• Conduction velocity is influenced by axon diameter and myelination. Larger diameter axons and myelinated axons generally conduct signals faster.

Factors Affecting Conduction

• Axon diameter, influencing the resistance to current flow.
• Myelination, increasing the speed of signal transmission by saltatory conduction.
• Temperature, affecting the rate of ion channel function. Higher temperatures lead to faster conduction.
• Presence of neurotoxins or drugs, which can interfere with ion channel function and disrupt conduction.