Neural 6. paragraph

Organization of the Nervous System

• The nervous system is organized into various components that allow it to function effectively.

• Receptor: Receives stimuli.

• Sensory Nervous System (SNS): Carries signals from receptors to the Central Nervous System (CNS).

• Motor Nervous System (MNS): Transmits signals from the CNS to Effectors (muscles and glands).

Cells of the Nervous System

• Neurons:

  • Primary signaling units that transmit action potentials and receive stimuli.

  • Composed of different parts:

    • Cell Body (Soma): Contains the nucleus and organelles.

    • Dendrites: Branch-like structures that receive messages from other neurons.

    • Axons: Long fibers that transmit impulses away from the cell body.

• Neuroglia (Glial Cells): Support and protect neurons by maintaining homeostasis, form myelin, and provide support/protection.

Electrical Signals in the Nervous System

• Neurons produce action potentials, which are electrical signals used for communication within the body.

• The generation of these signals relies on differences in ionic concentrations across the cell's plasma membrane and its permeability to those ions.

Resting Potential of the Membrane

• The resting membrane potential is maintained by the Na+/K+ pump and membrane permeability:

  • Sodium (Na+) and Chloride (Cl-) ions are predominantly outside the cell.

  • Potassium (K+) and proteins are found in higher concentrations inside the cell.

  • A steep concentration gradient exists for Na+ (high outside) and K+ (high inside), which is crucial for nerve impulses.

Sodium-Potassium Exchange Pump

• The Na+/K+ pump actively exchanges Na+ and K+ across the plasma membrane using ATP to maintain the resting potential.

• For every ATP consumed, it moves three Na+ ions out and two K+ ions into the cell.

Membrane Permeability

• Proteins: Negatively charged proteins remain in the cell because they are large and cannot diffuse through the phospholipid membrane.

• Chloride Ions (Cl-): Diffuse out due to being repelled by negative proteins through open ungated channels.

• Gated Ion Channels: Open in response to stimuli, changing the permeability of the membrane.

  • Ligand-gated channels open when specific molecules bind to receptor proteins.

Nongated (Leakage) Channels

• Nongated channels are always open and allow K+ and Cl- ions to move across the membrane, contributing to resting potential stability.

• They are crucial for maintaining the resting membrane potential due to the high permeability of these ions.

Gated Ion Channels

• Change membrane permeability in response to various stimuli.

• Ligand-gated channels open when neurotransmitters bind. For example, ACh binding opens channels that allow Na+ influx.

• Voltage-gated channels open in response to changes in membrane potential, crucial during action potentials.

Local Potentials and Action Potentials

• Local potentials are transient changes in membrane potential that can summate.

• A strong enough local potential triggers an action potential: an all-or-nothing response characterized by phases of depolarization followed by repolarization.

• During depolarization, Na+ channels open; during repolarization, K+ channels open to restore the resting state.

Refractory Period

• This period includes an absolute refractory phase, where no new action potentials can form, and a relative refractory phase, where a stronger-than-threshold stimulus can initiate a new action potential.

Action Potential Frequency

• The frequency of action potentials (number produced over time) is influenced by the stimulus strength:

  • Threshold stimulus: Causes sufficient depolarization to generate an action potential.

  • Subthreshold stimuli: Do not generate action potentials.

Action Potential Propagation

• In myelinated axons, action potentials jump between nodes of Ranvier, resulting in faster conduction (saltatory conduction). Unmyelinated axons propagate action potentials continuously.

Nerve Fiber Types

• Type A fibers: Large-diameter, myelinated (15-120 m/s).

• Type B fibers: Medium-diameter, lightly myelinated (3-15 m/s).

• Type C fibers: Small-diameter, unmyelinated (2 m/s or less).

The Synapse

• Synaptic transmission occurs at synapses, where action potentials from presynaptic neurons influence postsynaptic neurons. Types of synapses include:

  • Electrical Synapses: Gap junctions that allow direct ion flow between cells.

  • Chemical Synapses: Use neurotransmitters to convey signals across synaptic cleft.

Chemical Synapses

• Composed of presynaptic terminals, synaptic cleft, and postsynaptic membranes. Here, neurotransmitters are released and bind to specific receptors, inducing changes in the postsynaptic neuron.

Neurotransmitter Removal

• Various methods are employed to remove neurotransmitters from synaptic cleft, like the breakdown of ACh by acetylcholinesterase.

Receptor Molecules and Neurotransmitters

• Specific neurotransmitters bind to specific receptors, with some being excitatory and others inhibitory.

Neuromodulators

• Chemicals that regulate the release of neurotransmitters, often acting at axoaxonic synapses to influence the output of other neurons.

Postsynaptic Potentials

• EPSP and IPSP signify excitatory and inhibitory signals that affect neuronal firing rates. EPSPs may reach threshold for action potentials, while IPSPs decrease those chances.

Summation

• Postsynaptic potentials can summate to reach threshold for action potentials.

Senses and Sensation

• Sensation involves the detection and processing of stimuli. The sequence starts from sensory receptor activation to action potentials being processed in the CNS.

Types of Senses

• General senses (proprioception, pain) across the body, and special senses (vision, hearing) involving specialized receptors.

Cerebellar Comparator Function

• Integrates intended motor activity from the cortex with sensory feedback from muscles, adjusting movements as necessary for smooth functioning.

Brain Waves and Sleep

• EEGs record brain activity during different states, indicating mental and physical states through different wave patterns like alpha, beta, theta, and delta.

Structure and Function of the Retina

• The retina contains layers of neurons responsible for photoreception (rods and cones) and pigment cells for isolation and clarity of visual signals.

Rods and Cones

• Rods provide black-and-white vision; Cones enable color vision, with different pigments for detecting red, green, and blue light.

Neuronal Pathways for Vision

• Visual information from the retina travels through optic pathways to reach visual processing areas of the brain for interpretation.

Auditory Function

• Sound perception involves mechanical vibrations transferring to the cochlear structures, with auditory signals interpreted based on frequency and amplitude.

Olfaction

• Involves olfactory receptors responding to odorants, providing a rich discrimination in smell without relay through the thalamus.

Taste Perception

• Influenced greatly by texture, temperature, and olfaction, with quick adaptations occurring and varying sensitivity to different tastes.