Nervous system intro lecture notes Pt 1.

Functionality and Signaling in the Nervous System

Overview of Functions

Transitioning focus from structural aspects (muscles and bones) to functions and signaling in tissues, the nervous system plays a crucial role in maintaining homeostasis and enabling adaptation to external and internal changes. Its primary role is to send signals, allowing for fast communication between different body parts, thus facilitating coordination and response to stimuli.

Types of Signaling

Sensory Information

• Types of Senses:

  • Visual: Involves detection of light by photoreceptors in the retina.

  • Auditory: Sound waves are processed by the cochlea in the inner ear.

  • Taste: Gustatory receptors located on the tongue respond to chemical compounds.

  • Olfactory: Smell is detected by olfactory receptors in the nasal cavity.

  • Tactile: Touch and pressure are sensed via mechanoreceptors in the skin.

• Internal Sensors:

  • Carotid bodies monitor CO2 and O2 levels in the blood, triggering respiratory responses to maintain adequate gas exchange.

  • Baroreceptors in blood vessels detect changes in blood pressure, prompting cardiovascular adjustments.

Both external and internal sensory signals are critical for the nervous system's overall function, alerting the body to changes that require reaction.

Processing Signals

• Processing Levels:

  • Processing occurs at multiple levels, including unconscious functions (e.g., breathing during sleep), reflex actions, and complex cognitive activities.

  • Sensory pathways converge and diverge, integrating signals to form coherent responses involving multiple regions of the brain.

• Involved Mechanisms:

  • Neural pathways, comprising sensory and motor neurons, convert sensory information into appropriate responses, including reflexes and voluntary actions.

Motor Signals

• Governance of Muscle Actions:

  • The nervous system orchestrates movements, influencing both skeletal muscle (which is under conscious control) and smooth muscle (unconscious control, affecting functions such as blood vessel constriction, digestion, and peristalsis).

• Types of Motor Responses:

  • Motor signals can be voluntary (such as picking up an object) or involuntary (such as the heartbeat), impacting various activities including micturition (urination) and digestive reflexes.

Mechanisms of Signal Transmission

• Electrical and Chemical Signaling:

  • Coordination occurs between chemical signaling via neurotransmitters released from synaptic knobs and electrical signaling through action potentials along axons.

• Excitable Tissues:

  • Includes nerve tissue, muscle tissue, and endocrine/exocrine glandular tissues, which react to signals via chemical reception and electrical transmission, enabling a rapid and coordinated response throughout the body.

Nervous Tissue Composition

Neurons (Conduct Signals)

• Neurons, though crucial for signal conduction, are not the majority of cells in nervous tissue; they are outnumbered by glial cells that support them.

• Types of Neurons:

  • Multipolar Neurons: Most common, have numerous dendrites and a single axon; critical for motor pathways and higher-level cognitive functions.

  • Unipolar Neurons: Typically sensory neurons that transmit signals from peripheral sensory receptors to the spinal cord; mostly found in sensory ganglia of the peripheral nervous system.

  • Bipolar Neurons: Rare, involved in special senses like vision and hearing; possess one axon and one dendrite, allowing for one-directional sensory input.

Glial Cells (Support and Protect Neurons)

• Astrocytes: Provide metabolic support, regulate ion balance, monitor solute concentrations, and maintain the blood-brain barrier integrity.

• Microglia: Act as the immune cells of the nervous system, performing phagocytosis to eliminate damaged cells and pathogens.

• Oligodendrocytes: Responsible for myelinating axons in the central nervous system (CNS), which enhances transmission speed.

• Schwann Cells: Provide myelination for peripheral nervous system (PNS) axons, supporting regeneration after injury.

• Satellite Cells: Offer structural and metabolic support to neuron cell bodies located in the PNS.

Blood-Brain Barrier (BBB)

• Importance of the BBB:

  • Protects the brain from toxins and pathogens while allowing essential nutrients and gases to pass.

  • Serves a crucial role in maintaining the brain's stable environment, which is necessary for its proper functioning.

• Structure of the BBB:

  • Composed of endothelial cell tight junctions that limit permeability, a supporting basement membrane, and astrocytic foot processes that contribute to further restrict passage, ensuring a selective barrier to maintain neural homeostasis.

Cellular Anatomy of Neurons

Key Components of a Typical Neuron

• Cell Body (Soma): Contains the nucleus and organelles, responsible for the metabolic activities of the neuron.

• Dendrites: Branch-like structures that receive incoming signals from other neurons or sensory receptors, facilitating communication.

• Axon: A singular, longer projection that transmits signals away from the cell body to other neurons, muscles, or glands.

• Synaptic Knobs: Located at the axon terminal, these structures release neurotransmitters into the synaptic cleft to communicate with other neurons, facilitating signal transmission.

• Signal Directionality: Signals are received at dendrites, processed in the cell body, and transmitted through the axon, ultimately affecting another neuron or effector cell, thus ensuring precise coordination and control.

Nerve Cell Variants and Their Functions

• Unipolar Neurons: Primarily sensory, responding to stimuli from skin and muscles, essential for relaying information about the external environment.

• Bipolar Neurons: Found in specialized sensory areas (eyes and ears), facilitating one-directional sensory input crucial for perception.

• Multipolar Neurons: Most common in motor pathways and interneurons, integrating and processing signals from multiple sources to coordinate responses effectively.

Summary

Excitable tissues play critical roles in processing and transmitting information throughout the body. Understanding the structure and function of different types of neurons and glial cells is essential for grasping how signaling works in the nervous system, which underlines all aspects of physiology and behavior in the organism.