Bio250-12 Nervous System

Nervous System Functions

• Sensory: Detects changes in the external and internal environment, allowing organisms to perceive stimuli such as light, sound, touch, temperature, and pain. This detection is crucial for survival as it informs the organism about potential dangers and assists in navigation of their surroundings.

• Processing: Involves the analysis and interpretation of sensory information through complex neural networks. The brain integrates incoming data, leading to perception and cognition, which include decision-making, learning, and memory.

• Regulatory: Coordinates body functions and maintains homeostasis by regulating vital processes such as heartbeat, respiration, and thermoregulation. This facet of the nervous system ensures that the body systems operate within optimal ranges regardless of environmental changes.

• Communication via Electrical Signals: Neurons use action potentials to transmit messages rapidly across long distances within the body, facilitating immediate responses to stimuli.

• Coordination of Hormonal Signals: The nervous system works in tandem with the endocrine system to manage physiological activities, influencing processes like growth, metabolism, and stress responses.

• Limited Capacity for Regeneration: Neurons have a minimal ability to regenerate after injury, which poses challenges for recovery from nervous system damage.

• Excitable tissues

  • nervous tissues

  • muscle tissues

  • endocrine/exocrine

Cells of the Nervous System

Neurons

• Function: Conduct signals to other neurons, muscles, or glands, forming complex communication networks essential for all bodily functions.

• Characteristics: Neurons vary in shape (e.g., multipolar, unipolar) and size, adapted for their specific roles in transmitting information.

Glial Cells

• Support and Protect Neurons: These non-neuronal cells provide structural support, nutrition, and protection for neurons, playing a critical role in maintaining the health of the nervous system.

• Types of Glial Cells:

  • Oligodendrocytes: Myelinate axons in the Central Nervous System (CNS), facilitating faster signal transmission.

  • Astrocytes: Provide metabolic support, regulate blood flow, maintain the blood-brain barrier, and control the extracellular environment around neurons.

  • Microglia: Function as immune cells within the CNS, removing debris and dead neurons through phagocytosis.

  • Schwann Cells: Myelinate axons in the Peripheral Nervous System (PNS), which also aids in rapid signal conduction and supports nerve regeneration after injury.

  • Satellite Cells: Provide structural and metabolic support in ganglia of the PNS.

• Ependymal Cells: Line the ventricles of the brain and the central canal of the spinal cord, producing and circulating cerebrospinal fluid (CSF), which cushions and protects the brain and spinal cord.

Components of the Nervous System

• Central Nervous System (CNS): Comprises the brain and spinal cord, serving as the major control center for processing and directing responses to sensory information.

• Peripheral Nervous System (PNS): Includes all nerves branching from the brain and spinal cord, divided into afferent (sensory) and efferent (motor) pathways, enabling communication between the CNS and the rest of the body. Reflex arcs, critical for quick responses, are part of these pathways.

• Meninges: Three protective layers (dura mater, arachnoid, pia mater) enveloping the CNS, providing additional protection and stability.

  • Cerebrospinal Fluid (CSF): Fills the ventricles and surrounds the CNS, acting as a buffer to cushion the brain, remove waste, and supply nutrients.

Major Divisions of the Nervous System

• Neuron Anatomy:

  • Cell Body (Soma): Contains the nucleus and organelles, serving as the metabolic center of the neuron.

  • Axon: A long, singular projection that conducts electrical impulses. Axons can range significantly in length. Thicker than dendrites. They can be branched.

    • Axon Terminals: End points with synaptic vesicles that release neurotransmitters to communicate with other neurons or effector cells.

  • Dendrites: Short, branch-like extensions that receive signals from other neurons, facilitating input to the neuron.

  • Synapse: The junction connecting one neuron to another, where neurotransmitter signaling occurs, allowing for communication.

    • bind to dendrites when connecting to another neuron

  • Schwann cells: wrap themselves around axon. insulates the axon.

• signal travels through neuron electrically but to another neuron chemically.

Neuron Types

Structural Types

• Unipolar (Pseudo-Unipolar): Typically found in sensory neurons (skin, muscles, joints), having a single short process that divides into two branches.

• Multipolar: Characterized by one axon and multiple dendrites; this type is predominant in the CNS and allows for extensive connections. all motor neurons, all of neurons that are receiving signals from other neurons.

• Bipolar: With one axon and one dendrite, common in sensory pathways such as those for vision (retinal cells) and olfaction (smell). collects sensory info from one place.

Functional Types

• Sensory Neurons: Transmit sensory information from receptors to the CNS, allowing for the perception of sensory inputs.

• Motor Neurons: Convey signals from the CNS to muscles and glands to elicit responses or actions.

• Interneurons: Function within the CNS to modulate signals and facilitate communication between sensory and motor neurons.

Glial Cells in Detail

• Role: Act as neural "glue" providing various supportive functions essential for nerve health and repair.

• Specific Functions:

  • Astrocytes (CNS): Participate in neurotransmitter processing, nutrient exchange with blood vessels, and maintenance of the extracellular ion environment. secrete enzymes that break down neurotransmitters.

  • Satellite Cells (PNS): Envelop and support neuronal cell bodies in ganglia, contributing to homeostasis in the PNS. only in the PNS. wrap themselves around neurons and protect them.

  • Microglia (CNS & PNS): Serve as the first line of immune defense in the CNS, responding to injury or disease by removing waste and pathogens.

Blood-Brain Barrier**:

• Formation: Created by astrocytes and endothelial cells, this selective permeability barrier helps protect the brain from harmful substances while allowing essential nutrients to pass through.

• Components:

  • Tight Junctions: Prevent leakage of substances between endothelial cells.

  • Basement Membrane: Provides extra structural support to blood vessels in the brain.

  • Pericytes: Regulate blood flow and help maintain the integrity of the blood-brain barrier.

oligodendrocytes and Schwann cells

• Oligodendrocytes: Myelinate axons in the central nervous system, enhancing signal transmission.

• Schwann Cells: Myelinate axons in the peripheral nervous system, promoting faster nerve conduction.

Myelin**:

• Composition: Composed mainly of lipids (70-85%) and proteins (15-30%), forming a protective sheath around axons.

• Functions:

  • Protection: Insulates neurons, preventing signal loss and damage.

  • Increased Impulse Conduction Speed: Myelination enables faster transmission of electrical signals along axons through saltatory conduction.

  • Oligodendrocytes: Myelinate neurons in the CNS, while Schwann cells myelinate those in the PNS.

Neuronal Impulses**:

• Excitable Neurons: Have the capability to generate and conduct electrical impulses, essential for response to stimuli.

• Speed of Signal: Influenced by the diameter of the axon (larger diameter = faster signal) and the presence of myelin.

• Signal Arrangement: Include distinct pathways such as excitatory (stimulate) vs. inhibitory (inhibit) pathways, as well as divergence (where a signal spreads out) and convergence (multiple signals influencing one response).

• All-or-Nothing Response: Requires a specific threshold level of stimulation to trigger action potentials of uniform strength, regardless of the strength of the initial stimulus. Increased firing frequency occurs with stronger stimuli.

• Graded Potentials: Precede action potentials and can be either depolarizing (excitatory) or hyperpolarizing (inhibitory), setting the stage for neuronal firing.

Neuronal Signaling**:

Gated Channels

• Ion Channels: Critical for controlling the movement of ions (Na+, K+) across the neuronal membrane, influencing polarization and signaling.

  • Na+ Channel: Opens in response to depolarization, allowing sodium ions to influx, which propagates the action potential.

  • K+ Channel: Opens following an action potential, allowing potassium ions to exit, repolarizing the neuron.

  • Na+-K+ Pump: Restores resting potential by expelling Na+ from the neuron and bringing K+ back in, essential for maintaining the neuron’s readiness to fire again.

Synapse**:

• Function: Serve as sites for the transmission of electrical signals between neurons.

• Neurotransmitter Release: Synthesized in neurons, neurotransmitters are released into the synaptic cleft in response to action potentials, binding to receptors on adjacent neurons’ dendrites, thereby propagating further impulses. Following their action, neurotransmitters are usually broken down or reuptake into the presynaptic neuron, effectively terminating the signal and allowing for regulation of synaptic transmission.