Nervous System Overview

Main Functions and Components

The nervous system is responsible for receiving sensory input, interpreting information, and generating appropriate responses. It is composed of specialized cells and structures that coordinate bodily functions and behavior.

• Central Nervous System (CNS): Includes the brain and spinal cord; processes information and issues commands.

• Peripheral Nervous System (PNS): Consists of nerves outside the CNS; transmits signals between the CNS and the rest of the body.

• Main Functions: Sensory input, integration, motor output.

Example: Touching a hot surface triggers sensory neurons to send signals to the CNS, which then initiates a motor response to withdraw the hand.

Cells of the Nervous System

Neurons and Supporting Cells

Neurons are the functional units of the nervous system, specialized for transmitting electrical and chemical signals. Supporting cells (glial cells) provide structural and metabolic support.

• Neuron Structure: Cell body (soma), dendrites (receive signals), axon (transmits signals).

• Types of Neurons:

  • Sensory Neurons: Carry information from sensory receptors to the CNS.

  • Motor Neurons: Transmit commands from the CNS to muscles and glands.

  • Interneurons: Connect neurons within the CNS, facilitating communication.

• Supporting Cells:

  • Astrocytes: Maintain the environment around neurons.

  • Oligodendrocytes (CNS) and Schwann Cells (PNS): Form myelin sheaths for insulation.

  • Microglia: Act as immune cells within the CNS.

Example: The knee-jerk reflex involves sensory neurons, interneurons, and motor neurons working together.

Neuronal Communication

Membrane Potential and Sodium-Potassium Pump

Neurons maintain a resting membrane potential of approximately -70 mV, with the inside of the cell negative relative to the outside. This is achieved by the sodium-potassium pump, which actively transports ions across the membrane.

• Sodium-Potassium Pump: Moves 3 Na+ ions out and 2 K+ ions in per cycle, using ATP.

• Equation:

• Result: Maintains the electrochemical gradient necessary for action potentials.

Action Potential: Generation and Propagation

An action potential is a rapid change in membrane potential that travels along the axon, enabling signal transmission.

1. Resting State: Voltage-gated Na+ and K+ channels are closed; membrane potential is stable.

2. Depolarization: Stimulus opens Na+ channels; if threshold is reached, action potential is triggered.

3. Rising Phase: Additional Na+ channels open, causing rapid influx and further depolarization.

4. Falling Phase: Na+ channels inactivate; K+ channels open, K+ exits, repolarizing the cell.

5. Undershoot: K+ channels close slowly, causing a brief hyperpolarization.

Equation:

Example: Nerve impulses travel faster in myelinated axons due to saltatory conduction.

Impulse Transmission: Myelinated vs. Unmyelinated Neurons

Action potentials propagate differently depending on the presence of myelin.

• Unmyelinated Neurons: Action potential moves continuously along the axon.

• Myelinated Neurons: Action potential jumps between nodes of Ranvier (saltatory conduction), increasing speed.

Synapses and Synaptic Integration

Chemical Synapses

Synapses are junctions where neurons communicate with other neurons or effectors. Chemical synapses use neurotransmitters to transmit signals across the synaptic cleft.

• Neurotransmitters:

  • Acetylcholine

  • Glutamate (excitatory)

  • GABA (inhibitory)

  • Dopamine

  • Norepinephrine

• Drugs: Many drugs act at chemical synapses, affecting neurotransmitter release or receptor activity (e.g., antidepressants, benzodiazepines).

Synaptic Integration

Neurons integrate multiple excitatory and inhibitory signals to determine whether to fire an action potential.

• Excitatory Postsynaptic Potential (EPSP): Increases likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Decreases likelihood of action potential.

Evolution and Organization of Nervous Systems

Evolutionary Trends

The structure of nervous systems reflects evolutionary adaptations and symmetry.

Vertebrate Nervous System

Vertebrates have a centralized nervous system with distinct divisions.

• Central Nervous System (CNS): Brain and spinal cord

• Peripheral Nervous System (PNS): Sensory and motor neurons

Blood-Brain Barrier

Structure and Function

The blood-brain barrier is a selective barrier that protects the brain from harmful substances while allowing essential nutrients to pass.

• Structure: Tight junctions between endothelial cells in brain capillaries

• Function: Maintains brain homeostasis, restricts passage of toxins and pathogens

Peripheral Nervous System: Divisions and Functions

Parasympathetic vs. Sympathetic Divisions

The PNS is divided into functional components that regulate involuntary body functions.

Functional Areas of the Brain

Cerebral Cortex Mapping

The cerebral cortex contains distinct regions responsible for sensory perception, motor control, and higher cognitive functions.

• Frontal Lobe: Motor functions, decision-making

• Parietal Lobe: Somatosensory processing

• Temporal Lobe: Auditory processing

• Occipital Lobe: Visual processing

Example: The motor cortex controls voluntary movements; the somatosensory cortex processes touch and proprioception.

Communication: Body Language

Nonverbal Communication

Body language is a significant form of communication, often conveying more information than spoken words or tone of voice.

• Spoken Words: ~7% of communication

• Tone of Voice: ~38% of communication

• Body Language: ~55% of communication

Example: Facial expressions, gestures, and posture are interpreted by the brain to understand social cues.

Additional info: Some content and examples were inferred and expanded for clarity and completeness based on standard General Biology curriculum.