Chapter 11: The Nervous System

Overview of the Nervous System

The nervous system is the master control and communication system of the body. It is responsible for regulating and coordinating all bodily functions through rapid electrical and chemical signaling.

• Definition: The nervous system is a complex network of cells that transmits signals between different parts of the body.

• Three General Functions:

  1. Sensory Input: Gathering information from internal and external environments.

  2. Integration: Processing and interpreting sensory input to determine an appropriate response.

  3. Motor Output: Initiating actions in response to integrated information.

• Example: Touching a hot surface activates sensory receptors (input), the brain interprets the signal (integration), and the hand withdraws (motor output).

Organization of the Nervous System

The nervous system is organized into two main anatomical and functional divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS):

  • Consists of the brain and spinal cord.

  • Responsible for integration and command of sensory information and motor output.

  • The brain contains about 100 billion neurons; the spinal cord contains about 100 million neurons.

  • The spinal cord merges with the brain at the foramen magnum and continues to the first or second lumbar vertebra.

• Peripheral Nervous System (PNS):

  • Consists of cranial nerves (12 pairs) and spinal nerves (31 pairs) and their branches.

  • Carries signals to and from the CNS.

  • Includes bundles of long neuron axons surrounded by connective tissue sheaths.

Functional Divisions of the Nervous System

The PNS is further divided based on function:

• Sensory (Afferent) Division: Transmits sensory information from receptors to the CNS.

• Motor (Efferent) Division: Transmits motor commands from the CNS to effectors (muscles and glands).

• Motor Division Subdivisions:

  • Somatic Nervous System: Controls voluntary movements via skeletal muscles.

  • Autonomic Nervous System (ANS): Controls involuntary functions (e.g., heart rate, digestion).

    • Sympathetic Division: Prepares the body for 'fight or flight' responses.

    • Parasympathetic Division: Promotes 'rest and digest' activities.

Parts of the Nervous System

• Nerve Cells (Neurons):

  • Cell Body: Contains the nucleus and organelles.

  • Processes: Dendrites (receive signals) and axons (transmit signals).

• Supporting Cells (Neuroglia): Provide structural and metabolic support to neurons.

Structural and Functional Classification of Neurons

• Structural Classification:

  • Multipolar Neurons: Many dendrites, one axon (most common in CNS).

  • Bipolar Neurons: One dendrite, one axon (sensory organs).

  • Unipolar Neurons: Single process that splits into two branches (sensory neurons in PNS).

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward the CNS.

  • Motor (Efferent) Neurons: Transmit impulses away from the CNS to effectors.

  • Interneurons: Connect neurons within the CNS; involved in integration.

Action Potential and Membrane Physiology

Neurons communicate via electrical signals called action potentials, which depend on membrane channels and changes in membrane potential.

• Resting Membrane Potential: The voltage difference across the membrane at rest, typically -70 mV.

• Polarization: The membrane is at rest, with a negative inside relative to outside.

• Depolarization: The membrane potential becomes less negative (more positive).

• Hyperpolarization: The membrane potential becomes more negative than the resting potential.

• Action Potential: A rapid, temporary change in membrane potential that propagates along the axon.

  • Key Channels: Voltage-gated sodium and potassium channels.

Equation:

Synapses and Neuronal Communication

• Synapse: The junction between two neurons or between a neuron and an effector cell.

• Presynaptic Neuron: Sends the signal.

• Postsynaptic Neuron: Receives the signal.

• Types of Synapses:

  • Electrical Synapses: Direct flow of ions through gap junctions; rapid transmission.

  • Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; slower but more versatile.

Postsynaptic Potentials: EPSP vs. IPSP

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of action potential.

• Comparison: Action potentials are all-or-none, while EPSPs and IPSPs are graded and summate to influence neuron firing.

Neurotransmitters: Role and Types

• Role of Neurotransmitters:

  • Direct Action: Bind to ion channels and cause immediate changes in membrane potential.

  • Indirect Action: Activate second messenger systems for longer-lasting effects.

• Types of Neurotransmitters:

  • Acetylcholine (ACh): Excitatory at neuromuscular junctions.

  • Biogenic Amines: Dopamine, norepinephrine, serotonin.

  • Amino Acids: Glutamate (excitatory), GABA (inhibitory).

  • Neuropeptides: Substance P, endorphins.

Summary Table: Structural and Functional Divisions of the Nervous System

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.