Anatomical and Functional Divisions of the Nervous System

Overview of Nervous System Organization

The nervous system is divided into central and peripheral components, each with distinct anatomical structures and functions. Understanding these divisions is essential for grasping how the body processes and responds to information.

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for integrating and processing information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

Functional Divisions

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

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

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

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

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

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

Example: The sensory division detects pain in the skin, relays it to the CNS, which then sends a motor command to withdraw the hand.

Structure and Classification of Neurons

Typical Neuron Structure

Neurons are specialized cells that transmit electrical impulses. Their structure is adapted for communication within the nervous system.

• Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Nucleus: Houses genetic material; controls cell activities.

• Dendrites: Branch-like extensions; receive impulses from other neurons.

• Axon Hillock: Region where the axon originates; initiates action potentials.

• Axon: Long projection; transmits impulses away from the cell body.

• Axon Terminals (Synaptic Terminals): Endings that release neurotransmitters to communicate with other cells.

Example: Sensory neurons have long dendrites to receive signals from sensory receptors.

Neuron Classification

• Unipolar: One process extends from the cell body; found in sensory neurons of the PNS.

• Bipolar: One dendrite and one axon; found in sensory organs (e.g., retina).

• Multipolar: Multiple dendrites and one axon; most common type in CNS.

Types and Functions of Neurons

Sensory Neurons

Sensory neurons carry information from sensory receptors to the CNS.

• Interoceptors: Monitor internal systems (e.g., cardiovascular, digestive).

• Exteroceptors: Detect external stimuli (e.g., touch, temperature).

• Proprioceptors: Monitor body position and movement.

Motor Neurons

Motor neurons transmit instructions from the CNS to effectors.

• Somatic Motor Neurons: Control voluntary muscles.

• Visceral Motor Neurons: Regulate involuntary muscles and glands.

Interneurons

Interneurons connect sensory and motor neurons within the CNS, facilitating integration and coordination.

• Play roles in reflexes, learning, and memory.

Neuroglia: Locations and Functions

Types of Neuroglia

Neuroglia are supporting cells in the nervous system, each with specialized functions.

Additional info: Satellite cells (not shown) support neurons in PNS ganglia.

Myelination

• Myelin: Insulating sheath around axons; increases speed of impulse transmission.

• Oligodendrocytes: Myelinate CNS axons.

• Schwann cells: Myelinate PNS axons.

Example: Multiple sclerosis is a disease where myelin is damaged, impairing nerve function.

Transmembrane Potential: Origin and Significance

Definitions

• Cation: Positively charged ion (e.g., Na+).

• Anion: Negatively charged ion (e.g., Cl-).

• Transmembrane Potential: The voltage difference across a cell's plasma membrane due to unequal ion distribution.

Resting Potential

The resting potential is the membrane potential of a cell under normal conditions, typically negative inside relative to outside.

• Typical values (in mV): adipocyte/fat cell: -40 thyroid gland cell: -50 neuron: -70 skeletal muscle: -85 cardiac muscle: -90

All values are negative due to selective permeability and active transport of ions, creating an electrical gradient.

Creation and Maintenance of Resting Potential

Selective Permeability and Ion Distribution

The plasma membrane allows some substances to pass while blocking others, maintaining distinct ion concentrations inside and outside the cell.

• High Na+ concentration: Outside the cell

• High K+ concentration: Inside the cell

• Negatively-charged proteins: Inside the cell

Ion Movement and Gradients

• Na+ and K+: Cannot freely cross the membrane; require channels or pumps.

• Concentration Gradient: Drives ions from high to low concentration.

• Electrical Gradient: Drives ions toward opposite charge.

Example: K+ moves out of the cell, leaving behind negative proteins, making the inside more negative.

Na+/K+ Pump

The Na+/K+ pump uses ATP to maintain ion gradients, pumping 3 Na+ out and 2 K+ in per cycle.

• Maintains resting membrane potential.

• Creates electrical and concentration gradients.

Synapses and Signal Transmission

Structure of a Synapse

A synapse is the junction where a neuron communicates with another cell.

• Presynaptic cell: Sends the signal.

• Postsynaptic cell: Receives the signal.

• Synaptic cleft: Gap between cells where neurotransmitters are released.

• Axon terminal: Releases neurotransmitters.

Example: Neuromuscular junction is a synapse between a neuron and a muscle cell.

Types of Synapses

• Neuromuscular junction: Neuron to muscle cell.

• Neuroglandular junction: Neuron to gland cell.

Summary Table: Neuroglia Types and Functions

Key Equations

• Resting membrane potential: (for neurons)

• Na+/K+ pump:

Additional info:

• Neurons cannot divide; loss due to injury is permanent.

• Myelination increases conduction speed; loss of myelin impairs function.

• Interneurons are crucial for higher brain functions.