Nervous System Overview

Divisions of the Nervous System

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has specialized structures and functions essential for body regulation and response.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Subdivided into sensory (afferent)motor (efferent)

Subdivisions of the PNS:

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

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

• Somatic Division: Controls voluntary movements (skeletal muscles).

• Visceral Division (Autonomic): Regulates involuntary functions (smooth muscle, cardiac muscle, glands).

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

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

Example: Touching a hot surface activates sensory neurons in the skin, which send signals to the CNS. The CNS processes the information and sends motor commands via the PNS to withdraw the hand.

Neurons and Neural Pathways

Structure of a Neuron

Neurons are the basic functional units of the nervous system, specialized for transmitting electrical signals.

• Dendrites: Receive incoming signals from other neurons.

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

• Axon: Conducts electrical impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other neurons or effectors.

• Myelin Sheath: Insulates the axon, increasing the speed of impulse transmission.

Example: Motor neurons transmit signals from the spinal cord to muscles, causing contraction.

Neural Pathways: Afferent and Efferent

Neural pathways are classified based on the direction of signal transmission.

• Afferent (Sensory) Pathways: Carry information from sensory receptors to the CNS.

• Efferent (Motor) Pathways: Transmit commands from the CNS to muscles and glands.

Example: The withdrawal reflex involves afferent neurons sensing pain and efferent neurons activating muscle contraction.

Neuroglia (Glial Cells)

Types and Functions of Neuroglia

Neuroglia are supporting cells in the nervous system that provide structural and functional support to neurons.

Additional info: Myelin is produced by oligodendrocytes in the CNS and Schwann cells in the PNS.

Electrical Signaling in Neurons

Resting Membrane Potential

Neurons maintain a resting membrane potential due to the distribution of ions across their membrane.

• Polarized State: The inside of the neuron is negatively charged relative to the outside.

• Key Ions: Na+ (sodium) is concentrated outside the cell; K+ (potassium) is concentrated inside.

Equation:

Where is the membrane potential.

Action Potential Phases

An action potential is a rapid change in membrane potential that allows neurons to transmit signals.

• Depolarization: Na+ channels open, Na+ enters the cell, making the inside more positive.

• Repolarization: K+ channels open, K+ leaves the cell, restoring negative charge inside.

• Hyperpolarization: The membrane potential becomes more negative than the resting potential due to excess K+ outflow.

Equation:

Where is the ionic current, is conductance, is membrane potential, and is equilibrium potential.

Propagation of Action Potentials

Action potentials travel along axons by two main mechanisms:

• Continuous Conduction: Occurs in unmyelinated axons; the action potential moves along every part of the membrane.

• Saltatory Conduction: Occurs in myelinated axons; the action potential 'jumps' between nodes of Ranvier, increasing speed.

Additional info: Saltatory conduction is much faster and more energy-efficient than continuous conduction.

Types of Neurons

Classification by Conduction Velocity

Neurons are classified based on their diameter and myelination, which affect conduction speed.

• A Neurons: Large diameter, heavily myelinated, fastest conduction.

• B Neurons: Medium diameter, lightly myelinated, intermediate speed.

• C Neurons: Small diameter, unmyelinated, slowest conduction.

Example: Pain signals are often carried by C neurons, which conduct slowly.

Neuronal Communication

Synapses and Neurotransmitters

Neurons communicate at specialized junctions called synapses. The presynaptic neuron releases chemical messengers called neurotransmitters that bind to receptors on the postsynaptic neuron.

• Presynaptic Neuron: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Neuron: Receives and responds to neurotransmitters.

• Neurotransmitters: Chemicals that transmit signals across the synapse.

Types of Neurotransmitters:

• Biogenic Amines: Includes dopamine, norepinephrine, serotonin.

• Amino Acids: Includes glutamate, GABA, glycine.

• Peptides: Includes substance P, endorphins.

Additional info: Neurotransmitters can be excitatory or inhibitory, affecting the likelihood of action potential generation in the postsynaptic neuron.