Introduction to the Nervous System and Nervous Tissue

Overview of Nervous System Structures

The nervous system is a complex network responsible for coordinating body activities and processing information. Its main structures include the brain, spinal cord, nerves, and ganglia.

• Brain: Central organ for processing and integrating information.

• Spinal Cord: Connects the brain to peripheral nerves; involved in reflexes.

• Nerves: Bundles of axons transmitting signals between CNS and body.

• Ganglia: Clusters of neuron cell bodies in the PNS.

Functions of the Nervous System

The nervous system performs three primary functions: sensory, integrative, and motor.

• Sensory Function: Detects internal and external stimuli via receptors.

• Integrative Function: Processes and interprets sensory input, making decisions.

• Motor Function: Initiates responses by activating effectors (muscles/glands).

• Example: Touching a hot surface triggers sensory input, integration in the CNS, and a motor response (withdrawal).

Divisions of the Nervous System

The nervous system is divided into two main anatomical divisions:

• Central Nervous System (CNS): Includes the brain and spinal cord; responsible for integration and command.

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

Functional Divisions of the Peripheral Nervous System

The PNS is functionally divided into:

• Somatic Nervous System (SNS): Controls voluntary movements; under conscious control.

• Autonomic Nervous System (ANS): Regulates involuntary functions; under unconscious control.

• Subdivisions of ANS: Sympathetic ("fight or flight") and Parasympathetic ("rest and digest").

Neurons vs. Neuroglial Cells

Neurons are excitable cells that transmit electrical signals. Neuroglial cells (glia) support and protect neurons.

• Neurons: Have dendrites (receive signals), axons (transmit signals), and cell bodies.

• Neuroglial Cells: Six types, divided by location:

• Dendrites: Receive incoming signals.

• Axon: Conducts impulses away from the cell body.

Myelin Sheath and Action Potential Propagation

The myelin sheath is a fatty layer that insulates axons, increasing the speed of electrical signal transmission.

• Function: Prevents ion leakage, enables rapid signal conduction.

• Effect on Action Potential: Allows saltatory conduction (jumping between nodes of Ranvier), which is faster than continuous conduction.

Classification of Neurons

Neurons are classified by structure and function.

• Structural: Multipolar, bipolar, unipolar.

• Functional: Sensory (afferent), motor (efferent), interneurons.

Types of Ion Channels

Ion channels in neuron membranes allow selective ion movement, crucial for electrical signaling.

• Leak Channels: Always open; maintain resting potential.

• Voltage-Gated Channels: Open/close in response to membrane potential changes.

• Chemically-Gated Channels: Open/close in response to specific molecules (e.g., neurotransmitters).

• Mechanically-Gated Channels: Respond to physical deformation (e.g., touch).

Membrane Potentials: Polarization States

Neurons exhibit changes in membrane potential during signaling.

• Polarized: Resting state; inside is negative relative to outside.

• Depolarized: Membrane potential becomes less negative (more positive).

• Repolarized: Return to resting negative potential.

• Hyperpolarized: Membrane potential becomes more negative than resting.

Action Potentials vs. Graded Potentials

Action potentials are all-or-none electrical impulses; graded potentials are variable changes in membrane potential.

• Action Potential: Rapid, uniform, travels long distances.

• Graded Potential: Localized, varies in size, decays with distance.

Key Membrane Potential Values

Typical values for a neuron (in millivolts):

• Resting Potential: mV

• Threshold: mV

• Depolarization Peak: mV

• Repolarization: Returns toward mV

• Hyperpolarization: mV

Ions Involved in Action Potentials

Movement of sodium (Na+) and potassium (K+) ions generates action potentials.

• At Rest: Na+ higher outside, K+ higher inside.

• During Action Potential: Na+ moves in (depolarization), K+ moves out (repolarization).

Refractory Periods

The refractory period is when a neuron cannot fire another action potential.

• Absolute Refractory Period: No new action potential possible.

• Relative Refractory Period: Action potential possible with stronger stimulus.

Saltatory vs. Continuous Conduction

Action potentials travel differently depending on myelination.

• Saltatory Conduction: Jumps between nodes of Ranvier; fast, occurs in myelinated axons.

• Continuous Conduction: Moves along entire axon; slower, occurs in unmyelinated axons.

Axon Diameter and Propagation Speed

Larger axon diameter increases conduction speed due to lower resistance.

• Thicker axons: Faster signal transmission.

• Thinner axons: Slower transmission.

Synapses

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

• Presynaptic cell: Sends signal.

• Postsynaptic cell: Receives signal.

Chemical vs. Electrical Synapses

Synapses can be chemical or electrical.

• Chemical Synapse: Uses neurotransmitters to transmit signals across a gap.

• Electrical Synapse: Direct ion flow via gap junctions; faster, less common.

Synaptic Transmission

At a chemical synapse, the action potential triggers neurotransmitter release, which binds to receptors on the postsynaptic cell, causing a response.

• Steps: Action potential arrives, Ca2+ influx, neurotransmitter release, receptor activation, postsynaptic potential.

Neurotransmitters

Neurotransmitters are chemical messengers released by neurons to transmit signals across synapses.

• Examples: Acetylcholine, dopamine, serotonin, glutamate.

Fate of Neurotransmitters After Action Potential

After signal transmission, neurotransmitters are removed from the synaptic cleft by:

• Reuptake: Taken back into presynaptic neuron.

• Enzymatic degradation: Broken down by enzymes.

• Diffusion: Move away from synapse.