Nervous System Part II: Organization, Function, and Neural Transmission

Organization of the Nervous System

Main Divisions

The nervous system is a complex network responsible for coordinating body activities and responding to internal and external stimuli. It is divided into two main parts:

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and directs responses.

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

Subdivisions of the Peripheral Nervous System

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

• Autonomic Nervous System (ANS): Regulates involuntary functions such as heart rate, digestion, and glandular activity. It is further divided into:

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

  • Parasympathetic Nervous System: Promotes 'rest and digest' activities to conserve energy.

Synchronization of Body Activities

Pathways of Information Flow

• Sensory Input (Afferent Pathway): Sensory receptors detect changes in the environment (e.g., eyes, ears, skin) and send information to the CNS.

• Integration: The CNS processes and interprets sensory input, deciding on an appropriate response.

• Motor Output (Efferent Pathway): The CNS sends signals via motor neurons to effectors (muscles and glands) to produce a response.

Central Nervous System: Spinal Cord

Functions

• Conducts sensory information from the PNS to the brain.

• Conducts motor information from the brain to various effectors (muscles and glands).

Effectors include: Cardiac muscle, skeletal muscle, smooth muscle, and various organs (e.g., heart, lungs, digestive organs).

Peripheral Nervous System

Components and Subdivisions

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

• Motor Neurons: Carry commands from the CNS to effectors.

• Subdivided into the Somatic Nervous System and Autonomic Nervous System.

Somatic vs. Autonomic Nervous System

Somatic Nervous System (SNS)

• Controls voluntary responses involving skeletal muscles.

• Example: Moving your arm to pick up a glass.

Autonomic Nervous System (ANS)

• Controls involuntary responses involving smooth muscle, cardiac muscle, and glands.

• Subdivided into sympathetic and parasympathetic divisions.

Autonomic Nervous System

Structure and Function

• Consists of sensory and motor neurons connecting the CNS with internal organs (heart, lungs, glands, etc.).

• Main function: Monitoring and regulating internal environment conditions.

• Uses preganglionic (arise in CNS, run to ganglion) and postganglionic (run from ganglion to effector) neurons.

Sympathetic vs. Parasympathetic Systems

Sympathetic System	Parasympathetic System
Dilates pupils Inhibits salivation Accelerates heartbeat Inhibits digestion Stimulates glucose release Secretion of adrenaline/noradrenaline Relaxes bladder Stimulates orgasm	Constricts pupils Stimulates salivation Slows heartbeat Stimulates digestion Stimulates bile release Contracts bladder Stimulates sexual arousal

Physiological Effects

System	Sympathetic Effect	Parasympathetic Effect
Heart	Increases heart rate and force	Decreases heart rate
Lungs	Dilates bronchi	Constricts bronchi
Eyes	Dilates pupils (mydriasis)	Constricts pupils (miosis)
Digestive System	Inhibits digestion	Stimulates digestion
Glands	Stimulates adrenaline/noradrenaline release	Activates salivary/lacrimal glands
Urinary System	Constricts vessels, inhibits urination	Relaxes bladder sphincter, encourages urination

Electrical Properties of the Membrane

Background

• Neuronal electrical activity occurs in an aqueous medium (water-based environment).

• Water is a polar solvent, dissolving polar solids like NaCl.

• Polar molecules have charge separation, giving them electrical polarity.

The Importance of Ions

• Key ions: Sodium (Na+), Potassium (K+), Calcium (Ca2+), Chloride (Cl-).

• Cations are positively charged (Na+, K+, Ca2+); anions are negatively charged (Cl-).

Membrane Potential

• There is an unequal distribution of Na+, K+, Cl-, and organic anions across the membrane.

• At rest, there are 10x as many Na+ ions outside the axon as inside.

• Inside the axon are negatively charged ions; K+ is more concentrated inside (20x) than outside.

Maintaining Membrane Potential: Ion Channels and Ion Pumps

Ion Channels

• Pores in the membrane specific for certain ions (e.g., Na+, K+, Cl-).

• Allow passive movement of ions down their concentration gradients.

• Enable rapid changes in membrane potential, crucial for action potential generation and signal transmission.

• Example: Voltage-gated sodium channels open during an action potential, allowing Na+ influx.

Ion Pumps

• Enzymes that use ATP to actively transport ions against their gradients (e.g., Na+/K+-ATPase pump).

• Maintain and restore resting membrane potential by preserving ion gradients.

• Example: The sodium-potassium pump moves 3 Na+ out and 2 K+ in, sustaining the gradient.

Why Both Are Necessary

• Ion channels enable quick ionic movements for rapid signaling.

• Ion pumps ensure long-term stability by maintaining gradients for repeated signaling.

• Together, they balance fast signaling and steady-state maintenance in neurons.

Nerve Impulse Transmission

Overview

• Nerve impulses travel along neurons due to electrical changes across the membrane.

• These impulses are called action potentials.

• The membrane of an unstimulated neuron is polarized (difference in charge inside vs. outside).

Action Potential

• A change in membrane potential causes an action potential.

• Increase in membrane potential = Hyperpolarization

• Decrease in membrane potential = Depolarization

• Action potential is the conducting signal of the neuron.

Stages of Neural Transmission

1. Resting Potential (Polarization): Neuron maintains a negative charge inside the membrane. The sodium-potassium pump transports 3 Na+ out and 2 K+ in, keeping the interior negative.

2. Depolarization: Stimulus opens voltage-gated Na+ channels; Na+ enters, making the inside less negative and eventually positive.

3. Action Potential Formation: If threshold (~-55 mV) is reached, the neuron fires an action potential. Na+ channels open fully, K+ channels begin to open as the potential peaks (~+30 mV).

4. Repolarization: K+ channels open, K+ exits, restoring the negative membrane potential.

5. Hyperpolarization: Membrane potential becomes more negative than resting as K+ channels stay open a bit longer.

6. Refractory Period: Sodium-potassium pump restores ion balance, returning the neuron to resting potential.

7. Propagation of Action Potential: The action potential travels down the axon to the axon terminal, triggering synaptic transmission.

Graphical Representation

The action potential can be visualized as a spike in membrane potential, with phases corresponding to depolarization, repolarization, and hyperpolarization.

Synaptic Transmission

The Synapse

• The synapse is the junction between two neurons.

• The presynaptic neuron sends the signal; the postsynaptic neuron receives it.

Types of Synapses

• Electrical Synapses: Direct connections via pores (gap junctions); ions flow directly, allowing rapid transmission.

• Chemical Synapses: Small gaps where neurotransmitters are released from vesicles in the presynaptic neuron and bind to receptors on the postsynaptic neuron. Transmission is slower but allows for modulation.

Neurotransmitters

• Chemicals released from the presynaptic neuron that bind to specific receptors on the postsynaptic neuron.

• Examples: Acetylcholine, Norepinephrine, Gamma-aminobutyric acid (GABA).

Summary Table: Sympathetic vs. Parasympathetic Effects

Organ/System	Sympathetic Effect	Parasympathetic Effect
Pupils	Dilates	Constricts
Salivation	Inhibits	Stimulates
Heart Rate	Increases	Decreases
Digestion	Inhibits	Stimulates
Bladder	Relaxes	Contracts

Key Equations

• Sodium-Potassium Pump:

$3\text{Na}^+_{\text{in}} + 2\text{K}^+_{\text{out}} + \text{ATP} \rightarrow 3\text{Na}^+_{\text{out}} + 2\text{K}^+_{\text{in}} + \text{ADP} + \text{P}_i$

• Resting Membrane Potential (Nernst Equation):

$E_{\text{ion}} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]_{\text{outside}}}{[\text{ion}]_{\text{inside}}} \right)$

Additional info: The Nernst equation calculates the equilibrium potential for a particular ion based on its concentration gradient across the membrane.