Nervous System: Structure and Function

Nervous System

Structure and Function

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is essential for sensation, movement, cognition, and maintaining homeostasis.

• Sensory Function: Gathers input from the environment through sensory receptors.

• Integrative Function: Processes and interprets sensory input, determining the appropriate response.

• Motor Function: Activates effectors such as muscles and glands to produce a response.

Organization of the Nervous System

Main Divisions

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

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Divided into cranial nerves (12 pairs) and spinal nerves (31 pairs).

Functional Classification

• Sensory (Afferent) Neurons: Carry impulses toward the CNS from sensory receptors.

• Motor (Efferent) Neurons: Carry impulses away from the CNS to effectors (muscles and glands).

• Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Neuron Anatomy

Structure of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Short, branched extensions that receive signals from other neurons.

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

• Myelin Sheath: Insulating layer that speeds up impulse transmission; formed by Schwann cells in the PNS and oligodendrocytes in the CNS.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal conduction.

Structural Classification of Neurons

• Multipolar Neurons: Many processes; most common in CNS.

• Bipolar Neurons: Two processes; found in sensory organs (e.g., retina).

• Unipolar Neurons: One process; found in sensory ganglia of the PNS.

Neuroglia (Glial Cells)

Types and Functions

• Astrocytes: Maintain the blood-brain barrier, provide nutrients, and support neurons.

• Oligodendrocytes: Form myelin sheath in the CNS.

• Schwann Cells: Form myelin sheath in the PNS.

• Microglia: Act as phagocytes, removing debris and pathogens.

• Ependymal Cells: Line ventricles of the brain and central canal of the spinal cord; help circulate cerebrospinal fluid (CSF).

• Satellite Cells: Surround neuron cell bodies in the PNS, regulating the environment.

Cell Transport in the Nervous System

Cell Membrane Structure

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward. Acts as a selective barrier.

• Membrane Proteins: Function as channels, transporters, and pumps.

Types of Transport

• Passive Transport: No energy required; substances move down their concentration gradient.

• Active Transport: Requires ATP; substances move against their concentration gradient.

Passive Transport Mechanisms

• Simple Diffusion: Small, nonpolar molecules (e.g., O2, CO2) pass directly through the membrane.

• Facilitated Diffusion: Charged or polar molecules (e.g., Na+, K+, Ca2+, glucose) move through protein channels.

Ion Channels in Neurons

• Leak Channels: Always open; maintain resting membrane potential.

• Voltage-Gated Channels: Open/close in response to changes in membrane potential; essential for action potentials.

• Ligand-Gated Channels: Open when a specific chemical (ligand) binds; found at synapses.

Active Transport Example: Sodium-Potassium Pump

• Moves 3 Na+ out and 2 K+ into the cell per ATP molecule used.

• Maintains the electrochemical gradient necessary for nerve impulse transmission.

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

Nerve Impulses

Resting Membrane Potential (RMP)

• Difference in electrical charge across the neuron's membrane at rest (typically -70 mV).

• Maintained by the sodium-potassium pump and leak channels.

Action Potential

• Depolarization: Membrane potential becomes less negative; Na+ channels open and Na+ enters the cell.

• Repolarization: K+ channels open, K+ leaves the cell, restoring negativity.

• Hyperpolarization: Membrane potential becomes more negative than RMP before returning to baseline.

• Threshold: The critical level (usually -55 mV) that must be reached to trigger an action potential.

$\text{Resting potential} \rightarrow \text{Depolarization} \rightarrow \text{Repolarization} \rightarrow \text{Hyperpolarization} \rightarrow \text{Resting potential}$

Propagation of Action Potentials

• Action potentials travel along the axon as each segment depolarizes the next.

• Myelinated axons conduct impulses faster via saltatory conduction (jumping between nodes of Ranvier).

The Synapse

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

• Neurotransmitters: Chemical messengers released from synaptic vesicles in response to Ca2+ influx; bind to receptors on the postsynaptic cell to continue the signal.

Examples of Neurotransmitters

Name	Main Function	Effect
Acetylcholine (ACh)	Stimulates muscle contraction; attention, learning	Excitatory
Dopamine	Mood, motivation, reward; movement control	Excitatory/Inhibitory
Serotonin	Sleep, mood, appetite, calmness	Inhibitory
Endorphins	Pain relief, euphoria	Inhibitory
Glutamate	Learning, memory, major brain signal	Excitatory
GABA	Reduces neural activity; calming	Inhibitory

Central Nervous System: Brain and Spinal Cord

Brain Structure

• Cerebrum: Responsible for thinking, sensing, movement, memory, and reasoning. Divided into lobes: frontal, parietal, temporal, occipital, and insula.

• Diencephalon: Includes thalamus (sensory relay) and hypothalamus (homeostasis, hormone regulation).

• Cerebellum: Coordinates balance, posture, and skeletal muscle activity.

• Brainstem: Includes midbrain (reflexes), pons (breathing, relay), and medulla oblongata (heart rate, blood pressure, vital reflexes).

Spinal Cord

• Slender column of nervous tissue; center for spinal reflexes.

• Contains ascending (sensory) and descending (motor) tracts.

• Protected by vertebrae and meninges (dura mater, arachnoid mater, pia mater).

Gray Matter vs. White Matter

• Gray Matter: Neuronal cell bodies; processes information.

• White Matter: Myelinated axons; transmits information.

Cerebrospinal Fluid (CSF)

• Produced by the choroid plexus in brain ventricles.

• Cushions the brain and spinal cord, removes waste, and distributes nutrients.

Peripheral Nervous System

• Cranial Nerves: 12 pairs; connect to the brain.

• Spinal Nerves: 31 pairs; connect to the spinal cord.

Sensory Systems

General vs. Special Senses

• General Senses: Touch, temperature, pain; receptors distributed throughout the body.

• Special Senses: Vision, hearing, balance, taste, smell; have specialized organs.

Vision

• Eye has three layers: fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), and sensory (retina).

• Photoreceptors (rods and cones) in the retina convert light into action potentials.

Hearing and Equilibrium

• Ear divided into external, middle, and inner parts.

• Sound waves cause vibrations transmitted through the tympanic membrane and ossicles to the cochlea.

• Hair cells in the cochlea generate action potentials sent to the brain via the auditory nerve.

• Semicircular canals and vestibule in the inner ear detect head movement and maintain balance.

Summary Table: Main Parts of the Nervous System

Division	Main Components	Primary Function
Central Nervous System (CNS)	Brain, Spinal Cord	Integration, processing, command
Peripheral Nervous System (PNS)	Cranial Nerves, Spinal Nerves	Communication between CNS and body
Sensory (Afferent) Division	Sensory receptors, afferent neurons	Transmit sensory input to CNS
Motor (Efferent) Division	Motor neurons, effectors	Transmit commands from CNS to effectors

Additional info: This guide expands on the original notes by providing definitions, examples, and context for key terms and processes, ensuring a comprehensive overview suitable for exam preparation in General Biology.