Nervous System Overview

Functions of the Nervous System

The nervous system is responsible for monitoring the internal and external environment, integrating sensory information, and coordinating voluntary and involuntary responses with other organ systems.

• Monitoring: Detects changes inside and outside the body.

• Integration: Processes and interprets sensory input.

• Coordination: Directs responses by activating muscles or glands.

Organization of the Nervous System

Central Nervous System (CNS) vs. Peripheral Nervous System (PNS)

• CNS: Consists of the brain and spinal cord. Damage to the CNS typically does not heal well. Major cell types include astrocytes, ependymal cells, oligodendrocytes, and microglia.

• PNS: Communicates with the rest of the body via efferent (motor) and afferent (sensory) pathways. Key cell types are satellite cells and Schwann cells (which are crucial for myelination in the PNS).

Efferent vs. Afferent Pathways

• Efferent: Carries motor information from the CNS to muscles and glands.

• Afferent: Carries sensory information from the body to the CNS.

Neuron Structure and Function

Anatomy of a Neuron

Neurons are the fundamental units of the nervous system, specialized for communication. Each part of a neuron has a specific function:

• Dendrites: Receive incoming signals from other cells.

• Cell Body (Soma): Contains the nucleus and organelles; essential for cell function.

• Axon Hillock: Site where action potentials are initiated.

• Axon: Conducts action potentials away from the cell body.

• Synaptic Terminals (Axon Terminals): Communicate with other neurons or effectors.

• Nucleus: Contains DNA and RNA for protein synthesis.

• Nucleolus: Site of RNA synthesis.

• Collateral: Branches of the axon ending in synaptic terminals.

• Nissl Bodies: Clusters of rough ER and ribosomes, giving gray matter its color.

Structural Types of Neurons

• Multipolar: Several dendrites, one axon; most common, especially motor neurons.

• Unipolar: Dendrites and axon are continuous; most sensory neurons in the PNS.

• Bipolar: One dendrite, one axon; found in special sense organs (e.g., eyes, ears).

Functional Classes of Neurons

• Sensory Neurons (Afferent): Transmit sensory information to the CNS.

• Motor Neurons (Efferent): Transmit commands from the CNS to effectors.

• Interneurons: Connect neurons within the CNS, coordinate motor activity, and process information.

Visceral vs. Somatic Neurons

• Visceral Sensory Neurons: Monitor internal environment (organs).

• Visceral Motor Neurons: Control involuntary actions (cardiac, smooth muscle, glands).

• Somatic Sensory Neurons: Monitor external environment (e.g., proprioception).

• Somatic Motor Neurons: Control voluntary movements (skeletal muscle).

Spinal Cord Structure

Meningeal Layers

The meninges are protective membranes surrounding the brain and spinal cord, providing shock absorption, stability, and nutrient supply.

• Dura Mater: Outermost, tough layer.

• Arachnoid: Middle layer, reduces friction.

• Pia Mater: Innermost layer, contains blood vessels.

Ventral and Dorsal Roots

• Ventral Roots: Contain axons of motor neurons; join dorsal roots to form spinal nerves.

• Dorsal Roots: Contain axons of sensory neurons; dorsal root ganglia house sensory neuron cell bodies.

Gray Horns and White Columns

• Gray Horns: Projections of gray matter; posterior horns contain sensory nuclei, anterior horns control skeletal muscle.

• White Columns: Myelinated and unmyelinated axons; divided into posterior, lateral, and anterior columns carrying sensory or motor information.

Neuronal Signaling

Depolarization and Hyperpolarization

• Resting Membrane Potential: Typically -70 mV; maintained by sodium and potassium gradients.

• Depolarization: Interior becomes less negative (more positive), increasing likelihood of action potential.

• Hyperpolarization: Interior becomes more negative, decreasing likelihood of action potential.

Action Potentials: Saltatory vs. Continuous Propagation

• Continuous Propagation: Occurs in unmyelinated axons; slow, stepwise conduction (~2 mph).

• Saltatory Propagation: Occurs in myelinated axons; rapid, jumping conduction between nodes of Ranvier (up to 300 mph).

Divergence vs. Convergence

• Divergence: One input spreads to multiple targets (amplification/distribution).

• Convergence: Multiple inputs converge on one target (summation/integration).

Brain Structure and Function

Regions of the Cerebrum

• Frontal Lobe: Primary motor cortex (voluntary movement), gustatory cortex (taste).

• Parietal Lobe: Primary sensory cortex (touch, pain, pressure, temperature).

• Occipital Lobe: Visual cortex (vision).

• Temporal Lobe: Auditory and olfactory cortex (hearing, smell).

Cerebellum

Coordinates voluntary movements, balance, posture, and motor learning. Acts as a processing center for fine-tuning motor commands.

Association Areas

Interpret incoming sensory and motor information, coordinate responses, and allow recognition of sensory input.

Integrative Centers

• Broca's Area: Speech production and language processing (frontal lobe).

• Wernicke's Area: Language comprehension (temporal lobe).

• Prefrontal Cortex: Executive functions (decision-making, planning).

Diencephalon and Brain Stem

• Diencephalon: Includes epithalamus (melatonin production), thalamus (sensory relay), hypothalamus (autonomic and endocrine regulation).

• Brain Stem: Controls life-sustaining functions and relays information between brain and body.

  • Midbrain: Processes visual/auditory info, involuntary motor responses.

  • Pons: Connects cerebellum to brain stem, motor control.

  • Medulla Oblongata: Regulates heart rate, breathing, and relays sensory info.

Cranial Nerves

Names and Functions

1. Olfactory: Smell

2. Optic: Vision

3. Oculomotor: Eye movement, lens shape, pupil size

4. Trochlear: Eye movement

5. Trigeminal: Sensory from head, motor to chewing muscles

6. Abducens: Eye movement

7. Facial: Facial expression, taste, tears, salivation

8. Vestibulocochlear: Hearing

9. Glossopharyngeal: Tongue and pharynx movement, taste

10. Vagus: Sensory/motor to thoracic and abdominal organs

11. Accessory: Neck and back muscles

12. Hypoglossal: Tongue movement

Nerve Plexuses

• Cervical Plexus: Neck, diaphragm, upper chest, neck, ears

• Brachial Plexus: Shoulder girdle, arms

• Lumbar Plexus: Pelvic girdle, upper thigh

• Sacral Plexus: Hip, leg

Autonomic Nervous System

Sympathetic vs. Parasympathetic

• Sympathetic: "Fight or flight"; increases heart rate, respiration, releases energy, inhibits non-essential functions.

• Parasympathetic: "Rest and digest"; conserves energy, promotes digestion, slows heart rate.

Sensory Systems

General vs. Special Senses

• General Senses: Temperature, pain, touch, pressure, vibration, proprioception.

• Special Senses: Olfaction, vision, gustation, hearing, equilibrium.

Types of Sensory Receptors

• Nociceptors: Detect pain (temperature extremes, mechanical damage, chemicals).

• Thermoreceptors: Detect temperature changes; adapt quickly.

• Mechanoreceptors: Detect touch, pressure, vibration, proprioception (includes tactile receptors, baroreceptors, proprioceptors).

• Chemoreceptors: Detect chemical changes (pH, CO2 levels); usually subconscious.

Examples of Mechanoreceptors

• Tactile Receptors: Fine and crude touch, pressure, vibration (e.g., Merkel's disks, Meissner's corpuscles, Pacinian corpuscles).

• Baroreceptors: Monitor pressure in organs; regulate cardiac, respiratory, digestive, urinary functions.

• Proprioceptors: Monitor muscle and joint position (muscle spindles, Golgi tendon organs, joint capsule receptors).

Additional info:

Tickle and itch sensations are mediated by the same type of receptor but are perceived differently by the brain.