Fundamentals of the Nervous System, Central Nervous System, and Special Senses: Study Guide

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Fundamentals of the Nervous System and Nervous Tissue

Overview of Nervous System Function

The nervous system is responsible for receiving sensory input, integrating information, and producing motor output. It coordinates rapid responses to internal and external stimuli.

Receptors: Specialized cells or structures that detect changes in the environment.
Integration: Processing and interpretation of sensory input in the central nervous system (CNS).
Effectors: Muscles or glands that respond to nervous system commands.

Afferent vs. Efferent Pathways

Afferent (sensory) pathways carry information toward the CNS, while efferent (motor) pathways transmit commands from the CNS to effectors.

Afferent: Sensory neurons; input to CNS.
Efferent: Motor neurons; output from CNS.

Divisions of the Nervous System

The nervous system is divided into central and peripheral components, each with specialized functions.

CNS: Brain and spinal cord; integration and processing.
PNS: Nerves and ganglia; communication between CNS and body.
Somatic: Voluntary control (skeletal muscles).
Autonomic: Involuntary control (smooth muscle, glands).

Neurons: Structure and Function

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

Characteristics: Excitability, conductivity, longevity, amitotic (most do not divide), high metabolic rate.
Structure:
- Dendrites: Receive signals.
- Cell body (soma): Contains nucleus and organelles.
- Axon: Transmits signals away from cell body.

Synapses

Synapses are junctions where neurons communicate with other neurons or effectors.

Chemical synapses: Use neurotransmitters to transmit signals.
Electrical synapses: Direct electrical connection via gap junctions (less common).

Neuroglia (Glial Cells)

Neuroglia support and protect neurons. Different types are found in the CNS and PNS.

CNS:
- Oligodendrocytes: Form myelin sheaths.
- Microglia: Immune defense.
- Astrocytes: Support, blood-brain barrier, nutrient supply.
- Ependymal cells: Line ventricles, produce cerebrospinal fluid.
PNS:
- Schwann cells: Form myelin in PNS.
- Satellite cells: Support neuron cell bodies in ganglia.

Myelination

Myelin increases the speed of nerve impulse conduction.

CNS: Oligodendrocytes.
PNS: Schwann cells.
Loss of myelination: Leads to disorders like multiple sclerosis; slows conduction.
Conduction velocity: Faster in myelinated and larger diameter axons.

Chemical Synapses: Structure and Function

Chemical synapses transmit signals via neurotransmitters released in response to calcium influx.

Structure: Presynaptic terminal, synaptic cleft, postsynaptic membrane.
Mechanism: Action potential triggers Ca2+ influx, neurotransmitter release, postsynaptic response.

Action Potential Steps

Action potentials are rapid changes in membrane potential that propagate along axons.

Resting membrane potential: Typically -70 mV; maintained by Na-K pump.
Depolarization: Na+ influx.
Repolarization: K+ efflux.
Hyperpolarization: Excess K+ outflow.

Equation:

Graded vs. Action Potentials

Graded potentials are local changes in membrane potential; action potentials are all-or-none and propagate.

Graded: Vary in size, fade with distance.
Action: Fixed size, self-propagating.

EPSPs vs. IPSPs

Excitatory postsynaptic potentials (EPSPs) depolarize the membrane; inhibitory postsynaptic potentials (IPSPs) hyperpolarize it.

EPSP: Increases likelihood of action potential.
IPSP: Decreases likelihood.

Refractory Period

The refractory period is the time during which a neuron cannot fire another action potential.

Absolute: No action potential possible.
Relative: Action potential possible with stronger stimulus.

Signal Conduction: Myelinated vs. Unmyelinated Axons

Myelinated axons conduct signals via saltatory conduction; unmyelinated via continuous conduction.

Saltatory: Jumps between nodes of Ranvier; faster.
Continuous: Slower; along entire membrane.

Summation of EPSPs and IPSPs

Neurons integrate multiple inputs via summation.

Temporal: Multiple signals over time.
Spatial: Multiple signals from different locations.

Neurotransmitters

Neurotransmitters are chemical messengers; classified by structure.

Biogenic amines: Catecholamines (dopamine, norepinephrine, epinephrine).
Amino acids: Glutamate, GABA.
Peptides: Substance P, endorphins.

Direct vs. Indirect Neurotransmitter Action

Direct action involves receptor binding and immediate effect; indirect uses second messengers.

Direct: Ion channel opening.
Indirect: G-protein coupled, slower, longer-lasting.

Neural Circuits

Neural circuits are patterns of connectivity among neurons.

Diverging: One input, many outputs.
Converging: Many inputs, one output.
Reverberating: Feedback loop.
Parallel-after discharge: Multiple pathways, simultaneous output.

The Central Nervous System

Gray vs. White Matter

Gray matter contains neuron cell bodies; white matter consists of myelinated axons.

Gray matter: Processing centers.
White matter: Communication pathways.

Ventricles and Cerebrospinal Fluid (CSF) Flow

Ventricles are cavities in the brain filled with CSF, which cushions and nourishes the CNS.

CSF flow: Produced by choroid plexus, circulates through ventricles, central canal, and subarachnoid space.

Blood-Brain Barrier

The blood-brain barrier protects the brain from harmful substances while allowing essential nutrients to pass.

Formed by: Tight junctions between endothelial cells, astrocyte end-feet.

Areas of the Cerebrum

The cerebrum is divided into functional areas for motor, sensory, and association functions.

Primary motor cortex: Voluntary movement.
Premotor cortex: Planning movements.
Primary somatosensory: Sensory input.
Somatosensory association: Interpretation of sensory input.
Special senses areas: Vision, hearing, etc.
Multimodal areas: Anterior and posterior association areas; integrate information.
Lateralization: Functional differences between hemispheres.
White matter tracts:
- Association: Connect within hemisphere.
- Commissural: Connect hemispheres (e.g., corpus callosum).
- Projection: Connect cortex with lower CNS.

Diencephalon Areas

Thalamus: Relay station for sensory information.

Brain Stem Areas

Medulla oblongata: Autonomic functions (heart rate, breathing).
Midbrain: Visual and auditory reflexes.
Pons: Bridge between cerebrum and cerebellum; breathing regulation.

Cerebellum Function

The cerebellum coordinates movement and balance.

Limbic System and Reticular Formation

Limbic system: Emotion, memory.
Reticular formation: Alertness, sleep-wake cycles.

Memory

Relay during sleep: Hippocampus and thalamus.
Short-term to long-term: Repetition, emotional impact, synaptic potentiation.
Synaptic potentiation: Strengthening of synapses.

Brain Waves (EEG)

EEG measures electrical activity; different waves correspond to different states (alpha, beta, delta, theta).

Decussation

Decussation is the crossing of nerve fibers from one side to the other, often in the medulla.

Sensory vs. Motor Tracts

Sensory: Ascending; typically three neurons.
Motor: Descending; typically two neurons.
Main tracts: Spinothalamic (sensory), corticospinal (motor).

The Peripheral Nervous System and Special Senses

Sensory Cells and Receptors

Sensory receptors detect specific stimuli and convert them into electrical signals.

Types: Mechanoreceptors (touch), nociceptors (pain), thermoreceptors (temperature), photoreceptors (light), chemoreceptors (chemicals).
Adaptation: Decreased response to constant stimulus.

Pain

Pain is detected by nociceptors; modulated by endogenous opioids and other factors.

Vision: Anatomy and Physiology

The eye consists of three tunics, each with specialized structures and functions.

Fibrous tunic: Cornea, sclera.
Vascular tunic: Choroid, ciliary body, iris.
Neural tunic: Retina, macula lutea, fovea centralis.
Rods: Low-light vision; contain rhodopsin.
Cones: Color vision; three types (red, green, blue).
Pathway: Light → photoreceptors → bipolar cells → ganglion cells → optic nerve.
Rhodopsin: Visual pigment in rods; undergoes bleaching.
Glutamate: Neurotransmitter released by photoreceptors.

Olfaction (Smell)

Olfactory receptors detect odorants; signals travel via olfactory nerve to the brain.

Gustation (Taste)

Taste buds detect five basic tastes; signals travel via cranial nerves to the brain.

Tastes: Sweet, sour, salty, bitter, umami.

Hearing and Equilibrium

The ear detects sound and maintains balance.

Outer ear: Collects sound.
Middle ear: Transmits vibrations.
Inner ear: Cochlea (hearing), vestibular apparatus (balance).
Sound pathway: Vibration → fluid movement → hair cells → nerve impulses.
Hair cells: Stereocilia, tip links, potassium influx.
Equilibrium:
- Linear: Saccule, utricle, otolithic membrane.
- Rotational: Semicircular ducts, fluid movement.

Regeneration of Nerves

Peripheral nerves can regenerate; CNS nerves have limited regeneration.

Cranial Nerves and Nerve Plexuses

Cranial nerves serve sensory and motor functions; plexuses are networks of spinal nerves.

Cranial nerves: Optic, oculomotor, abducens, vestibulocochlear, glossopharyngeal, vagus.
Plexuses: Phrenic (cervical), brachial, lumbar.

Reflex Arcs and Reflexes

Reflexes are rapid, automatic responses; reflex arcs consist of sensory neuron, interneuron, motor neuron.

Receptor Type	Stimulus Detected
Mechanoreceptor	Touch, pressure
Nociceptor	Pain
Thermoreceptor	Temperature
Photoreceptor	Light
Chemoreceptor	Chemicals

Type of Equilibrium	Structure	Main Function
Linear	Saccule, Utricle	Detects straight-line acceleration
Rotational	Semicircular ducts	Detects rotational movement

Neuroglia Type	Division	Main Function
Oligodendrocytes	CNS	Myelination
Microglia	CNS	Immune defense
Astrocytes	CNS	Support, blood-brain barrier
Ependymal	CNS	CSF production
Schwann	PNS	Myelination
Satellite	PNS	Support neuron cell bodies