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Nervous Tissue, Spinal Cord, and Brain: Study Guide for Exam 3 (Chapters 12–14)

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Chapter 12: Nervous Tissue

Anatomical and Functional Divisions of the Nervous System

The nervous system is divided into distinct anatomical and functional regions, each with specialized roles in communication and control throughout the body.

  • Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integrating, processing, and coordinating sensory data and motor commands.

  • Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; delivers sensory information to the CNS and carries motor commands to peripheral tissues.

  • Functional Divisions:

    • Afferent Division: Transmits sensory information from receptors to the CNS.

    • Efferent Division: Carries motor commands from the CNS to effectors (muscles and glands).

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

    • Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion); subdivided into sympathetic and parasympathetic divisions.

Structure and Classification of Neurons

Neurons are the basic functional units of the nervous system, specialized for communication.

  • Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

  • Dendrites: Branchlike extensions that receive signals from other neurons.

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

  • Axon Terminals: Endings where neurotransmitters are released to communicate with other cells.

  • Structural Classification:

    • Multipolar: Many dendrites, one axon (most common in CNS).

    • Bipolar: One dendrite, one axon (sensory organs).

    • Unipolar: Single process with cell body off to the side (sensory neurons of PNS).

    • Anaxonic: No obvious axon (rare, found in brain and special sense organs).

  • Functional Classification:

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

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

    • Interneurons: Connect sensory and motor neurons; involved in processing.

Neuroglia: Types, Locations, and Functions

Neuroglia (glial cells) support and protect neurons. They differ between the CNS and PNS.

  • CNS Neuroglia:

    • Astrocytes: Maintain blood-brain barrier, provide structural support, regulate ion/nutrient concentrations.

    • Oligodendrocytes: Form myelin sheaths around CNS axons.

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

    • Ependymal Cells: Line ventricles, produce and circulate cerebrospinal fluid (CSF).

  • PNS Neuroglia:

    • Schwann Cells: Form myelin sheaths around PNS axons; aid in regeneration.

    • Satellite Cells: Surround neuron cell bodies in ganglia; regulate environment.

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane when the cell is at rest.

  • Established by: Unequal distribution of ions (Na+, K+, Cl-) and selective permeability of the membrane.

  • Maintained by: Sodium-potassium pump (Na+/K+ ATPase) and leak channels.

  • Typical Value: Approximately -70 mV in neurons.

  • Equation: (Nernst equation for a single ion)

Action Potentials: Generation and Propagation

An action potential is a rapid, temporary change in membrane potential that travels along the axon.

  • Phases: Depolarization, repolarization, hyperpolarization.

  • Steps:

    1. Threshold reached; voltage-gated Na+ channels open.

    2. Na+ influx causes depolarization.

    3. Na+ channels inactivate; K+ channels open.

    4. K+ efflux causes repolarization.

    5. Temporary hyperpolarization before return to resting potential.

  • Propagation: Action potentials move by continuous conduction (unmyelinated) or saltatory conduction (myelinated).

  • Speed Factors: Axon diameter (larger = faster), myelination (myelinated = faster).

Synapses and Synaptic Activity

A synapse is the junction between two neurons or a neuron and an effector cell.

  • Components: Presynaptic neuron, synaptic cleft, postsynaptic cell.

  • Mechanism: Action potential triggers neurotransmitter release, which binds to receptors on the postsynaptic membrane, causing a response.

  • Types: Electrical (rare, direct ion flow) and chemical (common, neurotransmitter-mediated).

Neurotransmitters and Neuromodulators

Neurotransmitters are chemicals that transmit signals across synapses; neuromodulators modify synaptic activity.

  • Major Neurotransmitters:

    • Acetylcholine (ACh): Excitatory at neuromuscular junctions.

    • Norepinephrine (NE): Excitatory or inhibitory; involved in attention and consciousness.

    • Dopamine: Excitatory or inhibitory; involved in motor control and reward.

    • Serotonin: Affects mood, appetite, and sleep.

    • Gamma-aminobutyric acid (GABA): Main inhibitory neurotransmitter in CNS.

  • Effects: Can be excitatory (depolarize) or inhibitory (hyperpolarize) the postsynaptic membrane.

Information Processing in Nervous Tissue

Neural circuits process information through patterns of connectivity and synaptic integration.

  • Summation: Integration of multiple synaptic inputs (spatial and temporal).

  • Neural Pools: Groups of interconnected neurons with specific functions (e.g., divergence, convergence, reverberation).

Chapter 13: Spinal Cord, Spinal Nerves, and Spinal Reflexes

Structure and Functions of the Spinal Cord

The spinal cord is a cylindrical structure that relays information between the brain and the body and coordinates reflexes.

  • Regions: Cervical, thoracic, lumbar, sacral.

  • Functions: Conducts sensory and motor information; mediates reflexes.

Meningeal Layers

The spinal cord and brain are protected by three connective tissue layers called meninges.

  • Dura Mater: Outermost, tough layer.

  • Arachnoid Mater: Middle, web-like layer.

  • Pia Mater: Innermost, delicate layer adhering to the spinal cord.

White Matter and Gray Matter

White and gray matter have distinct roles in the spinal cord.

  • White Matter: Myelinated axons; transmits signals up and down the cord.

  • Gray Matter: Neuron cell bodies; processes and integrates information.

Spinal Nerves and Nerve Plexuses

Spinal nerves connect the spinal cord to the body; plexuses are networks of nerves serving specific regions.

  • Spinal Nerve Components: Dorsal root (sensory), ventral root (motor), mixed nerve.

  • Nerve Plexuses: Cervical, brachial, lumbar, sacral plexuses.

  • Distribution: Each spinal nerve innervates a specific region (dermatome).

Neuronal Pools and Patterns of Interaction

Neuronal pools are groups of interneurons with specific patterns of synaptic connections.

  • Divergence: One neuron sends signals to many.

  • Convergence: Many neurons send signals to one.

  • Serial Processing: Sequential information processing.

  • Parallel Processing: Simultaneous processing along multiple pathways.

  • Reverberation: Feedback loops for prolonged response.

Neural Reflexes

Reflexes are rapid, automatic responses to stimuli.

  • Steps:

    1. Arrival of stimulus and activation of receptor.

    2. Activation of sensory neuron.

    3. Information processing in CNS.

    4. Activation of motor neuron.

    5. Response by effector.

  • Types: Monosynaptic (e.g., stretch reflex), polysynaptic (e.g., withdrawal reflex).

Chapter 14: The Brain and Cranial Nerves

Major Brain Regions and Ventricles

The brain is organized into several major regions, each with specialized functions.

  • Cerebrum: Largest part; responsible for higher functions (thought, memory, voluntary movement).

  • Diencephalon: Includes thalamus, hypothalamus, epithalamus; relays and processes sensory information, regulates autonomic functions.

  • Brainstem: Includes midbrain, pons, medulla oblongata; controls basic life functions.

  • Cerebellum: Coordinates movement and balance.

  • Ventricles: Four fluid-filled cavities (lateral ventricles, third ventricle, fourth ventricle) containing cerebrospinal fluid.

Protection and Support of the Brain; Cerebrospinal Fluid (CSF)

The brain is protected by the skull, meninges, blood-brain barrier, and CSF.

  • CSF Formation: Produced by choroid plexus in ventricles.

  • CSF Circulation: Flows through ventricles, central canal, and subarachnoid space; reabsorbed into venous blood.

  • CSF Functions: Cushions brain, supports weight, removes waste.

Components and Functions of Brainstem Regions

  • Medulla Oblongata: Regulates heart rate, blood pressure, respiration.

  • Pons: Relays information, regulates breathing.

  • Midbrain: Processes visual and auditory data, controls reflexes.

Cerebellum: Components and Functions

  • Cerebellar Cortex: Outer layer; coordinates voluntary movements.

  • Arbor Vitae: Internal white matter; relays information.

  • Cerebellar Nuclei: Deep clusters of neurons; modulate output.

Diencephalon: Components and Functions

  • Thalamus: Sensory relay station.

  • Hypothalamus: Regulates homeostasis, endocrine functions, emotions.

  • Epithalamus (including pineal gland): Secretes melatonin, regulates circadian rhythms.

Limbic System: Components and Functions

  • Amygdala: Emotion and memory.

  • Hippocampus: Learning and memory formation.

  • Fornix, cingulate gyrus, other structures: Emotional processing.

Cerebrum: Subdivisions and Brain Waves

  • Lobes: Frontal, parietal, temporal, occipital.

  • Functions: Sensory perception, voluntary movement, reasoning, language.

  • Brain Waves (EEG):

    • Alpha: Relaxed, awake state.

    • Beta: Active, alert state.

    • Theta: Light sleep, children.

    • Delta: Deep sleep.

Cranial Nerves: Names, Functions, and Classification

There are twelve pairs of cranial nerves, each with specific functions.

Number

Name

Main Function

Type

I

Olfactory

Smell

Sensory

II

Optic

Vision

Sensory

III

Oculomotor

Eye movement

Motor

IV

Trochlear

Eye movement

Motor

V

Trigeminal

Facial sensation, chewing

Mixed

VI

Abducens

Eye movement

Motor

VII

Facial

Facial expression, taste

Mixed

VIII

Vestibulocochlear

Hearing, balance

Sensory

IX

Glossopharyngeal

Taste, swallowing

Mixed

X

Vagus

Visceral sensation, parasympathetic control

Mixed

XI

Accessory

Neck muscles

Motor

XII

Hypoglossal

Tongue movement

Motor

  • Cranial Reflexes: Involve sensory and/or motor components; can be somatic (e.g., corneal reflex) or visceral (e.g., pupillary reflex).

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