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Nervous Tissue, Brain, and Spinal Cord: Study Guide for BIOL 203 Exam 3

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Chapter 11: Nervous Tissue

General Functions of the Nervous System

The nervous system is a complex network responsible for controlling and integrating all body activities. It detects changes, processes information, and coordinates responses.

  • Sensory Input: Detects internal and external stimuli via sensory receptors.

  • Integration: Processes and interprets sensory input, deciding what action is needed.

  • Motor Output: Activates effector organs (muscles and glands) to produce a response.

  • Homeostasis: Maintains internal balance by regulating physiological processes.

  • Mental Activity: Responsible for consciousness, memory, and learning.

Organization of the Nervous System

  • Central Nervous System (CNS): Consists of the brain and spinal cord; integrates and processes information.

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

  • Sensory (Afferent) Division: Transmits sensory information to the CNS.

  • Motor (Efferent) Division: Carries commands from the CNS to effector organs.

  • Somatic Sensory: Receives information from skin, skeletal muscles, and joints.

  • Visceral Sensory: Receives information from internal organs.

  • Control System Components: Sensory receptors → afferent pathways → integrating center → efferent pathways → effector organs.

Neurons

  • Major Components:

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

    • Nucleus & Nucleolus: Genetic control and ribosome production.

    • Chromatophilic Substance (Nissl Bodies): Rough ER for protein synthesis.

    • Axon Hillock: Initiates action potentials.

    • Dendrites: Receive input signals.

    • Axon: Transmits output signals.

  • Structural Types:

    • Unipolar (Pseudounipolar): Single process; sensory neurons.

    • Bipolar: Two processes; found in special senses (e.g., retina).

    • Multipolar: Many processes; most common, especially in CNS.

  • Functional Types:

    • Sensory (Afferent): Transmit impulses to CNS.

    • Interneurons (Association): Connect neurons within CNS.

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

Neuroglial (Glial) Cells

  • Types and Functions:

    • Astrocytes (CNS): Support, blood-brain barrier, regulate environment.

    • Oligodendrocytes (CNS): Myelinate CNS axons.

    • Microglia (CNS): Immune defense, phagocytosis.

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

    • Schwann Cells (PNS): Myelinate PNS axons.

    • Satellite Cells (PNS): Support neuron cell bodies in ganglia.

  • Myelination: Insulates axons, increases conduction speed. In CNS, oligodendrocytes myelinate multiple axons; in PNS, Schwann cells myelinate single axons.

Neurophysiology

  • Ion Channels:

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

    • Voltage-Gated Channels: Open/close in response to membrane potential changes.

    • Ligand-Gated Channels: Open in response to chemical signals.

    • Mechanically-Gated Channels: Open in response to physical deformation.

  • Resting Membrane Potential (RMP): Typically -70 mV; maintained by ion gradients and selective permeability.

    • High K+ inside, high Na+ outside.

    • Sodium-potassium ATPase pump maintains gradients:

  • Depolarization: Membrane potential becomes less negative.

  • Repolarization: Return to resting potential after depolarization.

  • Hyperpolarization: Membrane potential becomes more negative than resting.

  • Threshold: Minimum depolarization needed to trigger action potential.

  • Graded Potentials: Local changes, decrease with distance, occur in dendrites/cell body.

  • Action Potentials: All-or-none, propagate along axon, involve voltage-gated Na+ and K+ channels.

  • Phases of Action Potential:

    • Depolarization: Na+ influx

    • Repolarization: K+ efflux

    • Hyperpolarization: K+ channels remain open

  • Conduction:

    • Unmyelinated: Continuous conduction

    • Myelinated: Saltatory conduction (jumps between nodes of Ranvier)

    • Conduction velocity increases with axon diameter and myelination.

  • Refractory Periods:

    • Absolute: No new action potential possible.

    • Relative: Stronger stimulus needed for new action potential.

Neurotransmitters, Neuromodulators & Synaptic Transmission

  • Synapse: Junction between neurons.

    • Electrical Synapse: Direct ion flow via gap junctions.

    • Chemical Synapse: Neurotransmitter crosses synaptic cleft.

  • Chemical Synapse Structures: Axon terminal, voltage-gated Ca2+ channels, synaptic vesicles, synaptic cleft, postsynaptic receptors.

  • Synaptic Transmission Steps:

    1. Action potential arrives at axon terminal.

    2. Voltage-gated Ca2+ channels open; Ca2+ enters.

    3. Neurotransmitter released into synaptic cleft.

    4. Neurotransmitter binds to postsynaptic receptors.

    5. Postsynaptic potential generated (EPSP or IPSP).

  • EPSP (Excitatory Postsynaptic Potential): Depolarizes postsynaptic membrane.

  • IPSP (Inhibitory Postsynaptic Potential): Hyperpolarizes postsynaptic membrane.

  • Summation:

    • Temporal: Multiple signals from one neuron over time.

    • Spatial: Signals from multiple neurons at once.

  • Neurotransmitter Effects: Can vary by receptor type (e.g., acetylcholine excites skeletal muscle, inhibits cardiac muscle).

  • Fast (Ionotropic) vs. Slow (Metabotropic) Responses: Ionotropic receptors directly open ion channels; metabotropic use second messengers.

  • Common Neurotransmitters:

    • Excitatory: Glutamate, acetylcholine

    • Inhibitory: GABA, glycine

  • Termination Mechanisms: Reuptake, enzymatic breakdown, diffusion away from synapse.

Integration of Neural Information

  • Neural Circuit: Functional group of interconnected neurons.

  • Types:

    • Converging: Many inputs, one output.

    • Diverging: One input, many outputs.

Homeostatic Imbalance and Clinical Focus

  • Neurotoxins: Substances that disrupt nervous system function (e.g., tetrodotoxin blocks Na+ channels).

  • Local Anesthetics: Block voltage-gated Na+ channels, preventing pain signal transmission.

Chapters 12 & 13: Brain & Cranial Nerves

Division, Origin, & Function of Brain Regions

  • Five Developmental Regions:

    • Telencephalon: Cerebrum

    • Diencephalon: Thalamus, hypothalamus, epithalamus

    • Mesencephalon: Midbrain

    • Metencephalon: Pons, cerebellum

    • Myelencephalon: Medulla oblongata

  • Major Adult Brain Areas and Functions:

    • Cerebrum: Higher functions, voluntary movement, sensory perception.

    • Diencephalon: Sensory relay, autonomic control, hormone regulation.

    • Brainstem: Basic life functions (breathing, heart rate).

    • Cerebellum: Coordination, balance.

  • Lobes of Cerebral Cortex:

    • Frontal: Motor control, planning, speech.

    • Parietal: Sensory processing.

    • Temporal: Hearing, memory.

    • Occipital: Vision.

    • Insula: Taste, visceral sensation.

  • Limbic System: Emotion, memory, motivation; located in medial temporal lobe.

  • Reticular Activating System: Arousal, consciousness; runs through brainstem.

  • Cerebral Hemispheric Specialization: Left: language, logic; Right: spatial, creative. Corpus callosum connects hemispheres.

  • Memory Storage: Hippocampus, amygdala, cortex; consolidation involves synaptic changes.

  • Sensory and Motor Homunculi: Maps of body regions in cortex; clinically relevant for localizing brain lesions.

Protective Roles: Cranial Bones, Meninges, & Cerebrospinal Fluid

  • Meninges: Dura mater, arachnoid mater, pia mater; protect brain, enclose CSF.

  • Cerebrospinal Fluid (CSF): Cushions brain, removes waste, circulates in ventricles and subarachnoid space, reabsorbed via arachnoid granulations.

  • Blood-Brain Barrier: Tight junctions in capillaries restrict passage of substances, protect neural tissue.

Structure & Function of Cranial Nerves

  • Cranial Nerves: 12 pairs, numbered I-XII, each with specific sensory, motor, or mixed functions.

  • Clinical Application: Knowledge of cranial nerve nuclei helps localize brainstem lesions.

Number

Name

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

Viscera control

Mixed

XI

Accessory

Neck muscles

Motor

XII

Hypoglossal

Tongue movement

Motor

Clinical Focus: Brain Disorders

  • Meningitis: Inflammation of meninges.

  • Encephalitis: Inflammation of brain tissue.

  • Hydrocephalus: Excess CSF accumulation.

  • Epilepsy: Recurrent seizures.

  • Cerebrovascular Accident (Stroke): Disrupted blood flow to brain.

  • Amnesia: Memory loss.

  • Alzheimer’s Disease: Progressive dementia.

  • Concussion: Traumatic brain injury.

Chapters 12 & 13: Spinal Cord & Spinal Nerves

Anatomy of the Spinal Cord & Spinal Nerves

  • Gross Anatomy: Extends from foramen magnum to L1/L2; protected by vertebrae.

  • Gray Matter: Central, butterfly-shaped; contains neuron cell bodies.

  • White Matter: Surrounds gray matter; contains myelinated axons.

  • Dorsal Root Ganglia: Sensory neuron cell bodies.

  • Dorsal/Ventral Roots: Carry sensory/motor fibers, respectively.

  • Spinal Nerves: Mixed nerves formed by joining of dorsal and ventral roots.

  • Other Structures:

    • Ramus: Branch of spinal nerve.

    • Plexus: Network of nerves.

    • Tract: Bundle of axons in CNS.

    • Ganglion: Cluster of neuron cell bodies in PNS.

  • Spinal Nerve Plexuses:

    • Cervical: Phrenic nerve

    • Brachial: Median, ulnar, radial nerves

    • Lumbar: Femoral nerve

    • Sacral: Sciatic nerve

  • Ascending Tracts: Sensory information to brain.

  • Descending Tracts: Motor commands from brain.

  • Dermatomes: Skin regions supplied by specific spinal nerves; important for diagnosing nerve injuries.

Reflexes & Their Roles in Nervous System Function

  • Reflex: Rapid, automatic response to stimulus.

  • Reflex Arc Components: Receptor → sensory neuron → integration center → motor neuron → effector.

  • Types of Reflexes:

    • Intrinsic (Inborn) vs. Learned: Present at birth vs. acquired.

    • Somatic vs. Visceral: Skeletal muscle vs. internal organs.

    • Monosynaptic vs. Polysynaptic: One synapse vs. multiple synapses.

    • Ipsilateral vs. Contralateral: Same side vs. opposite side response.

  • Spinal Reflex: Processed in spinal cord.

  • Intersegmental Reflex: Involves multiple spinal cord segments.

  • Examples:

    • Stretch Reflex: Knee-jerk; monosynaptic.

    • Flexor (Withdrawal) Reflex: Pulling hand from hot object.

    • Crossed-Extensor Reflex: Balances body during withdrawal.

  • Clinical Assessment: Reflex testing helps evaluate nervous system function.

Physiology of Sensory & Motor Pathways

  • Sensory Pathways:

    • First-Order Neuron: Sensory receptor to spinal cord/brainstem.

    • Second-Order Neuron: Spinal cord/brainstem to thalamus.

    • Third-Order Neuron: Thalamus to cerebral cortex.

  • Motor Pathways:

    • Upper Motor Neuron: Cortex to spinal cord.

    • Lower Motor Neuron: Spinal cord to muscle.

  • Decussation: Crossing of fibers; explains contralateral effects of brain lesions.

Sensory Receptors & Their Roles

  • Types by Location:

    • Exteroceptors: External environment (skin).

    • Interoceptors: Internal organs.

    • Proprioceptors: Muscles, tendons, joints (body position).

  • Types by Stimulus:

    • Nociceptors: Pain; found in skin, organs.

    • Thermoreceptors: Temperature; skin.

    • Mechanoreceptors: Touch, pressure, proprioception; skin, muscles.

    • Baroreceptors: Pressure; blood vessels.

    • Chemoreceptors: Chemical changes; blood, nose.

    • Photoreceptors: Light; retina.

  • Generator/Graded Potentials: Local changes in membrane potential when receptors are stimulated.

  • Adaptation: Decreased response to constant stimulus over time.

Clinical Focus: Spinal Cord Disorders

  • Lumbar Puncture: Sampling CSF from lower back.

  • Polio: Viral infection damaging motor neurons.

  • Shingles: Reactivation of varicella-zoster virus in sensory ganglia.

  • Sciatica: Pain along sciatic nerve.

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