Exam 4 Student Self-Test - Nervous System Overview

Nervous System Overview

  • Primary Functions of the Nervous System:

    1. Sensory Input: Receiving stimuli from internal and external environments.

    2. Integration: Processing and interpreting sensory information to formulate responses.

    3. Motor Output: Sending signals to effectors to produce responses.

  • Difference Between CNS and PNS:

    • CNS (Central Nervous System): Composed of the brain and spinal cord; responsible for processing information and decision-making.

    • PNS (Peripheral Nervous System): Comprises all neural tissue outside the CNS, including sensory and motor nerves; links the CNS to the rest of the body.

  • Use of Electrical and Chemical Signals:

    • The nervous system employs electrical signals in the form of action potentials for rapid communication and chemical signals via neurotransmitters for transmitting messages across synapses.

Neurons

  • Parts of a Neuron & Their Functions:

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

    2. Dendrites: Receive incoming signals from other neurons.

    3. Axon: Transmits electrical impulses away from the cell body.

    4. Axon Terminals (Synaptic Buttons): Release neurotransmitters to communicate with other neurons.

  • Graded Potentials:

    • Typically received at the dendrites and cell body.

  • Types of Neurons:

    1. Sensory Neurons: Carry signals from sensory receptors to the CNS (e.g., photoreceptors in the eyes).

    2. Motor Neurons: Transmit impulses from the CNS to effectors (e.g., skeletal muscles).

    3. Interneurons: Connect sensory and motor neurons within the CNS (e.g., spinal cord interneurons).

Electrical Properties

  • Resting Membrane Potential:

    • Definition: The electrical potential difference across the cell membrane when a neuron is not transmitting signals.

    • Typical Value: Approximately -70 mV.

  • Establishment and Maintenance of Resting Membrane Potential:

    • Established primarily by the sodium-potassium pump (Na+/K+ ATPase), which exchanges 3 Na⁺ ions out of the cell for 2 K⁺ ions into the cell, creating a concentration gradient.

    • Maintained by the selective permeability of the membrane, primarily to K⁺ ions.

  • Graded Potential vs. Action Potential:

    • Graded Potential:

      • Result from sensory stimuli or synaptic inputs; vary in magnitude and do not propagate along the axon.

    • Action Potential:

      • A rapid, all-or-nothing depolarization transmitted along the axon.

  • Sequence of Events:

    1. Depolarization: Initial increase in membrane potential toward threshold.

    2. Threshold: The membrane potential reaches about -55 mV, triggering an action potential.

    3. Overshoot: Membrane potential exceeds 0 mV, becoming positive.

    4. Repolarization: Membrane potential returns to a more negative value after the peak.

    5. Hyperpolarization: Membrane potential briefly becomes more negative than resting potential due to increased permeability of K⁺ channels.

  • Blocking Sodium Channels (Lidocaine):

    • Prevents pain signals by inhibiting depolarization necessary for action potentials in pain neurons, thus stopping the transmission of pain signals to the brain.

Synapses & Neurotransmission

  • Parts of a Chemical Synapse:

    1. Presynaptic Neuron: The neuron sending the signal, containing synaptic vesicles filled with neurotransmitters.

    2. Synaptic Cleft: The gap between the presynaptic and postsynaptic neurons.

    3. Postsynaptic Neuron: The neuron receiving the signal; contains receptors for neurotransmitters.

  • Common Synapse Type:

    • The most common synapse type in the nervous system is chemical synapses, involving neurotransmitter release and receptor binding.

  • Role of Calcium (Ca²⁺) at the Axon Terminal:

    • Trigger the fusion of synaptic vesicles with the presynaptic membrane for neurotransmitter release into the synaptic cleft.

  • Removal of Neurotransmitters from the Synaptic Cleft:

    1. Reuptake: Transporters on the presynaptic neuron reabsorb neurotransmitters.

    2. Enzymatic Degradation: Enzymes in the cleft break down neurotransmitters (e.g., acetylcholine is broken down by acetylcholinesterase).

    3. Diffusion: Neurotransmitters diffuse away from the synaptic cleft.

  • Importance of Acetylcholine Removal:

    • Necessary to terminate the signal and prevent continuous stimulation of the postsynaptic neuron, ensuring proper communication.

Neuroglia

  • Glial Cell Functions:

    • Astrocyte: Support neurons, regulate blood-brain barrier, and maintain extracellular ion balance.

    • Microglia: Act as immune cells in the CNS, monitoring for pathogens and debris.

    • Oligodendrocyte: Myelinate CNS axons, insulating them for faster conduction.

    • Schwann Cell: Myelinate peripheral nervous system (PNS) axons.

    • Ependymal Cell: Line the ventricles of the brain and facilitate cerebrospinal fluid (CSF) circulation.

  • Commonality of Glial Cells:

    • All glial cells provide support and maintenance to neurons but have specialized functions in the nervous system.

Myelin & Nerves

  • Function of Myelin:

    • Myelin insulates axons, allowing for faster conduction of action potentials via saltatory conduction.

  • Appearance of White Matter:

    • Appears white due to the high lipid content of myelin sheaths surrounding the axons.

  • Definition of a Nerve:

    • A nerve is a bundle of myelinated and unmyelinated axons in the peripheral nervous system, wrapped in connective tissue.

  • Connective Tissue Layers of a Nerve (Outermost to Innermost):

    1. Epineurium: Outer layer that surrounds the entire nerve.

    2. Perineurium: Surrounds individual fascicles (bundles of axons).

    3. Endoneurium: Envelopes each axon.

Reflexes

  • Purpose of a Reflex:

    • To provide immediate and involuntary responses to stimuli for protection or rapid response without requiring conscious thought.

  • Components of a Reflex Arc (Correct Order):

    1. Receptor: Senses the stimulus.

    2. Sensory Neuron: Transmits sensory information to the CNS.

    3. Integration Center: Processes the information (e.g., spinal cord interneurons).

    4. Motor Neuron: Carries response signals from the CNS to effectors.

    5. Effector: The muscle or gland that responds.

  • Reason Reflexes Can Occur Without Conscious Involvement:

    • Reflexes are mediated by spinal circuits that do not require higher brain involvement for rapid response.

Spinal Cord Anatomy

  • Dorsal (Posterior) Horn:

    • Contains sensory neurons that receive sensory information.

  • Ventral (Anterior) Horn:

    • Contains cell bodies of motor neurons that send signals to muscles.

  • Axons in Posterior (Dorsal) Root:

    • Carry sensory axons that enter the spinal cord.

  • Axons in Anterior (Ventral) Root:

    • Carry motor axons that exit the spinal cord.

  • Definition of Ascending vs. Descending Tracts:

    • Ascending Tracts: Carry sensory information to the brain.

    • Descending Tracts: Conduct motor commands from the brain to the body.

CNS Protection

  • Three Meninges (Superficial to Deep):

    1. Dura Mater: The tough outer layer providing protection.

    2. Arachnoid Mater: Middle layer that contains cerebrospinal fluid (CSF).

    3. Pia Mater: Delicate inner layer adhering tightly to the brain and spinal cord.

  • Meningeal Space Containing CSF:

    • The subarachnoid space between the arachnoid mater and pia mater.

  • Production of CSF:

    • Produced in the choroid plexuses of the brain ventricles.

    • Functions of CSF:

    • Cushions the brain and spinal cord.

    • Provides buoyancy, reducing weight on the brain.

    • Removes waste products from the brain.

  • Structures Forming the Blood-Brain Barrier:

    • Mainly formed by endothelial cells of capillaries in the brain and astrocyte end-feet surrounding these capillaries.

Brain Anatomy & Function

  • Matching Brain Lobes with Primary Functions:

    • Frontal Lobe: Responsible for executive functions, decision making, and motor control.

    • Parietal Lobe: Process sensory information and spatial orientation.

    • Temporal Lobe: Involved in auditory processing and memory.

    • Occipital Lobe: Principal area for visual processing.

  • Brain Regions and Their Primary Responsibilities:

    • Conscious Thought: Primarily in the frontal lobe.

    • Sensory Relay: Functions in the thalamus.

    • Homeostasis: Regulated by the hypothalamus.

    • Balance and Posture: Governed by the cerebellum.

  • Parts of the Brainstem & Functions:

    1. Midbrain: Involved in vision, hearing, and motor control.

    2. Pons: Connects different parts of the brain; regulates sleep and arousal.

    3. Medulla Oblongata: Controls vital autonomic functions such as heart rate and respiration.

  • Persistent Vegetative State and Breathing/Digestion:

    • Individuals in a persistent vegetative state can still breathe and digest due to the autonomic control mediated by the brainstem, which continues to function independently of higher brain areas.

Peripheral Nervous System

  • Definitions:

    1. Afferent Division: Comprises sensory pathways that carry information from sense organs to the CNS.

    2. Efferent Division: Includes motor pathways that relay commands from the CNS to skeletal muscle or glands.

  • Effectors:

    • Structures that carry out responses (e.g., muscles and glands).

  • Nature of Spinal Nerves:

    • Spinal nerves are mixed nerves, containing both sensory and motor fibers; convey information to and from the CNS simultaneously.

Cranial Nerves (HIGH PRIORITY)

  • Cranial Nerves I–XII:

    1. Olfactory Nerve (I): Sense of smell.

    2. Optic Nerve (II): Vision.

    3. Oculomotor Nerve (III): Eye movement and pupil constriction.

    4. Trochlear Nerve (IV): Eye movement (superior oblique muscle).

    5. Trigeminal Nerve (V): Sensation of the face and motor functions for mastication.

    6. Abducens Nerve (VI): Eye movement (lateral rectus muscle).

    7. Facial Nerve (VII): Controls muscles of facial expression and taste sensation.

    8. Vestibulocochlear Nerve (VIII): Hearing and balance.

    9. Glossopharyngeal Nerve (IX): Taste and swallowing.

    10. Vagus Nerve (X): Regulation of internal organs and heart rate.

    11. Accessory Nerve (XI): Shoulder and neck movement.

    12. Hypoglossal Nerve (XII): Tongue movement.

Bonus / Exam-Level Questions

  • Action Potential Formation:

    • Begins with a graded potential that rapidly depolarizes the membrane to reach the threshold level, triggering voltage-gated Na⁺ channels to open, followed by the rapid inflow of Na⁺ ions, causing the action potential. - Subsequently, K⁺ channels open, allowing K⁺ to exit the cell, repolarizing the membrane. - The neurotransmitter is released when the action potential reaches the axon terminals, resulting in neurotransmitter release into the synaptic cleft.

  • Identifying Brain Lobe Damage:

    • In a patient exhibiting impulsive and aggressive behavior post-head trauma, the frontal lobe is likely damaged, as it governs personality, social behavior, and impulse control.