Nervous System and The Reflex Arc

Nervous System and The Reflex Arc

Objectives

  • Categorize major divisions of the nervous system.

  • Differentiate between the CNS (Central Nervous System) and the PNS (Peripheral Nervous System).

  • Distinguish between the Autonomic and Somatic nervous systems.

  • Create a model of a cross-section of the spinal cord and label its parts.

  • Label the parts of the spinal column.

  • Illustrate the Reflex Arc.

  • Describe the components of the Reflex Arc.

  • Explain how conscious sensations, voluntary movements, and memories relate to the reflex arc.

  • Analyze case studies for disruptions in the neural pathway.

  • Perform experiments with receptors and reflexes.

Introduction to the Nervous System

  • Response Example: When you place your hand on a hot stove, the nervous system mediates response by sensing the heat through receptors, evaluating it, and triggering the withdrawal of the hand promptly.

  • Function of the Nervous System: It senses changes in the environment, evaluates them, and elicits responses via electrical signals, being one of the body's two major control systems.

  • Core Functional Unit: The reflex arc is the fundamental operational unit within the nervous system.

Organization of the Nervous System

  • The nervous system can be organized based on several criteria:

    1. Functionally

    2. Structurally

    3. Direction of impulse transmission

Functional Classification

  • Functionally, the nervous system is categorized into three levels:

    • Sensory Level: Responsible for detecting changes in the environment.

    • Integrated Level: Involves decision-making processes.

    • Motor Level: Evokes a response (activation of muscles or glands).

Structural Classification

  • The overall organization includes two main divisions: A. Central Nervous System (CNS)

    1. Brain

    2. Spinal Cord
      B. Peripheral Nervous System (PNS)

    3. Encompasses everything else including nerves, ganglia, and associated structures.

    • Afferent Nervous System: Transmits information into the CNS.

    • Efferent Nervous System: Transmits information out of the CNS.

Divisions of the Nervous System based on Control

  • The PNS can further be divided into systems controlled in different ways:

    • Somatic Nervous System (SNS): Controls the skeletal muscle under voluntary control.

    • Divided into:

      • Sensory Nervous System

      • Motor Nervous System

    • Autonomic Nervous System (ANS): A motor system regulating glands, cardiac muscle, and smooth muscle (visceral motor system).

    • Sympathetic Division: Prepares the body for activity (affects functions like blood pressure, heart rate, blood glucose, and blood flow).

    • Parasympathetic Division: Calms the body and conserves energy (involved in resting and digesting).

The Spinal Cord

Longitudinal Section through Spinal Cord

  • The spinal cord is the second component of the CNS and is divided into regions based on the vertebral column level from which spinal nerves emerge.

  • Key Points:

    • Two significant swellings: Cervical and Lumbar Enlargements provide nerves for the upper and lower body.

    • Medullary Cone (Conus Medullaris): The true end of the spinal cord, positioned higher than anticipated.

    • Cauda Equina: Nerves that extend from the spinal cord through lower lumbar vertebrae.

    • Filum Terminale: A thin thread anchoring the spinal cord to the os coxae.

  • Structures to identify:

    1. Cervical Enlargement

    2. Lumbar Enlargement

    3. Subarachnoid Space

    4. Medullary Cone

    5. Cauda Equina

    6. Terminal Filum.

Activities

Dissection of the Spinal Cord

  1. Log into your E-Text.

  2. Select Module "7. Nervous" and choose the scalpel icon for dissection.

  3. Use the Gray “All Content” tab.

  4. Select "Spinal Cord" and view the overview.

  5. Identify parts of the spinal cord by clicking on terms and observing highlights.

  6. Utilize slider bars to dissect the spinal cord visually.

Cross-Section of the Spinal Cord

  • Making a transverse section through the spinal cord reveals structural organization:

    • Gray Matter: Organized into horns, composed of somas, dendrites, and some unmyelinated axons.

    • White Matter: Organized into funiculi or columns, has more myelination.

  • Structures to label in the cross-section:

    1. Gray Matter

    2. Posterior Horn

    3. Gray Commissure

    4. Lateral Horn

    5. Anterior Horn

    6. Anterior Median Fissure

    7. Central Canal

    8. Posterior Median Sulcus

    9. Posterior Column

    10. Lateral Column

    11. Anterior Column

    12. Spinal Nerve

    13. Pia Mater

    14. Arachnoid Mater

    15. Dura Mater.

Reflex Arc

  • The Reflex Arc represents the basic functional unit of the nervous system. Breakdown of any part leads to loss of function.

  • Five components of a Reflex Arc:

    1. Receptors

    2. Sensory Neurons

    3. Center (Interneurons in the CNS)

    4. Motor Neurons

    5. Effector Organ

  • Activity: Watch a video regarding the Reflex Arc.

    • Log into E-Text, select Module “7. Nervous,” and find the animation under “Anatomy and Physiology.”

Biology Brief: What is a Reflex?

  • A reflex is characterized as a rapid, unlearned, involuntary response to a stimulus.

  • Reflex actions like sneezing, coughing, blinking occur to protect the body and require immediate actions as they do not involve higher brain centers.

  • A reflex arc exists where nerve impulses travel through a simple pathway:

    • Receptors: Detect stimulus and generate impulses.

    • Sensory Neurons: Conduct impulses toward the CNS.

    • Interneurons: Connect sensory and motor neurons in the CNS.

    • Motor Neurons: Conduct impulses to effectors (muscles or glands).

    • Effectors: Respond by contracting or secreting substances.

Labeling and Questions

  1. Identify components in a reflex diagram:

    • Sensory Neuron

    • Motor Neuron

    • Interneuron

    • Effector (muscle)

    • Receptor

    • Stimulus

    • Spinal Cord

  2. Questions:

    • A. What is a reflex?

    • B. What is the purpose of most reflexes?

    • C. Why aren't higher brain centers involved in reflex responses?

    • D. Differentiate reflex actions from voluntary actions (e.g., walking, talking, driving).

Adding in the Brain

  • More complex behaviors necessitate partial brain involvement.

  • Conscious Sensations:

    • Sensation of pain travels from receptors through sensory neurons, synapsing with interneurons that relay messages to the sensory cortex of the cerebrum, where pain is consciously felt.

  • Voluntary Movements:

    • Initiated in the motor cortex and coordinated via the cerebellum before impulses move through the spinal cord to the motor neurons.

Memory Formation

  • Impulses from the sensory cortex propagate to the prefrontal cortex of the cerebrum, where they are recorded as memories to prevent future hazards.

Neurological Examination

  • Neurological assessments can reveal disruptions in neuron pathways leading to sensory response anomalies.

  • Examination includes patient stimuli exposure and observation of responses.

Activities and Exercises

  • Exercise 1: Receptors

    • Assess touch sensitivity distribution through two-point discrimination tests and tactile localization.

  • Exercise 2: Reflex

    • Conduct patellar reflex testing and analyze results.

    • Contrast Babinski reflex with plantar reflex to understand differences in responses based on age or condition.