Study Notes on Reflexes and Reflex Arcs

Reflex Definition and Characteristics

  • A reflex is defined as a fast, automatic, unplanned sequence of actions that occur in response to a specific stimulus.

  • The stimulus is applied to the periphery and is conducted to either the brain or the spinal cord.

  • Reflexes play a crucial role in restoring functions to homeostasis.

Neural Pathways in Reflex Actions

  • Nerve impulses propagate into, through, and out of the Central Nervous System (CNS) following specific pathways based on the kind of information, its origin, and its destination.

  • The pathway taken by nerve impulses that results in a reflex is referred to as a reflex arc (or reflex circuit).

Components of a Reflex Arc

  • A reflex arc consists of five functional components:

    1. Sensory Receptor

    • The distal end of a sensory neuron (dendrite) or an associated sensory structure that acts as a receptor.

    • It responds to a specific stimulus (change in internal or external environment) by producing a graded potential known as a generator (or receptor) potential.

    • If the generator potential reaches the threshold level of depolarization, it triggers one or more nerve impulses in the sensory neuron.

    1. Sensory Neuron

    • Once the sensory receptor generates a potential, the impulse travels along the axon of the sensory neuron to the axon terminals, which are located in the gray matter of the spinal cord or brain stem.

    1. Integrating Center

    • Located within the CNS, these regions of gray matter are where the sensory neuron makes a functional connection with one or more neurons.

    • In the simplest reflex type, the integrating center consists of a single synapse between a sensory neuron and a motor neuron, termed a monosynaptic reflex arc (mono = one).

    • A polysynaptic reflex arc (poly = many) involves more than two types of neurons and more than one synapse within the CNS.

    1. Motor Neuron

    • The impulses triggered by the integrating center propagate out of the CNS along a motor neuron that delivers the response to the body part designated to respond.

    1. Effector

    • The effector is the specific part of the body, which can either be a muscle or a gland, that responds to the motor nerve impulse.

Visual Representation

  • Label the components of a reflex arc as depicted in Figure 12.5 (specific figure not provided in the transcript).