CNS Physiology & Reflex Actions: Study Notes

CNS Physiology & Reflex Actions

Introduction to Reflexes

A reflex is an involuntary, predictable motor response to a specific stimulus. Reflexes are essential for rapid protective actions and basic bodily functions. The reflex arc refers to the neural pathway that mediates or controls a reflex action.

• Reflex: An automatic response to a stimulus, not under conscious control.

• Reflex arc: The circuit of neurons involved in producing a reflex.

• Example: The knee-jerk (patellar) reflex is a classic example of a simple reflex arc.

Components of a Reflex Arc

The basic components of a reflex arc ensure the transmission and processing of sensory information leading to a motor response.

• Sensory receptor: A nerve ending or specialized tissue that detects the stimulus (e.g., skin receptors for touch).

• Sensory neuron (afferent neuron): Transmits the stimulus information to the central nervous system (CNS).

• CNS (Spinal cord or brain tissue): Processes and integrates the information, often involving interneurons.

• Motor neuron (efferent neuron): Carries the response signal from the CNS to the effector.

• Effector organ/tissue: Performs the triggered action (e.g., muscle contraction or gland secretion).

• Examples of effectors: Skeletal muscle, gland.

Characteristics of Reflexes

Reflexes share several defining characteristics that distinguish them from voluntary actions.

• Stimulus required: A specific stimulus is necessary to initiate a reflex.

• Rapid response: Reflexes involve a chain of only a few neurons, allowing for quick reactions.

• Preprogrammed response: The response is always the same for a given stimulus.

• Involuntary action: Reflexes occur without conscious intent or awareness.

• Example: Touching a hot object triggers a withdrawal reflex before you consciously feel pain.

Functional Classification of Reflexes

Reflexes can be classified based on their structure, number of neurons involved, and the type of effector tissue.

• Structural classification:

  • Spinal reflexes: Integration occurs in the spinal cord (e.g., stretch reflex).

  • Cranial reflexes: Integration occurs in the brain (e.g., swallowing reflex).

• Number of neurons:

  • Monosynaptic reflex: Involves only one synapse between a sensory and a motor neuron (e.g., patellar reflex).

  • Polysynaptic reflex: Involves multiple synapses and interneurons (e.g., withdrawal reflex).

• Type of effector:

  • Somatic reflexes: Effector is skeletal muscle.

  • Autonomic (visceral) reflexes: Effector is smooth muscle, cardiac muscle, or gland.

Somatic Reflexes

Somatic reflexes involve skeletal muscle as the effector and are crucial for posture, movement, and protection.

• Stretch (knee-jerk/patellar) reflex: Maintains muscle tone and posture.

• Withdrawal (flexor) reflex: Protects the body from injury by withdrawing from harmful stimuli.

• Crossed-extensor reflex: Maintains balance during withdrawal actions.

Autonomic (Visceral) Reflexes

Autonomic reflexes are controlled by the autonomic nervous system and regulate involuntary functions.

• Effectors: Smooth muscle, cardiac muscle, glands.

• Examples: Heart rate regulation, respiration, digestion, urination.

• Response speed: Slower than somatic reflexes (often takes seconds).

Anatomy of Autonomic Reflex Arc

The autonomic reflex arc contains the same five basic components as the somatic reflex arc, but often includes a preganglionic and postganglionic neuron in the motor pathway.

• 1. Receptor

• 2. Sensory neuron

• 3. Integration center (often includes interneurons and autonomic ganglia)

• 4. Motor neuron (preganglionic and postganglionic)

• 5. Effector (smooth muscle, cardiac muscle, gland)

Comparison: Sympathetic vs Parasympathetic Nervous System

The autonomic nervous system has two branches with opposing effects on body systems.

Feature	Sympathetic	Parasympathetic
Location of preganglionic neuron cell body	Thoracolumbar spinal cord	Craniosacral regions
Location of ganglia	Near spinal cord	Near or within target organs
Neurotransmitter at effectors	Norepinephrine (main)	Acetylcholine
Main effects	"Fight or flight" (stimulates activity)	"Rest and digest" (inhibits activity)

Types of Reflexes: Cranial vs Spinal; Monosynaptic vs Polysynaptic

• Cranial reflex: Integration occurs in the brain; involves cranial nerves.

• Spinal reflex: Integration occurs in the spinal cord; involves spinal nerves.

• Monosynaptic reflex: One synapse, faster, simpler (e.g., patellar reflex).

• Polysynaptic reflex: Multiple synapses, slower, more complex (e.g., withdrawal reflex).

Summary Table: Monosynaptic vs Polysynaptic Reflexes

Feature	Monosynaptic	Polysynaptic
Number of synapses	One	Two or more
Speed	Faster	Slower
Complexity	Simpler	More complex
Example	Patellar reflex	Withdrawal reflex

Additional info:

• Reflex testing (e.g., using a reflex hammer) is a common clinical tool to assess nervous system function.

• Reflexes are important for maintaining homeostasis and protecting the body from harm.