The Peripheral Nervous System and Reflex Activity (Chapter 13): Study Notes

The Peripheral Nervous System and Reflex Activity

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS), each with distinct structures and functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and control centers.

• Peripheral Nervous System (PNS): Includes cranial nerves and spinal nerves; connects the CNS to the rest of the body.

The PNS is further divided into sensory (afferent) and motor (efferent) divisions:

• Sensory (Afferent) Division: Conducts impulses from receptors to the CNS.

• Motor (Efferent) Division: Conducts impulses from the CNS to effectors (muscles and glands).

• Somatic Nervous System (SNS): Voluntary control of skeletal muscles.

• Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. Subdivided into sympathetic and parasympathetic divisions.

Information Processing in the Nervous System

Information processing in the nervous system involves the reception, routing, and conscious perception of sensory input.

• Step 1: Information comes in from sensory receptors.

• Step 2: Information is routed to various parts of the CNS for processing.

• Step 3: Information is perceived consciously at the perceptual level.

Example: Touching a hot object activates sensory receptors in the skin, which send signals to the CNS for processing and perception, resulting in a motor response (withdrawal).

Anatomy of the Adult Spinal Cord

Spinal Nerves

The adult spinal cord gives rise to 31 pairs of spinal nerves, all of which are mixed nerves carrying both sensory and motor information.

• 8 cervical pairs (C1–C8)

• 12 thoracic pairs (T1–T12)

• 5 lumbar pairs (L1–L5)

• 5 sacral pairs (S1–S5)

• 1 coccygeal pair (Co1)

Each spinal nerve is formed by the combination of dorsal (sensory) and ventral (motor) roots, and branches into dorsal and ventral rami to supply different regions of the body.

Functions of the Nervous System

• Sensory Input: Monitoring stimuli occurring inside and outside the body.

• Integration: Interpretation of sensory input by the CNS.

• Motor Output: Response to stimuli by activating effector organs (muscles or glands).

Example: Seeing a glass of water (sensory input), deciding to pick it up (integration), and moving your arm to grasp it (motor output).

Reflexes and Reflex Arcs

Introduction to Reflexes

Reflexes are rapid, automatic responses to specific stimuli. They help maintain homeostasis and protect the body from harm.

• Neural reflexes involve sensory fibers carrying information to the CNS and motor fibers carrying commands to effectors.

Reflex Arc

The reflex arc is the basic wiring of a neural reflex, consisting of five essential steps:

1. Arrival of stimulus and activation of receptor

2. Activation of sensory neuron

3. Information processing in the CNS

4. Activation of motor neuron

5. Response by effector

Example: The withdrawal of a hand from a hot surface is mediated by a reflex arc.

Classification of Reflexes

Methods of Classifying Reflexes

Reflexes can be classified by development, processing site, response type, and complexity of circuit:

Classification	Types	Description	Examples
By Development	Innate	Genetically determined	Suckling, withdrawal from pain
By Development	Acquired	Learned, more complex	Braking for a hazard
By Processing Site	Spinal	Processing in the spinal cord	Patellar reflex
By Processing Site	Cranial	Processing in the brain	Pupillary reflex
By Response	Somatic	Controls skeletal muscle	Stretch reflex
By Response	Visceral (Autonomic)	Controls smooth/cardiac muscle, glands	Pupillary reflex
By Complexity	Monosynaptic	One synapse	Patellar reflex
By Complexity	Polysynaptic	Multiple synapses	Withdrawal reflex

Innate vs. Acquired Reflexes

• Innate Reflexes: Result from connections formed during development; e.g., suckling, withdrawal from pain.

• Acquired Reflexes: Learned through experience; e.g., braking a car for a hazard.

Spinal vs. Cranial Reflexes

• Spinal Reflexes: Interconnections and processing occur in the spinal cord.

• Cranial Reflexes: Reflexes processed in the brain.

Somatic vs. Visceral Reflexes

• Somatic Reflexes: Control skeletal muscle contractions.

• Visceral (Autonomic) Reflexes: Control activities of smooth muscle, cardiac muscle, and glands.

Monosynaptic vs. Polysynaptic Reflexes

• Monosynaptic Reflex: Sensory neuron synapses directly on a motor neuron; rapid response.

• Polysynaptic Reflex: At least one interneuron between sensory and motor neurons; longer delay and more complex response.

Examples of Reflexes

Monosynaptic Reflexes

• Stretch Reflex: Automatically monitors skeletal muscle length and tone; sensory receptors are muscle spindles.

• Patellar (Knee Jerk) Reflex: Classic example of a monosynaptic reflex; helps maintain posture.

• Postural Reflex: Maintains upright position by adjusting muscle tone.

Polysynaptic Reflexes

• Tendon Reflex: Involves Golgi tendon organs; prevents excessive tension in tendons.

• Withdrawal Reflexes: Move body parts away from a stimulus (e.g., pain).

• Flexor Reflex: Withdrawal of a limb in response to a painful stimulus.

• Crossed Extensor Reflex: Opposite limb extends to support body weight when the flexor reflex is activated.

Control of Spinal Reflexes

The brain can facilitate or inhibit motor patterns based in the spinal cord. Motor control involves a hierarchy of interacting levels:

• Lowest Level: Monosynaptic reflexes (simple, automatic responses).

• Highest Level: Brain centers that modulate or build on motor patterns.

Reinforcement and Inhibition of Reflexes

• Reinforcement: Facilitation that enhances spinal reflexes.

• Inhibition: Spinal reflexes can be suppressed; for example, the Babinski reflex is replaced by the plantar reflex in adults.

The Babinski Reflex

The Babinski reflex is a diagnostic sign in which stroking the lateral aspect of the sole of the foot causes extension of the big toe and fanning of the other toes. In adults, this is replaced by the plantar reflex (flexion of the toes), indicating normal descending control from the brain.

• Babinski Sign: Present in infants; abnormal in adults and may indicate CNS damage.

• Plantar Reflex: Normal adult response; toes flex downward.

Additional info: The presence of the Babinski sign in adults is often used clinically to assess the integrity of the corticospinal tract.