Chapter13-1

Chapter 13: The Peripheral Nervous System and Reflex Activity

Nervous System Organization

• Central Nervous System (CNS)

  • Comprises the brain and spinal cord

• Peripheral Nervous System (PNS)

  • Contains cranial and spinal nerves

• Divisions:

  • Afferent (Sensory) Division:

    • Transmits signals from receptors to the CNS

  • Efferent (Motor) Division:

    • Transmits signals from the CNS to effectors

    • Includes:

      • Somatic Nervous System:

        • Effectors are skeletal muscles

      • Autonomic Nervous System:

        • Effectors are smooth/cardiac muscles and glands

        • Divisions:

          • Sympathetic Division: Responsible for 'fight or flight' reactions

          • Parasympathetic Division: Responsible for 'rest and repose' responses

Peripheral Nervous System (PNS)

• Encompasses all neural structures outside the brain and spinal cord

• Provides connections to and from the external environment

• Includes sensory receptors, peripheral nerves, ganglia, and motor endings

From Sensation to Perception

• Sensation: Awareness of changes in the internal and external environment

• Perception: Conscious interpretation of those stimuli necessary for survival

Processing at the Receptor Level

• Receptor must have specificity for stimulus energy and must be stimulated

• Stimulus energy converted into a graded potential called receptor potential

• A generator potential in the associated sensory neuron must reach threshold

Adaptation of Sensory Receptors

• Adaptation is when sensory receptors are subjected to an unchanging stimulus

• Results in:

  • Receptor membranes become less responsive

  • Receptor potentials decline in frequency or stop entirely

• Types of Adaptation:

  • Rapid adaptation: receptors for pressure, touch, smell

  • Slow adaptation: Merkel’s discs, Ruffini’s corpuscles, chemical level interoceptors

  • Pain receptors and proprioceptors do not exhibit adaptation

Sensory Receptors

• Specialized structures responsive to stimuli

• Activation results in depolarizations that trigger impulses to the CNS, where sensation and perception occur in the brain

Receptor Classification

1. By Location:

   • Exteroceptors: Respond to external stimuli

   • Interoceptors: Respond to internal stimuli

   • Proprioceptors: Respond to body position and movement

2. By Stimulus Type:

   • Mechanoreceptors: Respond to touch, pressure, vibration

   • Thermoreceptors: Sensitive to temperature changes

   • Photoreceptors: Respond to light (found in retina)

   • Chemoreceptors: Respond to chemicals (smell and taste)

   • Nociceptors: Sensitive to pain-causing stimuli

3. By Structure:

   • Simple receptors detect general senses

   • Complex receptors are specialized sense organs (eyes, ears, etc.)

Anatomy of Peripheral Nerves

• Peripheral nerves consist of nerve fascicles encased in connective tissue

• Key components:

  • Endoneurium: Wraps individual axons

  • Perineurium: Covers fascicles

  • Epineurium: Covers the entire nerve

• Contains blood vessels and Schwann cells supporting myelinated axons

Cranial Nerves

• 12 pairs of cranial nerves with specific functions

• Each cranial nerve has designated sensory, motor, or mixed functions

• Examples include:

  • Olfactory (I): Sensory for smell

  • Optic (II): Sensory for vision

  • Oculomotor (III): Motor for extraocular eye muscles

  • Trigeminal (V): Mixed; sensory and motor functions for the face

Reflexes Overview

• Reflexes: Rapid, automatic responses to stimuli

• Involves neural pathways (reflex arcs) with sensory neurons, integration centers, and motor neurons

Reflex Arc Components

1. Arrival of stimulus and receptor activation

2. Activation of sensory neuron

3. Information processing in CNS

4. Activation of motor neuron

5. Response by effector

Reflex Classification

1. By Development:

   • Innate (genetically determined)

   • Acquired (learned)

2. By Processing Site:

   • Spinal reflexes

   • Cranial reflexes

3. By Response Nature:

   • Somatic reflexes (control skeletal muscles)

   • Visceral (autonomic) reflexes (control smooth and cardiac muscles, glands)

4. By Circuit Complexity:

   • Monosynaptic (simple)

   • Polysynaptic (complex)

Summary of Key Reflexes

• Monosynaptic Reflex: Direct sensory neuron to motor neuron connection

  • E.g., Stretch reflex (patellar reflex)

• Polysynaptic Reflex: More complex, involve interneurons

  • E.g., Withdrawal reflex

Control of Spinal Reflexes

• Brain can facilitate or inhibit spinal reflexes

• Reinforcement: Facilitation that enhances reflexes

• Inhibition: Ability to suppress reflex response

This outline provides detailed information on the peripheral nervous system, its classification, cranial nerves, and reflex activity. Be sure to study each section for a comprehensive understanding of the subject.

• Reflex Arc: The pathway that a reflex impulse travels, consisting of a sensory neuron, interneuron, and motor neuron.

  General Sensory Receptors

  • Classification by Stimulus Detected:

    • Mechanoreceptors: respond to mechanical pressure or distortion

    • Thermoreceptors: respond to temperature changes

    • Nociceptors: respond to pain

    • Chemoreceptors: respond to chemical stimuli

    • Photoreceptors: respond to light

  • Classification by Body Location:

    • Exteroceptors: located at or near body surface, detect external stimuli

    • Interoceptors: located in blood vessels and organs, monitor internal conditions

    • Proprioceptors: located in muscles and joints, sense body position and movement

  • Classification by Structure:

    • Free Nerve Endings: bare dendrites, respond to pain and temperature

    • Encapsulated Nerve Endings: enclosed in connective tissue, respond to various stimuli

    • Specialized Receptors: such as hair cells and photoreceptors, respond to specific stimuli

  Sensation and Perception

  • Outline the Events Leading to Sensation and Perception:

    1. Stimulus Detection: sensory receptors detect environmental stimuli

    2. Transduction: conversion of stimulus into electrical signals

    3. Transmission: sensory signals travel to the central nervous system (CNS) via afferent nerve fibers

    4. Processing: signals processed in CNS, leading to perception

    5. Response: interpretation leads to a response based on perception

  • Receptor and Generator Potentials:

    • Receptor Potential: change in membrane potential of a receptor cell in response to a stimulus

    • Generator Potential: change in membrane potential in sensory neurons

  • Sensory Adaptation:

    • Decreased sensitivity to a continuous stimulus over time, allows for detection of changes in stimuli

  Sensory Perception

  • Main Aspects of Sensory Perception:

    • Quality: identification of distinct characteristics of the stimulus

    • Intensity: strength of the stimulus perceived

    • Duration: length of time the stimulus is present

    • Localization: ability to determine the location of the stimulus

  Structure of a Nerve

  • General Structure of a Nerve:

    • Composed of bundles of axons (nerve fibers)

    • Surrounded by connective tissue:

      • Epineurium: outer layer

      • Perineurium: middle layer around fascicles

      • Endoneurium: innermost layer surrounding each axon

  Ganglia

  • Definition: clusters of neuron cell bodies located in the peripheral nervous system

  • General Body Location:

    • Dorsal root ganglia: near the spinal cord

    • Autonomic ganglia: throughout the body, associated with autonomic pathways

  Cranial Nerves

  • Name and Structure Innervated:

  1. Olfactory (I): smell

  2. Optic (II): sight

  3. Oculomotor (III): eye movements

  4. Trochlear (IV): eye movements

  5. Trigeminal (V): facial sensation and chewing

  6. Abducens (VI): eye movements

  7. Facial (VII): facial expression, taste

  8. Vestibulocochlear (VIII): hearing and balance

  9. Glossopharyngeal (IX): taste, swallowing

  10. Vagus (X): autonomic functions, swallowing

  11. Accessory (XI): shoulder movement, swallowing

  12. Hypoglossal (XII): tongue movements

  Spinal Nerves

  • General Structure of a Spinal Nerve:

    • Consist of dorsal and ventral roots to form the spinal nerve

    • Dorsal roots contain sensory fibers; ventral roots contain motor fibers

  • Distribution of Rami:

    • Dorsal Ramus: innervates back muscles and skin

    • Ventral Ramus: innervates anterior and lateral trunk and limbs

    • Meningeal Ramus: reenters vertebral canal to innervate meninges and blood vessels

  Plexus

  • Definition: a network of intersecting nerves

  • Major Plexuses and Functions:

    • Cervical Plexus: innervates neck and diaphragm

    • Brachial Plexus: innervates upper limb

    • Lumbar Plexus: innervates lower limb and pelvic region

    • Sacral Plexus: innervates pelvic organs and lower limb

  Motor Endings

  • Comparison of Motor Endings:

    • Somatic Nerve Fibers: innervate skeletal muscles directly, using neuromuscular junctions

    • Autonomic Nerve Fibers: innervate cardiac and smooth muscles, and glands, using varicosities for neurotransmitter release

  Reflex Arc

  • Components of a Reflex Arc:

    1. Receptor

    2. Sensory neuron

    3. Integration center (spinal cord)

    4. Motor neuron

    5. Effector (muscle or gland)

  • Distinction between Autonomic and Somatic Reflexes:

    • Autonomic Reflexes: regulate involuntary responses

    • Somatic Reflexes: control voluntary skeletal muscle responses

  Reflex Types

  • Comparison of Reflexes:

    • Stretch Reflex: maintains muscle tone and posture

    • Flexor Reflex: protects body by withdrawing from harmful stimuli

    • Crossed-Extensor Reflex: maintains balance when one limb is withdrawn

    • Tendon Reflex: protects muscles and tendons from excessive tension

  • Superficial Reflexes:

    1. Plantar Reflex

    2. Abdominal Reflex

Chapter 13

Check Your Understanding

1. In addition to nerves, the PNS also consists of sensory receptors, motor endings, and ganglia.

2. Nociceptors respond to painful stimuli. They are exteroceptors that are nonencapsulated (free nerve endings).

3. Syrenthia is likely to suffer from numerous undetected injuries. These could lead to infection (and possibly amputation).

4. The sensory structures are as follows:

   1. epithelial tactile complexes, which detect light pressure;

   2. tactile corpuscle, which also detects light pressure (usually on hairless skin);

   3. lamellar corpuscle, which detects deep pressure, stretch, and vibration;

   4. bulbous corpuscle, which detects deep pressure and stretch;

   5. free nerve endings, which detect pain and temperature; and

   6. hair follicle receptor, which detects hair movement.

5. The three levels of sensory integration are receptor level, circuit level, and perceptual level.

6. Phasic receptors adapt, whereas tonic receptors exhibit little or no adaptation. Pain receptors are tonic so that we are reminded to protect the injured body part.

7. Hot and cold are conveyed by different sensory receptors that are parts of separate “labeled lines.” Cool and cold are two different intensities of the same stimulus, detected by frequency coding—the frequency of APs would be higher for a cold stimulus than for a cool one. Action potentials arising in the fingers and foot arrive at different locations in the somatosensory cortex via their own “labeled lines” and in this way the cortex can determine their origin.

8. An end-plate potential occurs in a muscle cell, where it triggers an action potential that leads to muscle contraction. A postsynaptic potential is produced when the stimulus is a neurotransmitter released by another neuron. For an excitatory postsynaptic potential (EPSP), the membrane potential moves toward the threshold for generating an action potential. For an inhibitory postsynaptic potential (IPSP), the membrane potential moves away from threshold. A generator potential occurs in the receptor region of a sensory neuron (as with free dendrites or the encapsulated receptors of most general sense receptors). A receptor potential occurs in a separate receptor cell (as in most special senses).

9. Ganglia are collections of neuron cell bodies in the PNS.

10. Nerves also contain connective tissue, blood vessels, lymphatic vessels, and the myelin surrounding the axons.

11. Schwann cells, macrophages, and the neurons themselves were all important in healing the nerve.

12. The oculomotor (III), trochlear (IV), and abducens (VI) nerves control eye movements. Sticking out your tongue involves the hypoglossal nerve (XII). The vagus nerve (X) influences heart rate and digestive activity. The accessory nerve (XI) innervates the trapezius muscle, which is involved in shoulder shrugging.

13. Roots lie medial to spinal nerves, whereas rami lie lateral to spinal nerves. Dorsal roots are purely sensory, whereas dorsal rami carry both motor and sensory fibers.

14. The ventral rami were C3–C5, the nerve plexus was the cervical plexus, and the major nerve was the phrenic nerve. The phrenic nerve is the sole motor nerve supply to the diaphragm, the primary muscle for respiration.

15. Varicosities are the series of knob-like swellings that are the axon endings of autonomic motor neurons. You would find them on axon endings serving smooth muscle or glands.

16. The cerebellum and basal nuclei, which form the precommand level of motor control, plan and coordinate complex motor activities.

17. The effector (muscle or gland) brings about the response.

18. See the labeled diagram. Make sure that in your drawing (as in the diagram), the cell body of the sensory neuron is in the dorsal root ganglion, the cell body of the interneuron is in the dorsal horn, and the cell body of the motor neuron is in the ventral horn.

The components of the reflex arc are as follows: 1. Receptor. A skin is injured by the nail prick. A nerve ending as a receptor carries a signal. 2. Sensory neuron. The cell body of a sensory neuron is present in the dorsal root ganglion. 3. Integration center. The cell body of an interneuron is present in the dorsal horn. 4. Motor neuron. The cell body of the motor neuron is present in the ventral horn. 5. Effector. The axon terminal of the motor neuron leads to the effector.

19. The stretch reflex is important for maintaining muscle tone and adjusting it reflexively by causing muscle contraction in response to increased muscle length (stretch). It maintains posture. The flexor, or withdrawal, reflex is initiated by a painful stimulus and causes automatic withdrawal of the painful body part from the stimulus. It is protective.

20. This response is called Babinski’s sign and it indicates damage to the corticospinal tract or primary motor cortex.

21. The spinothalamic pathway carries pain signals to the somatosensory cortex in the parietal lobe. The pyramidal pathways carry voluntary motor information and would be involved in inhibiting the flexor reflex.

Review Questions

1. b;

2. c;

3. d;

4. c;

5. e;

6. 1. d,

   2. c,

   3. f,

   4. b,

   5. e,

   6. a;

7. 1. f,

   2. i,

   3. b,

   4. g, h,

   5. e,

   6. i,

   7. c,

   8. k,

   9. l,

   10. c, d, f, k;

8. 1. b 6;

   2. d 8;

   3. c 2;

   4. c 5;

   5. a 4;

   6. a 3, 9;

   7. a 7;

   8. a 7;

   9. d 1;

   10. a 3, 4, 7, 9;

9. c;

10. c;

11. The PNS includes all nervous tissue outside the CNS, and consists of the sensory receptors that detect specific stimuli, the peripheral nerves (cranial and spinal) that conduct impulses to and from the CNS, the ganglia that contain synapses or cell bodies outside the CNS, and motor nerve endings that innervate effector organs.

12. 1. A plexus is a network. A nerve plexus is a branching nerve network formed by ventral rami from several spinal nerves.

    2. The cervical plexus originates from the ventral rami of C1 to C4 (and some C5) spinal nerves; the brachial plexus originates from the ventral rami of C5 to T1; the lumbar plexus originates from the ventral rami of L1 to L4; and the sacral plexus originates from L4 to S4.

13. Ipsilateral reflexes involve a reflex initiated on and affecting the same side of the body; contralateral reflexes involve a reflex that is initiated on one side of the body and affects the other side.