Chapter 13: Peripheral Nervous System and Reflex Activity

Organization and Requirements for Sensation

Peripheral Nervous System (PNS) Overview: The PNS collects information from sensory receptors (like your skin, eyes, and ears) and sends signals to muscles and organs through the Central Nervous System (CNS), which is your brain and spinal cord.
Criteria for Perception of Sensation: For you to feel something, there are three steps that have to happen:
    1. Receptor Level: Sensory receptors need to react to a stimulus (like heat or touch).
    2. Circuit Level: Nerve pathways carry these signals to the CNS via action potentials (electrical impulses).
   * 3. Perceptual Level: Finally, this information is interpreted in specific areas within your brain.

Structure and Classification of Nerves

Definition of a Nerve: Think of nerves as cables that carry electrical signals. They are made up of tightly packed and insulated nerve fibers (called axons) that are bundled together and wrapped in protective tissue.
Nerve Classification by Origin: Nerves can be grouped based on where they come from:
Spinal Nerves: Come from the spinal cord.
Cranial Nerves: Come directly from the brain.
Nerve Classification by Direction of Impulse Transmission:
    Sensory (Afferent): These fibers carry signals to the CNS.
    Motor (Efferent): These fibers send signals out from the CNS to your muscles.
   * Mixed Nerves: Most nerves are mixed, meaning they carry both sensory and motor signals, allowing for two-way communication.
Types of Mixed Nerve Fibers:
1. Somatic Afferent and Somatic Efferent: Messages related to your body's movement and sensations.
2. Visceral Afferent and Visceral Efferent: Messages related to internal organs and bodily functions.

Regeneration of Peripheral Nerve Fibers

Mitotic Status: Mature nerve cells usually don't divide. If the cell body of a nerve remains undamaged, some nerve fibers in the PNS may heal after injury; the same is not true in the CNS.
Process of PNS Axon Regeneration:
    1. Axon Fragmentation: If a nerve is cut, the ends seal off and swell up, causing the axon and surrounding tissue to break down in a process known as Wallerian degeneration.
    2. Cleaning Debris: Support cells (Schwann cells) help clear away the mess and signal the immune cells to help.
    3. Regeneration Tube Formation: Schwann cells create a pathway for the regenerating axon to follow and release chemicals to spur growth.
    4. Axon Regeneration and Remyelination: The Schwann cells nurture the new growth and eventually form a new protective covering around the regenerating axon.
Key Participants in Regeneration:
    Macrophages: Clean up the injury site.
    Schwann Cells: Aid in regrowth and provide support.
   * Axons: These are the structures that are being repaired and rebuilt.
Gross Anatomy of Spinal Nerves and the Spinal Cord

Spinal Nerve Pairs: There are $31$ pairs of spinal nerves, all of which are mixed nerves. They are named for their point of issue from the spinal cord: * Cervical Nerves: $8$ pairs ( $C1-C8$ ). * Thoracic Nerves: $12$ pairs ( $T1-T12$ ). * Lumbar Nerves: $5$ pairs ( $L1-L5$ ). * Sacral Nerves: $5$ pairs ( $S1-S5$ ). * Coccygeal Nerve: $1$ pair ( $C0$ ).
   Epidural Space: Area filled with fat located outside the dura mater (the outer protective layer).
    Subarachnoid Space: Contains cerebrospinal fluid (CSF), providing cushioning around the spinal cord.
   * Denticulate Ligaments: Connective tissue strands that hold the spinal cord in place and protect it.
Key Spinal Cord Structures:
    Conus Medullaris: The end point of the spinal cord.
    Filum Terminale: A thread-like tissue anchoring the spinal cord to the coccyx.
    Cervical and Lumbar Enlargements: Areas where nerves responsible for the arms and legs exit the spinal cord.
    Cauda Equina: A bundle of nerves that looks like a horse's tail near the bottom of the spinal cord.
Segmental Organization: The spinal cord itself isn’t divided into sections, but it gives the appearance of being segmented due to the 31 pairs of attached spinal nerves.

Distribution of Spinal Nerves: Rami and Plexuses

Spinal Nerve Roots: Each nerve connects to the spinal cord via two roots:
Posterior (Dorsal) Root: Contains sensory fibers that send information to the spinal cord and has a Dorsal Root Ganglion (a cluster of nerve cell bodies).
Anterior (Ventral) Root: Contains motor fibers sending commands to your muscles.
Spinal Nerve Branching: Spinal nerves are short (only about 1-2 cm) and immediately split into branches:
    Posterior (Dorsal) Ramus: Smaller branch that goes to the back.
    Anterior (Ventral) Ramus: Larger branch that goes to the front.
    Meningeal Branch: A tiny branch that goes back to the meninges (the protective coverings of the spinal cord).
    Rami Communicantes: Branches that include autonomic fibers found in the thoracic region.
Nerve Plexuses: These are networks of nerves formed from the front roots of spinal nerves, except for T2-T12. Found in regions like the cervical (neck), brachial (arms), lumbar (lower back), and sacral (pelvic) areas. They ensure multiple nerves supply the same muscle, which is crucial so that damage to one nerve usually won’t paralyze a muscle completely.

Specific Plexuses and Major Nerves

Cervical Plexus (C1-C4): - Mostly provides sensation to the skin of the neck, ear, back of the head, and shoulders. - Phrenic Nerve: The most critical nerve that originates from C3-C5 and controls the diaphragm, enabling you to breathe.
Brachial Plexus (C5-C8, T1): Responsible for the nerves of the upper limbs, forming five major branches: - 1. Axillary Nerve: Controls the deltoid muscle and provides sensation to the outer arm. - 2. Median Nerve: Controls the muscles in the forearm and hand and provides sensation to most fingers except the pinkie. - 3. Musculocutaneous Nerve: Controls anterior arm muscles (like biceps) and provides sensation to the lateral forearm. - 4. Radial Nerve: Controls the posterior arm (like triceps) and forearm, providing sensation to the back side of the arm and hand. - 5. Ulnar Nerve: Controls muscles in the forearm and hand, providing sensation to the inner two fingers.
Lumbar Plexus (L1-L4): Supplies nerves to the thigh and abdominal muscles. - Femoral Nerve: Controls the quadriceps and sensation in the front of the thigh and inner leg. - Obturator Nerve: Controls the inner thigh muscles.
Sacral Plexus (L4-S4): Supplies nerves to the buttocks, legs, pelvic organs, and perineum. - Sciatic Nerve: The largest nerve in the body, composed of two nerves (tibial and common fibular) and innervates most leg/foot muscles.
Anterolateral Thorax (T2-T12): These rami don’t form plexuses, but become Intercostal Nerves that control rib muscles, thoracic muscles, and the abdominal wall.

Clinical Nerve Conditions and Pathology

Phrenic Nerve Damage: Irritation can cause hiccups, while severe damage can result in loss of diaphragm function, making breathing difficult.
Ulnar Nerve Injury: This nerve is vulnerable at the elbow, leading to symptoms like the "funny bone" sensation or weakness in hand function (often called 'clawhand').
Sciatica: A painful condition from nerve irritation (often from a slipped disc) that can lead to leg weakness and issues like foot drop, where the foot cannot lift properly.
Shingles: A painful rash caused by a reactivated chickenpox virus. It follows a nerve path, affecting specific regions on the skin, often treated with antivirals.

Dermatomes and Joint Innervation

Dermatomes: Specific areas of skin connected to individual spinal nerves; useful for diagnosing spinal injuries based on sensory loss in these areas. - Clinical Utility: Testing these dermatomes can help pinpoint spinal cord injuries. - Overlap: Dermatomes often overlap, meaning you may need to anesthetize several nerves to block sensation completely in one area.
Hilton’s Law: This principle states that nerves controlling a muscle also innervate the joint it moves and the skin over that joint.

Peripheral Motor Endings and Effector Innervation

Function: Motor endings allow nerve signals to activate muscles and glands by releasing chemical signals called neurotransmitters.
Innervation of Skeletal Muscle: This process happens at neuromuscular junctions, where the neurotransmitter Acetylcholine (ACh) is released to stimulate muscle contraction.
Innervation of Visceral Muscle and Glands: This is simpler and often involves varicosities where neurotransmitters like ACh and norepinephrine (NE) interact with target cells differently than in skeletal muscle, leading to slower responses.

Reflex Activity: Principles and Components

Definition: Reflexes are quick and automatic responses by muscles to stimuli.
Properties of Reflexes: - 1. Require Stimulation: They don't happen on their own. - 2. Rapid: They involve minimal neural connections, so they happen quickly. - 3. Involuntary: You don’t have to think about them; they happen automatically. - 4. Stereotyped: They occur the same way each time.
Functional Classifications: - Somatic Reflexes: Engage skeletal muscles. - Autonomic (Visceral) Reflexes: Control smooth muscles, heart, or gland activity.
Reflex Origin Classifications: - Inborn Reflexes: Present from birth (like pulling away from pain). - Learned Reflexes: Result from practice (like driving).
Five Components of a Reflex Arc: - 1. Receptor: Senses the stimulus. - 2. Sensory Neuron: Delivers the signal to the CNS. - 3. Integration Center: Processes the information in the CNS. - 4. Motor Neuron: Sends signals from the CNS to the muscle or gland. - 5. Effector: The muscle or gland that responds.

Specific Spinal Reflexes

Proprioceptor Input: Helps the brain coordinate muscle movement by sensing muscle length and tension.
Stretch Reflex (e.g., Knee-Jerk/Patellar Reflex): - Components: Involves muscle spindle sensors that trigger quick contractions when muscles are stretched. This reflex protects the muscle from excessive stretching.
Tendon Reflex: Prevents muscle injury from excessive force and promotes smooth muscle contraction.
Flexor (Withdrawal) Reflex: A protective reflex that causes you to withdraw from pain, such as pulling your hand away from a hot stove.
Crossed-Extensor Reflex: Helps maintain balance during a withdrawal reflex by activating opposing limb muscles.
Superficial Reflexes and Clinical Testing

Plantar Reflex: Tests cord integrity from $L4$ to $S2$ . Stimulus is stroking the lateral sole of the foot. * Normal Response: Downward flexion of toes. * Babinski’s Sign: Abnormal response where the hallux (big toe) dorsiflexes and smaller toes fan out. Indicates lesion in the corticospinal tract if present after the age of $1.5$ years. It is normal in infants under $1-1.5$ years because myelination is incomplete.
Abdominal Reflexes: Tests cord integrity from $T8$ to $T12$ . Stimulus is stroking the skin of the lateral abdomen. * Response: Contraction of abdominal muscles and movement of the umbilicus toward the stimulus. Absent in corticospinal tract lesions.

Questions & Discussion

Nerve Fascicle: Bundle of nerve fibers surrounded by protective tissue.
Functions of Posterior Rami: Control the muscles and skin on the back.
Biceps Brachii Innervation: Controlled by the Musculocutaneous nerve.
Lumbar Plexus Origins: Comes from anterior rami of spinal nerves L1 to L4.
Reflex Characteristics: The flexion withdrawal reflex is an ipsilateral response (same side); the main monosynaptic reflex is the stretch reflex (knee-jerk).