3.1 Nerves: Central & Peripheral Nervous Systems

Overview of the Nervous System

• The nervous system is responsible for enabling reactions to a continually changing environment and controlling various bodily activities such as circulation and respiration.

• Composed of two major cell types:

  • Neurons: The functional units of the nervous system that transmit signals.

  • Neuroglia: Support and protect neurons.

Classification of the Nervous System

• Central Nervous System (CNS): Brain and spinal cord.

• Peripheral Nervous System (PNS): Nerves that connect the CNS to the rest of the body. Divided into:

  • Somatic Nervous System: Controls voluntary movements (skin, joints, skeletal muscles).

  • Autonomic Nervous System: Regulates involuntary functions (cardiac & smooth muscle, glands).

Nerve Fiber Types in the PNS

• Sensory Fibers (Afferent): Carry signals from the body to the CNS.

  • Sensory Somatic Fibers: From skin, joints, and skeletal muscles.

  • Sensory Visceral Fibers: From internal organs.

• Motor Fibers (Efferent): Carry signals from the CNS to muscles and glands.

  • Motor Somatic Fibers: Activate skeletal muscles.

  • Motor Visceral Fibers: Control involuntary actions in smooth muscles and glands.

Cranial and Spinal Nerves

• Cranial Nerves (CN I-XII): Emerge directly from the brain and primarily control functions of the head and neck.

• Spinal Nerves:

  • Cervical (C1-C8)

  • Thoracic (T1-T12)

  • Lumbar (L1-L5)

  • Sacral (S1-L5)

  • Coccygeal (C0)

Anatomy of Nerves

• Nerves consist of bundles of nerve fibers surrounded by connective tissues:

  • Epineurium: Outer layer surrounding the entire nerve.

  • Perineurium: Surrounds fascicles (bundles of nerve fibers).

  • Endoneurium: Surrounds individual nerve fibers.

• White Matter vs. Gray Matter:

  • White matter consists of myelinated axons.

  • Gray matter consists of unmyelinated nerve cell bodies.

Plexuses of the Somatic Nervous System

SPINAL PLEXUSES: = braids - A network of intersecting nerves

• Cervical Plexus (C1-C4): Innervates neck and diaphragm.

• Brachial Plexus (C5-T1): Innervates upper limb.

• Lumbar Plexus (L1-L4): Innervates lower limb.

• Sacral Plexus (L4-S4): Innervates pelvic organs, buttocks, and lower limbs.

• Coccygeal Plexus: Supplies sensation to the skin around the coccyx.

Nerve example C6

• C6 spinal nerve. In between C5-C6, the C6 spinal nerve exits the spinal
  cord through a small bony opening on the left and right sides of the spinal
  canal called the intervertebral foramen.

• This C6 nerve has a sensory root and a motor root.

• The C6 dermatome (relay on many nerve connections) is an area of skin that receives sensations through the
  C6 nerve. This dermatome includes the skin over the ‘thumb’ side of the
  forearm and the thumb.

• The C6 myotome(single nerve root) is a group of muscles controlled by the C6 nerve. These
  muscles include the wrist extensor muscles, which allow the wrist to bend
  backward; and the biceps and supinator muscles of the upper arm, which
  serve to bend the elbow and rotate the forearm.

Dermatome and Myotome Mapping

• Dermatome: Area of skin innervated by sensory fibers of a single spinal nerve.

  • Example: C6 dermatome corresponds to the skin over the thumb and forearm.

• Myotome: Group of muscles innervated by motor fibers of a single spinal nerve.

  • Example: C6 myotome includes wrist extensors and biceps.

Autonomic Nervous System (ANS)

• Autonomic Nervous System: Involuntary component controlling visceral functions:

  • Sympathetic Division: Prepares the body for 'fight-or-flight' responses.

  • Parasympathetic Division: Promotes 'rest and digest' functions.

Sympathetic Division

• Originates from thoracic and lumbar regions (T1-L2).

• Utilizes a two-neuron pathway: preganglionic neuron in the spinal cord and postganglionic neuron in a ganglion.

• Functions include increasing heart rate, dilating bronchi, and inhibiting digestion

  Pathways

• Sympathetic C8; T1-L2
  • Parasympathetic CN X – vagus
  – Sacral nerves S2-S4

Parasympathetic Division

• Originates from cranial nerves (e.g., CN III, VII, IX, X) and sacral nerves (S2-S4).

• Effectively opposes sympathetic functions by decreasing heart rate, stimulating digestion, and promoting energy conservation.

Enteric Nervous System (ENS)

• Regulates gastrointestinal functions independently, composed of two major plexuses (myenteric and submucosal).

• Controls digestive reflexes, secretion, and absorption.

Neurotransmitters in the ANS

• Sympathetic pathways: Typically use norepinephrine as a neurotransmitter.

• Parasympathetic pathways: Typically use acetylcholine (ACh) as their neurotransmitter.

• ENS employs multiple neurotransmitters including ACh and serotonin.

Summary of Key Structures and Functions

• *Main Components of the Nervous System:

  • CNS (brain, spinal cord)

  • PNS (cranial and spinal nerves)

• Divisions of the ANS:

  • Sympathetic (fight or flight)

  • Parasympathetic (rest and digest)

  • ENS (controls digestive processes)

• Understanding the structure and functions of these systems is crucial for comprehending how the body responds to stimuli and maintains homeostasis.