THE GENERAL ORGANIZATIOIN OF NERVOUS SYSTEM

Learning Objectives

After studying this topic, students will be able to:

1. Identify the main divisions of the nervous system.
2. Describe the basic structure of the central and peripheral nervous systems.
3. Explain the organization and functions of cranial and spinal nerves.
4. Differentiate between the somatic and autonomic nervous systems.
5. Compare the sympathetic and parasympathetic divisions.
6. Describe the role of visceral afferent nerves.

Introduction

• The nervous system is the body’s main control and coordination system.
• It works together with the endocrine system to control the activities of different body parts.
• It is divided into two main parts:
  • Central Nervous System (CNS): Consists of the brain and spinal cord.
  • Peripheral Nervous System (PNS): Consists of cranial nerves, spinal nerves, and their ganglia.
• Functionally, the nervous system has two divisions:
  • Somatic Nervous System: Mainly controls voluntary activities of skeletal muscles.
  • Autonomic Nervous System (ANS): Mainly controls involuntary activities of smooth muscle, cardiac muscle, and glands.
• The ANS has two main divisions:
  • Sympathetic Division: Prepares the body for emergencies.
  • Parasympathetic Division: Helps the body recover.
• Together, these systems control, coordinate, and regulate body activities to maintain normal body function.

• The nervous system and endocrine system work together to control and coordinate the activities of different body parts.
• The nervous system is divided into two main parts (Fig. 1.18):
  • Central Nervous System (CNS): Consists of the brain and spinal cord.
  • Peripheral Nervous System (PNS): Consists of paired cranial nerves, spinal nerves, and their associated ganglia.
• Functionally, the nervous system is divided into two parts:
  • Somatic Nervous System:
    • Controls the body’s interaction with the external environment.
    • Mainly acts through skeletal muscles.
    • Mostly produces voluntary responses to consciously felt sensations from the body wall and limbs.
  • Autonomic Nervous System (ANS):
    • Controls the body’s internal environment.
    • Mainly acts through smooth muscle, cardiac muscle, and glands.
    • Mostly produces involuntary responses to sensory signals that are not consciously felt.

CENTRAL NERVOUS SYSTEM

• The CNS contains many nerve cells and their processes.
• These cells are supported by specialized tissue called neuroglia.
• A neuron is a single nerve cell, including all its processes.
• Each neuron has three main components:
  • Cell body: Main part of the neuron.
  • Dendrites: Usually short processes that carry nerve impulses toward the cell body.
  • Axon: Usually the longest process that carries nerve impulses away from the cell body and dendrites (Fig. 1.19A).
• Groups of neuron cell bodies within the CNS are called nuclei.

– The CNS contains two types of matter:

• Gray Matter:
  • Mainly contains nerve cell bodies supported by neuroglia.
  • Appears dull gray because it contains many nonmyelinated cell bodies.
• White Matter:
  • Mainly contains axons and blood vessels supported by neuroglia.
  • Appears white and shiny because it contains many myelinated nerve fibers.

• In the spinal cord, gray matter forms an H-shaped or butterfly-shaped pattern (Fig. 1.19B,C).
• This gray matter contains:
  • Posterior (dorsal) gray horns: Extend along the spinal cord.
  • Anterior (ventral) gray horns: Also extend along the spinal cord.
  • Lateral gray horns: Present in the thoracic and upper lumbar regions.
• A central canal containing cerebrospinal fluid runs along the central part of the spinal cord.

• The entire CNS is surrounded by three protective membranes called meninges:
  • Dura mater: Outermost membrane.
  • Arachnoid mater: Middle membrane.
  • Pia mater: Innermost membrane.
• The meninges:
  • Protect the brain and spinal cord.
  • Hold and stabilize the CNS.
  • Form a surrounding sac containing cerebrospinal fluid.

PERIPHERAL NERVOUS SYSTEM

• The PNS consists of:
  • Cranial nerves.
  • Spinal nerves.
  • Their associated ganglia.
• A ganglion is a group of neuron cell bodies located outside the CNS.
• Cranial and spinal nerves appear as grayish-white cords during dissection.
• These nerves consist of bundles of axons supported by delicate areolar connective tissue.

CRANIAL NERVES

• There are 12 pairs of cranial nerves.
• They arise from the brain and upper spinal cord and pass through openings in the skull called foramina.
• Most cranial nerves supply structures in the head and neck.
• The exception is cranial nerve X (vagus nerve), which also supplies structures in the thorax and abdomen.
• Cranial nerves are described in Chapter 12.

SPINAL NERVES

• There are 31 pairs of spinal nerves.
• They leave the spinal cord through openings between vertebrae called intervertebral foramina (Fig. 1.20; Fig. 1.18).

– Spinal nerves are named according to their vertebral regions:

• 8 cervical pairs
• 12 thoracic pairs
• 5 lumbar pairs
• 5 sacral pairs
• 1 coccygeal pair

– Each spinal nerve begins as two groups of small nerve fibers called rootlets:

• Anterior rootlets: Connected to the spinal cord near the anterior gray horn.
• Posterior rootlets: Connected near the posterior gray horn (Figs. 1.19 and 1.20).

• The rootlets join to form:
  • An anterior root.
  • A posterior root.
• The anterior and posterior roots unite within the intervertebral foramen to form a spinal nerve.
• Each posterior root contains a posterior root ganglion, located near the intervertebral foramen.
• After passing through the foramen, each spinal nerve divides into:
  • Smaller posterior ramus
  • Larger anterior ramus

– Anterior rootlets and roots (Motor/Efferent Fibers):

• Carry nerve impulses away from the CNS (Fig. 1.21; Fig. 1.19).
• Somatic motor neuron cell bodies are located in the anterior gray horn.
• These neurons supply skeletal muscles and cause contraction.
• Autonomic motor neuron cell bodies are located in the lateral gray horn.
• These neurons supply smooth muscle, cardiac muscle, and glands, causing contraction or secretion.

– Posterior rootlets and roots (Sensory/Afferent Fibers):

• Carry sensory impulses toward the CNS (Figs. 1.19 and 1.21).
• Carry information about touch, pain, temperature, and other sensations.
• The cell bodies of these sensory neurons are located in the posterior root ganglia.

– Mixed Spinal Nerves:

• Spinal nerves and their anterior and posterior rami contain both motor and sensory fibers.
• Therefore, damage to roots and damage to rami cause different problems.
• Anterior root injury: Affects motor fibers only.
• Anterior ramus injury: Affects both motor and sensory fibers.

– Anterior Rami:

• Pass forward and sideways.
• Supply muscles and skin of the anterolateral body wall.
• Also supply muscles and skin of the limbs.

– Posterior Rami:

• Pass backward around the vertebral column.
• Supply muscles and skin of the back.

– Other Branches of Spinal Nerves:

• Meningeal branch: Supplies vertebrae and spinal meninges.
• Rami communicantes: Found in thoracic and upper lumbar spinal nerves and are associated with the sympathetic nervous system.

– The anterior rami follow two main distribution patterns (Fig. 1.18):

1. Simple Segmentation

• Each anterior ramus remains separate from neighboring rami.
• Each nerve passes around the trunk independently.
• Example: Intercostal nerves.
• Each peripheral nerve remains identifiable by its spinal nerve of origin.
• Example: Intercostal nerve T5 comes from spinal nerve T5.
• Neighboring anterior rami may supply overlapping areas.

2. Plexuses

• Several nearby anterior rami join together.
• Their nerve fibers mix and form networks called nerve plexuses.
• Peripheral nerves arising from a plexus contain fibers from multiple spinal nerves.
• Therefore, these peripheral nerves cannot be identified as branches of only one spinal nerve.
• Example: The median nerve contains fibers from spinal nerves C5–T1.
• Major plexuses include:
  • Cervical plexus: Neck.
  • Brachial plexus: Root of the upper limb.
  • Lumbosacral plexus: Root of the lower limb.

• The separation of the nervous system into CNS and PNS is mainly for easier description.
• In reality, nerve processes pass continuously between the CNS and PNS.

– Example 1: Motor Neuron

• A motor neuron begins in the anterior gray horn at spinal cord level T1.
• Its axon leaves through the anterior root of the first thoracic nerve (Fig. 1.21).
• It then passes through the brachial plexus.
• The axon travels through the arm and forearm in the ulnar nerve.
• Finally, it reaches the motor end plates of a small hand muscle.
• The total length may be about 90 cm (3 ft).

– Example 2: Sensory Neuron

• Touch sensation from the lateral side of the little toe is carried by a sensory neuron associated with spinal segment S1.
• Sensory fibers begin at receptors in the skin.
• The axon travels upward through:
  • Sural nerve.
  • Tibial nerve.
  • Sciatic nerve.
  • Lumbosacral plexus.
• It then passes through the posterior root of the first sacral nerve.
• Its cell body is located in the S1 posterior root ganglion.
• The central axon enters the posterior white column of the spinal cord.
• It travels upward to the nucleus gracilis in the medulla oblongata.
• The total distance may be about 1.5 m (5 ft).
• Thus, a single neuron may extend from the little toe to inside the skull.

– Both examples show that a single neuron can be extremely long.

AUTONOMIC NERVOUS SYSTEM

• The autonomic nervous system (ANS) controls the motor activities of:
  • Smooth muscle.
  • Cardiac muscle.
  • Glands.
• The hypothalamus of the brain controls the ANS.
• It coordinates autonomic and neuroendocrine activities to maintain homeostasis.
• The ANS is distributed throughout the central and peripheral nervous systems.
• The main differences between the somatic and autonomic nervous systems are summarized in Table 1.5 and Fig. 1.22.

– Main Anatomical Difference:

• Somatic Nervous System: Uses a one-neuron pathway from the CNS to skeletal muscle.
• Autonomic Nervous System: Uses a two-neuron pathway to reach its target organs.

• The ANS has two main divisions:
  • Sympathetic Division: Prepares the body for emergency situations.
  • Parasympathetic Division: Helps the body recover.
• Their general structure and functions are shown in Fig. 1.23 and Table 1.6.

– The sympathetic and parasympathetic divisions often produce opposite effects.

Organ or activity	Sympathetic	Parasympathetic
Heart rate	Increases	Decreases
Bronchi	Dilate	Constrict
Intestinal peristalsis	Decreases	Increases
Sphincters	Close	Open
Bladder wall	Relaxes	Contracts
Pupils	Dilate	Constrict

• Sometimes both divisions work together.
• Example: During normal sexual function:
  • Parasympathetic activity: Produces erection.
  • Sympathetic activity: Produces ejaculation.
• Sometimes one division works independently.
• Example:
  • Sympathetic stimulation activates sweat glands.
  • Parasympathetic nerves do not control sweat gland activity.

SYMPATHETIC DIVISION

– The sympathetic division supplies all parts of the body through a complex but organized nerve network.

– Origin of Sympathetic Fibers:

• Preganglionic neuron cell bodies are located in the lateral gray horn of spinal cord segments T1–L2 (Fig. 1.22).
• Their fibers leave through the anterior roots and spinal nerves at these levels.
• Therefore, the sympathetic division is traditionally called the thoracolumbar outflow.

– Sympathetic Trunks:

• Myelinated preganglionic fibers leave the spinal nerves through white rami communicantes.
• They enter the sympathetic trunks (Figs. 1.22 and 1.23).
• Two sympathetic trunks lie along the vertebral column, one on each side.
• Each trunk contains a chain of connected ganglia.
• These ganglia are called paravertebral ganglia because they lie beside the vertebral column.

– Distribution of Sympathetic Ganglia:

• The sympathetic trunks extend above and below T1–L2, along the vertebral column and sacrum (Fig. 1.23).
• They usually contain:
  • 11–12 thoracic ganglia
  • 3 cervical ganglia, formed by fusion of smaller embryonic ganglia.
  • Variable numbers of lumbar and sacral ganglia, which may also fuse.
• At the lower end, a single midline coccygeal ganglion is present.
• It is called the ganglion impar.
• It forms from the fusion of two embryonic coccygeal ganglia.

– Sympathetic Pathways:

• Sympathetic fibers follow four main pathways to four target regions (Table 1.7; Fig. 1.23).
• Preganglionic fibers from T1–L2 enter the sympathetic chains.
• They synapse with postganglionic neurons in ganglia associated with their pathways.
• Synapses may occur in chain ganglia along the length of the sympathetic trunk.
• Postganglionic fibers leave the ganglia.
• They then follow branches of spinal nerves to reach the body wall and limbs.
• Further details of sympathetic distribution are described in later chapters.

PARASYMPATHETIC DIVISION

• The parasympathetic division has a more limited and less complex distribution than the sympathetic division.
• It mainly supplies:
  • Head.
  • Internal organs within body cavities.
  • External genitalia.
• The body wall and limbs do not receive parasympathetic innervation.

– Origin of Parasympathetic Fibers:

• Preganglionic neurons arise from the brain and sacral spinal cord (Fig. 1.23).

– Cranial Origin:

• Parasympathetic neurons arise from nuclei associated with four cranial nerves:
  • CN III: Oculomotor nerve.
  • CN VII: Facial nerve.
  • CN IX: Glossopharyngeal nerve.
  • CN X: Vagus nerve.
• Their axons leave the brain through the corresponding cranial nerves.

– Sacral Origin:

• Preganglionic neurons arise from the lateral gray matter of spinal segments S2–S4.
• These segments do not contain a distinct lateral gray horn because the number of neurons is relatively small.
• Myelinated axons leave through the anterior roots of S2–S4.
• They then leave the spinal nerves to form the pelvic splanchnic nerves.
• Therefore, the parasympathetic division is traditionally called the craniosacral outflow.

– Parasympathetic Ganglia and Distribution:

• Preganglionic neurons synapse with postganglionic neurons in peripheral ganglia.
• These ganglia are usually located near the organs they supply.

– CN III, VII, and IX:

• Preganglionic fibers travel to autonomic ganglia in the head.
• Their distribution is limited to the head.

– CN X (Vagus Nerve):

• Preganglionic fibers have a wide distribution.
• They reach ganglia near internal organs in the neck, thorax, and much of the abdomen.

– Pelvic Splanchnic Nerves:

• Preganglionic fibers reach ganglia in the hypogastric plexuses.
• They also reach ganglia in the walls of lower abdominal and pelvic organs.

– Postganglionic Fibers:

• Are usually nonmyelinated.
• Are relatively short compared with sympathetic postganglionic fibers.
• Further details are described in later chapters.

– Recent View of Sacral Autonomic Outflow:

• Traditionally, sacral autonomic fibers have been classified as parasympathetic.
• However, recent studies discussed in the provided text suggest that these fibers may anatomically belong to the sympathetic nervous system.
• These fibers show a cholinergic phenotype, similar to sympathetic fibers supplying sweat glands.
• According to this proposed classification:
  • Sympathetic outflow: Spinal outflow, T1–L2 and S2–S4.
  • Parasympathetic outflow: Cranial outflow through CN III, VII, IX, and X.
• This proposed change is based on neurophysiological features and does not change the basic anatomy described above.

VISCERAL AFFERENTS

• The ANS is mainly a motor system.
• However, sensory neurons also travel closely with autonomic nerve fibers.
• These sensory neurons are called visceral afferents.
• They carry information from the body’s internal environment.

– Functions of Visceral Afferents:

• Carry sensory information for visceral reflexes.
• Help regulate blood pressure and blood chemistry.
• Help control:
  • Heart rate.
  • Respiratory rate.
  • Blood vessel resistance.

– Important Visceral Sensory Receptors:

• Chemoreceptors
• Baroreceptors
• Osmoreceptors

– Normal Visceral Sensations:

• Normal intestinal movement and distension usually do not produce conscious sensations.

– Abnormal Visceral Sensations:

• Extreme stretching.
• Chemical irritation.
• Ischemia.
• These conditions may produce conscious sensations of pain or nausea.

– Pathways of Visceral Afferent Fibers:

• Myelinated visceral afferent fibers from thoracic, abdominal, and pelvic organs travel with sympathetic and parasympathetic nerves.
• They pass through autonomic plexuses and ganglia without forming synapses.

– Visceral Afferents Traveling with Sympathetic Nerves:

• Many visceral sensory fibers enter the sympathetic trunks.
• They then pass through the white rami communicantes into spinal nerves.
• From there, they enter the posterior roots of spinal nerves.
• Finally, they reach the spinal cord (Fig. 1.22).
• Their cell bodies are located in the posterior root ganglia.

– Visceral Afferents Traveling with the Vagus Nerve:

• Other visceral sensory fibers travel through the vagus nerve.
• They enter the brainstem.
• Their cell bodies are located in the vagal sensory ganglia at the base of the skull.

HIGH-YIELD POINTS – NERVOUS SYSTEM

1. Nervous System: Works with the endocrine system to control and coordinate body activities.

2. Two Main Divisions:

• CNS: Brain and spinal cord.
• PNS: Cranial nerves, spinal nerves, and ganglia.

3. Functional Divisions:

• Somatic Nervous System: Mainly controls voluntary skeletal muscle activities.
• Autonomic Nervous System: Mainly controls involuntary activities of smooth muscle, cardiac muscle, and glands.

4. Neuron: Basic nerve cell consisting of a cell body, dendrites, and an axon.

• Dendrites: Carry impulses toward the cell body.
• Axon: Usually carries impulses away from the cell body.

5. Nucleus vs. Ganglion:

• Nucleus: Collection of neuron cell bodies inside the CNS.
• Ganglion: Collection of neuron cell bodies outside the CNS.

6. Gray Matter: Mainly contains neuron cell bodies supported by neuroglia.

7. White Matter: Mainly contains axons, many of which are myelinated, giving it a white appearance.

8. Spinal Cord Gray Matter: Has an H-shaped or butterfly-shaped arrangement.

9. Three Meninges:

• Dura mater: Outer layer.
• Arachnoid mater: Middle layer.
• Pia mater: Inner layer.

10. Cranial Nerves: There are 12 pairs. The vagus nerve (CN X) also supplies thoracic and abdominal structures.

11. Spinal Nerves: There are 31 pairs:

• 8 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 1 coccygeal

12. Anterior Root = Motor (Efferent): Carries impulses away from the CNS.

13. Posterior Root = Sensory (Afferent): Carries sensory impulses toward the CNS.

14. Anterior Gray Horn: Contains cell bodies of somatic motor neurons supplying skeletal muscles.

15. Lateral Gray Horn: Contains autonomic motor neuron cell bodies in the thoracic and upper lumbar spinal cord.

16. Posterior Root Ganglion: Contains cell bodies of sensory neurons.

17. Spinal Nerves Are Mixed Nerves: They contain both motor and sensory fibers.

18. Nerve Injury:

• Anterior root lesion: Affects motor fibers only.
• Anterior ramus lesion: Affects both motor and sensory fibers.

19. Anterior Rami: Supply the anterolateral body wall and limbs.

20. Posterior Rami: Supply the muscles and skin of the back.

21. Major Nerve Plexuses:

• Cervical plexus — Neck.
• Brachial plexus — Root of upper limb.
• Lumbosacral plexus — Root of lower limb.

22. Somatic vs. Autonomic Motor Pathway:

• Somatic: One-neuron pathway.
• Autonomic: Two-neuron pathway.

23. Hypothalamus: Controls the ANS and coordinates autonomic and neuroendocrine functions to maintain homeostasis.

24. Sympathetic Division:

• Traditionally called thoracolumbar outflow (T1–L2).
• Prepares the body for emergencies.
• Preganglionic neuron cell bodies are located in the lateral gray horn.

25. Parasympathetic Division:

• Traditionally called craniosacral outflow.
• Arises from CN III, VII, IX, X and spinal segments S2–S4.
• Helps the body recover.

26. Sympathetic vs. Parasympathetic Effects:

Sympathetic	Parasympathetic
Increases heart rate	Decreases heart rate
Dilates bronchi	Constricts bronchi
Decreases gut peristalsis	Increases gut peristalsis
Relaxes bladder wall	Contracts bladder wall
Dilates pupils	Constricts pupils

27. Sympathetic Trunks: Two chains of connected paravertebral ganglia lie along the vertebral column.

28. White Rami Communicantes: Carry myelinated sympathetic preganglionic fibers from spinal nerves into sympathetic trunks.

29. Parasympathetic Ganglia: Usually lie close to the organs they supply. Their postganglionic fibers are relatively short and nonmyelinated.

30. Important Autonomic Actions:

• Parasympathetic activity produces erection.
• Sympathetic activity produces ejaculation.
• Sympathetic activity stimulates sweat gland secretion.

31. Sacral Autonomic Outflow: Traditionally classified as parasympathetic (S2–S4). The provided text also discusses a proposed sympathetic classification based on newer research.

32. Visceral Afferents: Carry sensory information from internal organs.

33. Important Visceral Sensory Receptors:

• Chemoreceptors.
• Baroreceptors.
• Osmoreceptors.

34. Visceral Sensations:

• Normal gut movements usually do not produce conscious sensations.
• Extreme stretching, chemical irritation, and ischemia may produce pain or nausea.

35. Visceral Sensory Pathways:

• Sensory fibers traveling with sympathetic nerves may reach the spinal cord through posterior roots.
• Their cell bodies lie in posterior root ganglia.
• Other visceral sensory fibers travel through the vagus nerve to the brainstem, with cell bodies in vagal sensory ganglia.

Most Important Concept: The nervous system receives sensory information, coordinates body activities, and sends motor signals. The somatic system mainly controls skeletal muscles, while the autonomic system regulates internal organs to maintain homeostasis.