Nervous System Overview

Nervous System Overview

• The nervous system is functionally divided into somatic and autonomic systems.
• The somatic nervous system controls voluntary movements of skeletal muscles and relies on somatic fibers.
• The autonomic nervous system is involuntary and controls smooth and cardiac muscles, as well as gland secretions. It relies on autonomic nerve fibers.

Nervous System Divisions

• Central Nervous System (CNS):

  • Brain and Brain Stem:
    • Composed of two cerebral hemispheres and the cerebellum.
    • Includes the midbrain, pons, and medulla oblongata, part of the brain stem.
  • Spinal Cord:
    • Contains 31 segments (Cervical, Thoracic, Lumbar, Sacral, Coccygeal).
    • Specific segment counts vary. e.g., Cervical contains 8 segments

• Peripheral Nervous System (PNS):

  • Cranial Nerves: 12 pairs.
  • Spinal Nerves: 31 pairs.

Reflex Arc Components

• Receptors: The initial point of stimulation.
• Afferent Neuron: Carries signals from receptors to the center.
• Center: The integration point in the CNS (e.g., spinal cord).
• Efferent Neuron: Carries signals from the center to the effector.
• Effector Organ: Responds to the signal (e.g., muscle).

Autonomic Ganglia

• Definition: Collections of nerve cells outside the CNS, where preganglionic and postganglionic autonomic fibers synapse.
• Chemical Transmitter: Acetylcholine (ACh) is the primary chemical transmitter in all ganglia.
• Types of autonomic ganglia:
  • Lateral Ganglia (paravertebral):
    • Primarily sympathetic.
    • Lie alongside the vertebral column.
    • An exception for cervical segments: They’re unified into three pairs (superior, middle, and inferior).
    • Postganglionic fibers are long.
  • Collateral Ganglia:
    • Located between the sympathetic chain and the internal organs (viscera).
    • Named based on the artery they're near (e.g., celiac, renal, superior/inferior mesenteric ganglia).
  • Terminal Ganglia:
    • Primarily parasympathetic.
    • Situated near or within the target organ.
    • Postganglionic fibers are short.

Functions of Autonomic Ganglia

• Relay station between preganglionic and postganglionic fibers.
• Distributing centers. Each preganglionic sympathetic neuron branches out to 8-12 postganglionic neurons.
• Sites at which many drugs (e.g., ganglion blockers) act.

Sympathetic Nervous System (SNS)

• SNS to the head & neck, origin: from thoracic (T1-T2) segments, relay: superior cervical ganglia, chemical transmitters: Acetylcholine (pre) & Norepinephrine (post).
• SNS to the thorax, origin: upper 4 thoracic segments, relay in thoracic and cervical ganglia, activities: heart stimulation (contractility, conduction, excitability), dilation of coronary blood vessels, widening of bronchi & bronchioles, decreased mucous secretions in lungs, vasoconstriction of pulmonary vessels
• SNS to the abdomen, origin: thoracic (T6-T12) segments, relay in collateral ganglia (celiac, renal, superior/inferior mesenteric), the part supplying the gut (enteric nervous system) acts on the GIT by relaxing smooth muscle of stomach, small intestine, and proximal colon. Stimulating glycogenolysis in liver and contraction of sphincters. Vasoconstriction of splanchnic arteries.
• SNS to the pelvis, origin: from the upper 2–3 lumbar segments (L1-L3), relay in inferior mesenteric ganglia, functions: Relaxation of bladder wall, contraction of internal urethral sphincter, relaxation of rectum wall and contraction at the internal anal sphincter, participation in sexual acts.
• SNS to the upper and lower limbs and thoracic abdominal walls, origin: Thoracic (T5-T9) upper limbs, thoracic and lumbar segments for lower limbs, thoracic and abdominal walls, relay in sympathetic chain ganglia. Functions: Sweat secretion, hair erection, vasoconstriction for the skin, vasodilation for skeletal muscles.

Orbeli Phenomenon

• Sympathetic stimulation leads to enhanced skeletal muscle contraction, delayed fatigue, and accelerated recovery from fatigue.
• This results from improved blood flow and enhanced sensitivity of motor end plates.

Mass Discharge of SNS

• SNS often discharges as a unit during emergencies/stress.
• This is commonly seen in circumstances like extreme exercise, fear, hemorrhage, or cold exposure.
• The response is widespread, preparing the body for fight-or-flight reactions.

Cranial Nerves

• The 12 pairs of cranial nerves have specific functions like olfactory (smell), optic (vision), oculomotor (eye movement), and more.
• Cranial nerves have both sensory and motor components.

Spinal Nerves

• Spinal nerves also have both motor and sensory functions.
• They consist of dorsal and ventral roots.

Enteric Nervous System (ENS)

• Part of the autonomic nervous system specifically for the gastrointestinal tract (GIT).
• Has its own intricate network of neurons to control digestive processes, independently regulating motility, secretions, and blood flow.

Horner's Syndrome

• A group of symptoms resulting in one-sided interruption of sympathetic nerve supply to the head and neck.
• Symptoms include ptosis (drooping of the eyelid), miosis (constricted pupil), enophthalmos (sunken eyeball), anhydrosis (lack of sweating), and warmth and redness of the skin.
• Usually due to lesions in cervical and/or first two thoracic ganglia

Autonomic Control of Organs

• Specific examples of autonomic control are presented for various organs (eyes, salivary glands, skin, heart, lungs, etc.). This describes the effects of sympathetic and parasympathetic stimulation on different bodily functions.