Nervous System: Divisions and Functions

• The nervous system is a network of neurons.
• Its main function is generating, modulating, and transmitting information.
• The nervous system regulates vital functions like heartbeat, breathing, digestion, sensation, and movement.
• It is fundamental to consciousness, cognition, behavior, and memories.

Nervous System Divisions

• The central nervous system (CNS) serves as the integration and command center.
• The peripheral nervous system (PNS) acts as a conduit between the CNS and the body, subdivided into:
  • Somatic nervous system (SNS).
  • Autonomic nervous system (ANS).

Key Facts

• Definition: The nervous system is a network of neurons transmitting neural impulses.
• Divisions: CNS and PNS
• Central Nervous System: Brain and spinal cord.
• Peripheral Nervous System Functional Divisions:
  • Somatic nervous system.
  • Autonomic nervous system includes sympathetic, parasympathetic, and enteric divisions.

Cells of the Nervous System

• Two primary cell types: neurons and glial cells.

Neurons

• Neurons/nerve cells, are the main structural/functional units.
• Consist of: - Soma (cell body): Contains organelles and generates neural impulses (action potentials). - Neurites (processes): Connect neurons/body cells, enabling neural impulse flow. - Axons: Conduct impulses away from the cell body. - Dendrites: Receive impulses and conduct electrical signals towards the cell body.
• Every neuron has an axon, dendrite number varies.
• Four structural types: multipolar, bipolar, pseudounipolar, and unipolar.
• They generate, receive, and send neural impulses to other neurons/non-neural tissues.

Neuron Types

• Neurons are classified based on the direction of electrical signal transmission.
  • Efferent neurons (motor/descending): Transmit signals from the CNS to peripheral tissues.
  • Afferent neurons (sensory/ascending): Conduct impulses to the CNS, carrying sensory information.
• Synapse: The site where an axon connects to another cell to pass neural impulses, releasing neurotransmitters.

Glial Cells

• Neuroglia/glia, are non-excitatory cells supporting neurons.
• They myelinate neurons.
• Maintain homeostatic balance.
• Provide structural support, protection, and nutrition.
• Four types of glial cells carry out these roles:
  • Myelinating glia: Produce myelin sheath, called oligodendrocytes (CNS) and Schwann cells (PNS).
    • COPS mnemonic: Central - Oligodendrocytes; Peripheral - Schwann
  • Astrocytes (CNS) and satellite glial cells (PNS): Support and protect neurons.
  • Microglia (CNS): Phagocytes.
  • Ependymal cells (CNS): Line the ventricular system.
• PNS equivalent to microglia: Macrophages assume the phagocytic role.

Myelin Sheath

• Most axons are insulated by this white substance.
• It is produced by oligodendrocytes and Schwann cells.
• Myelin encloses an axon segmentally, with myelin sheath gaps (nodes of Ranvier).
• Neural impulses jumping through these gaps increases transmission speed.

White and Gray Matter

• Myelinated axons appear white, contrasting with the gray of neuronal bodies and dendrites.
• Nervous tissue has white and gray matter, each with a unique distribution:
  • White matter- outer spinal cord layer, inner brain part.
  • Gray matter- central spinal cord part, outer brain layer (cerebral cortex), and subcortical nuclei.

Nervous System Organization

• Nervous tissue forms organs (brain, nerves).
• These organs are the evolutionary perfection.
• Structurally divided into:
  • Central nervous system (CNS): brain and spinal cord
  • Peripheral nervous system (PNS): neural tissue outside the CNS
• Functionally into: sensation, integration, and response.

Functional Divisions

• Sensory (afferent) nervous system: Detects stimuli via receptors, transmits info to the CNS.
  • Input is divided into somatic, visceral, and special senses.
• Integration occurs in the brain at low and high levels, including functions from basic bodily actions to complex decision-making.
• Motor (efferent) nervous system: Carries signals from the brain to effectors for responses.
  • Actions such as muscle movement or glandular secretion are responses.
  • Includes somatic (voluntary) and autonomic (involuntary) systems:
    • Autonomic system further divides into sympathetic (stress) and parasympathetic (resting) responses.

Afferent and Efferent

• Neural pathways or tracts in the CNS connect with each other.
• Those that connect with peripheral tissues belong to the PNS.
• Bundles in the PNS are called afferent and efferent peripheral nerves.

Central Nervous System

• The CNS includes the brain and spinal cord.
• These are protected by the skull and vertebral column.
• The brain components entail: cerebrum, diencephalon, cerebellum, and brainstem.
• These parts process incoming information and generate commands for tissue responses that manage involuntary functions and complex processes like thinking.
• The spinal cord continues from the brainstem, generating commands for involuntary processes (reflexes) and transmitting information between the CNS and periphery.

Peripheral Nervous System

• The PNS has:
  • 12 cranial nerve pairs.
  • 31 spinal nerve pairs.
  • Neuronal clusters (ganglia).
• Peripheral nerves can be sensory (afferent), motor (efferent), or mixed.
• Peripheral nerves have modalities based on innervated structures:
  • Special - special senses (afferent fibers).
  • General - everything (excluding special senses).
  • Somatic - skin and skeletal muscles.
  • Visceral - internal organs.

Cranial Nerves

• Cranial nerves are peripheral nerves from the brainstem/spinal cord cranial nerve nuclei.
• They innervate the head/neck.
• Numbered from I to XII by their exit order through skull fissures:
  • I: Olfactory
  • II: Optic
  • III: Oculomotor
  • IV: Trochlear
  • V: Trigeminal
  • VI: Abducens
  • VII: Facial
  • VIII: Vestibulocochlear
  • IX: Glossopharyngeal
  • X: Vagus
  • XI: Accessory
  • XII: Hypoglossal
• Nerves are motor (III, IV, VI, XI, XII), sensory (I, II, VIII) or mixed (V, VII, IX, X).

Spinal Nerves

• Emerge from the spinal cord segments.
• Numbered by origin segment, with 31 pairs:
  • 8 cervical.
  • 12 thoracic.
  • 5 lumbar.
  • 5 sacral.
  • 1 coccygeal.
• All spinal nerves contain both sensory and motor fibers (mixed).
• Spinal nerves innervate the body (except the head);
  • Directly synapsing with target organs.
  • Interlacing and forming plexuses, four major plexuses supply body regions.
  • Cervical plexus (C1-C4) supplies the neck.
  • Brachial plexus (C5-T1) supplies the upper limb.
  • Lumbar plexus (L1-L4) supplies the lower abdominal wall, anterior hip, and thigh.
  • Sacral plexus (L4-S4) supplies the pelvis and lower limb.

Ganglia

• Ganglia are neuronal cell body clusters outside the CNS that are similar to subcortical nuclei within the CNS.
• Ganglia can be sensory or visceral motor (autonomic) with clear distributions throughout the body.
• Dorsal root ganglia- sensory nerve cell body clusters adjacent to the spinal cord, being a component of the posterior nerve root.
• Autonomic ganglia- can be sympathetic or parasympathetic.
• Sympathetic ganglia- can be grouped into paravertebral and prevertebral ganglia in the thorax/abdomen.
• Paravertebral ganglia- reside on either side of the vertebral column, comprising two ganglionic chains extending from skull base to the coccyx.
• Sympathetic trunks- are the ganglia.
• Prevertebral ganglia are anterior to the vertebral column near target organs branching from abdominal aorta (celiac, aorticorenal, superior/inferior mesenteric ganglia).
• Parasympathetic ganglia can be found in the head and pelvis.
• Ganglia in the head are associated with cranial nerves (ciliary, pterygopalatine, otic, submandibular), while the pelvic ganglia near reproductive organs consist of autonomic plexuses for innervation of pelvic viscera (prostatic, uterovaginal plexuses).

Somatic Nervous System

• Voluntary component of the PNS.
• It has fibers of cranial/spinal nerves allowing voluntary movements (efferent nerves) and sensations from skin, muscles, and joints (afferent nerves).
• Somatic sensation covers touch, pressure, vibration, pain, temperature, stretch, and position sense related to structure types.
• The autonomic nerves convey sensations from the glands, smooth, and cardiac muscles.

Autonomic Nervous System

• Involuntary aspect of the PNS.
• Is divided into sympathetic (SANS) and parasympathetic (PANS) systems, exclusively comprised of visceral motor fibers.
• Nerves from these systems innervate all involuntary structures;
  • Cardiac muscle.
  • Glandular cells.
  • Smooth muscles in blood vessel/hollow organ walls.
• Balanced SANS and PANS functioning maintains homeostasis.
• They augment one another through potentializing organ activities under specific situations (e.g., PSNS stimulates intestine activity after food, SANS heart output during exercise).

Autonomic Nerves

• These synapse within autonomic ganglia prior to reaching target organs, thereby demonstrating presynaptic and postsynaptic components.
• Presynaptic fibers start from the CNS and synapse with neurons of the peripheral autonomic ganglia.
• Postsynaptic fibers are ganglion neuron axons extending to peripheral tissues.
• Sympathetic nerves feature short preganglionic fibers (due to ganglia proximities to the spinal cord), and longer postganglionic fibers traversing towards their target organ.
• The contrary occurs in parasympathetic nerves, where the preganglionic fiber is generally longer than its postganglionic counterpart.

Sympathetic Nervous System

• SANS adjusts physiology for more activity.
• Fight-or-flight response triggers faster breathing, heightened heart rate, high blood pressure, wider pupils, and redirected blood flow from skin, kidneys, stomach, and intestines to the heart and muscles.
• Sympathetic nerve fibers stem from the T1-L2/L3 spinal cord segments.
• They synapse with prevertebral and paravertebral ganglia, where from postsynaptic fibers supply the target viscera.

Parasympathetic Nervous System:

• This system uses energy conservation.
• A state of “rest and digest" or "feed and breed” ensues.
• Actions of PSNS slow the actions of the cardiovascular system.
• The system diverts blood from muscles, and increasing peristalsis and gland secretion.
• Parasympathetic fibers come from the brainstem (cranio-) and S2-S4 spinal cord segments (-sacral).
• These travel to thoracic and abdominal organs, where they synapse in ganglia near or within the target organ.

Enteric Nervous System

• Comprises SANS/PANS fibers regulating activity of the gastrointestinal tract.
• The enteric nervous system has parasympathetic fibers from the vagus nerve (CN X) plus sympathetic fibers from thoracic splanchnic nerves.
• Fibers form two plexuses inside intestinal tube walls for modulating peristalsis such as;
  • Submucosal plexus (of Meissner): found in the submucosa, and only has parasympathetic fibers.
  • Myenteric plexus (of Auerbach): this is in the muscularis externa, plus has both sympathetic/parasympathetic nerve fibers.

Mnemonic

• SMP & MAPS for two plexuses such as:
  • Submucosal
  • Meissner's
  • Parasympathetic
  • Myenteric
  • Auerbach's
  • Parasympathetic
  • Sympathetic

Clinical Notes

Vagotomy

• It is an older method for treating gastric ulcers due to diet and medication shortcomings.
• The vagus nerve increases secretion of gastric acid.
• Types performed would diminish effect.

Cranial Nerve Palsies

• The 12 cranial nerves exit/enter the skull through foramina.
• Narrowing/constriction along nerves results in palsy.
• Facial nerve palsy causes:
  • Hemiplegia
  • Dry eyes
  • Absent corneal reflex
  • Overloud hearing
  • Affected taste (anterior 2/3 of tongue)

Limb Nerve Lesions

• Palsies from fracture, constriction/overuse.
• In the carpal tunnel, the median nerve gets compressed, caused by flexor tendon enlargement/swelling.
• This syndrome appears in pregnancy and acromegaly.

Hirschsprungs Disease

• Colonic atony because of failure in migration of ganglion cells (part of enteric NS) to the enteric NS, constipation, and malnourishment, needing corrective surgery.

Spina bifida

• There is a failure in neural arch development, that produces lumbar spine defects.
• Contributing influences are environmental/genetic, however, there are folate supplements given.

Parkinson’s Disease

• Cognition and movement are controlled by dopamine and basal ganglia.
• Those with Parkinson's have degradation of dopaminergic neurons in the substantia nigra.
  • Difficulty initiating movement
  • Shuffling gait
  • Masked facies
  • Cog-wheel/lead-pipe rigidity (limbs)