Nervous System Overview and Structure

Nervous System Overview

• The nervous system is composed of chapters 16, 17, 18, and 19.
• It controls and integrates all body activities within limits needed to maintain life.
• It has three basic functions:
  • Sensing changes in the surroundings with sensory receptors.
  • Interpreting and remembering the changing sensory information.
  • Reacting to changes with effectors (muscle contractions and glandular secretions)

Nervous System Divisions

• The central nervous system (CNS) is composed of the brain and spinal cord.
• The peripheral nervous system (PNS) consists of cranial and spinal nerves.
• The PNS connects the CNS to muscles, glands, and sensory receptors.

Organization of the Nervous System

• The CNS is the brain and spinal cord.
• The PNS includes everything else.

Structures of the Nervous System

• The nervous system includes the brain, cranial nerves (and branches), spinal cord, spinal nerves (and branches), ganglia, enteric plexuses, and sensory receptors.

Subdivisions of the PNS

• Somatic (voluntary) nervous system (SNS)
  • Neurons from receptors to the CNS
  • Motor neurons to skeletal muscle tissue
• Autonomic (involuntary) nervous system
  • Sensory neurons from visceral organs to the CNS
  • Motor neurons to smooth and cardiac muscle and glands
    • Sympathetic division speeds up heart rate.
    • Parasympathetic division slows down heart rate
• Enteric nervous system (ENS)
  • Involuntary sensory and motor neurons controlling the GI tract.
  • Function independently from the ANS and CNS.

Neurons

• Nerve cells form the functional unit of the nervous system.
• Nerve cells have electrical excitability.
• The nerve cell body contains a single nucleus with a prominent nucleolus.
• Cell processes include dendrites and axons.

Parts of a Neuron

• Axons or dendrites conduct impulses toward the cell body.
• Surfaces specialized for contact with other neurons.

Axons

• Conduct impulses away from the cell body.
• Synaptic end bulbs contain vesicles with neurotransmitters.

Functional Classification of Neurons

• Sensory (afferent) neurons transport sensory information to the CNS.
• Motor (efferent) neurons send motor nerve impulses to muscles and glands.

Structural Classification of Neurons

• Multipolar neurons have several dendrites and one axon.
• Bipolar neurons have one main dendrite and one axon.
• Unipolar neurons have one process only; they are always sensory neurons.

Neuroglia

• Neuroglia or glial cells constitute about half the volume of the CNS.
• Neuroglia are smaller than neurons and are 50x more numerous.
• They do not generate or propagate nerve impulses.
• They have the ability to multiply and divide in the mature nervous system.
• Six types of neuroglia: astrocytes, oligodendrocytes, microglia, and ependymal cells are found only in the CNS.
• Schwann cells (neurolemmocytes) and satellite cells are present in the PNS.

Astrocytes

• Star-shaped cells.
• Form blood-brain barrier (BBB) by covering blood capillaries.
• Provide strength.
• Maintain the unique permeability characteristics of the endothelial cells.
• Secrete chemicals that regulate growth, migration, and interconnections among neurons in the brain.

Oligodendrocytes

• Most common glial cell type.
• Each forms myelin sheath around more than one axons in the CNS.
• Analogous to Schwann cells of the PNS.

Microglia

• Small cells found near blood vessels.
• Phagocytic role -- clear away dead cells, remove cellular debris, and phagocytize microbes and damaged nervous tissue.

Ependymal cells

• Line the ventricles of the brain and central canal of the spinal cord.
• Functionally, ependymal cells produce, possibly monitor, and assist in the circulation of cerebrospinal fluid (CSF).

Schwann Cell

• Schwann cells (neurolemmocytes) encircle PNS axons, forming the myelin sheath.
• Participate in axon regeneration, which is more easily accomplished in the PNS.
• Each cell produces part of the myelin sheath surrounding an axon in the PNS.

Satellite Cell

• Satellite cells surround the cell bodies of neurons of PNS ganglia.
• Regulate the exchange of materials between neuronal cell bodies and interstitial fluid.

Myelination in the CNS

• Oligodendrocytes myelinate axons in the CNS.

Gray and White Matter

• White matter = myelinated processes (white in color)
• Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons, and neuroglia (gray color)

Multiple Sclerosis (MS)

• Autoimmune disorder causing destruction of myelin sheaths in CNS.
• Sheaths become scars or plaques.
• Symptoms include muscular weakness, abnormal sensations, or double vision

Regeneration & Repair

• Plasticity is maintained throughout life through sprouting of new dendrites, synthesis of new proteins.
• Changes in synaptic contacts with other neurons.
• Limited ability for regeneration (repair) of PNS.
• PNS can repair damaged dendrites or axons if the cell body remains intact.
• CNS has little to no repair even if the cell body is intact.

Neurogenesis in the CNS

• Formation of new neurons from stem cells was not thought to occur in humans.
• A growth factor was found in 1992 to stimulate adult mice brain cells to multiply.
• New neurons were found in 1998 to be formed in the adult human hippocampus.
• Factors preventing neurogenesis in the CNS, such as inhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and rapid formation of scar tissue.

Hippocampus location

Repair within the PNS

• Axons and dendrites can be repaired if the neuron cell body remains intact.
• Schwann cells remain active and form a tube.
• Scar tissue does not form too rapidly.
• Chromatolysis: 24-48 hours after injury, Nissl bodies break up into fine granular masses.
• By 3-5 days, Wallerian degeneration occurs (breakdown of axon & myelin sheath distal to injury).
• Retrograde degeneration occurs one node back.

The Spinal Cord & Spinal Nerves

• Together with the brain forms the CNS.
• Functions include spinal cord reflexes.
• Spinal cord is a highway for upward and downward travel of sensory and motor information.

Spinal Cord

• Structures covering the spinal cord include vertebrae, epidural space (filled with fat), dura mater (dense irregular CT tube), subdural space (filled with interstitial fluid), arachnoid (spider web of collagen fibers), subarachnoid space (CSF), and pia mater (thin layer).

External Anatomy of Spinal Cord

Spinal Nerves

• 31 pairs of spinal nerves numbered by cord level of origin.
• Named and numbered as C1 to C8 for cervical, T1 to T12 for thoracic, L1 to L5 for lumbar, S1 to S5 for sacral, and 1 pair of coccygeal nerves.
• Mixed sensory and motor nerves.

A Nerve Plexus

• Joining of spinal nerves to form nerve networks or plexuses.
• Found in the neck, arm, low back, and sacral regions.
• No plexus in the thoracic region (intercostals).

Cervical Plexus & Phrenic Nerve

C3, C4, C5, & Diaphragm

Branches off Brachial Plexus

Sacral Plexus

Branches of Sacral Plexus

Sciatic Nerve Branches

Dermatomes & Myotomes

• Each spinal nerve contains both sensory and motor nerve fibers.
• Dermatome: area of skin supplied by one spinal nerve; overlaps to prevent loss of sensation if one is damaged.
• Knowing spinal cord segments responsible for supplying each dermatome helps locate damaged regions of the spinal cord.

Dermatomes

Referred Pain

• Skin area and organ are served by the same segment of the spinal cord.

The Brain and Cranial Nerves

• Largest organ in the body, approximately 3 lbs.
• Brain functions in sensations, memory, emotions, decision-making, and behavior.

Cranial Nerves

• The cranial nerves are olfactory (I, sensory), optic (II, sensory), oculomotor (III, motor), trochlear (IV, motor), trigeminal (V, mixed), abducens (VI, motor), facial (VII, mixed), vestibulocochlear (VIII, sensory), glossopharyngeal (IX, mixed), vagus (X, mixed), accessory (XI, motor), and hypoglossal (XII, motor).

Cranial Nerves and Functions

• Mnemonic devices exist to remember the functions of the cranial nerves.

Olfactory (I) Nerve

Optic (II) Nerve

Oculomotor (III), Trochlear (IV), and Abducens (VI) Nerves

Trigeminal (V) Nerve

Facial (VII) Nerve

Vestibulocochlear (VIII) Nerve

Glossopharyngeal (IX) Nerve

Vagus (X) Nerve

Accessory (XI) Nerve

Hypoglossal (XII) Nerve

Principal Parts of the Brain

Protective Coverings of the Brain

• Bone, meninges, and fluid protect the brain, similar to the spinal cord.
  • Dura mater
  • Arachnoid mater
  • Pia mater

Blood Supply to Brain

• Internal carotid and vertebral arteries supply most of the brain.
• The circle of Willis (or cerebral arterial circle) provides alternate routes.
• The brain uses 20% of the body's oxygen and glucose.
• The blood-brain barrier (BBB) protects cells from toxins and pathogens.

Cerebrospinal Fluid (CSF)

• 80-150 ml (3-5 oz).
• Clear liquid containing glucose, proteins, and ions.
• Functions: mechanical protection, chemical protection, and circulation.

Origin of CSF

Ventricles

• The ventricular system consists of four ventricles.

Flow of Cerebrospinal Fluid

Reabsorption of CSF

• Reabsorbed through arachnoid villi, at a rate equal to production (20 ml/hour).

Hydrocephalus

• Blockage of CSF drainage leads to increased pressure.
• Neurosurgeons implant drains to shunt the CSF.

Brain Stem

• Composed of midbrain, pons, and medulla oblongata.

Medulla Oblongata

• Continuation of the spinal cord.
• Contains nuclei of 5 cranial nerves.
• Has cardiovascular centers (force and rate of heart beat, diameter of blood vessels).
• Respiratory centers (medullary rhythmicity area sets the rhythm of breathing).
• Reflex centers (coughing, sneezing, swallowing, vomiting, hiccuping).

Pons

• One inch long.
• Pneumotaxic and apneustic areas help control breathing.
• Middle cerebral peduncles carry sensory information to the cerebellum.
• Cranial nerves 5 through 8.

Midbrain

• One inch in length; extends from the pons to the diencephalon.
• Cerebral aqueduct connects the 3rd and 4th ventricles.
• Relays motor output from the cerebral cortex to the pons: relays sensory input from the spinal cord to the thalamus.
• Coordinates eye and head/trunk movements, especially in response to auditory stimuli.

Cerebellum

• 2 hemispheres and vermis.
• Correct voluntary muscle contraction and posture, based on sensory data about actual movements and equilibrium.

Cerebellum

Diencephalon Surrounds 3rd Ventricle

• Superior part of walls: thalamus
• Inferior part of walls & floor: hypothalamus

Thalamus

Hypothalamus

• Dozen or so nuclei in 4 major regions.
• Mammillary bodies are relay stations for olfactory reflexes, infundibulum suspends pituitary gland).
• Major regulator of homeostasis (receives somatic and visceral input, taste, smell & hearing information; monitors osmotic pressure, temperature of blood).

Functions of Hypothalamus

Epithalamus

Cerebrum (Cerebral Hemispheres)

• Cerebral cortex: gray matter overlying white matter.
• Cerebral cortex: 2-4mm thick with billions of cells; formed folds (gyri) and grooves (sulci).
• Longitudinal fissure separates left & right cerebral hemispheres.
• Corpus callosum: band of white matter connecting left and right hemispheres.
• Each hemisphere subdivided into 4 lobes.

Lobes and Fissures

Primary Motor Cortex

• Precentral gyrus initiates voluntary movements; upper motor neurons.
• Muscles represented unequally, depending on the number of motor units.

Somatosensory Map of Postcentral Gyrus

• Relative sizes of cortical areas depend on the number of sensory receptors and sensitivity of part of the body.
• Cortical area can be modified by learning (e.g., an enlarged somatosensory area for fingertips in Braille readers).

Limbic System

• Parahippocampal & cingulate gyri & hippocampus.
• Emotional brain--intense pleasure & intense pain; strong emotions increase efficiency of memory.

The Autonomic Nervous System

• Regulates activity of smooth muscle, cardiac muscle and certain glands.
• Receives input from limbic system and other regions of the cerebrum.

Autonomic vs Somatic NS

Autonomic versus Somatic NS

Divisions of the ANS

• Two major divisions of the ANS (parasympathetic and sympathetic).
• Dual innervation with one speeding up an organ and one slowing it down.

Sources of Dual Innervation

Pathways of Sympathetic Fibers

ANS Neurotransmitters

• Cholinergic neurons release ACh.
• Adrenergic neurons release NE.

Sympathetic Responses

Parasympathetic Responses

Comparison: PNS Somatic

Comparison: PNS Autonomic (Visceral)

Autonomic Motor (Efferent) Pathway

Brachial Plexus

Lumbar and Sacral Plexus

Sacral Plexus

Cranial Nerve Review

• Each cranial nerve has specific functions that can be remembered via mnemonics.

This quiz covers the major aspects of the nervous system, including its overview, divisions, and structural organization. You'll explore the roles of the central and peripheral nervous systems, and how they interact to maintain bodily functions. Test your knowledge on sensory receptors, brain, and spinal structures!