Nervous System Review

• Both the nervous and endocrine systems transmit information to control bodily functions.
• Proper function of control and integration networks is essential for homeostasis.

Nervous System Organization

• Central Nervous System (CNS): brain and spinal cord.
• Peripheral Nervous System (PNS): all nerves outside the CNS.
• Autonomic Nervous System (ANS): regulates involuntary functions.

Cells of the Nervous System

• Neurons and glia are the two main cell types in the nervous system.

Neuron Structure

• Dendrites: Branching projections that conduct impulses to the neuron's cell body.
• Axon: Elongated projection that conducts impulses away from the neuron's cell body.
• Myelin: A white, fatty substance formed by glia, that surrounds some axons as a sheath.
• Nodes of Ranvier: Gaps in the myelin sheath that expose the axon membrane.
• Neurilemma: The outer layer of the myelin sheath, aids in the repair of damaged axons,

Neuron Types

• Sensory neurons (afferent): Carry impulses to the spinal cord and brain.
• Motor neurons (efferent): Carry impulses away from the brain and spinal cord to muscles and glands.
• Interneurons (central or connecting): Conduct impulses between sensory and motor neurons, or within networks of interneurons.

Glia (Neuroglia) Function

• Support cells that hold nervous tissue together structurally and functionally.

Central Glia

• Astrocytes: Star-shaped cells that anchor small blood vessels to neurons.
• Microglia: Small, mobile cells that perform phagocytosis in inflamed brain tissue.
• Oligodendrocytes: Produce myelin sheaths on axons in the CNS; Schwann cells perform this function in the PNS.

Peripheral Glia

• Schwann cells form myelin sheaths on axons in the PNS.

Disorders of Nervous Tissue

• Multiple sclerosis: Myelin loss in central nerve fibers leads to impaired nerve conduction.
• Neuroma: General name for nervous system tumors, most of which are gliomas (glial tumors).
• Multiple neurofibromatosis: Characterized by numerous benign tumors.

Nerves and Tracts

• Nerve: A bundle of peripheral axons.
• Tract: A bundle of central axons.
• Endoneurium: Connective tissue surrounding individual nerve fibers within a nerve.
• Perineurium: Connective tissue surrounding a group (fascicle) of nerve fibers.
• Epineurium: Connective tissue surrounding the entire nerve.
• White matter: Tissue primarily composed of myelinated axons (tracts).
• Gray matter: Tissue composed primarily of cell bodies and unmyelinated fibers.

Nerve Signals

• Reflex arcs are neuron pathways that conduct nerve impulses from receptors to effectors.
• A reflex is a contraction by a muscle or secretion by a gland caused by conduction by a reflex arc.
• Two-neuron arcs: Simplest type, consisting of sensory neurons synapsing with motor neurons in the spinal cord.
• Three-neuron arcs: Sensory neurons synapse with interneurons in the spinal cord, which then synapse with motor neurons.

Nerve Impulses

• Nerve impulse: A self-propagating wave of electrical disturbance that travels along the neuron membrane, also known as action potential.
• At rest, a neuron membrane is slightly positive on the outside (polarized) due to a slight excess of Na+ ions.
• A stimulus opens Na+ channels, causing inward movement of Na+ and depolarizing the membrane, making the inside more positive.

Conduction of Nerve Impulses

• Continuous conduction: Depolarization propagates along the entire membrane as each adjacent section is stimulated.
• Saltatory conduction: In myelinated fibers, impulses "jump" from one node of Ranvier to the next, greatly increasing conduction speed.

The Synapse

• Synapse: The site where impulses are transmitted from one neuron (presynaptic) to another (postsynaptic).
• Synapse Structures: synaptic knob, synaptic cleft, and plasma membrane
• Neurotransmitters: Bind to receptors on the postsynaptic neuron, opening ion channels and stimulating impulse conduction.
• Neurotransmitter Removal: Reuptake into presynaptic neurons, breakdown in the synaptic cleft, or breakdown by glia
• Neurotransmitter examples: Acetylcholine, catecholamines (norepinephrine, dopamine, serotonin), endorphins, enkephalins, nitric oxide (NO).
• Receptors are specific to neurotransmitters, with possible variations for a single neurotransmitter in different body locations.
• Parkinson's disease is characterized by low dopamine levels.
• Individuals with Parkinson's typically experience involuntary trembling and muscle rigidity.

Central Nervous System (CNS)

• The CNS consists of the brain and spinal cord, protected by bones and meninges.

Brain

• Brainstem: Consists of the medulla oblongata, pons, and midbrain, featuring white matter with scattered gray matter
• All three parts of the brainstem are two-way conduction paths.
  • Sensory tracts conduct impulses to higher brain regions.
  • Motor tracts conduct impulses from higher brain regions to the spinal cord.
  • Gray matter areas function as reflex centers.

Cerebellum

• Second largest part of the brain
• Gray matter outer layer is highly folded
• Arbor vitae: Internal, treelike network of white matter tracts
• Functions: Controls muscle contractions for coordinated movement, balance, smooth motion, and posture; assists cerebrum and other brain regions.

Diencephalon

• Hypothalamus: Contains the posterior pituitary gland, pituitary stalk, and gray matter.
  • Functions as the major center for controlling the ANS.
  • Controls hormone secretion by anterior and posterior pituitary glands.
  • Contains centers for controlling appetite, wakefulness, and pleasure centers.
• Thalamus: Dumbbell-shaped mass of gray matter extending toward each cerebral hemisphere.
  • Relays sensory impulses to cerebral cortex sensory areas
  • Produces emotions associated with sensations
  • Regulates consciousness through arousal mechanisms
  • Plays a role in reflex movements
• Pineal Gland: Small body resembling a pine nut behind the thalamus.
  • Adjusts melatonin output in response to changing light

Cerebrum

• Largest part of the brain
• Cerebral cortex: Outer layer of gray matter; composed of lobes
• Interior is composed of white matter
• Functions: Mental processes, sensations, consciousness, memory, and voluntary control of movements

Brain Disorders

• Concussion: A type of traumatic brain injury (TBI) caused by a jolt to the head, characterized by changes in thinking, physical symptoms, and mood or sleep changes
• Cerebrovascular accident (CVA): Hemorrhage or cessation of blood flow through cerebral blood vessels, also refers to a stroke.
• Cerebral palsy (CP): Damage to motor control areas of the brain before, during, or shortly after birth, causing paralysis.
• Dementia: Syndrome with progressive memory loss, shortened attention span, personality changes, reduced intellectual capacity, and motor control deficit
  • Alzheimer disease (AD): Brain disorder characterized by dementia
  • Chronic traumatic encephalopathy (CTE): Results from repeated head trauma, causes memory loss etc
  • Huntington disease (HD): Inherited disorder characterized by chorea progressing to severe dementia
  • HIV can infect neurons and cause dementia.
• Seizure disorders
  • Seizure: Sudden burst of abnormal neuron activity that results in temporary changes in brain function
  • Epilepsy: Recurring seizures
  • Electroencephalogram: Graphic representation of voltage changes in the brain

Spinal Cord

• Columns of white matter form the outer portion
• Interior is gray matter
• Spinal cord tracts provide two-way conduction paths
• Functions as the primary center for all spinal cord reflexes
• Sensory tracts conduct impulses to the brain, and motor tracts conduct impulses from the brain

Coverings and Fluid Spaces of the Brain and Spinal Cord

• Meninges and bone
  • Dura mater: Tough outer membrane
  • Arachnoid mater: Cobweblike middle layer
  • Pia mater: Delicate inner layer
  • Cranial bones and vertebrae form hard outer covering
• Cerebrospinal fluid (CSF) spaces:
  • Subarachnoid spaces of meninges
  • Central canal inside cord
  • Ventricles in brain
  • Hydrocephalus can result from blocked CSF circulation

Peripheral Nervous System

• Cranial nerves: Twelve pairs attached to the undersurface of the brain
• Connect brain with the neck and structures in the thorax and abdomen
• Spinal nerves: Thirty-one pairs that contain dendrites of sensory neurons and axons of motor neurons
• Conduct impulses necessary for sensations and voluntary movements
• Dermatome: Skin surface area supplied by a single cranial or spinal nerve

Peripheral Nerve Disorders

• Neuritis: General term referring to nerve inflammation.
• Sciatica: Inflammation of the sciatic nerve that innervates the legs.
• Neuralgia: Muscle pain, often accompanies neuritis.
• Trigeminal neuralgia: Recurring episodes of stabbing pain along one or more branches of the trigeminal nerve in the head.
• Bell palsy: Paralysis of facial features resulting from damage to the facial nerve.
• Herpes zoster (shingles): Viral infection caused by chickenpox virus that has invaded the dorsal root ganglion and remained dormant until stress or reduced immunity precipitates an episode of shingles.

Autonomic Nervous System

• Motor neurons that conduct impulses from the CNS to cardiac muscle, smooth muscle, and glandular epithelial tissue.
• Regulates involuntary functions
• Autonomic neurons: Preganglionic autonomic neurons conduct from spinal cord or brainstem to an autonomic ganglion; postganglionic neurons conduct from autonomic ganglia to cardiac muscle, smooth muscle, and glandular epithelial tissue
• Autonomic or visceral effectors: Tissues to which autonomic neurons conduct impulses
• Composed of two divisions: sympathetic and parasympathetic

Autonomic Conduction Paths

• Consist of two-neuron relays
• Somatic motor neurons conduct all the way from the CNS to somatic effectors with no intervening synapses

Sympathetic Division

• Located in of the thoracic and upper lumbar segments of the spinal cord
• Axons leave the spinal cord in the ventral roots of spinal nerves, extend to sympathetic, or collateral ganglia and synapse with several postganglionic neurons whose axons extend to spinal or autonomic nerves to terminate in visceral effectors
• Functions: Serves as the emergency or stress system, controlling visceral effectors during strenuous exercise and strong emotions
• Fight-or-flight response: Group of changes induced by sympathetic control

Parasympathetic Division

• Preganglionic neurons have dendrites and cell bodies in the gray matter of the brainstem and the sacral segments of the spinal cord
• Preganglionic neurons terminate in parasympathetic ganglia located in the head and the thoracic and abdominal cavities close to visceral effectors
• Each preganglionic neuron synapses with postganglionic neurons to only one effector
• Function: Dominates control of many visceral effectors under normal, everyday conditions; counterbalances sympathetic function

Autonomic Neurotransmitters

• Cholinergic fibers: Preganglionic axons of parasympathetic and sympathetic systems and parasympathetic postganglionic axons release acetylcholine
• Adrenergic fibers: Axons of sympathetic postganglionic neurons release norepinephrine (noradrenaline)

Autonomic Nervous System (Overall)

• Regulates the body’s automatic functions in ways that maintain or quickly restore homeostasis
• Many visceral effectors are doubly innervated

Disorders of the Autonomic Nervous System

• Stress-induced disease: Prolonged or excessive response to stress can disrupt normal functioning throughout the body
  • Examples include heart disease, digestive problems, reduced resistance to disease, and spread of cancer
• Neuroblastoma: Highly malignant tumor of the sympathetic division, primarily affecting young children

Test your knowledge of the nervous system. Questions cover the parasympathetic and sympathetic divisions, neurotransmitters, and nervous tissue. Assess your understanding of anatomical locations, interactions, stress responses, and neurological conditions.