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The Nervous System- Yatziri Carmona Per.1 (The Central Nervous System…
The Nervous System- Yatziri Carmona Per.1
There are 2 divisions of the nervous system: the peripheral nervous system (PNS) and the central nervous system (CNS)
The PNS: cranial and spinal nerves
afferent division: conveys impulses to the CNS
efferent division: conveys impulses from the CNS
somatic (voluntary) system: serves skeletal muscles
autonomic (involuntary) system: innervates smooth and cardiac muscle and glands
Sympathetic division: mobilizes body systems during activity
Parasympathetic division: conserves energy & promotes house keeping functions during rest
The CNS: Brain and spinal cord
Neuroglia in the CNS (nerve glue): segregate and insulate neurons and assist neurons in various other ways
Neuroglia in the PNS
satellite cells: have many of the same functions in the PNS as astrocytes do in the CNS
schwann cells: surround all nerve fibers in the PNS and form myelin sheaths around the thicker nerves ; function similarly to oligodendrocytes; vital to regeneration of damaged peripheral nerve fibers
microglial cells: oval shaped defensive cells in the CNS
ependymal cells/ "wrapping garment": line cerebrospinal fluid-filled CNS cavities, range from squamous to columnar
astrocytes/"star cells": the most abundant CNS neuroglia; support and brace the neurons and anchor them to their nutrient supply lines
oligodendrocytes: have processes that form myelin sheaths around CNS nerve fivers
Neuron
Axon hillock: the action potential is generated here
Axon: impulsing-generating and conducting region
Nucleolus
Schwann cell: known for their roles in supporting nerve regeneration
Nucleus
Myelin sheath gap/ Node of Ranvier: serves to facilitate the rapid conduction of nerve impulses
Dendrites: receptive regions
Terminal branches
Cell body: biosynthetic center and metabolic center
Axon terminals: secretory region
Classification of Neurons
Structural classification
Functional classification
Motor/ efferent neurons: carry impulses away from the CNS to the effector organs (muscles and glands) of the body
Interneurons: lie between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs
Sensory/ afferent neurons: transmit impulses from sensory receptors in the skin or internal organs toward or into the CNS
Bipolar neurons: have 2 or more process- an axon and a dendrite; extend from opposite sides of the cell body, rare; found in some of the special sense organs
Multipolar neurons: have 3 or more processes-an axon and the rest dendrites; most common neuron type; major neuron type in the CNS
Unipolar neurons: a single short process that emerges from the cell body and divides T-like into proximal and distal branches
Gated channels
Voltage gated channels
Open in response to changes in membrane potential
Mechanically gated channels
Open in response to physical deformation of the receptor
Chemically gated channels
Open in response to binding of the appropriate neurotransmitter
The direction an ion moves is determined by the electrochemical gradient
concentration gradient: ions move along chemical concentration gradients from an area of their higher concentration to an area of lower concentration
electrical gradient: ions move toward an area of opposite electrical charge
Resting membrane potential
The potential difference in a resting neuron
Sodium-potassium pump
stabilizes the resting membrane potential by maintaining the concentration gradients for sodium and potassium
Changes in the resting membrane potential
Action potential-long distance signals of axons that always have the same strength, an all-or-none phenomenon; impulse rate depends on axon diameter and the degree of myelination
Depolarization: Voltage-gated Na+ channels open
Repolarization: Na+ channels are inactivating, and the voltage-gated K+ channels open
Resting state: All voltage-gated Na+ and K+ channels are closed
Hyperpolarization: Some K+ channels remain open, and Na+ channels reset
Continuous conduction: relatively slow
Saltatory conduction: electrical signals jump from gap to gap along the axon; about 30x faster than continuous conduction
Graded potentials- usually incoming signals operating over short distances that have variable( graded) strength, triggered by some change in the neuron's environment that opens gated ion channels
receptor potential: produced when a sensory receptor is excited by its stimulus
postsynaptic potential: produced when the stimulus is a neurotransmitter released by another neuron
Depolarization: a decrease in membrane potential
Hyperpolarization: an increase in membrane potential
Synapse: a junction the mediates information transfer from one neuron to the next or from the a neuron to an effector cell
Electrical synapses
less common than chemical synapses; ions flow directly between cells.
Chemical synapses
most common type of synapse; use chemical messengers called neurotransmitters to transmit signals
Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal
Ca2+ entry causes synaptic vesicles to release neurotransmitters
Action potential arrives at axon terminal
Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane
Binding of neurotransmitter opens ion channels, creating graded potentials
Neurotransmitter effects are terminated
Classification of neurotransmitters by chemical structure
biogenic amines: serotonin, histamine, dopamine, norepinephrine, epinephrine
amino acids glutamate: aspartate, D-serine, γ-aminobutyric acid (GABA), glycine.
acetylocholine
peptides: susbtance P (pain signals), endorphins (pain perception), somatostatin, cholecystokinin
purines: susbtance P (pain signals), endorphins (pain perception), somatostatin, cholecystokinin
gases: freely diffuse across membrane; involved in regulation of vasodilation peripherally & neurally; i.e. NO and CO
lipids: derived from arachidonic acid; endocannabinoids: anandamide, 2-arachidonoglycerol (2-AG)
Classification of neurotransmitters by function
Effects
Excitatory: cause depolarization
Inhibitory: cause hyperpolarization
Actions
Direct: those that bind to and open ion channels
Indirect: promote broader, longer lasting effects by acting through intracellular second messenger molecules
Neurotansmitter receptors
Channel-linked receptors: are cell membrane bound receptors. They act through synaptic signaling on electrically excitable cells and convert chemical signals (ligand) to electrical ones
G Protein-Coupled Receptors: act like an inbox for messages in the form of light energy, peptides, lipids, sugars, and proteins. Such messages inform cells about the presence or absence of life-sustaining light or nutrients in their environment, or they convey information sent by other cells.
The Central Nervous System
Neural tube (brain and spinal cord) -> Primary brain vesicles -> Secondary brain vesicles -> Adult brain structures -> Adult neural canal regions
Gray matter
: consists of short, nonmyelinated neurons and neuron cell bodies.
White matter
: consists mostly of myelinated axons with some nonmyelinated axons, primarily in fiber tracts. The dense coating of fatty myelin is what gives white matter its color
cerebral hemispheres: form superior part of the brain
gyri: elevated ridges
sulci: shallow grooves
longitudinal fissure: separates the cerebral hemispheres
transverse cerebral fissure: separates the cerebral hemispheres from the cerebellum below
Cerebral Cortex: executive suite of the NS; enables us to be aware of ourselves and sensations, to communicate, remember, understand, and initiate voluntary movements
sensory areas
Primary Somatosensory Cortex: Cortex
Processes sensory input from general somatic senses (touch, pain, pressure, temp, proprioception)
Somatosensory Association Cortex: integrates sensory inputs and relays it via the primary somatosensory cortex to produce an understanding of an object being felt
Auditory Areas: the perception of sound stimulus
Vestibular Cortex: responsible for conscious awareness of balance
Olfactory Cortex: Processes smells
Gustatory Cortex: Processes taste stimuli from the tongue
Visceral Sensory Area: involved in conscious perception of visceral sensations
Visual areas: receives visual information that originates on the retina of the eye
association areas
Posterior Association Area: plays a role in recognizing patterns and faces
Limbic Association Area: : provides the emotional impact that makes the a scene important to us
Anterior Association Area: involved with intellect, complex learning abilities, recall, and personality
motor areas
premotor cortex: helps plan movement
broca's area: directs the muscles involved in speech production
primary motor cortex: skilled voluntary movement
frontal eye field: controls voluntary movement of the eyes
Hypothalamus
initiate physical responses to emotions
regulate body temperature
Control the autonomic nervous system
regulate sleep-wake cycle
regulate water balance and thirst
control endocrine system function
regulate food intake
brain stem: consists of the the midbrain, pons, medulla oblongata
medulla oblongata: cardiovascular center, respiratory centers, and various other centers
cerebellum= "tiny brain"
By processing inputs received from the cerebral motor cortex, various brain stem nuclei, and sensory receptors, the cerebellum provides the precise timing and appropriate patterns of skeletal muscle contraction for smooth, coordinated movements and agility needed for our daily living
Brain Wave Patterns
Beta waves
(14–30 Hz) are also rhythmic, but less regular than alpha waves and with a higher frequency. Beta waves occur when we are mentally alert, as when concentrating on some problem or visual stimulus
Theta waves
(4–7 Hz) are still more irregular. Though common in children, theta waves are uncommon in awake adults but may appear when concentrating
Alpha waves
(8–13 Hz) are relatively regular and rhythmic, low-amplitude, synchronous waves. In most cases, they indicate a brain that is “idling”—a calm, relaxed state of wakefulness
Delta waves
(4 Hz or less) are high-amplitude waves seen during deep sleep and when the reticular activating system is suppressed, such as during anesthesia. In awake adults, they indicate brain damage
Stages of Sleep
REM sleep-rapid eye movement
NREM sleep- non-rapid eye movement
During the first 30 to 45 minutes of the sleep cycle, EEGs show that we pass through the first two stages of NREM sleep and into NREM stages 3 and 4, also called slow-wave sleep
About 90 minutes after sleep begins, after reaching NREM stage 4, the EEG pattern changes abruptly. It becomes very irregular and backtracks quickly through the stages until alpha waves (more typical of the awake state) reappear, indicating the onset of REM sleep.
Components of a Reflex Arc
Integration center: either monosynaptic or polysynaptic region within CNS
Motor neuron: conducts efferent impulses from integration center to effector organ
sensory neuron: transmits afferent impulses to CNS
Effector: muscle fiber or gland cell that responds to efferent impulses by contracting or
secreting
Receptor: site of stimulus action
Reflexes are classified functionally as:
• Somatic reflexes
• Activate skeletal muscle
• Autonomic (visceral) reflexes
• Activate visceral effectors (smooth or cardiac muscle or glands)
Cranial nerves
12 pairs of cranial nerves are associated with brain
• Two attach to forebrain, rest with brain stem
• Most are mixed nerves, but two pairs purely sensory
• Each numbered (I through XII) and named from rostral
(front) to caudal (back)