Chapter 9: Nervous System
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Transcript of Chapter 9: Nervous System
Chapter 9:
Nervous System
Functions Sensors
› Monitor external and internal environment Processing
› Receives information, integrates it, and decided what to do
Effectors› Carries messages to effectors and tells
them what to do
Types of Nervous Cells Neurons
› Main cell of nervous tissue› Relay and process messages
Neuroglial› Provide support to the neurons› Several types known› Ex: Schwann, microglial
Neuroglial Cells Microglial cells
› Scattered throughout CNS› Phagocytize bacteria or cellular debris
Oligodendrocytes› Along nerve fibers› Provide myelin sheath (made of a fatty
substance called myelin) around axon in CNS Schwann cells
› Same as oligodendrocytes but in PNS
Neuroglial Cells (cont) Astrocytes
› Provide connection between a neuron and a blood vessel
› Provide support, help regulate ion concentrations in tissue, make-up scar tissue after injury
Ependymal cells› Forms epithelial-like linings on the outsides
of specialized parts or lining cavities within the CNS
Neurons Remember: It’s a cell! Body of neuron
› Cell Body – contains cell organelles› Dendrites- carry messages to cell body› Axons – carry messages away from cell body
Cell Structures› Large nucleus with easily seen nucleolus› Chromatophilic substance – similar to rough ER
Scattered throughout cytoplasm, membranous› Neurofibrils- help support cell shape
Axons Can be myelinated or unmyelinated PNS
› Schwann cells form myelin sheath› Nodes of Ranvier- small breaks in myelin
sheath CNS
› Oligodendrocytes form myelin › Myelinated neurons form white matter › Unmyelinated neurons form gray matter
Structural Classification of Neurons
Multipolar› Many small branched dendrites› One axon › Found in CNS
Bipolar› Two processes off of cell membrane (one axon and
one dendrite)› Neurons in special sense organs
Unipolar› One process off of cell body (one axon)› Found throughout PNS
Functional Classification of Neurons
Sensory (afferent) neurons› Have sensitive dendrites that are stimulated by
changes in environment› Message is taken into CNS› Usually unipolar or sometimes bipolar
Interneurons› Transfer, direct, and process messages within CNS› Usually multipolar
Motor (efferent) neurons› Carries message out of CNS to effectors› Usually multipolar
A Neuron at Rest Inside the neuron
› High in K+
› High in negative ions Outside the neuron
› High in Na+
› High in positive ions Result
› K+ tends to diffuse out› Na+ tends to diffuse in› Negative ions cannot cross
A Neuron at Rest (cont) Na/K pump - helps to restore
concentration gradient across the cell membrane
Resting potential - difference is electrical charge across the membrane› Established by concentration gradients of
various ions› Inside of the membrane has a negative
charge of 70 mv› Membrane is said to be polarized
Potential Changes Stimuli cause changes to the resting
potential by making the inside of the membrane less negative
Once a stimulus happens:› If stimulus is not strong enough to reach
threshold potential = cell membrane will return to resting potential
› If stimulus is strong enough to reach threshold potential = start an action potential
Summation - when additive effect of stimuli causes action potential
Action Potential Starts at trigger zone of axon Threshold stimulus open sodium channels Sodium moves into axon
› Because of the concentration gradient› Because of the negative charge that attracts the
positive ions Depolarizes the membrane as negative
charge diminishes Potassium channels open and potassium
moves out of the axon, repolarizing the membrane
Animation #1 Animation #2
Action Potential Animation
Nerve Impulse Action potential at the trigger zone
stimulates the next part of the axon to do a action potential
Potentials spread along the axon like a wave
Unmyelinated axons › Wave continues uninterrupted; relatively slow
Myelinated axons› Wave goes through saltatory conduction (jump
from one node to the next); very fastAnimation
Neuron Responses All-or-nothing effect
› Neuron does not react until a threshold stimulus is applied, but once it is applied it reacts fully
Stimuli greater than threshold levels don’t change the size of the response but changes its frequency
Refractory period:› After a action potential› Brief period of time › The nerve cannot be stimulated again.
The Synapse The connection between two neurons Don’t touch, separated by synaptic
cleft One-way communication between axon
of presynaptic neuron and dendrite of postsynaptic neuron
Neurotransmitters are made in the synaptic knob of the axon, stored in synaptic vesicles, and cross the cleft when needed
Neurotransmitter Actions
Excitatory Action:› A neurotransmitter that puts a neuron
closer to an action potential (facilitation) or causes an action potential
Inhibitory Action:› A neurotransmitter that moves a neuron
further away from an action potential Response of neuron:
› Responds according to the sum of all the neurotransmitters received at one time
Neurotransmitters Acetylcholine Monoamines – modified amino acids Amino acids Neuropeptides- short chains of amino
acids Depression:
› Caused by the imbalances of neurotransmitters
Many drugs imitate neurotransmitters› Ex: Prozac, zoloft, alcohol, drugs, tobacco
Release of Neurotransmitters
When an action potential reaches the end of an axon, Ca+ channels in the neuron open
Causes Ca+ to rush in› Cause the synaptic vesicles to fuse with the cell
membrane› Release the neurotransmitters into the synaptic
cleft After binding, neurotransmitters will either:
› Be destroyed in the synaptic cleft OR› Taken back in to surrounding neurons (reuptake)
Animation
Neuronal Pools Groups of highly interconnected
neurons that work together in the CNS Convergence
› Axons from different parts of the nervous system connect to the same neuron combining their affects
Divergence› A message from one neuron is sent to
many neurons at once; amplifies message
Convergence
Divergence
Nerves Nerves are made of bundled axons,
called nerve fibers Nerve fibers
› Sensory (afferent)- carry messages to CNS› Motor (efferent)- carry messages from CNS to
effectors Nerves
› Same definitions hold true› Most nerves contain both types of fibers and
are called mixed nerves
Structure of a nerve A nerve fiber (axon) is surrounded with
endoneurium Nerve fibers are bundled together and
surrounded with perineurium to form a fascicle
Fascicles are bundled together and surrounded with epineurium to form a nerve
Nerve Pathways
Path that the message takes through the body
Includes:› Sensor› Sensory neuron› Interneuron› Motor neuron› Effector
Reflex Arc Simplest nerve pathway is a reflex Reflexes without pain
› Involve only sensory and motor neurons› Ex: knee-jerk reflex
Reflexes with pain› Involve interneurons
in CNS› Ex: withdrawal reflex
The CNS Central nervous system Consists of brain and spinal cord Made of both gray and white matter Covered in meningies
The Meninges (Brain) Cranial Bone Dura mater
› First layer; tough, fibrous connective tissue › Forms inner periosteum of cranial bone › Folds into the cranium in some places to
form division walls in the brain Arachnoid mater
› Web-like membrane over CNS › Does not dip into crevices
The Meninges (Brain) Subarachnoid space
› Below arachnoid layer› Contains cerebrospinal fluid
Pia mater› Lower layer of meninges› Forms a tight covering over brain › Does dip into crevaces
Meningies (Spinal Cord) Same except:
› Vertebrae bones - protection› Epidural space- filled with loose connective
and adipose tissue› All other are the same
Cerebrospinal fluid Flows through ventricles (spaces in
brain) in the subarachnoid space, and through the central canal of the spinal cord
Fluid is made by the choroid plexus
Spinal Cord Stretches from brain to intervertebral
disk between first and second lumbar vertebrae
31 pairs of spinal nerves come of the cord
Gray matter core surrounded by white matter
Spinal Cord (cont) Responsible for communication
between brain and body and spinal reflexes
Ascending tracts› Nerves that send info to brain
Descending tracts› Nerves that send into to effectors
The Brain Made up off about 100 billion neurons Four main sections:
› 1) Cerebrum Nerves for processing sensory and motor
function Higher functions (like reasoning)
› 2) Diencephalon Processes sensory information
› 3) Brainstem Regulates certain body functions like breathing
› 4) Cerebellum Coordinates skeletal muscle movements
Cerebrum Divided into two hemispheres: right
and left Corpus callosum
› Connects the two sides Other structures
› Convolutions - ridges› Sulcus - shallow groove› Fissure - deep groove
Areas of the Cerebrum Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula
Each lobe has unique functions
Areas of the Cerebrum (cont)
Cerebral cortex› Thin layer of gray matter surrounding
cerebral hemisphere; contain most of the cell bodies in the cerebrum
Inner part of the cerebrum is mainly made of white matter
Functions of the Cerebrum Motor areas
› Primarily in frontal lobe› Send information out to effectors
Sensory areas› Interpret information from sensors› Area in parietal, temporal, and occipital
lobes Association areas
› Analyze information from sensors› Located in areas in all lobes mentioned
above
Diencephalon Located between the cerebral
hemispheres above the brainstem Contains:
› Thalamus› Hypothalamus› Pituitary gland› Pineal gland
Functions of the Diencephalon
Thalamus› Routes sensory impulses to the correct
region of the cerebrum Hypothalamus
› Monitors many internal conditions, link between nervous and endocrine system
Limbic system › Thalamus, hypothalamus, and basal nuclei› Controls experience and expression
(feelings)
Brainstem Connection between spinal cord and
the rest of the brain Contains:
› Midbrain› Pons› Medulla oblongata
Midbrain Located between diencephalon and
pons Contains some visual and auditory
reflexs Serves as the main connection for
motor neurons between spinal cord and upper part of brain
Pons Rounded bulge between midbrain and
medulla oblongata Relays impulses between medulla and
cerebrum or between cerebrum and cerebellum
Medulla oblongata Lowest part of brain, connect to spinal
cord All ascending and descending tracts
run through the oblongata Serves as a control center for many
vital function like heart rate, blood pressure, and respiratory center
Cerebellum Located in the lower back part of the
brain Structured liked cerebrum with inner
white matter core and gray matter covering
Controls posture and complex skeletal movements
The PNS Peripheral Nervous system Includes:
› 12 pairs of cranial nerves › 31 pairs of spinal nerves
Divided into:› 1) Somatic nervous system
Controls conscious activities; connects to skin and skeletal muscles
› 2) Autonomic nervous system Controls unconscious activities; connects to
internal organs or structures
Autonomic Nervous System
Two branches:› 1) Parasympathetic
Control under more normal conditions› 2) Sympathetic
Control under stress or emergency conditions (fight or flight)
Usually these have antagonistic effects› Work to counteract each other (one
increases, while other decreases) Either might be utilized to maintain
homeostasis