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Chapter 44 ~ Nervous System
Effector cells~ muscle or gland cells
Nerves~ bundles of neurons wrapped in connective tissue
Central nervous system (CNS)~ brain and spinal cord
Peripheral nervous system (PNS)~ sensory and motor neurons
Neuron~ structural and functional unit Cell body~ nucelus and organelles Dendrites~ impulses from tips to neuron Axons~ impulses toward tips Myelin sheath~ supporting, insulating layer Schwann cells~PNS support cells Oligodendrocytes- CNS support cells Synaptic terminals~ neurotransmitter releaser Synapse~ neuron junction
Sensory neuron: convey information to spinal cord
Interneurons: information integration
Motor neurons: convey signals to effector cell (muscle or gland)
Reflex: simple response; sensory to motor neurons
Ganglion (ganglia): cluster of nerve cell bodies in the PNS
Supporting cells/glia: nonconductiong cell that provides support, insulation, and protection
Membrane potential (voltage differences across the plasma membrane)
Intracellular/extracellular ionic concentration difference K+ diffuses out (Na+ in); large anions cannot
follow….selective permeability of the plasma membrane Net negative charge of about -70mV
Excitable cells~ cells that can change membrane potentials (neurons, muscle) Resting potential~ the unexcited state of excitable cells Gated ion channels (open/close response to stimuli): photoreceptors; vibrations
in air (sound receptors); chemical (neurotransmitters) & voltage (membrane potential changes)
Graded Potentials (depend on strength of stimulus): 1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes
more negative 2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes
less negative
Threshold potential: if stimulus reaches a certain voltage (-50 to -55 mV)….
The action potential is triggered…. Voltage-gated ion channels (Na+; K+) 1-Resting state •both channels
closed 2-Threshold •a stimulus opens some
Na+ channels 3-Depolarization •action potential
generated •Na+ channels open; cell becomes positive (K+ channels closed)
4-Repolarization •Na+ channels close, K+ channels open; K+ leaves •cell becomes negative
5-Undershoot •both gates close, but K+ channel is slow; resting state restored
Refractory period~ insensitive to depolarization due to closing
of Na+ gates
“Travel” of the action potential is self-propagating Regeneration of “new” action potentials only after refractory
period Forward direction only Action potential speed: 1-Axon diameter (larger = faster; 100m/sec) 2-Nodes of Ranvier (concentration of ion channels); saltatory
conduction; 150m/sec
Video 1 Video 2 Video 3
Presynaptic cell: transmitting cell
Postsynaptic cell: receiving cell Synaptic cleft: separation gap Synaptic vesicles:
neurotransmitter releasers Ca+ influx: caused by action
potential; vesicles fuse with presynaptic membrane and release….
Neurotransmitter
Acetylcholine (most common) •skeletal muscle
Biogenic amines (derived from amino acids)•norepinephrine
•dopamine (Parkinson/Schizophrenia)•serotonin (SSRI)
Amino acids◦ GABA- IPSP
Neuropeptides (short chains of amino acids)•endorphin
Table 48.1
Drug Addiction and Neurotransmitters
1. Cocaine2. Nicotine
Nervous systems consist of circuits of neurons and supporting cells
All animals except sponges◦ Have some type of nervous system
What distinguishes the nervous systems of different animal groups◦ Is how the neurons are organized into circuits
The simplest animals with nervous systems, the cnidarians◦ Have neurons arranged in nerve nets
Figure 48.2a
Nerve net
(a) Hydra (cnidarian)
Sea stars have a nerve net in each arm◦ Connected by radial nerves to a central nerve ring
Figure 48.2b
Nervering
Radialnerve
(b) Sea star (echinoderm)
In relatively simple cephalized animals, such as flatworms◦ A central nervous system (CNS) is evident
Figure 48.2c
Eyespot
Brain
Nerve cord
Transversenerve
(c) Planarian (flatworm)
Annelids and arthropods◦ Have segmentally arranged clusters of neurons
called ganglia These ganglia connect to the CNS
◦ And make up a peripheral nervous system (PNS)
Brain
Ventral nervecord
Segmentalganglion
Brain
Ventralnerve cord
Segmentalganglia
Figure 48.2d, e (d) Leech (annelid) (e) Insect (arthropod)
Anteriornerve ring
Longitudinalnerve cords
Ganglia
Brain
Ganglia
Figure 48.2f, g (f) Chiton (mollusc) (g) Squid (mollusc)
Nervous systems in molluscs◦ Correlate with the animals’ lifestyles
Sessile molluscs have simple systems◦ While more complex molluscs have more
sophisticated systems
In vertebrates◦ The central nervous system consists of a brain and
dorsal spinal cord◦ The PNS connects to the CNS
Figure 48.2h
Brain
Spinalcord(dorsalnervecord)
Sensoryganglion
(h) Salamander (chordate)
The three stages of information processing◦ Are illustrated in the knee-jerk reflex
Figure 48.4
Sensory neurons from the quadriceps also communicatewith interneurons in the spinal cord.
The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps.
The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward.
4
5
6
The reflex is initiated by tapping
the tendon connected to the quadriceps
(extensor) muscle.
1
Sensors detecta sudden stretch in the quadriceps.
2 Sensory neuronsconvey the information to the spinal cord.
3
Quadricepsmuscle
Hamstringmuscle
Spinal cord(cross section)
Gray matter
White matter
Cell body of sensory neuronin dorsal root ganglion
Sensory neuron
Motor neuron
Interneuron
The vertebrate nervous system is regionally specialized
In all vertebrates, the nervous system◦ Shows a high degree of cephalization and distinct
CNS and PNS components
Figure 48.19
Central nervoussystem (CNS)
Peripheral nervoussystem (PNS)
Brain
Spinal cord
Cranialnerves
GangliaoutsideCNSSpinalnerves
Cranial nerves (brain origin) Spinal nerves (spine origin) Sensory division Motor division
•somatic system voluntary, conscious control •autonomic system
√parasympathetic
conservation of energy √sympathetic
increase energy consumption
In all vertebrates◦ The brain develops from three embryonic regions:
the forebrain, the midbrain, and the hindbrain
Figure 48.23a
Forebrain
Midbrain
Hindbrain
Midbrain Hindbrain
Forebrain
(a) Embryo at one month
Embryonic brain regions
By the fifth week of human embryonic development◦ Five brain regions have formed from the three
embryonic regions
Figure 48.23b
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
(b) Embryo at five weeks
MesencephalonMetencephalon
Myelencephalon
Spinal cord
Diencephalon
Telencephalon
Embryonic brain regions
As a human brain develops further◦ The most profound change occurs in the forebrain,
which gives rise to the cerebrum
Figure 48.23c
Brain structures present in adult
Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)
Diencephalon (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem), cerebellum
Medulla oblongata (part of brainstem)
(c) Adult
Cerebral hemisphereDiencephalon:
Hypothalamus
ThalamusPineal gland(part of epithalamus)
Brainstem:
Midbrain
Pons
Medullaoblongata
Cerebellum
Central canal
Spinal cord
Pituitarygland
The brainstem consists of three parts◦ The medulla oblongata, the pons, and the midbrain
The medulla oblongata◦ Contains centers that control several visceral
functions The pons
◦ Also participates in visceral functions The midbrain
◦ Contains centers for the receipt and integration of several types of sensory information
A diffuse network of neurons called the reticular formation◦ Is present in the core of the brainstem
Figure 48.24
Eye
Reticular formation
Input from touch, pain, and temperature receptors
Input from ears
A part of the reticular formation, the reticular activating system (RAS)◦ Regulates sleep and arousal
The cerebellum◦ Is important for coordination and error checking
during motor, perceptual, and cognitive functions
The cerebellum ◦ Is also involved in learning and remembering motor
skills
The embryonic diencephalon develops into three adult brain regions◦ The epithalamus, thalamus, and hypothalamus
The epithalamus◦ Includes the pineal gland and the choroid plexus
The thalamus◦ Is the main input center for sensory information
going to the cerebrum and the main output center for motor information leaving the cerebrum
The hypothalamus regulates◦ Homeostasis◦ Basic survival behaviors such as feeding, fighting,
fleeing, and reproducing
The hypothalamus also regulates circadian rhythms◦ Such as the sleep/wake cycle
Animals usually have a biological clock◦ Which is a pair of suprachiasmatic nuclei (SCN)
found in the hypothalamus
The cerebrum ◦ Develops from the embryonic telencephalon
The cerebrum has right and left cerebral hemispheres◦ That each consist of cerebral cortex overlying white
matter and basal nuclei
Left cerebralhemisphere
Corpuscallosum
Neocortex
Right cerebralhemisphere
Basalnuclei
Figure 48.26
The basal nuclei◦ Are important centers for planning and learning
movement sequences In mammals
◦ The cerebral cortex has a convoluted surface called the neocortex
In humans, the largest and most complex part of the brain ◦ Is the cerebral cortex, where sensory information is
analyzed, motor commands are issued, and language is generated
A thick band of axons, the corpus callosum◦ Provides communication between the right and left
cerebral cortices
The cerebral cortex controls voluntary movement and cognitive functions
Each side of the cerebral cortex has four lobes◦ Frontal, parietal, temporal, and occipital
Frontal lobe
Temporal lobe Occipital lobe
Parietal lobe
Frontalassociationarea
Speech
Smell
Hearing
Auditoryassociationarea
Vision
Visualassociationarea
Somatosensoryassociationarea
Reading
Speech
TasteSom
atos
enso
ry c
orte
x
Mot
or c
orte
x
Figure 48.27
Each of the lobes◦ Contains primary sensory areas and association
areas
Specific types of sensory input◦ Enter the primary sensory areas
Adjacent association areas◦ Process particular features in the sensory input and
integrate information from different sensory areas
In the somatosensory cortex and motor cortex◦ Neurons are distributed according to the part of the
body that generates sensory input or receives motor input
Figure 48.28
Tongue
JawLips
Face
Eye
Brow
Neck
Thumb
Fingers
HandW
ristForearmE
lbowShoulder
Trunk
Hip
Knee
Primarymotor cortex Abdominal
organs
Pharynx
Tongue
TeethGumsJaw
Lips
Face
Nose
Eye
Fingers
HandForearm
Elbow
Upper arm
Trunk H
ip
Leg
Thumb
Neck
Head
Genitalia
Primarysomatosensory cortex
Toes
Parietal lobeFrontal lobe
During brain development, in a process called lateralization◦ Competing functions segregate and displace each
other in the cortex of the left and right cerebral hemispheres
The left hemisphere◦ Becomes more adept at language, math, logical
operations, and the processing of serial sequences The right hemisphere
◦ Is stronger at pattern recognition, nonverbal thinking, and emotional processing
Studies of brain activity◦ Have mapped specific areas of the brain responsible
for language and speech
Figure 48.29
Hearingwords
Seeingwords
Speakingwords
Generatingwords
Max
Min
Portions of the frontal lobe, Broca’s area and Wernicke’s area◦ Are essential for the generation and understanding
of language
The limbic system ◦ Is a ring of structures around the brainstem
Figure 48.30
HypothalamusThalamus
Prefrontal cortex
Olfactorybulb
Amygdala Hippocampus
This limbic system includes three parts of the cerebral cortex◦ The amygdala, hippocampus, and olfactory bulb
These structures interact with the neocortex to mediate primary emotions ◦ And attach emotional “feelings” to survival-related
functions
Structures of the limbic system form in early development◦ And provide a foundation for emotional memory,
associating emotions with particular events or experiences
The frontal lobes◦ Are a site of short-term memory◦ Interact with the hippocampus and amygdala to
consolidate long-term memory
Many sensory and motor association areas of the cerebral cortex◦ Are involved in storing and retrieving words and
images
In the vertebrate brain, a form of learning called long-term potentiation (LTP)◦ Involves an increase in the strength of synaptic
transmission
Figure 48.32
PRESYNAPTIC NEURON
NO
Glutamate
NMDAreceptor
Signal transduction pathways
NO
Ca2+
AMPA receptor
POSTSYNAPTIC NEURON
Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.
5
Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).
6
The presynapticneuron releases glutamate.
1
Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.
2
Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.
3
Ca2+ diffuses into thepostsynaptic neuron.
4
NO diffuses into thepresynaptic neuron, causing it to release more glutamate.
7
P
Modern brain-imaging techniques ◦ Suggest that consciousness may be an emergent
property of the brain that is based on activity in many areas of the cortex
CNS injuries and diseases are the focus of much research
Unlike the PNS, the mammalian CNS◦ Cannot repair itself when damaged or assaulted by
disease Current research on nerve cell development
and stem cells◦ May one day make it possible for physicians to
repair or replace damaged neurons
Signal molecules direct an axon’s growth ◦ By binding to receptors on the plasma membrane of
the growth cone
Forebrain (Prosencephalon)•cerebrum~memory,
learning, emotion •cerebral cortex~sensory and motor nerve cell bodies •corpus callosum~connects left and right hemispheres
•thalamus; hypothalamus Midbrain ( Mesencephalon)
•inferior (auditory) and superior (visual) colliculi
Hindbrain(Rhombencephalon) •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions