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Electrical Signals in Animals. Chapter 45. Nerve net. (a) Hydra (cnidarian). Nervous Systems. Consist of circuits of neurons and supporting cells All animals except sponges have some type of nervous system Differ in the way that the neurons are organized into circuits

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nervous systems

Nerve net

(a) Hydra (cnidarian)

Nervous Systems
  • Consist of circuits of neurons and supporting cells
  • All animals except sponges have some type of nervous system
    • Differ in the way that the neurons are organized into circuits
  • The simplest animals with

nervous systems, the cnidarians

    • Have neurons arranged in nerve nets
nervous systems3

Radialnerve

Nervering

(b) Sea star (echinoderm)

Nervous Systems
  • More complex animals contain nerve nets as well as nerves
    • Bundles of fiber-like extensions of neurons
  • Sea stars have a nerve net in each arm
    • Connected by radial nerves to a central nerve ring
nervous systems4

Eyespot

Brain

Nerve cord

Transversenerve

(c) Planarian (flatworm)

Nervous Systems
  • Cephalization evolved with greater complexity in nervous systems
    • Clustering of neurons in a brain near the anterior end in worms
    • Small brain, longitudinal nerve chords constitute the simplest central nervous system
nervous systems5

Brain

Brain

Ventralnerve cord

Ventral nervecord

Segmentalganglia

Segmentalganglion

(d) Leech (annelid)

(e) Insect (arthropod)

Nervous Systems
  • Annelids and arthropods have more complicated brains
    • Have segmentally arranged clusters of neurons called ganglia
  • These ganglia connect to the

CNS

    • And make

up a peripheral

nervous system

(PNS)

nervous systems6

Anteriornerve ring

Ganglia

Brain

Longitudinalnerve cords

Ganglia

(g) Squid (mollusc)

(f) Chiton (mollusc)

Nervous Systems
  • Nervous systems in mollusks
    • Correlate with the animals’ lifestyles
  • Sessile mollusks have simple systems
    • While more complex mollusks have more sophisticated systems
      • Can complete complicated tasks
nervous systems7

Brain

Sensoryganglion

Spinalcord

(dorsalnerve

cord)

(h) Salamander (chordate)

Nervous Systems
  • In vertebrates
    • The central nervous system consists of a brain and dorsal spinal cord
    • The PNS connects to the CNS
neuron structure
Neuron Structure
  • The ability of the neuron to receive and transmit information depends on their structure. Consists of:
  • Cell body- where most of the organelles are housed
  • Numerous dendrites- highly branched extensions that receive signals from other neurons
  • Axon- larger extension that transmits signals, may be covered with a myelin sheath
dendrite
Dendrite
  • Receives electrical signals from the axons of adjacent cells.
  • Axon then sends the signal to the dendrites of other neurons
  • Cell body, or soma, which includes the nucleus, integrates the incoming signals and generates an outgoing signal in the axon
neurons

Dendrites

Axon

Cell body

(c) Motor neuron

(b) Interneurons

(a) Sensory neuron

Neurons
  • Neurons have a wide variety of shapes
    • That reflect their input and output interactions
    • Reflects the number of synapses it has with other neurons
slide12
Glia
  • Glia are supporting cells
    • That are essential for the structural integrity of the nervous system and for the normal functioning of neurons
  • In the CNS astrocytes regulate extracellular ion concentrations
slide13

Node of Ranvier

Layers of myelin

Axon

Schwann

cell

Schwann

cell

Nodes of Ranvier

Nucleus of Schwann cell

Axon

Myelin sheath

0.1 µm

  • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS)
    • Are glia that form the myelin sheaths around the axons of many vertebrate neurons
    • Act as insulators
    • Multiple Sclerosis is deterioration of the myelin sheath
resting potential of a cell
Resting Potential of a Cell

Ion pumps and ion channels maintain the resting potential of a neuron

  • The resting potential
    • Is the membrane potential of a neuron that is not transmitting signals
  • Across its plasma membrane, every cell has a voltage
    • Called a membrane potential
  • The inside of a cell is negative
    • Relative to the outside
resting potential of a cell15
Resting Potential of a Cell
  • The concentration of Na+ is higher in the extracellular fluid than in the cytosol
    • While the opposite is true for K+
  • A neuron that is not transmitting signals
    • Contains many open K+ channels and fewer open Na+ channels in its plasma membrane
  • The diffusion of K+ and Na+ through these channels
    • Leads to a separation of charges across the membrane, producing the resting potential
gated ion channels
Gated Ion Channels
  • Ungated ion channels are always open, results in the resting potential of the cell
  • Gated ion channels open or close
    • In response to membrane stretch or the binding of a specific ligand
    • In response to a change in the membrane potential
    • Neurons have gated ion channels
  • Responsible for generating the signals of the nervous system
starting an action potential
Starting An Action Potential?
  • An action potential is a rapid, temporary change in a membrane potential
  • Has three phases: depolarization, repolarization, and the undershoot
  • The initial event is a rapid depolarization of the membrane
    • Membrane potential must shift from its resting potential of –70 mV to about –55 mV
slide20
If the threshold potential is reached, channels in the axon membrane open and ions rush into the axon, following their electrochemical gradients
  • The current flow causes further depolarization
  • When the membrane potential reaches about +40 mV the membrane experiences a rapid repolarization as ions flow out of the axon
slide21
The repolarization event results in the membrane becoming more negative than the resting potential
    • Called the undershoot
  • All phases occur in about a millisecond
regeneration of the action potential
Regeneration of the Action Potential
  • An action potential can travel long distances
    • By regenerating itself along the axon
  • At the site where the action potential is generated
    • An electrical current depolarizes the neighboring region of the axon membrane
regeneration of the action potential23

+

+

+

+

+

+

+

+

+

+

+

+

Axon

Actionpotential

An action potential is generated as Na+ flows inward across the membrane at one location.

1

+

+

+

+

+

+

+

+

Na+

+

+

Actionpotential

The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward.

2

K+

+

+

Na+

+

+

+

+

+

+

+

+

K+

Actionpotential

The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon.

3

K+

+

+

+

+

+

+

+

+

Na+

+

+

+

+

+

+

+

+

K+

Regeneration of the Action Potential
action potentials
Action Potentials
  • The speed of an action potential
    • Increases with the diameter of an axon
  • In vertebrates, axons are myelinated
    • Also causing the speed of an action potential to increase
    • Causes the membranes to have a simulated wider width
  • Action potentials in myelinated axons
    • Jump between the nodes of Ranvier in a process called saltatory conduction
synapses
Synapses
  • Neurons communicate with other cells at synapses
  • In an electrical synapse
    • Electrical current flows directly from one cell to another via a gap junction
    • Synchronize the activity of neurons responsible for rapid responses (flight response)
  • The vast majority of synapses
    • Are chemical synapses
    • Not as fast as electrical synapsis
chemical synapses

Postsynapticneuron

Synapticterminalof presynapticneurons

5 µm

Chemical Synapses
  • In a chemical synapse, a presynaptic neuron
    • Releases chemical neurotransmitters, which are stored in the synaptic terminal
synapses34
Synapses
  • The interface between two neurons is called a synapse.
  • Just inside the synapse, the axon contains synaptic vesicles that serve as storage sites for neurotransmitters
  • The sending cell is called the presynaptic neuron and the receiving cell is called the postsynaptic neuron
what do neurotransmitters do
What Do Neurotransmitters Do?
  • There are several categories of neurotransmitters
  • Many neurotransmitters function as ligands that bind to ligand-gated ion channels
  • Cause the ion channels to open, generating a postsynaptic potential
    • Causes the start of a new action potential
the vertebrate nervous system

Central nervous

system (CNS)

Peripheral nervous

system (PNS)

Brain

Cranial

nerves

Spinal cord

Ganglia

outside

CNS

Spinal

nerves

The Vertebrate Nervous System
  • 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
the central nervous system
The Central Nervous System
  • The brain provides the integrative power
    • That underlies the complex behavior of vertebrates
  • The spinal cord integrates simple responses to certain kinds of stimuli
    • And conveys information to and from the brain
  • The central canal of the spinal cord and the four ventricles of the brain
    • Are hollow, since they are derived from the dorsal embryonic nerve cord
the central nervous system40
CNS consists of spinal chord and four ventricles in the brain

Contain cerebrospinal fluid

Assists in supply of nutrients and hormones to parts of the brain and in removal of wastes

Grey matter contains mostly dendrites

White matter contains long axons with great bundles of myelin sheaths

Gray matter

White

matter

Ventricles

The Central Nervous System
the peripheral nervous system
The Peripheral Nervous System
  • The PNS transmits information to and from the CNS and plays a large role in regulating a vertebrate’s movement and internal environment
  • The cranial nerves originate in the brain
    • And terminate mostly in organs of the head and upper body
  • The spinal nerves originate in the spinal cord
    • And extend to parts of the body below the head
the peripheral nervous system42

Peripheral

nervous system

Somatic

nervous

system

Autonomic

nervous

system

Sympathetic

division

Parasympathetic

division

Enteric

division

The Peripheral Nervous System
  • The PNS can be divided into two functional components
    • The somatic nervous system and the autonomic nervous system
the peripheral nervous system43
The Peripheral Nervous System
  • The somatic nervous system
    • Carries signals to skeletal muscles
    • Often considered voluntary
  • The autonomic nervous system
    • Regulates the internal environment, in an involuntary manner
    • Is divided into the sympathetic, parasympathetic, and enteric divisions
brain structures in an adult

Brain structures present in adult

Cerebrum (cerebral hemispheres; includes cerebral

cortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

Diencephalon:

Cerebral hemisphere

Hypothalamus

Thalamus

Pineal gland

(part of epithalamus)

Brainstem:

Midbrain

Pons

Pituitary

gland

Medulla

oblongata

Cerebellum

Spinal cord

Central canal

(c) Adult

Brain Structures in an Adult
parts of the brain
The brainstem consists of three parts

The medulla oblongata, the pons, and the midbrain

Parts of The Brain
brainstem
Brainstem
  • 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
the cerebellum
The Cerebellum
  • 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

diancephalon
Diancephalon
  • The embryonic diencephalon develops into three adult brain regions
    • The epithalamus, thalamus, and hypothalamus
  • The epithalamus
    • Includes the pineal

gland and the

choroid plexus

    • Produce cerebro-

spinal fluid

diancephalon50
Diancephalon
  • 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
cerebrum

Right cerebral

hemisphere

Left cerebral

hemisphere

Corpus

callosum

Basal

nuclei

Neocortex

Cerebrum

The cerebrum has right and left cerebral hemispheres

  • That each consist of cerebral cortex overlying white matter and basal nuclei
cerebrum52
Cerebrum
  • The basal nuclei
    • Are important centers for planning and learning movement sequences
  • In mammals
    • The cerebral cortex has a convoluted surface called the neocortex
    • Sensory information is analyzed , motor commands are issued, and language is generated
  • Corpus callosum allows communication between hemispheres
cerebral cortex
Cerebral Cortex
  • 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
  • Controls voluntary movement and cognitive functions
  • Each side of the cerebral cortex has four lobes
    • Frontal, parietal, temporal, and occipital
cerebral cortex54

Frontal lobe

Parietal lobe

Motor cortex

Somatosensory cortex

Somatosensory

association

area

Speech

Frontal

association

area

Taste

Reading

Speech

Hearing

Visual

association

area

Smell

Auditory

association

area

Vision

Temporal lobe

Occipital lobe

Cerebral Cortex
lateralization
Lateralization
  • 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
cns injuries
CNS Injuries
  • 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
diseases of the nervous system
Diseases of the Nervous System
  • About 1% of the world’s population suffers from schizophrenia
  • Schizophrenia is characterized by
    • Hallucinations, delusions, blunted emotions, and many other symptoms
  • Available treatments have focused on
    • Brain pathways that use dopamine as a neurotransmitter
diseases of the nervous system58
Diseases of the Nervous System
  • Two broad forms of depressive illness are known
    • Bipolar disorder and major depression
  • Bipolar disorder is characterized by
    • Manic (high-mood) and depressive (low-mood) phases
  • In major depression
    • Patients have a persistent low mood
  • Treatments for these types of depression include
    • A variety of drugs such as Prozac and lithium
diseases of the nervous system59

20 m

Senile plaque

Neurofibrillary tangle

Figure 48.35

Diseases of the Nervous System
  • Alzheimer’s disease (AD)
    • Is a mental deterioration characterized by confusion, memory loss, and other symptoms
  • AD is caused by the formation of
    • Neurofibrillary tangles and senile plaques in the brain
diseases of the nervous system60
Diseases of the Nervous System
  • Parkinson’s disease is a motor disorder
    • Caused by the death of dopamine-secreting neurons in the substantia nigra
    • Characterized by difficulty in initiating movements, slowness of movement, and rigidity
  • There is no cure for Parkinson’s disease
    • Although various approaches are used to manage the symptoms