Physiology: The central nervous system

Physiology

The Central
Nervous System
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CNS
• Consists of:

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• Brain.
• Spinal cord.

• Receives input from
sensory neurons.
• Directs activity of motor
neurons.
• Association neurons
maintain homeostasis in
the internal environment.

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Embryonic Development
• Groove appears in ectoderm to fuse to form neural tube by 20th
day after conception. Neural tube eventually forms the CNS.
• During 5th week, modified:
• Forebrain: telencephalon and diencephalon.

• Midbrain: unchanged.
• Hindbrain: metencephalon and myelencephalon.

• Part of ectoderm where fusion occurs becomes neural crest.
• Neural crest forms ganglia of PNS.

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Embryonic Development

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Embryonic Development

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• Telencephalon grows disproportionately forming 2
the hemispheres of the cerebrum.
• Ventricles and central canal become filled with
cerebral spinal fluid (CSF).
• CNS composed of gray and white matter.
• Gray matter consists of neuron cell bodies and dendrites.
• White matter (myelin) consists of axon tracts.

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Cerebrum
• Only structure of the telencephalon.
• Largest portion of brain (80% mass).
• Responsible for higher mental functions.
• Corpus callosum:
• Major tract of axons that functionally interconnects right
and left cerebral hemispheres.

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Cerebrum

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Cerebral Cortex
 Characterized by numerous convolutions.
◦ Elevated folds: gyri.
◦ Depressed groves: sulci.

 Frontal lobe:
◦ Anterior portion of each cerebral hemisphere.
◦ Precentral gyri:
• Contains upper motor neurons.

• Involved in motor control.

 Body regions with the greatest number of motor
innervation are represented by largest areas of motor
cortex.
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Cerebral Cortex

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Cerebral Cortex

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• Parietal lobe:
• Primary area responsible for perception of somatesthetic
sensation.
• Body regions with highest densities of receptors are
represented by largest areas of sensory cortex.

• Temporal lobe:
• Contain auditory centers that receive sensory fibers from
cochlea.
• Interpretation and association of auditory and visual
information.

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Cerebral Cortex

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• Occipital Lobe:
• Primary area responsible for vision and coordination of
eye movements.

• Insula:
• Implicated in memory encoding.
• Integration of sensory information with visceral
responses.
• Coordinated cardiovascular response to stress.

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Visualizing the Brain
• X-ray computed tomography (CT):
• Complex computer manipulations of data obtained from x-ray
absorption by tissues of different densities.
• Soft tissue.

• Positron-emission tomography (PET):
• Radioisotopes that emit positrons are injected into blood stream.
• Collision of positron and electron result in emission of gamma rays.

• Pinpoint brain cells that are most active.
• Brain metabolism, drug distribution.

• Magnetic resonance imaging (MRI):
• Protons (H+) respond to magnetic field, which align the protons.
• Emit a radio-wave signal when stimulated.
• Brain function.
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Electroencephalogram (EEG)
• Measures synaptic
potentials produced at
cell bodies and
dendrites.
• Create electrical
currents.

• Used clinically do
diagnose epilepsy and
brain death.
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EEG Patterns
• Alpha:

▫ Recorded from parietal and occipital regions.
 Person is awake, relaxed, with eyes closed.
 10-12 cycles/sec.

• Beta:
▫ Strongest from frontal lobes near precentral gyrus.
 Produced by visual stimuli and mental activity.
 Evoked activity.
 13-25 cycles/sec.

• Theta:

▫ Emitted from temporal and occipital lobes.
 Common in newborn.
 Adult indicates severe emotional stress.
 5-8 cycles/sec.

• Delta:
▫ Emitted in a general pattern.
 Common during sleep and awake infant.
 In awake adult indicate brain damage.
 1-5 cycles/sec.

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EEG Sleep Patterns
• 2 types of EEG patterns during sleep:
• REM (rapid eye movement):
• Dreams occur.
• Low-amplitude, high-frequency oscillations.
• Similar to wakefulness (beta waves).

• Non-Rem (resting):
• High-amplitude, low-frequency waves (delta waves).
• Superimposed on these are sleep spindles:
• Waxing and waning bursts of 7-14 cycles/sec.
• Last for 1-3 sec.

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Basal Nuclei (basal ganglia)
• Masses of gray matter
composed of neuronal cell
bodies located deep within
white matter.
• Contain:
• Corpus striatum:
• Caudate nucleus.
• Lentiform nucleus:
• Putman and globus
pallidus.

• Functions in the control of
voluntary movements.
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Cerebral Lateralization
• Cerebral dominance:
▫ Specialization of one
hemisphere.

• Left hemisphere:
▫ More adept in language and
analytical abilities.
▫ Damage:
 Severe speech problems.

• Right hemisphere:
▫ Most adept at visuospatial
tasks.
▫ Damage:
 Difficulty finding way around
house.

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Language
• Broca’s area:
• Involves articulation of speech.
• In damage, comprehension of speech in unimpaired.
• Wernicke’s area:
• Involves language comprehension.
• In damage, language comprehension is destroyed, but speech is
rapid without any meaning.
• Angular gyrus:
• Center of integration of auditory, visual, and somatesthetic
information.
• Damage produces aphasias.

• Arcuate fasciculus:
• To speak intelligibly, words originating in Wernicke’s area must be
sent to Broca’s area.
• Broca’s area sends
fibers to the motor cortex which directly controls
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the musculature of speech.


Emotion and Motivation
• Important in the neural basis of
emotional states are
hypothalamus and limbic
system.
• Limbic system:
▫ Group of forebrain nuclei and
fiber tracts that form a ring
around the brain stem.
 Center for basic emotional drives.

• Closed circuit (Papez circuit):
▫ Fornix connects hippocampus

to hypothalamus, which projects
to the thalamus which sends
fibers back to limbic system.

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Emotion and Motivation

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• Areas or the hypothalamus and limbic system are
involved in feelings and behaviors.
• Aggression:
• Amygdala and hypothalamus.

• Fear:
• Amygdala and hypothalamus.

• Feeding:
• Hypothalamus (feeding and satiety centers).

• Sexual drive and behavior:
• Hypothalamus and limbic system.

• Goal directed behavior (reward and punishment):
• Hypothalamus and frontal cortex.
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Memory
• Short-term:
• Memory of recent events.

• Medial temporal lobe:
• Consolidates short term into long term memory.

• Hippocampus is critical component of memory.
• Acquisition of new information, facts and events
requires both the medial temporal lobe and
hippocampus.

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Long-Term Memory
• Consolidation of short-term memory into long-term
memory.
• Requires activation of genes, leading to protein synthesis and
formation of new synaptic connections.
• Altered postsynaptic growth of dendritic spines in area of contact.

• Cerebral cortex stores factual information:
• Visual memories lateralized to left hemisphere.
• Visuospatial information lateralized to right hemisphere.

• Prefrontal lobes:
• Involved in performing exact mathematical calculations.
• Complex, problem-solving and planning activities.

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Long-Term Potentiation
• Type of synaptic learning.
▫ Synapses that are 1st stimulated at high frequency will
subsequently exhibit increased excitability.

• In hippocampus, glutamate is NT.
▫ Requires activation of the NMDA receptors for glutamate.
 Glutamate and glycine or D-serine binding and partial

depolarization are required for opening of channels for Ca2+ and Na+.

• May also involve presynaptic changes:
▫ Binding of glutamate to NMDA receptors and simultaneous
depolarization, open receptor channels for Ca2+.
 Ca2+ causes long-term potentiation in postsynaptic neuron, release of
NO from postsynaptic neuron.
 NO acts as a retrograde messenger, causing release of NT.

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Neuronal Stem Cells in Learning
and Memory
• Neural stem cells:

• Cells that both renew themselves through mitosis and
produce differentiated neurons and neuroglia.

• Hippocampus has been shown to contain stem
cells (required for long-term memory).
• Neurogenesis:
• Production of new neurons.

• Indirect evidence that links neuogenesis in

hippocampus with learning and memory.

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Thalamus and Epithalamus
• Thalamus:

▫ Composes 4/5 of the diencephalon.
▫ Forms most of the walls of the 3rd ventricle.
▫ Acts as relay center through which all sensory information (except
olfactory) passes to the cerebrum.
 Lateral geniculate nuclei:
 Relay visual information.

 Medial geniculate nuclei:
 Relay auditory information.

 Intralaminar nuclei:
 Activated by many sensory modalities.
 Projects to many areas.
▫ Promotes alertness and arousal from sleep.

• Epithalamus contains:

▫ Choroid plexus where CSF is formed.
▫ Pineal gland which secretes melatonin.

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