Chapter 16
I.
INTRODUCTION
A. The components of the brain interact
to receive sensory input, integrate and store the information,
and transmit motor responses.
B.
To
accomplish the primary functions of the nervous system there are neural
pathways to transmit impulses from receptors to the circuitry of the brain,
which manipulates the circuitry to form directives that are transmitted via
neural pathways to effectors as a response.
II. SENSATION
A. Sensation is a conscious or unconscious
awareness of external or internal stimuli. Perception is the conscious
awareness and interpretation of sensations.
B.
Sensory
Modalities
1.
Sensory Modality is the property by which one sensation is distinguished from another.
2.
In
general, a given sensory neuron carries only one modality.
3.
The
classes of sensory modalities are general senses and special senses.
a.
The
general senses include both somatic and visceral senses, which provide
information about conditions within internal organs.
b.
The
special senses include the modalities of smell, taste, vision, hearing,
and equilibrium.
C.
The
Process of Sensation
1.
For
a sensation to arise, four events must occur.
2.
These
are stimulation, transduction, conduction, and translation.
a.
A
stimulus, or change in the environment, capable of initiating a nerve
impulse by the nervous system must be present.
b.
A
sensory receptor or sense organ must pick up the stimulus and transduce (convert) it to a nerve impulse by
way of a generator potential.
c.
The
impulse(s) must be conducted along a neural pathway from the receptor or
sense organ to the brain.
d.
A
region of the brain or spinal cord must translate the impulse into a
sensation.
D. Sensory Receptors
1.
Types
of Sensory Receptors
a.
On
a microscopic level, sensory receptors are free nerve endings, encapsulated
nerve endings at the dendrites of first-order sensory neurons, or separate
cells that synapse with first order sensory neurons (Figure 15.1).
1)
When
stimulated the dendrites of free nerve endings, encapsulate nerve endings, and
the receptive part of olfactory receptors produce generator potentials.
2)
The
specialized cells that act as receptors for the special senses of vision,
hearing, equilibrium, and taste produce receptor potentials in response
to stimuli.
3)
Generator
and receptor potentials are graded, local potentials; refer to lecture notes for
material on generator and receptor potentials.
4)
According
to location, receptors are classified as exteroceptors,
interoceptors (visceroceptors),
and proprioceptors.
b.
On
the basis of type of stimulus detected, receptors are classified as mechanoreceptors,
thermoreceptors, nociceptors,
photoreceptors, and chemoreceptors.
c.
Table
15.1 summarizes the classification of sensory receptors.
2.
Adaptation
in Sensory Receptors
a.
A
characteristic of many sensations is adaptation, i.e., a change in
sensitivity (usually a decrease) to a long-lasting stimulus.
b.
The
receptors involved are important in signaling information regarding steady
states of the body.
III. SOMATIC SENSATIONS (Receptors for somatic sensation are
summarized in Table 15.2)
A. Tactile Sensations
1.
Tactile sensations are touch, pressure, and vibration plus itch and tickle.
2.
Tactile
receptors include corpuscles of touch (Meissner’s
corpuscles), hair root plexuses, type I (Merkel’s discs) and type
II cutaneous (Ruffini’s
corpuscles)
mechanoreceptors, lamellated (Pacinian) corpuscles, and free nerve endings
(Figure 15.2).
3.
Touch
a.
Crude touch
refers to the ability to perceive that something has simply touched the skin; fine
touch provides specific information about a touch sensation such as
location, shape, size, and texture of the source of stimulation.
b.
Receptors
for touch include corpuscles of touch (Meissner’s
corpuscles) and hair root plexuses; these are rapidly adapting
receptors.
c.
Type I cutaneous mechanoreceptors (tactile or Merkel discs)
and type II cutaneous mechanoreceptors (end
organs of Ruffini) are slowly adapting
receptors for touch (Figure 15.2).
4.
Pressure
and Vibration
a.
Pressure sensations generally result from stimulation of tactile receptors in deeper tissues
and are longer lasting and have less variation in intensity than touch
sensations; pressure is a sustained sensation that is felt over a larger area
than touch.
1)
Receptors
for pressure are type II cutaneous mechanoreceptors
and lamellated (Pacinian)
corpuscles.
2)
Like
corpuscles of touch, lamellated corpuscles adapt
rapidly.
b.
Vibration
sensations result from rapidly repetitive sensory signals from tactile
receptors; the receptors for vibration sensations are corpuscles of touch and lamellated corpuscles, which detect low-frequency and
high-frequency vibrations, respectively.
5.
Itch
and Tickle
a.
Itch and tickle
receptors are free nerve endings.
b.
Tickle
is the only sensation that you may not elicit on yourself.
6.
Phantom
pain is the sensations of pain in a limb that has been amputated; the brain
interprets nerve impulses arising in the remaining proximal portions of the
sensory nerves as coming from the nonexistent (phantom) limb. (Clinical
Application)
B.
Thermal
Sensations
1.
Thermoreceptors are free nerve endings.
2.
Separate
thermoreceptors respond to hot and cold stimuli.
C.
Pain
Sensations
1.
Pain is a
vital sensation because it provides us with information about tissue-damaging
stimuli and with signs that may be used for diagnosis of disease or injury.
2.
Pain receptors
(nociceptors) are free endings that are
located in nearly every body tissue; adaptation is slight if it occurs at all.
3.
Two
kinds of pain are recognized in the parietal lobe of the cortex: somatic
(superficial and deep) and visceral; visceral pain, unlike
somatic pain, is usually felt in or just under the skin that overlies the
stimulated organ – the pain may also be felt in a surface area far from the
stimulated organ in a phenomenon known as referred pain (Figure 15.3).
4.
Anesthesia
blocks sensations while still maintaining the stability of the patient’s organ
systems.
D. Proprioceptive Sensations
1.
Receptors
located in skeletal muscles, in tendons, in and around joints,
and in the internal ear convey nerve impulses related to muscle tone, movement
of body parts, and body position. This awareness of the activities of muscles,
tendons, and joints and of balance or equilibrium is provided by the proprioceptive or kinesthetic sense.
2.
The
receptors include muscle spindles, tendon organs (Golgi
tendon organs), and joint kinesthetic receptors. You should know where these are
located, but not the specific structure of the receptors.
A. Specific areas of the cerebral cortex
receive somatic sensory input from particular parts of the body and other areas
of the cerebral cortex provide output instructions for movement of particular
parts of the body (Figure 15.5)
B.
Precise
location of somatic sensations occurs at the primary somatosensory
area (Figures 14.15 & 15.5a).
C.
The
primary motor area in the precentral gyrus of the frontal lobe is a major control region for
voluntary movements (Figures 14.15 & 15.5b)
A. Somatic sensory pathways relay information from somatic
receptors to the primary somatosensory area in the
cerebral cortex.
1.
The
pathways consist of first-order, second-order, and third-order neurons.
2.
Axon
collaterals of somatic sensory neurons simultaneously carry signals into the
cerebellum and the reticular formation of the brain stem.
B.
Posterior
Column-Medial Lemniscus Pathway to the Cortex
1.
The
nerve impulses for conscious proprioception and most
tactile sensations ascend to the cortex along a common pathway formed by
three-neuron sets. Refer to lecture notes and Figure
15.16a for more info.
2.
Impulses
conducted along this pathway are concerned with discriminative touch, stereognosis, proprioception, and
vibratory sensations.
C.
Anterolateral Pathways to the Cortex
1.
The
anterolateral (spinothalamic)
pathways carry mainly tickle, itch, some tactile impulses (anterior), pain
and temperature (lateral). Refer to lecture notes and Figure 15.16b for
more info.
D. Somatic Sensory Pathways to the
Cerebellum
1.
The
posterior spinocerebellar and the anterior spinocerebellar tracts are the major routes whereby proprioceptive impulses reach the cerebellum.
E.
Table
15.3 summarizes the major sensory tracts in the spinal cord and pathways in the
brain.
A. Lower motor neurons extend from the
brain stem and spinal cord to skeletal muscles.
B.
Four
distinct neural circuits (somatic motor pathways) participate in control of
movement by providing input to lower motor neurons (Figure 15.7).
1.
Local
circuit neurons are located close to lower motor neuron cell bodies in the
brain stem and spinal cord.
2.
Local
circuit neurons and lower motor neurons receive input from upper motor neurons.
3.
Neurons
of the basal ganglia provide input to upper motor neurons.
4.
Cerebellar neurons also control activity of upper motor neurons.
C.
Voluntary
motor impulses are propagated from the motor cortex to somatic efferent neurons
(voluntary motor neurons) that innervate skeletal muscles via the direct or
pyramidal pathways. The simplest pathways consist of upper and lower motor
neurons.
1.
Direct
motor pathways provide input to lower motor neurons via axons that extend
directly from the cerebral cortex.
2.
The
direct pathways include the lateral and anterior corticospinal tracts and corticobulbar
tracts. Refer to Figure 15.8 and lecture
notes for more info.
3.
The
various tracts of the pyramidal system convey impulses from the cerebral cortex
that result in precise muscular movements.
4.
The
lateral corticospinal, anterior corticospinal,
and corticobulbar tracts contain axons
of upper motor neurons (Figure 15.8 and Table 15.4).
C.
Indirect
Pathways
1.
Indirect or extrapyramidal pathways include all somatic
motor tracts other than the corticospinal and corticobulbar tracts.
a.
Indirect
pathways involve the motor cortex, basal ganglia, thalamus,
cerebellum, reticular formation, and nuclei in the brain stem
(Figure 15.8).
b.
Major
indirect tracts are the rubrospinal, tectospinal, vestibulospinal,
and reticulospinal tracts.
D. Table 15.4 summarizes the major motor
tracts, their functions, and pathways in the brain.
E.
Modulation
of Movement by the Basal Ganglia
1.
The
basal ganglia help program habitual or automatic sequences and set an
appropriate level of muscle tone.
2.
They
also selectively inhibit other motor neuron circuits that are intrinsically
active or excitatory.
3.
Damage
to the basal ganglia results in uncontrollable, abnormal body movements, often
accompanied by muscle rigidity and tremors.
Parkinson disease results from damage to the basal ganglia.
F.
Modulation
of Movement by the Cerebellum
1.
The
cerebellum is active in both learning and performing rapid, coordinated,
highly skilled movements and in maintaining proper posture and equilibrium.
2.
The
four aspects of cerebellar function are monitoring
intent for movement, monitoring actual movement, comparing
intent with actual performance, and sending out corrective
signals (Figure 15.9)
A. The integrative functions
include sleep and wakefulness, memory, and emotional responses (discussed in
Chapter 14).
B.
Wakefulness
and Sleep
1.
Reticular
Activating System (RAS)
a.
Sleep
and wakefulness are integrative functions that are controlled by the reticular
activating system (Figure 15.10).
b.
Arousal, or
awakening from a sleep, involves increased activity of the RAS.
1)
Once
the RAS is activated, the cerebral cortex is also activated and arousal occurs.
2)
The
result is a state of wakefulness called consciousness.
2.
Sleep
a.
During
sleep, a state of altered consciousness or partial unconsciousness from
which an individual can be aroused by different stimuli, activity in the RAS is
very low.
b.
Normal
sleep consists of two types: non-rapid eye movement sleep (NREM)
and rapid eye movement sleep (REM).
1)
Non-rapid eye movement or slow wave sleep consists of four stages, each of
which gradually merges into the next. Each stage has been identified by EEG
recordings. Refer to lecture notes and figure 15.11 for more info.
2)
Most
dreaming occurs during rapid eye movement sleep.
C.
Learning
and Memory
1.
Learning is
the ability to acquire new knowledge or skills through instruction or
experience. Memory is the process by which that knowledge is retained
over time.
a.
For
an experience to become part of memory, it must produce persistent functional
changes that represent the experience in the brain.
b.
The
capability for change with learning is called plasticity.
2.
Memory
is the ability to recall thought and is generally classified into two kinds
based on how long the memory persists: short-term and long-term
memory.
a.
Short-term memory lasts only seconds or hours and is the ability to recall bits of
information; it is related to electrical and chemical events.
b.
Long-term memory lasts from days to years and is related to anatomical and biochemical
changes at synapses.
Please note: Lecture
on the cerebellum, basal ganglia, and limbic system was given with
Chapter 14.