Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_123_библиотеки_им_акад_М_И_Перельмана
.pdf
CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 161
»
Polysynaptic activation in multi-muscle coordination: Alpha motor
neurons receive substantial input from nearby and distant spinal interneu-
rons. These interneuron circuits are involved inlocomotion and complex,
multi-muscle coordination, and reexes.
Reecting on the basic spinal reex
Spinal reexes are rapid responses that don’t require brain input or awareness. In
Chapter7, we explain that a reex is the basic unit of sensory motor integration.
In spinal reexes, a stimulus (such as to a pain receptor on the nger or stretch of
a muscle) leads to a direct movement (such as withdrawing the nger or contrac-
tion of that muscle).
Primary somatosensory neurons extend their axons into the spinal cord where
they branch. One branch synapses on a secondary sensory (relay) neuron to relay
sensation. The other branch may synapse on an alpha motor neuron or on a spinal
interneuron that provides input to an alpha motor neuron. Sensory information
from the skin, muscles, and joints provides input for both conscious sensation and
rapid reex responses for posture and balance or to protect limbs from injury.
The inputs to and outputs from the majority of spinal interneurons occur within a
single spinal cord segment. A few spinal interneurons, called propriospinal inter-
neurons, project to other segments and are important for coordinating move-
ments and locomotion.
Focusing on spinal central
pattern generators
Early in the 20th century, research showed that animals could walk and run on
treadmills even if the spinal cord connections to their brains were severed. This
means that the spinal cord can produce basic rhythmic patterns for locomotion
without the need for any descending commands from the cortex. The spinal cord
neuronal circuits that produce rhythmic patterns of neural activity are called cen-
tral pattern generators (CPGs).
CPGs normally produce rhythmic patterned outputs in response to sensory feed-
back; however, sensory feedback isn’t required for CPGs to produce these outputs.
Sensory feedback helps coordinate ecient locomotion in the limbs, for instance.
The brain normally initiates and controls the activity of spinal CPGs through
descending axonal projections that make up high-level commands, such as
changing your gait.
https://t.me/med1917
162 PART 2 Controlling Muscles, Movement, and Survival
Locomoting with alternate limb movement
Walking and other forms of locomotion involve moving all four limbs. CPGs are
distributed throughout the lower thoracic and lumbar spinal cord. These circuits
generate motor neuron ring patterns that synchronize rhythmic contractions of
exor-extensor muscles of the arms and legs. As the legs move in antiphase with
each other, the arms move in the opposite antiphase— so your left arm goes for-
ward at the same time as your right leg.
Balance is mediated by having lumbar pattern generators aect pattern genera-
tors in cervical segments. It’s also subject to top-down control from inputs from
the visual and vestibular (your sense of balance) systems. Your visual system gen-
erates commands to keep you from running into trees. Your vestibular system
prevents you from falling if you step in a hole and allows you to run on a slope.
CPGs require neural circuits that interact in such a way that half of the circuit
increases in activity while the other half decreases. Scientists believe this alter-
nating phase activity is based on what are called half-center oscillators. In a reci
-
procal network, two groups of neurons reciprocally inhibit each other, which is a
core feature of many CPGs.
Changing pace, walking, and running
Our immediate needs for balance and central commands to change gaits— such
as from walking to running— modulate CPGs. Virtually all gaits involve alternat-
ing lower limbs, and simultaneously alternating upper limbs in antiphase. A gait
is normally established by a command from the brain to begin the gait by setting
parameters for the CPGs, followed by a hierarchical set of reex circuits. These
circuits move the limbs and make many adjustments according to the ground
surface and muscle fatigue during the movement.
Feeling and Acting: Connecting
the CNS and PNS
The nerves and ganglia outside the spinal cord (and brain) constitute the periph-
eral nervous system (PNS). The PNS connects the CNS to the limbs and organs. The
PNS is divided into the somatic nervous system and the autonomic nervous sys-
tem. (We discuss the autonomic nervous system in Chapter10.) Here we take a
quick look at the ascending sensory tracts and descending motor tracts within the
spinal cord for the somatic nervous system.
https://t.me/med1917
CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 163
Somatosensory neurons of the dorsal root ganglia and axons of the motor neurons
leaving the ventral root are part of the somatic PNS.Most (10 of 12) of the cranial
nerves are also part of the PNS. The somatosensory system is involved in the
senses of touch, temperature, and various types of pain, and the sense of limb
position and movement (called proprioception and kinesthesia).
Getting stimulated by neural sensors
The skin consists of an outer layer of cells called the epidermis and an inner layer
of cells called the dermis (see Chapter14 for more details). Within the dermis are
somatosensory receptor cell endings. These include touch receptors (mechanore-
ceptors), hot and cold temperature receptors (thermoreceptors), and pain receptors
(nociceptors). Within the muscles tendons and joints are proprioceptors. These sen-
sory neuron cell bodies are located in the dorsal root ganglia. They send input to
the spinal cord where the sensory information travels to the thalamus or cerebel-
lum through three major spinal cord white matter tracts (see Figure9-4):
»
Dorsal column medial lemniscus tracts: The large axons transmit the
various kinds of discriminative touch sense (pressure, utter, vibration) and
conscious proprioception.
»
Spinothalamic tracts: The smaller axons of the spinothalamic pathway
transmit pain, temperature, itch, and crude touch information.
»
Spinocerebellar tracts: The sensors for limb position, movement, and
muscle tension (proprioceptors) located in skin, muscles, and joints transmit
unconscious information to the cerebellum.
FIGURE9-4:
Spinal cord tracts.
© John Wiley & Sons, Inc.
https://t.me/med1917
164 PART 2 Controlling Muscles, Movement, and Survival
Moving with motor neuron inputs
Upper motor neurons project to the spinal cord via pyramidal tracts and extrapy-
ramidal tracts to control voluntary and involuntary muscle contraction. The tracts
are as follows:
»
Pyramidal tracts: Upper motor neurons in the motor cortex extend axon
tracts called the corticobulbar and corticospinal tracts. Corticobulbar tracts
end at the brainstem and control movements of the head, face, and mouth.
Corticospinal tracts are further divided:
•
Lateral corticospinal tracts cross over at the pyramids of the medulla and
control appendicular muscles that mediate limb movements.
•
Anterior corticospinal tracts don’t cross over at the pyramids and control
axial muscles of the trunk.
»
Extrapyramidal tracts: Upper motor neurons in brainstem nuclei project to
the spinal cord via the extrapyramidal tracts: vestibulospinal, reticulospinal,
and tectospinal tracts (refer to Chapter8). They provide involuntary control of
axial muscles in the trunk and neck involved in posture and balance and head
movements. The rubrospinal tract is a fourth extrapyramidal tract that
provides minor control over voluntary upper limb movements.
Zeroing in on motor neurons-eectors
Alpha motor neurons typically innervate muscles on the same side of the body on
which they exit the spinal cord. One alpha motor neuron and all the muscle bers
it projects to are called a motor unit. Gamma motor neurons innervate muscle
spindles, which are involved in sensory feedback from muscles. The set of all the
alpha and gamma motor neurons that project to a single muscle is called the motor
neuron pool.
Fine muscle control is generally achieved by having a large number of alpha motor
neurons, each with fewer muscle outputs, compared to a small number of alpha
motor neurons that innervate hundreds of muscle bers.
Alpha motor neurons are located in lamina IX in the middle of the ventral horn of
the spinal cord gray matter. Alpha motor neurons that innervate more distal mus-
cles are located more laterally in the gray matter than those that innervate proxi-
mal muscles. Alpha motor neurons that innervate extensors tend to be located
more ventrally than those that innervate exors.
https://t.me/med1917
CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 165
Doing the heavy lifting: Muscle cells
Chapter7 describes the neuromuscular junction and mechanisms of muscle con-
traction. When the muscle ber is relaxed, the protein troponin binds to actin la-
ments, preventing actin laments from interacting with myosin laments. In a
relaxed muscle ber, myosin binds and hydrolyzes ATP to activate it.
Increased calcium levels produced by the muscle cell action potential cause cal-
cium to bind to troponin, releasing it from actin, so the actin can bind to myosin
forming the cross-bridges. When activated myosin interacts with actin, it acts like
a molecular ratchet. After it binds to actin, the myosin proteins rotate in unison
and pull the actin lament, so the muscle ber shortens to produce contraction or
tension.
Concluding with the spinal cord
With its complex circuitry the spinal cord receives motor commands from the
brain and integrates those with sensory input from skin, muscles, and joints to
produce voluntary movement and spinal reexes, coordinate actions, ensure pos-
ture and balance, and generate locomotion. It also transmits somatosensory and
visceral sensory information to the brain and mediates autonomic output. It works
closely with the brainstem, the other evolutionary ancient part of the CNS.
Moving and Surviving with the Brainstem
Like the spinal cord, the brainstem mediates motor and sensory functions. It con-
trols voluntary movements of the mouth and tongue for speech, singing, and
chewing. It generates facial expressions and eye movements. It’s an origin of
autonomic control, and it relays somatic and visceral sensory information to the
brain.
In addition, the brainstem is always working behind the scenes to control essen-
tial survival and homeostatic functions, like breathing, and control of heart rate,
blood pressure, temperature, and sleep. It’s also involved in the modulation of
pain transmission, emotion, and cognition. This following sections explore the
functional anatomy of the brainstem.
https://t.me/med1917
166 PART 2 Controlling Muscles, Movement, and Survival
Looking closer at the brainstem
Together, the medulla, pons, and midbrain form the brainstem, which is the tran-
sition between the brain and spinal cord. These are phylogenetically old brain
structures that originated in primitive sh more than 500 million years ago and
have evolved since then.
The brainstem contains white and gray matter regions. White matter regions con-
tain axons traveling within and from the brainstem and to and from the brain and
spinal cord. Groups of neurons whose cell bodies are in the brainstem are called
nuclei. Cranial nerve nuclei are the origins or targets of the cranial nerves. Other
nuclei are involved in survival, homeostasis, and behavior.
All the cranial nerves except cranial nerves I and II originate in the brainstem.
Figure9-5 shows the brainstem in relation to the rest of the brain and spinal cord.
The medulla, pons, and midbrain have very dierent anatomy and functions.
FIGURE9-5:
The brainstem:
medulla, pons,
and midbrain.
© John Wiley & Sons, Inc.
https://t.me/med1917
CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 167
In humans, the reticular formation is a group of more than a hundred intercon-
nected nuclei found along the entire brainstem with connections to the hypo-
thalamus, thalamus, forebrain, and cerebellum. It’s a major integration and relay
center for many vital brain systems to coordinate functions necessary for
survival.
The reticular formation mediates both ascending pathways involved in sleep,
arousal, consciousness, emotion, and descending extrapyramidal pathways. The
reticular formation is also important in homeostasis, which we cover more in
Chapter10. Several regions that we describe in the next sections contribute to the
reticular formation.
Meeting the medulla
The most posterior and ventral area of the brain is called the medulla (or medulla
oblongata). The lower border of the medulla is the top of the spinal cord. The tran-
sition occurs at a hole in the occipital bone at the base of the skull called the fora-
men magnum.
Carrying information
In the medulla (refer to Figure9-5) white matter regions contain axonal tracts
that carry sensory information from and motor information to the spinal cord.
Several white matter tracts are located around the lateral regions of the medulla
and include the pyramids and inferior cerebellar peduncles. One prominent medial
tract is the medial lemniscus.
Two gray areas in the medulla, called the gracile and cuneate nuclei, are the relays
for axons in the dorsal column medial lemniscus pathway. Those axons originate
with somatosensory neurons in the skin and transmit discriminative touch, vibra-
tion, and conscious proprioception.
Examining sensory input and motor output
The medulla receives sensory information from taste receptors in the tongue,
somatosensory receptors in the skin of the head, and visceral sensory input from
the heart, lungs, blood vessels, and digestive system. Sensory axons in several
cranial nerves transmit this information to the medulla. Motor functions of the
medulla include the following:
»
Voluntary control of muscles of the tongue, the soft palate, and larynx
associated with producing speech and to the glandular portions of
these organs.
https://t.me/med1917
168 PART 2 Controlling Muscles, Movement, and Survival
»
Regulation of important survival functions such as breathing and homeostasis
by controlling heart rate, digestion processes, and blood vessels.
»
Involuntary control of brainstem reexes such as vomiting, coughing,
sneezing, and swallowing.
Because the medulla controls so many body functions that are crucial to life—
such as breathing and heart rate— any extensive damage to it is typically fatal.
Noting essential nuclei
Figure9-6 shows several nuclei in the medulla. The medulla is a long structure
and not all the nuclei shown would appear in every cross section.
The nuclei and their functions in the medulla are as follows:
»
Nucleus of the solitary tract: This is also called the nucleus tractus solitarius
(NTS). The NTS is a column of many sensory nuclei that span the length of the
medulla. It includes the gustatory nucleus that receives taste inputs. The NTS
also receives sensory input from baroreceptors, chemoreceptors, and stretch
receptors for the control of both somatic reexes (breathing, gag, and cough
reexes) and autonomic responses.
FIGURE9-6:
Medulla cross
section.
© John Wiley & Sons, Inc.
https://t.me/med1917
CHAPTER 9 Focusing On the Spinal Cord and Brain Stem 169
»
Respiratory centers: The NTS sends signals to the two output respiratory
centers located in the medulla:
•
Dorsal respiratory group: The dorsal respiratory group initiates the
inhalation phase of breathing. It’s located partially within and adjacent to
the NTS.
•
Ventral respiratory group:The ventral respiratory group controls both
inhalation and exhalation. It is located near the nucleus ambiguus.
»
Arcuate nucleus: The arcuate nucleus in the medulla receives input from
chemoreceptors and contributes sensory input to the respiratory centers.
(Note there is another arcuate nucleus in the hypothalamus involved in
hunger and satiety.)
»
Vagus nucleus: Also called the dorsal nucleus of the vagus, this gives rise to
CN X.Axons from this nucleus are preganglionic parasympathetic axons that
emanate from the brainstem near the inferior olive. This nucleus controls
parasympathetic functions in the heart, lungs, gastrointestinal tract, and
abdomen.
»
Hypoglossal nucleus: The hypoglossal nucleus gives rise to CN XII.This nerve
innervates the muscles of the tongue that control chewing, swallowing, and
speaking.
»
Nucleus ambiguus: The inputs to this nucleus come from the corticobulbar
tract, and its outputs are motor neurons that travel via the vagus nerve (CN X)
to the pharynx, larynx, and esophagus.
»
Inferior olive: The inferior olive receives inputs from the dorsal column
medial lemniscus nuclei. Its output is to the cerebellum for motor coordina-
tion and learning. Within the cerebellum, the olivary axons are called climbing
bers that synapse on Purkinje cells in the cerebellar cortex.
Tracts of ascending (from spinal sensory neurons) and descending (from motor
cortex) axons that go through the medulla cross over to the opposite side of the
centerline of the body at what’s called the decussation of the pyramids. This crosso-
ver causes each of the two cerebral hemispheres to receive sensory input from,
and control muscles on the opposite side of, the body. The crossing of sensory
input at the midline of the medulla is called the sensory decussation.
Presenting the pons
Above the medulla lies the pons. You can see in Figure9-5 that the pons bulges
out in front of the ventral surface of the medulla below it. Projecting from the back
of the pons is the cerebellum. Above the pons is the midbrain. The pons includes
white matter tracts that transmit signals to the cerebellum, medulla, and
https://t.me/med1917
170 PART 2 Controlling Muscles, Movement, and Survival
forebrain. It contains the nuclei for cranial nerves V through VIII.The pons con-
tains tracts that connect the forebrain with the cerebellum, medulla, and spinal
cord. It also contains nuclei involved in receiving and relaying sensory informa-
tion and controlling motor functions:
»
Hearing and balance: Within the pons is a sensory nucleus for vestibuloco-
chlear nerve (CN VIII). The cochlear nucleus receives input from the inner ear
and transmits sound information and localization through additional nuclei to
the thalamus. Circuits for balance involve vestibular nuclei input from the
inner ear. Outputs from vestibular nuclei project to the cerebellum and
frontal lobes.
»
Making faces: Facial expressions are controlled by sensory and motor nuclei
for the facial nerve (CN VII). The pons receives some sensory input from the
face and sends motor output to facial muscles to control both voluntary and
involuntary facial expressions.
»
Looking around: Pontine nuclei for the control of eye movements are
mediated by the motor nucleus for abducens nerve (CN VI).
»
Sensing and chewing: The motor and sensory nuclei of the trigeminal nerve
(CN V) provides the majority of facial sensations and controls jaw movements.
»
Secreting: The superior salivary nucleus in the pons helps produce tears and
secrete saliva.
»
Working with the medulla: Neural circuits involving both the pons and
medulla control respiration, swallowing, sleep, and bladder emptying.
Monitoring the midbrain
The top level of the brainstem is called the midbrain. The midbrain works with the
pons to control eye movements and relay sound information.
Moving the eyes
In mammals, the major visual area in the midbrain is called the superior colliculus.
In lower vertebrates this midbrain region is called the optic tectum and is the
major visual processing center. Mammals analyze visual input in the visual cortex
and use the superior colliculus to control rapid eye movements called saccades and
direct visual attention.
Cranial nerve nuclei for the oculomotor nerve (CN III) are located in the superior
colliculus. The oculomotor nerve controls the eyelid and most eye movements.
The area below the superior colliculus is called the inferior colliculus. The trochlear
https://t.me/med1917
Соседние файлы в папке Библиотека им академика М.И. Перельмана