Spinal cord physiology
1. The white matter tracts in the spinal cord are highways for nerve impulse propagation. Along
these tracts, sensory input travels toward the brain, and motor output travels from the brain
toward skeletal muscles and other eff ector tissues. Sensory input travels along two main routes
in the white matter of the spinal cord: the posterior column and the spinothalamic tract. Motor
output travels along two main routes in the white matter of the spinal cord: direct pathways and
indirect pathways.
2. A second major function of the spinal cord is to serve as an integrating center for spinal
reflexes. This integration occurs in the gray matter.
3. A reflex is a fast, predictable sequence of involuntary actions, such as muscle contractions or
glandular secretions, which occurs in response to certain changes in the environment. Reflexes
may be spinal or cranial and somatic or
autonomic (visceral).
4. The components of a reflex arc are sensory receptor, sensory neuron, integrating center, motor
neuron, and eff ector.
5. Somatic spinal reflexes include the stretch reflex, the tendon reflex, the flexor (withdrawal)
reflex, and the crossed extensor reflex; all exhibit reciprocal innervation
.
6. A two-neuron or monosynaptic reflex arc consists of one sensory neuron and one motor
neuron. A stretch reflex, such as the patellar reflex, is an example.
7. The stretch reflex is ipsilateral and is important in maintaining muscle tone.
8. A polysynaptic reflex arc contains sensory neurons, interneurons, and motor neurons. The
tendon reflex, flexor (withdrawal) reflex, and crossed extensor reflexes are examples
9. The tendon reflex is ipsilateral and prevents damage to muscles and tendons when muscle
force becomes too extreme. The flexor reflex is ipsilateral and moves a limb away from the
source of a painful stimulus. The crossed extensor reflex extends the limb contralateral to a
painfully stimulated limb, allowing the weight of the body to shift when a supporting limb is
withdrawn
.
10. Several important somatic reflexes are used to diagnose various disorders. These include the
patellar reflex, Achilles reflex, Babinski sign, and abdominal reflex.
The Cerebrum
1. The cerebrum is the largest part of the brain. Its cortex contains gyri (convolutions), fissures,
and sulci.
2. The cerebral hemispheres are divided into four lobes: frontal, parietal, temporal, and occipital.
3. The white matter of the cerebrum is deep to the cortex and consists primarily of myelinated
axons extending to other regions as association, commissural, and projection fibers
.
4. The basal nuclei are several groups of nuclei in each cerebral hemisphere They help initiate
and terminate movements, suppress unwanted movements, and regulate muscle tone.
.
5. The limbic system encircles the upper part of the brainstem and the corpus callosum. It
functions in emotional aspects of behavior and memory.
14.7 Functional Organization of the Cerebral Cortex
1. The sensory areas of the cerebral cortex allow perception of sensory information. The motor
areas control the execution of voluntary movements. The association areas are concerned with
more complex integrative functions such as memory, personality traits, and intelligence.
2.The primary somatosensory area (areas 1, 2, and 3) receives nerve impulses from somatic
sensory receptors for touch, pressure, vibration, itch, tickle, temperature, pain, and
proprioception and is involved in the perception of these sensations. Each point within the area
receives impulses from a specific part of the face or body. The primary visual area (area 17)
receives visual information and is involved in visual perception. The primary auditory area (areas
41 and 42) receives information for sound and is involved in auditory perception. The primary
gustatory area (area 43) receives impulses for taste and is involved in gustatory perception and
taste discrimination. The primary olfactory area (area 28) receives impulses for smell and is
involved in olfactory perception
3. Motor areas include the primary motor area (area 4), which controls voluntary contractions of
specific muscles or groups of muscles, and Broca’s speech area (areas 44 and 45), which controls
production of speech.
4. The somatosensory association area (areas 5 and 7) permits you to determine the exact shape
and texture of an object simply by touching it and to sense the relationship of one body part to
another. It also stores memories of past somatic sensory experiences
.
5.The visual association area (areas 18 and 19) relates present to past visual experiences and is
essential for recognizing and evaluating what is seen. The facial recognition area (areas 20, 21,
and 37) stores information about faces
and allows you to recognize people by their faces. The auditory association area (area 22) allows
you to recognize a particular sound as speech, music, or noise
6.The orbitofrontal cortex (area 11) allows you to identify odors and discriminate among diff
erent odors. Wernicke’s area (area 22 and possibly 39 and 40) interprets the meaning of speech
by translating words into thoughts. The
common integrative area (areas 5, 7, 39, and 40) integrates sensory interpretations from the
association areas and impulses from other areas, allowing thoughts based on sensory inputs.
7. The prefrontal cortex (areas 9, 10, 11, and 12) is concerned with personality, intellect,
complex learning abilities, judgment, reasoning, conscience, intuition, and development of
abstract ideas. The premotor area (area 6)
generates nerve impulses that cause specific groups of muscles to contract in specific sequences.
It also serves as a memory bank for complex movements. The frontal eye field area (area 8)
controls voluntary scanning movements of
the eyes.
8. Subtle anatomical diff erences exist between the two hemispheres, and each has unique
functions. Each hemisphere receives sensory signals from and controls movements of the
opposite side of the body. The left hemisphere
is more important for language, numerical and scientific skills, and reasoning. The right
hemisphere is more important for musical and artistic awareness, spatial and pattern perception,
recognition of faces, emotional content
of language, identifying odors, and generating mental images of sight, sound, touch, taste, and
smell
9. Brain waves generated by the cerebral cortex are recorded from the surface of the head in an
electroencephalogram (EEG). The EEG may be used to diagnose epilepsy, infections, and
tumors.
SIGNAL TRANSMISSION AT SYNAPSE
A nerve impulse arrives at a synaptic end bulb (or at a varicosity) of a presynaptic axon.
2 The depolarizing phase of the nerve impulse opens voltage-gated Ca2+ channels, which are
present in the membrane of synaptic end bulbs. Because calcium ions are more concentrated in
the extracellular fluid, Ca2+ flows inward through the opened channels
An increase in the concentration of Ca2+ inside the presynaptic neuron serves as a signal that
triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma
membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft .
Each synaptic vesicle contains several thousand molecules of neurotransmitter.
The neurotransmitter molecules diff use across the synaptic cleft and bind to neurotransmitter
receptors in the postsynaptic neuron’s plasma membrane. The receptor shown in is part of a
ligand-gated channel you will soon learn that this type of neurotransmitter receptor is called an
ionotropic receptor. Not all neurotransmitters bind to ionotropic receptors; some bind to
metabotropic receptors.
Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the
channels and allows particular ions to flow across the membrane.
6 As ions flow through the opened channels, the voltage across the membrane changes. This
change in membrane voltage is a postsynaptic potential. Depending on which ions the channels
admit, the postsynaptic potential may be a depolarization (excitation) or a hyperpolarization
(inhibition). For example, opening of Na+ channels allows inflow of Na+, which causes
depolarization. However, opening of Cl– or K+ channels causes hyperpolarization. Opening Cl−
channels permits Cl− to move into the cell, while opening the K+ channels allows K+ to move
out—in either event, the inside of the cell becomes more negative.
When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in
the axon of the postsynaptic neuron.
.
