Neurons & the Nervous System - Part 2
The Human Nervous System consists of the Central Nervous System & the Peripheral Nervous System.
Central Nervous System:
2 - Spinal cord
Central Nervous System
Divisions of Peripheral Nervous System -
Used with permission of John Kimball
Divisions of the Human Brain:
1 - Myelencephalon, which includes the medulla
2 - Metencephalon, which includes the pons and cerebellum
3 - Mesencephalon, which includes the midbrain (tectum and tegmentum)
4 - Diencephalon, which includes the thalamus and hypothalamus
5 - Telencephalon, which includes the cerebrum (cerebral cortex, basal ganglia, & medullary body)
Used with permission of John W. Kimball
Human brain (coronal section). The divisions of the brain include the (1) cerebrum, (2) thalamus, (3) midbrain,
(4) pons, and (5) medulla oblongata. (6) is the top of the spinal cord (Source: Wikipedia).
Structures of the Brain:
Medulla (also called medulla oblongata) -
2 - contains ascending & descending tracts that communicate between the spinal cord & various parts of the brain
3 - contains 3 vital centers:
2 - Origin of four cranial nerves (V or trigeminal, VI or abducens, VII or facial, & VIII or vestibulocochlear)
3 - contains pneumotaxic center (a respiratory center)
|The brain stem is the region between the diencephalon (thalamus and hypothalamus) and the spinal cord. It consists of three parts: midbrain, pons, and medulla oblongata. The midbrain is the most superior portion of the brain stem. The pons is the bulging middle portion of the brain stem. This region primarily consists of nerve fibers that form conduction tracts between the higher brain centers and spinal cord. The medulla oblongata, or simply medulla, extends inferiorly from the pons. It is continuous with the spinal cord at the foramen magnum. All the ascending (sensory) and descending (motor) nerve fibers connecting the brain and spinal cord pass through the medulla (Source: training.seer.cancer.gov).|
2 - Cerebral peduncles - ascending & descending fiber tracts
3 - Origin of two cranial nerves (III or oculomotor & IV or trochlear)
2 - Reception of sensory impulses from viscera
3 - Intermediary between nervous system & endocrine system
4 - Control of body temperature
6 - Thirst center
7 - Part of limbic system (emotions such as rage and aggression)
8 - Part of reticular formation
Reticular formation -
2 - needed for arousal from sleep & to maintain consciousness
2 - consists of 2 hemispheres divided by a fissure
3 - includes cerebral cortex, medullary body, & basal ganglia:
'Forward' (a) and 'inverse' (b) model control systems for movement. According to 'instructions' from the premotor cortex (P), an area in the motor cortex (controller, or CT) sends impulses to the controlled object (CO; a body part). The visual cortex (VC) mediates feedback from the body part to the motor cortex. The dashed arrow indicates that the body part is copied as an 'internal model' in the cerebellum. In the forward-model control system, control of the body part (CO) by the motor cortex (CT) can be precisely performed by referring to the internal feedback. In the inverse-model control system, feedback control by the motor cortex (CT) is replaced by the inverse model itself (Ito 2008).
Cortex matures faster in youth with superior IQs -- Children and teens with superior IQ's are distinguished by how fast the thinking part of their brains thickens and thins as they grow up. Magnetic resonance imaging (MRI) scans showed that their brain’s outer mantle, or cortex, thickens more rapidly during childhood, reaching its peak later than in their peers — perhaps reflecting a longer developmental window for high-level thinking circuitry. It also thins faster during the late teens, likely due to the withering of unused neural connections as the brain streamlines its operations. Although most previous MRI studies of brain development compared data from different children at different ages, Shaw et al. (2006) controlled for individual variation in brain structure by following the same 307 children and teens, ages 5-19, as they grew up. Most were scanned two or more times at two-year intervals. The resulting scans were divided into three equal groups and analyzed based on IQ test scores: superior (121-145), high (109-120), and average (83-108). The researchers found that the relationship between cortex thickness and IQ varied with age, particularly in the prefrontal cortex, seat of abstract reasoning, planning, and other “executive” functions. The smartest 7-year-olds tended to start out with a relatively thinner cortex that thickened rapidly, peaking by age 11 or 12 before thinning. In their peers with average IQ, an initially thicker cortex peaked by age 8, with gradual thinning thereafter. Those in the high range showed an intermediate trajectory (see below). Although the cortex was thinning in all groups by the teen years, the superior group showed the highest rates of change. “Brainy children are not cleverer solely by virtue of having more or less gray matter at any one age,” explained co-author J. Rapoport. “Rather, IQ is related to the dynamics of cortex maturation.” The observed differences are consistent with findings from functional magnetic resonance imaging, showing that levels of activation in prefrontal areas correlates with IQ, note the researchers. They suggest that the prolonged thickening of prefrontal cortex in children with superior IQs might reflect an “extended critical period for development of high-level cognitive circuits.” Although it’s not known for certain what underlies the thinning phase, evidence suggests it likely reflects “use-it-or-lose-it” pruning of brain cells, neurons, and their connections as the brain matures and becomes more efficient during the teen years. “People with very agile minds tend to have a very agile cortex,” said co-author P. Shaw.
Limbic System -
2 - located in part in cerebral cortex, thalamus, & hypothalamus
3 - Functions:
The spinal cord extends from the skull (foramen magnum) to the first lumbar vertebra. Like the brain, the spinal cord consists of gray matter and white matter. The gray matter (cell bodies & synapses) of the cord is located centrally & is surrounded by white matter (myelinated axons). The white matter of the spinal cord consists of ascending and descending fiber tracts, with the ascending tracts transmitting sensory information (from receptors in the skin, skeletal muscles, tendons, joints, & various visceral receptors) and the descending tracts transmitting motor information (to skeletal muscles, smooth muscle, cardiac muscle, & glands). The spinal cord is also responsible for spinal reflexes.
Reflex- rapid (and unconscious) response to changes in the internal or external environment needed to maintain homeostasis
Reflex arc - the neural pathway over which impulses travel during a reflex. The components of a reflex arc include:
There are 31 pair of spinal nerves & each has a dorsal root and a ventral root. The dorsal root is sensory (all neurons conduct impulses into the spinal cord) while the ventral root is motor (all neurons conduct impulses out of the spinal cord). The dorsal root has a ganglion that contains the cell bodies of the sensory neurons that pass through the dorsal root. Each spinal nerve includes numerous sensory, or afferent, & motor, or efferent, neurons. Some of these neurons are classified as somatic, and these neurons conduct impulses to or from 'somatic' structures (skin, skeletal muscles, tendons, & joints). Other neurons are 'visceral', and these conduct impulses to or from 'visceral' structures (smooth muscle, cardiac muscle, and glands). Thus, all neurons in spinal nerves (& the peripheral nervous system) can be placed in one of four categories:
Somatic efferent neurons are motor neurons that conduct impulses from the spinal cord to skeletal muscles. These neurons are multipolar neurons, with cell bodies located in the gray matter of the spinal cord. Somatic efferent neurons leave the spinal cord through the ventral root of spinal nerves.
Visceral afferent neurons are sensory neurons that conduct impulses initiated in receptors in smooth muscle & cardiac muscle. These neurons are collectively referred to as enteroceptors or visceroceptors. Visceral afferent neurons are unipolar neurons that enter the spinal cord through the dorsal root & their cell bodies are located in the dorsal root ganglia.
Visceral efferent neurons are motor neurons that conduct impulses to smooth muscle, cardiac muscle, & glands. These neurons make up the Autonomic Nervous System. Some visceral efferent neurons begin in the brain; others in the spinal cord. Because we're focusing on spinal nerves right now, we'll focus on those that begin in the spinal cord. It always takes two visceral efferent neurons to conduct an impulse from the spinal cord (or brain, in some cases) to a muscle or gland:
The 4 types of peripheral neurons: somatic afferent (top
right), somatic efferent (bottom right),
visceral afferent (top left), and visceral efferent (bottom left).
Autonomic Nervous System:
Used with permission of John W. Kimball
Autonomic Nervous System - control of involuntary muscle
5 - Functions of the Autonomic Nervous System:
6 - Control of Autonomic Nervous System - primary control center is the hypothalamus
Back to Neurons & the Nervous System I
Development of Transmembrane Resting Potential
The Physical Factors Behind the Action Potential
Nerve Action Potentials
Saltatory Conduction of Action Potentials
Neurons: Our Internal Galaxy
The Autonomic Nervous System
The Nervous System
Explore the Brain and Spinal Cord
The Animated Brain
Ito, M. 2008. Internal-model control systems for voluntary movement and mental activity. Nature Reviews Neuroscience 9: 304-313.
Mignot, E., S. Taheri, and S. Nishino. 2002. Sleeping with the hypothalamus: emerging therapeutic targets for sleep disorders. Nature Neuroscience 5: 1071-1075.
Shaw, P., D. Greenstein, J. Lerch, L. Clasen, R. Lenroot, N. Gogtay, A. Evans, J. Rapoport and J. Giedd. 2006. Intellectual ability and cortical development in children and adolescents. Nature 440: 676-679.
Back to BIO 301 syllabus