BIO 301
Human Physiology

Neurons & the Nervous System - Part 2

The Human Nervous System consists of the Central Nervous System & the Peripheral Nervous System.

Central Nervous System:Drawing of brain and spinal cord

Peripheral Nervous System:

Divisions of the nervous system

Central Nervous System

Image showing the 12 cranial nerves

Colored drawing of the different sections of the vertebral column

Divisions of Peripheral Nervous System -

Diagram showing relationships between the central nervous system and the internal and external environments

Used with permission of John Kimball

Flow chart showing different divisions of the human nervous system

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)

Side view of a human brain
Used with permission of John W. Kimball

Frontal view of a section through a human brain
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) -

Pons -
Drawing of a side view of the brain stem 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:


Midbrain -

Drawing of the medulla, pons, and cerebellum
1- posterior medullary velum, 2 - choroid plexus, 3 - cisterna cerebellodellaris of subarachnoid cavity, 4 - central canal,
5 - corpora quadrigemina , 6 - cerebral peduncle, 7 - anterior medullary, 8 - ependymal lining of ventricle, & 9 - cisterna pontis of subarachnoid cavity
(Source: Wikipedia).

Brain stem

Thalamus -

Drawing of the thalamus



Hypothalamus -

Functions of the hypothalamus

Reticular formation -

Drawing of the ascending reticular activation system
The ascending reticular activation system. During periods of wakefulness, impulses from the brainstem activate neurons in the thalamus that are crucial for transmitting information to the cerebral cortex. Impulses also travel to the hypothalamus and throughout the cerebral cortex. A key switch in the hypothalamus (SCN, or suprachiasmic nucleus) that serves as the brain's 'master clock' shuts off this arousal system during sleep (Figure from: Mignot et al. 2002).

Reticular formation

Cerebellum -

Structures of the brain

Cerebrum -

Photo of a frontal section through the human brain showing white and grey matter
Animated gif showing the four lobes of the cerebrum

Drawing showing the functional areas of the human cerebrum

Drawings showing the movement control systems of the brain
'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).

Sensory cortex

Motor cortex

Graph showing relationship between a human's age and the change in thickness of the cerebral cortex
The rate of change in cortical thickness in children and teens of varying intelligence. Positive values indicate increasing cortical thickness, negative values indicate cortical thinning. The point of intersection on the x axis (0) represents the age of maximum cortical thickness (5.6 yr for average, 8.5 yr for high, and 11.2 yr for the superior intelligence group).

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.

Drawing of a sagittal section of a human brain

Image showing location of the basal ganglia in the brain


Basal ganglia

Limbic System -

Limbic system of a human brain

Limbic system

Emotion and memory

Spinal cord

Cross-section through the spinal cord

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.

Drawing showing examples of spinal tracts in the spinal cord

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:

Drawing showing the components of a reflex arc

Reflex arc

Spinal Nerves:

Drawing of a section of spinal cord showing spinal nerves

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 afferent neurons are sensory neurons that conduct impulses initiated in receptors in the skin, skeletal muscles, tendons, & joints. Receptors in the skin are responsible for sensing such things as touch, temperature, pressure, & pain and are called exteroceptors. Receptors in the skeletal muscles, tendons, & joints provide information about body position & movement and are called proprioceptors. Somatic afferent neurons are unipolar neurons that enter the spinal cord through the dorsal root & their cell bodies are located in the dorsal root ganglia.

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.

Drawing of the spinal cord and a spinal nerve showing the different types of neurons

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:

Drawing showing a small portion of the sympathetic chain
Drawing of the spinal cord, a spinal nerve, and a section of the sympathetic chain

Drawing showing the four types of peripheral neurons

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:

Drawing showing organs and structures innervated by the Autonomic Nervous System
Used with permission of John W. Kimball

Autonomic Nervous System - control of involuntary muscle

Fight-or-flight response


Illustration showing the areas of the brain that regulate the Autonomic Nervous System

Back to Neurons & the Nervous System I

Related links:

Development of Transmembrane Resting Potential

The Physical Factors Behind the Action Potential

Nerve Action Potentials

Saltatory Conduction of Action Potentials

Neurons: Our Internal Galaxy

Synaptic Transmission

The Autonomic Nervous System

The Nervous System

Explore the Brain and Spinal Cord

The Animated Brain

Literature cited:

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.

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