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:

1 - Brain

2 - Spinal cord

Peripheral Nervous System: 1 - Cranial nerves (12 pair) & their branches

2 - Spinal nerves (31 pair) & their branches

Source: http://mail.med.upenn.edu/~hessd/Lesson3.htm

Divisions of Peripheral Nervous System -

1 - Somatic - supplies & receives fibers (neurons) to & from the skin, skeletal muscles,

joints, & tendons

Used with permission of John Kimball

2 - Visceral - supplies & receives fibers to & from smooth muscle, cardiac muscle, and

glands. The visceral motor fibers (those supplying smooth muscle, cardiac muscle, &

glands) make up the Autonomic Nervous System. The ANS has two divisions:

o Parasympathetic division - important for control of 'normal' body functions, e.g.,

normal operation of digestive system

o Sympathetic division - also called the 'fight or flight' division; important in

helping us cope with stress

Source: http://faculty.washington.edu/chudler/nsdivide.html

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

Structures of the Brain:

Medulla (also called medulla oblongata) -

1 - continuous with spinal cord

2 - contains ascending &

descending tracts that

communicate between the spinal

cord & various parts of the brain

3 - contains 3 vital centers:

cardioinhibitory

center, which

regulates heart rate

respiratory center,

which regulates the

basic rhythm of

breathing

vasomoter center, which regulates the diameter of blood vessels

4 - origin of five cranial nerves (VIII or vestibulocochlear, IX or glossopharyngeal, X or

vagus, XI or accessory, & XII or hypoglossal)

Pons -

1 - Bridge connecting spinal cord w/ brain & parts of brain w/ each other

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)

Midbrain -

1 - Corpora quadrigemina - visual reflexes & relay center for auditory information. Two pairs of rounded knobs on the upper surface of the midbrain mark the location of four nuclei, which

are called collectively the "corpora quadrigemina." These masses contain the centers for certain visual

reflexes, such as those responsible for moving the eyes to view something as the head is turned. They also

contain the hearing reflex centers that operate when it is necessary to move the head so that sounds can be

heard better.

2 - Cerebral peduncles - ascending & descending fiber tracts

3 - Origin of two cranial nerves (III or oculomotor & IV or trochlear)

Thalamus -

1 - Relay station for nearly all sensory impulses (except olfaction)

Source: http://songweaver.com/brain/index.html

Hypothalamus -

1 - Control of Autonomic Nervous System

2 - Reception of sensory impulses from viscera

3 - Intermediary between nervous system & endocrine system

4 - Control of body temperature

5 - Regulation of food intake

6 - Thirst center

7 - Part of limbic system (emotions such as rage and

aggression)

8 - Part of reticular formation

Reticular formation -

1 - portions located in the spinal cord, medulla, pons,

midbrain, & hypothalamus

2 - needed for arousal from sleep & to maintain

consciousness

Cerebellum -

1 - functions in coordination, maintenance of posture, & balance

Cerebrum -

1 - largest portion of the human brain

2 - consists of 2 hemispheres divided by a fissure

Source: http://faculty.washington.edu/chudler/split.html

3 - includes cerebral cortex, medullary body, & basal ganglia:

o Cortex:

outer 2 - 4 mm of the cerebrum

consists of gray matter (cell bodies & synapses; no myelin)

'folded', with upfolded areas called gyri & depressions or grooves called

sulci

consists of four primary lobes

Source: http://faculty.washington.edu/chudler/lobe.html

functional areas include motor areas (initiate impulses that will cause

contraction of skeletal muscles) (see A Map of the Motor Cortex), sensory

areas (receive sensory impulses from throughout the body), and

association areas (for analysis)

Source: http://faculty.washington.edu/chudler/functional.html

o Medullary body:

the 'white matter' of the cerebrum; consists of myelinated axons

types of axons include:

commissural fibers - conduct impulses between cerebral

hemispheres (and form the corpus callosum)

Source: http://faculty.washington.edu/chudler/sagittal.html

projection fibers - conduct impulses in & out of the cerebral

hemispheres

association fibers - conduct impulses within hemispheres

o Basal ganglia:

masses of gray matter in each cerebral hemisphere

important in control of voluntary muscle movements

Limbic System -

1 - consists of a group of nuclei + fiber tracts

2 - located in part in cerebral cortex, thalamus, & hypothalamus

3 - Functions:

o aggression

o fear

o feeding

o sex (regulation of sexual drive & sexual behavior)

Spinal cord

Used with permission of John W. Kimball

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:

1 - receptor - responds to the stimulus

2 - afferent pathway (sensory neuron) - transmits impulse into the spinal cord

3 - Central Nervous System - the spinal cord processes information

4 - efferent pathway (motor neuron) - transmits impulse out of spinal cord

5- effector - a muscle or gland that receives the impulse from the motor neuron & carries

out the desired response

Spinal Nerves:

Source: http://nanonline.org/nandistance/nanneuro/modules/cranial/cranial.html

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

Somatic efferent

Visceral afferent

Visceral efferent

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.

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:

Visceral efferent 1 (also called the preganglionic neuron) is a multipolar neuron that

begins in the gray matter of the spinal cord, which is where its cell body is located. This

neuron leaves the cord through the ventral root of a spinal nerve, leaves the spinal nerve

via a structure called the white ramus, then ends in an autonomic ganglion (either

sympathetic or parasympathetic). In the ganglion, the visceral efferent 1 neuron synapses

with a visceral efferent 2 neuron.

Visceral efferent 2 (also called the postganglionic neuron) is also a multipolar neuron and

it begins in the sympathetic ganglion (which is where its cell body is located). Visceral

efferent 2 neurons may exit the ganglion through the gray ramus, then proceed to some

visceral structure (smooth muscle, cardiac muscle, or gland).

Source: http://www.mmi.mcgill.ca/Unit2/Mandl/lect20autonomicnervoussystem.htm

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:

1 - entirely motor (consisting of the visceral efferent fibers)

2 - has two divisions: sympathetic & parasympathetic

o sympathetic neurons leave the central nervous system through spinal nerves in the

thoracic & lumbar regions of the spinal cord

o parasympathetic neurons leave the central nervous system through cranial nerves

plus spinal nerves in the sacral region of the spinal cord

Used with permission of John W. Kimball

3 - impulses always travel along two neurons: preganglionic & postganglionic

o sympathetic division - preganglionic neurons are relatively short & postganglionic

neurons are relatively long

o parasypathetic division - preganglionic neurons are relatively long &

postganglionic neurons are very short

4 - Chemical transmitters - all autonomic neurons are either cholinergic or adrenergic

o cholinergic neurons - use acetylcholine as a neurotransmitter

includes all preganglionic neurons (both sympathetic & parasympathetic

divisions), all parasympathetic postganglionic neurons, plus the

sympathetic postganglionic neurons that supply the sweat glands

o adrenergic neurons - used norepinephrine (also called noradrenalin) as a

neurotransmitter

includes all postganglionic sympathetic neurons (except those that go to

the sweat glands)

5 - Functions of the Autonomic Nervous System:

o sympathetic division - prepares the body for strenuous physical activity in

stressful situations. This response is often referred to as the 'fight-or-flight'

response because the sympathetic division prepares the body to fight against or

flee from a threat.

o parasympathetic division - regulates important body functions such as digestion &

'slows down' the body after a 'flight-or-flight' response ('rest & digest')

6 - Control of Autonomic Nervous System - primary control center is the hypothalamus

Source: http://www.mmi.mcgill.ca/Unit2/Mandl/lect20autonomicnervoussystem.htm

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

Back to BIO 301 syllabus