Nervous System and the Somatic Senses

...that the human brain contains more than 100,000,000,000 (100 billion) neurons?...that nerve impulses can travel at 120

meters/second?...that Prozac acts by inhibiting reuptake of serotonin

in brain neurons?...that some neurons are over a meter long?...that the cerebellum (the region behind the

brainstem) contains half of the neurons in the brain?...that when areas of the brain are damaged other

areas can actually take over the job of the damaged area?

Nervous System FactsDid you know……

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The basic nerve cell

Neurons are electrically excitable cells that

function to process and transmit information. Neurons are the core components of the

brain, spinal cord, and peripheral nerves.

Neurons are typically composed of a

1. soma, or cell body, a 2. dendritic tree and an 3. axon. Neurons receive input on the

cell body and dendritic tree, and transmit output via the axon.

Neurons communicate via chemical and electrical

synapses. The fundamental process that triggers synaptic

transmission is the action potential, an electrical signal

that is generated by exploiting the electrically excitable membrane of the neuron.

Schwann cells and the Myelin sheath mainly provide myelin insulation to axons in the peripheral nervous system and increase impulse speed.

Node of Node of RanvierRanvier are are

the gaps the gaps (about 1 (about 1

micrometer micrometer in diameter) in diameter)

formed formed between between myelin myelin

sheath cells sheath cells along axons along axons

or nerve or nerve fibers. fibers.

Chemical synapses are specialized junctions through

which cells of the nervous system signal to one another

and to non-neuronal cells such as muscles or glands. A

chemical synapse between a motor neuron and a muscle

cell is called a neuromuscular junction.

Chemical synapses allow the neurons of the CNS to form

interconnected neural circuits. They are thus crucial to the biological processes that underlie perception and

thought. They also provide the means through which the

nervous system connects to and controls the other systems

of the body.

Events leading to the Conduction of a Nerve Impulse (Neuron membrane via ion channels)

1. Neuron membrane maintains resting potential ( + outside, - inside = polarized, No nerve impulse,)

2. Threshold stimulus is received. (Stimulus)

3. Na+ channels open (Rising Phase)

4. Na+ diffuses inward, depolarizing the membrane (Peak = inside less -, outside less +)

5. K+ channels in membrane open. (Falling Phase)

6. K+ ions diffuse outward, repolarizing the membrane. (Undershoot )

7. Resulting action potential causes bioelectric current that stimulates adjacent portions of the membrane.

8. A wave of action potentials travel the length of axon as a nerve impulse.

Action Potential Animation

Neurotransmitters are chemicals that are used to relay, amplify and modulate electrical signals between

a neuron and another cell. Some examples of neurotransmitter action:Acetylcholine - voluntary movement of the muscles

Norepinephrine - wakefulness or arousal Dopamine - voluntary movement and emotional arousal Serotonin - memory, emotions, wakefulness, sleep and

temperature regulation GABA (gamma aminobutyric acid) - motor behavior

Glycine - spinal reflexes and motor behaviour Neuromodulators - sensory transmission-especially pain

Epinephrine (or adrenaline) - is a "fight or flight" hormone which is released from the adrenal glands

whenever danger threatens. Oxytocin (Greek: "quick birth") In the brain, oxytocin is involved in social recognition and bonding, and might be

involved in the formation of trust between people. Endorphins work as "natural pain killers”.

Glucagon helps maintain the level of glucose in the blood

In the nervous system, afferent nerves (sensory

nerves) carry nerve impulses from receptors or sense organs toward

the central nervous system.

Efferent nerves (motor nerves) carry nerve

impulses away from the central nervous system to effectors such as muscles

or glands.

The cell body of the efferent neuron is found in

the central nervous system where it is

connected to a single, long axon and several

short dendrites projecting out of the cell body itself.

The structure of an afferent neuron contains a single long dendrite and a short

axon; the shape of the cell body of an afferent neuron is smooth and rounded.

The nervous system is divided into the 1. central nervous system (CNS) The CNS consists of the brain and spinal cord.

2. peripheral nervous system (PNS). The PNS consists of all other nerves and neurons that do not lie within the CNS. The large majority of what are commonly called nerves are considered to be PNS. The peripheral nervous system is divided into the

1. somatic nervous system and the 2. autonomic nervous system. The Autonomic is further divided

into the 1. Sympathetic, 2. Parasympathetic, and 3. Enteric Nervous systems

Organization of the nervous system

Peripheral Somatic

Autonomic Sympathetic

Parasympathetic

Enteric

Central

The brain has three main parts that interact with the nervous system: the cerebrum, the cerebellum, and the medulla.

Central Nervous System (brain and spinal Cord)

Examples of the cerebrum's tasks include high-order thinking and learning, while the cerebellum manages learned automatic bodily functions, including walking, jumping, and running. The medulla processes simple body functions, such as breathing and digestion.

The spine is the area where reflexes are made. Split-

second decisions do not go back to the brain and then

back to the organ or body part.

For instance, if a ball was thrown at an individual's head,

the reflex to move out of the way would come from the

spine, not the brain, increasing reaction time. The spine is also

the "highway" which passes orders from the brain to motor

nerves.

A diagram showing the CNS:1. Brain2. Central nervous system    (brain and spinal cord)3. Spinal cord

Development of the CNSIn the developing fetus, the CNS originates from the neural plate, a specialized region of the ectoderm, the most external of the three

embryonic layers. During embryonic development, the neural plate folds and forms the neural tube.

First, the whole neural tube will differentiate into its two major subdivisions: spinal cord (caudal) and brain (rostral/cephalic).

Consecutively, the brain will differentiate into the brainstem (pons, medulla oblongata, and midbrain) and prosencephalon (forebrain).

Later, the brainstem will subdivide into rhombencephalon and mesencephalon, and the prosencephalon into diencephalon and telencephalon.

***The rhombencephalon gives rise to the pons, the cerebellum and the medulla oblongata.

***The mesencephalon (midbrain) gives rise to the cerebral peduncle.

***The diencephalon give rise to the subthalamus, hypothalamus, thalamus, and epithalamus.

***Finally, the telencephalon (cerebrum) gives rise to the hippocampus and the neocortex.

The CNS is covered by the meninges. The brain is protected by the skull and the spinal cord by the vertebrae.

Rhombencephalon (pons, cerebellum and the medulla oblongata)

Pons- is part of the CNS, and relays sensory information between the cerebellum and cerebrum.Some theories suspect that it has a role in dreaming.

Cerebellum (“little brain") is a region that plays an important role in the integration of sensory perception and motor output. Because of this function, disorders in fine movement, equilibrium, posture, and motor learning are associated with the cerebellum.

Medulla oblongata -controls autonomic functions and relays nerve signals between the brain and spinal cord.The Medulla oblongata is responsible for controlling several major autonomic functions of the body:*respiration *blood pressure *heart rate *vomiting

Mesencephalon (midbrain, cerebral peduncle)The mesencephalon is considered part of the brain stem. Its substantia nigra (responsible for dopamine production in the brain, and

therefore plays a vital role in reward and addiction) is closely associated with motor system pathways.

Cerebral Peduncle -this area contains many nerve tracts conveying motor

information to and from the brain to the

rest of the body.

Diencephalon (subthalamus, hypothalamus, thalamus, epithalamus)

The Thalamus also plays an important role in regulating states of sleep and wakefulness and consciousness. A human with a severely damaged or severed thalamus suffers permanent coma.

Many different functions are linked to the system to which thalamic parts belong. The major function relates to sensory systems (except olfactory

function) auditory, somatic, gustatory and visual systems A major role of the thalamus is devoted to "motor" systems.

The hypothalamus links the nervous system to the endocrine system via the pituitary gland, also known as the "master gland," The major function is in

synthesizing and secreting neurohormones received from the pituitary gland.

Telencephalon Cerebrum (hippocampus and the neocortex)

Hippocampus- plays a part in memory and spatial navigation.

The neocortex is the top layer of the cerebral hemispheres. The neocortex is part of the

cerebral cortex. It is involved in higher functions such as

sensory perception, generation of motor

commands, spatial reasoning, conscious thought, and language. The neocortex consists of gray matter

surrounding the deeper white matter of the cerebrum.

The Telencephalon is the name for the large region of the brain (2 hemispheres- Corpus Callosum communicates b/ 2 hemispheres) referred to as the cerebrum.

In Alzheimer’s disease, the hippocampus becomes one of the first regions of the brain to suffer damage; memory problems and disorientation appear among the first symptoms. Damage to the hippocampus can also result from oxygen starvation (anoxia) and encephalitis.

Hippocampus

The Frontal lobes have been found to play a part in impulse control, judgment, language, memory, motor function, problem solving,

socialization and spontaneity. The Frontal lobes assist in planning, coordinating, controlling and executing behavior.

People that have damaged frontal lobes may experience problems with these aspects of cognitive function, being at times impulsive; impaired in their ability to plan and execute complex sequences of actions; perhaps persisting with one course of action or pattern of behavior when a change would be appropriate.

The Parietal lobe plays important roles in integrating sensory information from various parts of the body, and in the manipulation of objects. Portions of the parietal lobe are involved with visuospatial processing. Much less is known about this lobe than the other three in the cerebrum.

The Temporal lobes enclose the hippocampi and amygdala. The posterior and lateral parts of the temporal lobes are involved in high-

level auditory processing (auditory cortex)

The amygdala perform primary roles in the formation and storage of memories associated with emotional events. Research indicates that during fear conditioning, (including freezing or

immobility, tachycardia or rapid heartbeat, increased respiration, and stress-hormone

release) sensory stimuli reach the amygdalae.

Damage to the amygdala impairs

both the acquisition and

expression of fear conditioning.

amygdalae

The Occipital lobe is the visual processing center of the brain, containing most of the

anatomical region of the visual cortex.

Retinal sensors convey stimuli

through the optic tracts to visual

cortex. Each visual cortex receives raw sensory information from the outside half of the retina on the

same side of the head and from the inside half of the

retina on the other side of the head.

White matter is one of the two main solid components of the central nervous system.

It is composed of myelinated nerve cell processes, or axons, which connect various gray matter areas of the brain to each

other and carry nerve impulses between neurons.

Generally, white matter can be understood as the parts of the brain and spinal cord responsible for information transmission (axons); whereas, gray matter is mainly responsible for information processing (neuron bodies).

White matter forms the bulk of the deep parts of

the brain and the superficial parts of the

spinal cord.

Multiple Sclerosis is one of the most common diseases which affects white matter. In MS lesions, the myelin shield around the axons has been destroyed by inflammation.

Gray matter is a major component of the CNS, consisting of nerve cell bodies, capillaries, and short nerve cell extensions/processes (axons

and dendrites).Gray matter is composed of unmyelinated neurons as opposed to white matter (myelinated neurons).

The function of gray matter is to route sensory or motor stimulus to the CNS for creation of

response to stimulus through chemical synapse activity.

Spinal cord segmentsThe spinal cord is divided into 31 different segments, with motor

nerve roots exiting in the ventral aspects and sensory nerve roots entering in the dorsal aspects. The ventral and dorsal roots

later join to form paired spinal nerves, one on each side of the

spinal cord.There are 31 spinal cord segments:

8 cervical segments8 cervical segments Supply muscles and skin of neck, motor impulses to diaphragm, arm and hand.

12 thoracic segments12 thoracic segments Supply motor impulses to upper abdominal wall muscles and receive impulses from skin and abdomen

5 lumbar segments Muscles and skin of lower abdominal wall, genitalia, thighs, legs and feet (includes femoral and sciatic nerves)

5 sacral segments See lumbar

1 coccygeal segment See lumbar

After leaving the spinal cord, all nerves are considered part of the PNS.

Spinal cord segments – another look!

Peripheral nervous system The peripheral nervous system (or PNS) consists of the nerves and neurons that reside or extend outside the CNS to serve the limbs and organs. Unlike the central nervous system, however, the PNS is not protected by bone or the blood-brain barrier, leaving it exposed to

toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and

the autonomic nervous system.

Nerve Function

Olfactory Purely Sensory; carries signals to the brain related to olfaction

(smell).

Optic Purely Sensory; carries signals to the brain related to vision.

Occulomotor Mixed Nerves; chiefly motor nerves of the eye controlling not

only the eye muscles, but also the iris and ciliary muscles.

Trochlear Mixed Nerves; serves some somatic motor fibers.

Trigeminal

Purely Sensory; Conveys impulses from the skin of the scalp, upper eyelid, nose,

lacrimal gland (tear duct), teeth, cheeks, upper lip, tongue, and chin.

Abducens Mixed Nerves; primarily supplies somatic

motor fibers to lateral rectus muscle (an eye muscle).

Facial

Mixed Nerves; They are the chief motor nevers of the face. There are five major branches - temporal, zygomatic, buccal,

mandibular, and cervical.

Vestibulocochlear

(Auditory)

Purely Sensory; carries signals to the

brain related to hearing and equilibrium.

Glossopharyngeal

Mixed Nerves; The tongue and pharynx and contain sensory

receptors for taste and general touch

(touch/pressure/pain).

Vagus

Mixed Nerves; The smooth muscle of the

esophagus, trachea, the pharynx and the palate. It

is connected to the external ear, eardrum,

and the epiglotus.

Accessory

Mixed Nerves; The sternocleidomastoid muscle

as well as the trapezius muscle.

Hypoglossal

Mixed Nerves; motor nerves of the tongue.

The autonomic nervous system (ANS) is the part of the peripheral nervous system that controls homeostasis. It does so mostly by controlling cardiovascular, digestive and respiratory functions, salivation, perspiration, diameter of the pupils, and micturition - (the discharge of urine). Many of the activities of

the ANS are involuntary. However, breathing, for example, can be in part consciously controlled.

Zygote Media Group, www.3DScience.com

Sympathetic and parasympathetic divisions typically function in opposition to each other. But this

opposition is better termed complementary in nature rather than antagonistic.

For an analogy, one may think of the sympathetic division as the accelerator and the parasympathetic division as the brake. The

sympathetic division typically functions in actions requiring quick responses. The parasympathetic division functions with actions

that do not require immediate reaction. Consider sympathetic as "fight or flight" and parasympathetic as "rest and digest". The

purpose of these two complementary systems is to maintain homeostasis.

The parasympathetic system conserves energy as it slows

the heart rate, increases intestinal and gland activity,

and relaxes sphincter muscles in the gastrointestinal

tract.

The enteric nervous system (ENS) is the part of the nervous system that directly controls the gastrointestinal system. It has as many as one billion neurons. The ENS is

sometimes referred to as the “second brain”.

Through intestinal muscles, the motor

neurons control peristalsis and

churning of intestinal contents. The enteric nervous system also makes

use of the same neurotransmitters

as the CNS, such as acetylcholine, dopamine, and

serotonin.

There are several reasons why the enteric nervous system may be regarded as a second

brain. The enteric nervous system can

operate autonomously. It normally

communicates with the CNS through the

parasympathetic (via the vagus nerve) and sympathetic nervous

systems. Studies show that when the vagus nerve is severed, the

enteric nervous system continues to function.

The somatic nervous system is responsible

for coordinating the body's

movements, and also for

receiving external

stimuli. It is the system that regulates

activities that are under conscious control.

Neuromuscular junction 1. Axon 2. Synaptical junction

3. Muscle fiber 4. Myofibrils