BPCC/NSU Title III Cooperative Project
Nervous System

Nervous System

The human body maintains homeostasis (maintenance of constant internal environment ), effective function, and coordination of trillions of cells by using the endocrine system and nervous system.  The Nervous System employs electrical and chemical means to send messages very quickly from cell to cell.  

Nervous system has two major anatomical subdivisions:

1.       The Central Nervous System (CNS) consists of the brain and spinal cord, which are enclosed and protected by the cranium and vertebral column.

2.       The Peripheral Nervous System (PNS) consists of the remainder of the nervous system such as nerves, ganglia, and sensory organs.

The peripheral nervous system is divided into sensory and motor divisions, and each of these is further divided into somatic division (pertaining to the body as a whole, e.g., skin, bones, and muscle)

and visceral division (pertaining to the inner or deeper layer) divisions.

Sensory neurons are nerve cells (neurons) that respond to a stimulus and conduct signals to the central nervous system.

Motor neurons are neurons that transmit signals from the CNS to any muscle or gland cells.

SUBDIVISIONS OF NERVOUS SYSTEM

The Autonomic Nervous System

The visceral motor division, also known as autonomic nervous system, carries signals to glands, cardiac muscle, and smooth muscle. The autonomic system has two divisions:

1. The sympathetic division tends to arouse the body for action by accelerating the heart beat and increasing respiratory airflow and inhibiting digestion.

2. The parasympathetic division is responsible for energy intake and conservation by stimulating digestion and slowing down the heartbeat and reducing respiratory airflow.

Structure of a Neuron:

1.  The control center of neuron is its soma or cell body.

2.  The dense mesh of microtubules and neurofibrils (actin filaments) that compartmentalize the rough ER are called Nissl bodies.

3.  The primary site for receiving signals are called dendrites.

4.  The cylindrical nerve fiber originating from soma responsible for rapid conduction of nerve signals to other cells is called the axon.

5.  The small swellings that form a junction (synapse) with a muscle cell or another neuron are called synaptic knobs.

Classification of Neurons

Neurons are classified structurally according to the number and arrangement of processes extending from the soma.

NEURONAL VARIETY

Supporting cells (neuroglia)

Neuroglial cells protect the neurons and aid their function.

Types of Neuroglia (Glial Cells)

CNS

•Oligodendrocyte

•Ependymal cells

•Microglia cells

•Astrocytes

PNS

•Schwann cells (neurilemmocytes)

•Satellite cells

Myelin

Myelin Is an insulating layer around a nerve fiber, it is formed by oligodendrocytes in the CNS and Schwann cells in the PNS.

The speed at which a signal travels along a nerve fiber depends on two factors: the diameter of the fiber and the presence or absence of myelin. Myelination of nerve fibers speed up the conduction of the electrical signal on the axon of the nerve fibers.   Also, because signal conduction occurs along the surface of a nerve fiber not deep within its axoplasm, large fibers conduct signals more rapidly than small fibers.

What is a SYNAPSE?

The junction between a neuron and any other cell is termed a synapse.  In the cerebral cortex, the main information–processing tissue of the brain, there are as many as 100 trillion synapses. Simply, the more synaptic contact a neuron has the greater the amount of information it is able to process at any given time.  There two different types of synapses: Chemical synapses and electrical synapses. In a chemical synapse, the presynaptic neuron releases a neurotransmitter to stimulate the post synaptic cell. In the neuromuscular junction, the synapse between a neuron and skeletal muscle fiber, the presynaptic neuron releases acetylcholine as a neurotransmitter, which reacts with the post synaptic muscle fiber resulting in muscle contraction. In an electrical synapse, adjacent cells are joined by gap junctions. Instead of releasing a neurotransmitter to activate the post synaptic cell,  ions diffuse from the presynaptic cell to the postsynaptic cell. This is a quick transmission of signal from one cell to the next.

Test Yourself

The Spinal Cord and Brain

The spinal cord serves as a pathway for messages between the brain and the rest of the body. The spinal cord is the information highway that connects the brain with the lower body. It contains the neural routes that explain why a lesion to a specific part of the brain results in a functional loss in a specific locality in the lower body. It is a cylinder of nervous tissue that arises from the brain stem at the foramen magnum of the skull. It passes down the vertebral canal to the inferior margin of the first lumber vertebra (L1). The spinal cord gives rise to 31 pairs of spinal nerves. 

Meninges of the spinal cord

The spinal cord and brain are enclosed in three connective tissue membranes called meninges.

1. The dura mater is a tough collagenous membrane surrounding the spinal cord. The space between the dura mater and vertebral bone is called the epidural space, which contains blood vessels, adipose tissue, and loose connective tissue
2. The arachnoid  mater, also called the arachnoid membrane, consists of simple squamous epithelium. The space between the arachnoid membrane and the third layer is filled with cerebrospinal fluid (CSF).
3. The pia mater is a delicate translucent membrane. It forms part of the coccygeal ligament that anchors the spinal cord and forms the denticulate ligaments which prevent spinal cord lateral movement.

Gray Matter and White Matter

The spinal cord consists of two kinds of nervous tissue called gray and white matter. The spinal cord has a central core of gray matter which consists of two dorsal horns and two ventral horns and two lateral horns. The horns contain neuoron somas, dendrites, and proximal axons, but little meylin. The white matter of the spinal cord surrounds the gray matter and containes myelinated axons arranged in bundles termed tracts.

Spinal Tracts

Ascending tracts carry sensory information up the spinal cord and descending tracts conduct motor impulses down the spinal cord. All the nerve fibers in a given tract have a similar origin, destination, and function. Many of the spinal tracts have their origin or destination in the brainstem.

The Spinal Nerves

Each spinal nerve has a dorsal (posterior) root that passes dorsally toward the back of the spinal cord and a ventral (anterior) root that passes ventrally toward the front of the spinal cord.

Each spinal nerve is composed of numerous nerve fibers (axons) bound together by connective tissues. Each nerve fiber is surrounded by a basal lamina and connective tissue called endoneurium. The nerve fibers are in bundles and are wrapped in a sheath called the perineurium. The perineurium is composed of up to 20 layers of squamous epithelial-like cells. Several nerve bundles (fascicles) are then bundled together and wrapped in a dense irregular fibrous connective tissue called epineurium.

Ganglia

A ganglion is a cluster of cell bodies (somas) outside the CNS. It is enveloped in an epineurium.  Among the somas are bundles of nerve fibers leading into and out of the ganglion

Spinal Nerves

There are 31 pairs of spinal nerves:

8 cervical (C1 - C8)

12 thoracic (T1 - T12)

5 lumbar (L1 - L5)

5 sacral (S1 - S5)

1 coccygeal (Co1)

Cutaneous Innervation and Dermatomes

Each spinal nerve except C1 receives sensory input from a specific area of skin called a dermatome as demonstrated below. Spinal nerve damage is assessed by testing the dermatomes with pinpricks and noting areas in which the patient has no sensation.

Plexuses

There are four major spinal nerve plexuses: the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus.

Cervical Plexus

The cervical plexus is formed by the ventral rami (branches) of C1–C4. Most of these rami are branches of the cutaneous nerves of the neck, ear, back of head and shoulders.

It also contains the major motor and sensory nerves of the diaphragm.

Brachial Plexus

The brachial plexus is formed by C6 - C8 and T1, although C4 and T2 may also contribute. This plexus gives rise to nerves that innervate the upper limbs.

Lumbar Plexus

The lumbar plexus arises from L1 - L4 and innervates the thigh, the abdominal wall and psoas muscles. It also forms the femoral nerve and obturator nerve.

Sacral Plexus

The sacral plexus arises from L4 - S4 and serves the buttock, lower limb, pelvis structures and the perineum. Its major nerve is the sciatic nerve, the longest and thickest nerve of the body.

Reflexes

• require stimulation
• are quick
• are involuntary
• are stereotyped

Visceral Reflexes

• responses of glands, cardiac muscle, and smooth muscle
• controlled by the autonomic nervous system

Somatic Reflexes

• responses of skeletal muscle
• controlled by the somatic nervous system

Reflex Arc

Identify on the spinal cord the various spinal nerve plexus

Test Yourself

Brain and Cranial Nerves

 The human brain is the most sophisticated brain compared to other species.

Gray and White Matter

Cortex

Superficial layer

Composed of gray matter - site of neuron cell bodies, dendrites and synapses

Medulla

Deep to cortex

Composed of white matter - composed primarily of tracts or bundles of myelinated axons, which connect one part of the brain to another and to the spinal cord

Meninges

Layers of connective tissue for protection of the brain

Ventricles

Several large 'spaces' filled with cerebrospinal fluid (CSF). CSF forms a liquid cushion in and around the brain and spinal cord that is constantly drained and replaced.

Cerebrospinal Fluid  

 Blood-Brain Barrier

Seals nearly all of the blood capillaries throughout the brain tissue.

Important protective device

Highly permeable to water, glucose, lipid-soluble substances such as oxygen and carbon dioxide, and drugs such as alcohol, caffeine, nicotine, and anesthetics.

Place the following components in order from internal to external layers of the brain

Medulla Oblongata

• corticospinal tracts descend from the cerebral cortex carrying motor signals to the spinal cord
• reticular formation begins in the upper spinal cord and ascends through the medulla, pons and midbrain
• cranial nerves IX, X, part of I, XII begin or end in the medulla oblongata

Pons

• cranial nerves V through VIII begin or end in the pons
• part of the fourth ventricle bordered the pons

Midbrain

• central gray matter surround the aqueduct and functions with the reticular formation
• substantia nigra - motor center that relays inhibitory signals to the thalamus and basal nuclei
• cranial nerves III and IV originate in the midbrain

Diencephalon - thalamus, hypothalamus and epithalamus. All surround the third ventricle of the brain

Thalamus

• gateway to the cerebral cortex - the relay station
• key role in motor control
• nearly all sensory input and other going to the cerebrum passes by way of synapses in the thalamic nucleus

Hypothalamus

• major control center of the autonomic nervous system and endocrine systems
• plays an essential role in the homeostatic regulation of nearly all organs of the body

Epithalamus

• consists mainly of the pineal gland which regulates sleep/wake cycle

Reticular Formation consists of more than 100 small neural networks which include

1. somatic motor control
2. cardiovascular control
3. pain modulation
4. sleep and consciousness
5. habituation

Cerebellum - largest part of the hindbrain and second largest part of the brain as a whole

The Cerebrum

Largest and most conspicuous part of the human brain.

Cerebral medulla

Most of cerebrum is white matter.

Consists of nerve fibers conducting signals from one region of the cerebrum to another and between the cerebrum and lower brain centers.

Cerebral cortex

Neural integration is carried out in the gray matter of the cerebrum.

Basal nuclei

The basal nuclei are clusters of gray matter found deep within the cerebral medulla

The three pair of basal nuclei are

• caudate nucleus
• putamen
• globus pallidus

Limbic system

Series of interconnected rings that are an important center of emotion and learning.

Higher Forebrain Functions

Primary cortex

regions that receive input directly from the sense organs or brainstem, or issue motor commands directly to the brainstem for distribution to the cranial and spinal nerves.

Primary Somesthetic (Somatosensory) Cortex

Located on the anterior edge of the parietal lobe

Functions in the interpretation of the sensation of touch from areas of the body

Primary Motor Cortex

Located on the posterior edge of the frontal lobe

Functions in the initiation of skeletal muscle contractions

This is where we plan our behavior

Association cortex

Consists of all regions other than the primary cortex, involved in integrative functions.

75% of the mass of the cerebral cortex is association cortex.

Motor Control

Upper motor neurons

These tract decussate (cross from one side of the body to the other) in the pyramids of the medulla oblongata and then continue into the spinal cord.

Lower motor neurons

The fibers from the upper motor neurons synapse with the lower motor neurons whose axons innervate the skeletal muscle.

Other areas of the brain important for muscle control

Basal nuclei

The basal nuclei assume control of highly practiced behaviors that one carries out with little thought - writing, typing, driving a car, etc.

Cerebellum

Highly important in motor coordination

Aids in learning motor skills

Maintains muscle tone and posture as well as smooth muscle contractions

Coordinates eye and body movements

Coordinates the motions of different joints with each other

Language

Includes several abilities - reading, writing, speaking and understanding spoken and printed words

Assigned to different regions of the cerebral cortex

 Emotion

Result from an interaction between areas of the prefrontal cortex and the diencephalon

The hypothalamus and amygdala play especially important roles in emotion

Cognition

Acquisition and use of knowledge - sensory perception, thought, reasoning, judgment, memory, imagination, and intuition.

The prefrontal cortex is concerned with many of our most distinctive abilities, such as abstract thought, foresight, judgment, responsibility, a sense of purpose and a sense of socially appropriate behavior.

Cerebral lateralization

• in women, the left temporal lobe is longer than the right
• In left-handed people, the left frontal, parietal and occipital lobes are usually wider than those on the right
• the two hemispheres are also different in some of their functions
• Neither hemisphere is "dominant" but each is specialized for certain tasks

Are You Right or Left Brained? Take the Quiz.

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The Cranial Nerves

Scenario and Activities

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CRANIAL NERVES

SPINAL NERVES