Nervous SystemChapter 8

The nervous system is the body’s control and communication center.

It serves to organize incoming data into useful information that can be used for internal functions and, by interpreting external dangers and initiating movement, physical activities necessary for survival

Structure and function: The Neuron

Neuron: is designed to transmit various levels of information from one cell to another.

There are three main types of neuron:› Sensory neurons: Transmit impulses back

toward the brain and spinal cord› Motor neurons: Carry impulses away from

the brain and spinal cord to the muscles and glands of the body

› Associative neurons: Carry impulses from sensory neurons to motor neurons

Neurons

Parts of neurons:› Dendrites: receive information from another cell and transmit the message to the cell body.

› Axon: conducts messages away from the cell body.

Multipolar neurons: neurons with many dendrites and one axon. EX: brain and spinal cord

Bipolar neurons: neurons with only one dendrite and one axon. EX: found only in the inner ear, the olfactory area of the nose, and the retina of the eye

Unipolar neuron: Most sensory neurons are unipolar meaning that they have only one extension

The axon is covered by an insulating material called the neurilemma, also known as the myelin sheath.

It speeds up the electrical signal and prevents it from interacting with other signals.

Nodes of Ranvier: refers to the gaps in the myelin sheath.› These gaps allow ions to flow freely from

extracellular fluid to the axons. 

Neuroglial Cells

These cells do not transmit impulses like the neurons, but instead insulate, support, and protect the neurons.› Astrocyte – Serve as part of the blood brain

barrier, which helps to prevent certain substances from entering the brain.

› Oligodendroglia – Provide support by forming rigid connections between neurons, also produce myelin for insulation.

› Microglial – Protect the neurons through phagocytosis(destroying foreign materials)

Cell Membrane Potential

Nerve cells generate electrical impulses through a process called membrane excitability

The membrane that separates the cytoplasm on the inside of the cell from the extracellular fluids on the outside of the cell separates two areas of different chemical composition.

Synapse

Synapse: is an area between the terminal branches of an axon and the ends of a branched dendrite

Reflex Arcs

Reflex: an involuntary reaction to an external response is called a reflex.

The reflex arc has the following components:› A sensory receptor on the skin› An afferent neuron(Sensory)› Associated neurons within the spinal cord› An efferent neuron (motor)› An effector organ

Ex: Pg. 173

Divisions of the Nervous System

The nervous system of the human body is divided into two systems:

Central nervous system: composed of the brain and spinal cord.

Peripheral nervous system: composed of the nerves that link the various of the body to the CNS. Includes the cranial nerves.

The primary functions of the nervous system are:› Sensory: detects alterations of internal and

external stimuli› Integrative: sensory information is

analyzed and appropriate behaviors are selected in response

› Motor: the appropriate behaviors are implemented

Division of the Peripheral nervous system

The PNS can be divided into the somatic nervous system and the autonomic nervous system.

The SNS connects the CNS to skin and skeletal muscles via the cranial and spinal nerves, initiating voluntary responses.

The ANS connects the CNS to visceral organs via the cranial and spinal nerves, initiating involuntary responses.

A sympathetic response prepares the body to deal with emergencies through the expenditure of energy

Parasympathetic response restores homeostatic balance and conserves energy.

The Brain

Approximately 100 billion neurons that communicate with each other through electrochemical pulses.

These complex interactions are ultimately responsible for all human physical and mental function

Neurons are the same as other body cells except they are specialized to gather and evaluate information from the internal and external environment, and then coordinate a response to that information.

Meninges

Meninges: Three layers of protective tissue; cover the brain and spinal cord

Dura mater: outermost layer; composed of tough fibrous connective tissue

Arachnoid mater: thin weblike membrane lacking blood vessels

Pia mater: it contains blood vessels and nerves for the nourishment of the underlying neural tissue.

Meninges cont’d

Epidural space: a space between bone and dura.

Subarachnoid space: The space created between the arachnoid mater and the pia mater; contains CSF

Cerebral Cortex

Is a layer of neurons on the surface of the brain that is approximately 2-4mm thick.

These neurons are specialized functions, are distributed across layers in the cortex.

Cerebrum Represents that largest portion of the human

brain Divided into separate halves, or hemispheres,

by a prominent central groove called longitudinal fissure.

The transverse fissure separates the cerebrum from the cerebellum.

Left Hemisphere generally handles speech functions, whereas the Right Hemisphere deals with nonverbal, intuitive behaviors.

90% of the population the left hemisphere is the dominate hemisphere because it controls verbal, computational and analytical skills.

Cerebrum

The two hemispheres are connected by a thick bundle of commissural nerve fibers referred to as the corpus callosum and anterior and posterior commissures.

These structures allow for communication between the brain halves.

Refer to Pg. 178

Ventricles and Cerebrospinal Fluid

Ventricles: A series of interconnected canal and cavities within the brain.

The first two ventricles are referred to as the lateral ventricles(right and left)

The large ventricles, which are located in each cerebral hemisphere, connect to the smaller third ventricle, located between the halves of the thalamus, by way of interventricular foramen

Ventricles and Cerebrospinal Fluid cont’d

The third ventricle connects to the even smaller fourth ventricle, located by way of cerebral aqueduct.

The fourth ventricle is continuous with the central canal of the spinal cord.

The ventricles are filled with a clear, colorless fluid containing small amounts of protein, glucose, lactic acid, urea, and potassium, as well as a relatively large amount of sodium chloride.

The fluid, known as CSF, helps to support and cushion the brain and spinal cord, and stabilizes the ionic concentration of the CNS.

It also acts to filter the waste products of metabolism and other substances that diffuse into the brain from blood.

Basal Ganglia

Basal ganglia: collection of nuclei embedded deep within the white matter of the cerebral hemispheres.

They act with the cerebellum to modify movement from moment to moment.

Cerebellum

The second largest structure of the brain and is located posterior to the medulla oblongata and inferior to the cerebrum’s occipital lobe.

Separated from the cerebrum by the transverse fissure.

Acts to coordinate skeletal muscle movement by comprising input from the motor cortex of the frontal lobe with proprioceptive feedback from the extremities, and correcting any perceived problems.

Diencephalon

Located between the midbrain and the cerebrum.

Surrounds the third ventricle Primary structures include:

› Thalamus› Hypothalamus› Posterior pituitary gland› Pineal gland

Diencephalon cont’d

Thalamus: acts as a relay station to the cerebral cortex for all sensory data from the cerebellum, brainstem, spinal cord, and other parts of the cerebrum

Hypothalamus: collection of nuclei located just inferior to the thalamus. It regulates homeostasis of the body through coordination of activities of the ANS. Serves as a link between the endocrine system and the nervous system.

Mesencephalon (midbrain)

Section of the brainstem located between the diencephalon and the pons.

Brainstem

Connects the diencephalon to the spinal cord and consist of the following:› Medulla Oblongata:

connect the brain with the spinal cord. Five of the 12 cranial nerve nuclei Responsible for breathing rhythm, heart rate, and blood

pressure› Pons:

Connects the brain with the spinal cord and other brain parts.

Four of the 12 cranial nerves Works with the medulla for regulation of breathing

› Midbrain: Inferior to the thalamus Contains center for visual reflexes( movement of the head

and eyes)

Spinal Cord Column of nervous tissue that begins at the

level of the foramen magnum and terminates at the level of the first and second lumbar disks.

Protected by the same three meningeal coverings as the brain(dura, arachnoid, and pia mater)

Function is to conduct impulses and serve as a spinal reflex center

At the level of the first lumbar vertebra, a collection of spinal roots descends from the inferior spinal cord, resembling the hairs of a horse’s tail. These nerves are known collectively as the cauda equina.

Cranial Nerves

There are 12 pairs of cranial nerves Originate in the brainstem with the

exception of the 1st and 2nd

Roman numerals that indicate the order in which they arise from the brainstem(from the front to the back)

Cranial Nerves I-VI Olfactory (I) - (instrumental in the sense of smell) Optic (II) - (transmits visual information from the

retina to the brain) Ocularmotor (III) - (controls most of the eye's

movement, constriction of the pupil, and maintains an open eyelid)

Trochlear (IV) - (“somatic efferent” that innervates a single muscle: the superior oblique muscle of the eye)

Trigeminal (V) - (responsible for sensation in the face)Pg. 186

Abducens (VI) - (“somatic efferent” nerve that controls the movement of a single muscle, the lateral rectus muscle of the eye)

Cranial Nerves VII-XII Facial (VII) - muscles of facial expression,

conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity

Vestibulocochlear (VIII) – sound and equilibrium (balance) information from the inner ear to the brain

Glossopharyngeal (IX) – visceral sensory ears, pharynx, back one thirds of tongue

Vagus(X) - innervates the viscera, conveys sensory information about the state of the body's organs

Accessory (XI) – control the sternocleidomastoid and trapezius muscles of the neck

Hypoglossal (XII) - innervates the muscles of the tongue

Spinal Nerves

The spinal cord is made up of continuous nerve tracts and cell columns that can be divided into segments.

31 spinal nerve roots:› 8 cervical› 12 thoracic› 5 lumbar› 5 sacral› 1 coccygeal