nervous system part 1

The nervous system

The nervous system contains all neural tissue in the body and is compromised of two different cell types

1. Neurons

2. Neuroglia

There are also two main divisions of the nervous system

1. Central nervous system (CNS)

2. Peripheral nervous system (PNS)

Neurons

Cells that send and receive signals

Neuroglia

Cells that support and protect neurons

Central nervous system (CNS)

Contains the brain and spinal cord

Peripheral nervous system

Delivers sensory information to the CNS; transmits motor info from the CNS

The neuron

Can be split up into four general regions or areas

1. Cell body

2. Dendrites

3. Axons

4. Telodendria

Cell body

Contains the perikaryon (cytoplasm) and nucleus

Dendrites

Receive info from other neurons

Axon

Carries electrical signal (action potential) towards targets

Telodendria

Extensions of the axons that end in synaptic terminals

Four distinct neuron types

1. Anaxonic neuron

2. Bipolar neuron

3. Unipolar neuron

4. Multipolar neuron

The synapse

Where the action potential is converted into a chemical signal

When this involves a muscle, we call the synapse the neuromuscular junction

When a gland is involved, we might say the neuroglandular junction

Myelination

Increases the velocity of an action potential

• The most important Neuroglia are involved with myelination

Oligodendrocytes

Myelinate axons of the CNS

• without myelin, the action potential wouldn’t travel quick enough from one side of a neuron to the other side of the neuron, leading to a loss of signal

Shwann cells

Myelinate axons of the PNS

Action potential

The electrical propagation through a neuron in order to initiate a response

Two factors that action potentials use to work

1. Permeability

2. Electrochemical gradient

Two channels that action potentials use to work

1. Sodium voltage gated ion channel

2. Potassium voltage gated ion channel

Depolarization

Making the membrane potential less negative

Polarization

Making the membrane potential more negative

Action potential process

At rest, the membrane of a neuron isn’t permeable to sodium, and slightly permeable to potassium

When an electrical stimulation causes a threshold to be reached, first sodium voltage gated channels open, allowing an infux of sodium

After the peak of an action potential, sodium voltage gated channels open, causing a release of potassium

After a short afterhyperpolarization, the resting potential re-establishes

Absolute refractory period in action potential

An amount of time in which under no circumstances another action potential can be triggered

Relative refractory period in action potential

An amount of time in which an increased stimulus can trigger an action potential

When is an action potential termed an all or none phenomenon

1. If the threshold is reached, an action potential has to occur

2. If the threshold is not reached, an action potential never occurs

Saltatory propagation

The velocity of the action potential is quicker in myelinated segments, with restoration and slowing of the action potential in unmyelinated segments called the Nodes of Ranvier

Propagation along a myelinate nerve is often termed saltatory propagation

An action potential undergoing salutatory propagation

Action potential loss with demyelinating disease

Synapse

When the action potential reaches a synapse, the electrical signal is changed into a chemical signal with the help of vestibular transport. Chemicals are used in the synaptic cleft to introduce the desired effect on the target tissue.

Brain

Contains 97% of the body’s neural tissue in the adult; exerts centralized control of the other organs in the human body

Spinal cord

Continuation of neural tissue off of the medulla oblongata; transmits information to and from the brain; independently responsible for numerous reflexes

Begins past the medulla oblongata and tapers at the conus medulla ribs (L1 or L2)

Frontal lobe

Involves the ability to recognize future consequences resulting from current actions and helps with the determination of similarities and differences between things or events

Temporal lobe

Involves the retention of visual memories, the processing of sensory input, the comprehension of language, emotion, and the storage of new memories

Parietal lobe

Helps integrate sensory information from various parts of the body; involved with the knowledge of numbers and their relations

Occipital lobe

Primarily involved with processing visual information

Precentral gurus

Directs voluntary voluntary movement

Postcentral gurus

Receives somatic sensory information

Meninges

Membranes that surround the central nervous system

Dura matter

Thick membrane and outermost of the meninges

Arachnoid matter

Middle, web-like meningeal layer

Pia mater

The innermost and delicate meningeal layer; clings on to the brain and spinal cord

Dura folds

Brain ventricles

Create and help transport cerebrospinal fluid (CSF)

Cerebrospinal fluid

Clear, colorless, bodily fluid that helps protect the brain and the spinal cord mechanically and chemically; found in the subarachnoid space (between pia mater and arachnoid mater)

Arachnoid granulations

Projections of arachnoid mater that allow cerebrospinal fluid to enter the rural venous sinuses

Rural venous sinus

Channel found between layers of the dura mater that receive blood from the veins of the brain and cerebrospinal fluid from the subarachnoid space; empty into the internal jugular veins

Pituitary gland

Endocrine gland responsible for interfacing the nervous system and the endocrine system

Cauda equine

Bundle of spinal nerves that continue off of the conus medulla ribs

Filum terminate

Modification of pia mater that attaches the conus medullaris to the coccyx

Spinal cords seven branches

• 8 cervical spinal nerve pairs

• 12 thoracic spinal nerve pairs

• 5 lumbar nerve pairs

• 5 sacral nerve pairs

• 1 coccygeal nerve pairs

Efferent nerves

Those that carry motor information from the CNS

Automatic nervous system

• sympathetic division

• Parasympathetic division

Sympathetic division

Generally, causes stimulatory effects on the body

Parasympathetic division

Generally, causes relaxing effects on the body

• the cranial nerves are 12 pairs of nerves that arise from the inferior aspect of the brain

• The cranial nerves are numbered from anterior to posterior as they arise from the brain

• The functions of each cranial nerve varies considerably from nerve to nerve

(1) CN I - Olfactory nerve

Function to facilitate smell (sensory)

To move from the olfactory to epithelium, the nerves pass through the foramina of the cribriform plate and back towards the brain to convey sensory information

(2) CN II - Optic nerve

Facilities vision (sensory)

Nerve travels from the retina of each eye and through the optic canals of the skull in order to reach the optic chasm and optics tracts before finally reaching the brain

(3) CN III - Oculomotor nerve

Innervates muscles associated with eye movement, accommodation, and pupil constriction (motor)

• inferior rectus muscle

• Superior rectus muscle

• Medial rectus muscle

• Interior oblique muscle

• Ciliary muscle- controls accommodation

• Iris- responsible for controlling the diameter of the pupil

• Lavatory palpebrae superioris muscle- elevates the superior eyelid

(4) CN IV- trochlear nerve

Innervates the superior oblique muscle (motor)

(5) CN V- trigeminal nerve

Has both sensory and motor function (mixed)

There are three sensory branches of this nerve1

1. Opthalmic (V1)- conveys information from the scalp, forehead, upper eyelid, nasal mucosa, tip of the nose, and frontal sinus

2. Maxillary nerve (V2)- conveys information from the lower eyelid, cheek, upper teeth and gums, the palate, and the sphenoid, ethmoid, and maxillary sinuses

3. Mandibular (V3)- conveys information from the lower lip, lower teeth and gums, chin, jaw, and areas of the ear

(5) CN V- Trigeminal nerve pt.2

The motor aspect of the trigeminal nerve controls eight muscles

1. Tensor veil palatine muscle

2. Tensor tympani muscle

3. Mylohoid muscle

4. Anterior belly of the digastric

5. Temporalis muscle

6. Masseter muscle

7. Lateral pterygoid

8. Medial pterygoid

(6) CN VI- abducens nerve

Innervates the lateral rectus muscle (motor)

(7) CN VII- facial nerve

Carries both sensory and motor information (mixed) and leaves the cranium via the stylomastoid foramen

Carries sensory information concerning taste from the anterior 2/3 of the tongue, specifically from taste receptors which send information to the chorda tympani

The facial nerve sends motor (somatic) information to the muscles of the facial expression

The facial nerve sends motor (visceral) information to the sublingual and submandibular glands

(8) VIII- vestibulocochlear nerve

Made of a vestibular branch that conveys information about balance (sensory) and a cochlear branch that conveys information about hearing (sensory)

(9) CN IX- Glossopharyngeal nerve

Mixed nerve that involves the head and neck; travels through the jugular foramen

This nerve sends sensory information back to the brain, including information concerning taste from the posterior 1/3 of the tongue (lingual branch), information from the pharynx and palate, and information about blood pressure from the carotid arteries

Motor (somatic) information travels via pharyngeal branches to control muscles associated with the pharynx

Motor (visceral) information travels towards the parotid gland in order to stimulate salivation

(10) CN X- Vagus nerve

The longest of the cranial nerves which happens to be a mixed nerve; travels through the jugular foramen

Sends info to the brain concerning the pharynx and epiglottis, along with information concerning the visceral organs (sensory)

Additionally, the nerve transmits motor information to the palate and pharynx (swallowing and coughing) and the viscera

(11) CN XI- spinal cord accessory

A motor nerve; sends and internal branches towards voluntary muscles of the palate, pharynx, and larynx; sends an external branch to the sternocleidomastoid muscle and trapezius muscle; travels through the jugular foramen

(12) CN XII- hypoglossal nerve

A motor nerve associated with movement of the tongue; travels through the hypoglossal canal