CHAPTER 10

Central Nervous System, Spinal and Cranial Nerves

INTRODUCTIONThe Central Nervous System

(CNS) consists of the brain and spinal cord

The Peripheral Nervous system (PNS) consists of the spinal nerves and cranial nerves

I. SPINAL CORD STRUCTURE A. Protection and Coverings: Vertebral Canal and

Meninges I. The Spinal cord is located within the vertebral

cavity of the vertebral column. The cord is well protected by vertebral bones, ligaments, meninges, and cerebrospinal fluid. A. Meninges are 3 layers of CT coverings that extend around

the spinal cord and the brain. 1. Dura Mater (= tough mother) is the outer-most layer. It

is made of tough dense CT that protect the delicate structures of the CNS.

2. Arachnoid Mater (= spider –like) is the middle layer that resembles a spider’s web. It is made of collagen and elastic fibers.

3. Pia Mater (=delicate) is the inner-most layer. It adheres to the spinal cord and brain. It contains numerous blood vessels

B. The are 2 spaces between the meninges that help protect the spinal cord 1. Epidural space – Space between the vertebral

column and the dura mater. It contains a cushion of fat and CT.

2. Subarachnoid space – Space between the arachnoid space and the pia mater. It contains cerebrospinal fluid (CSF). In a spinal tap a local anesthetic is given and a long

needle is inserted into the subarachnoid space. This procedure is used to withdraw cerebrospinal fluid for diagnostic purposes; introduce antibiotics, anesthetics, chemotherapy; or measure CSF pressure and evaluate diseases such as meningitis.

SPINAL CORD AND MENINGES

C. ANATOMY OF THE SPINAL CORD 1. The spinal cord ranges from 42-45 cm (16-18

inches) in length. It extends from the lowest part of the brain (medulla oblongata) to the upper border of the second lumbar vertebra in the vertebral column.

2. The nerves at the end of the spinal cord angle down the vertebral cavity like wisps of flowing hair. They are named the cauda equina = horse’s tail.

3. There are two enlargements of the spinal cord . a. cervical enlargement – contains nerves that supply

the upper limbs b. lumbar enlargement – contains nerves that supply

the lower limbs

4. There are 31 spinal segments of the spinal cord that give rise to 31 pair of spinal nerves. 8 cervical, 12 Thoracic, 5 Lumbar, 5 sacral, and 1 coccygeal.

5. Two grooves divide the spinal cord into right and left halvesa. anterior median fissure – deep groove in

the frontb. posterior median sulcus – shallow groove in

the back of the spine.6. In the center of the spinal cord there is a

small space that extends the length of the cord called the central canal. It contains CSF.

SPINAL CORD AND SPINAL NERVES

D. INTERNAL STRUCTURE OF THE SPINAL CORD 1. The spinal cord consists of two types of

matter. a. Gray matter – The darker inner matter contains

unmyelinated axons. 3 horns – anterior, posterior, and lateral

Anterior – contain cell bodies of somatic motor neurons which provide nerve impulses that contract skeletal muscles.

Posterior – contain somatic and autonomic sensory neurons

Lateral – contain autonomic motor neurons that regulate activity of smooth muscle, cardiac muscle, and glands

b. White matter consists primarily of myelinated axons and also has 3 regions called anterior, lateral and posterior columns. Each column contains one or more tracts, which are distinct bundles of axons having a common origin or destination and carrying similar information. There are two types of tracts: Sensory (ascending ) tracts – axons that conduct

nerve impulses toward the brain Motor (descending) tracts – carry nerve

impulses down the spinal cord.

INTERNAL STRUCTURES OF THE SPINAL CORD

SPINAL NERVES E. Spinal Nerves are the paths of

communication between the spinal cord and the nerves that serve specific regions of the body. They belong to the peripheral nervous system (PNS) 1. Two bundles of axons, called roots, connect

each spinal nerve to a segment of the cord Posterior (dorsal)root – contains only sensory axons

and conduct impulses for sensations from the skin, muscles , and internal organs.

Anterior (ventral) root – contains somatic motor neurons that conduct impulses to the skeletal muscles and autonomic motor neurons that conduct impulses to smooth & cardiac muscle and also to glands.

2. The main function of the spinal nerves is to connect the CNS to sensory receptors, muscles, and glands in all parts of the body.

3. Spinal nerves leave the vertebral column by passing through the intervertebral foramina which are the holes between vertebrae.

Many of the spinal nerve branches do not extend directly to the body structures they supply. Instead, they form networks called a plexus. There are 4 major plexuses: Cervical - head, neck. Upper shoulders Brachial – upper limbs, neck, and shoulders Lumbar – abdominal wall, external genitals, and parts of

the lower limbs Sacral – buttocks and lower limbs

SPINAL CORD FUNCTIONS AND REFLEXES F. Functions of the white and gray matter

1. The white matter of the spinal cord consists of tracts that serve as highways for nerve impulse conduction.a. Sensory impulses travel toward the brain

b. Motor impulses travel from the brain toward skeletal muscles and other effector tissues such as glands.

c. The routes these impulses take are called a pathway

2. The gray matter of the spinal cord receives and integrates incoming and outgoing information and is a site for integration of reflexes. A reflex is a fast, involuntary sequence of actions

that occurs in response to a particular stimulus. Some relflexes are inborn and others are learned or acquired such as driving skills a. spinal reflex occurs in the spinal gray matter b. cranial reflex occurs in the brain stem c. examples: patellar reflex, withdrawal reflex

B. The pathway followed by nerve impulses that produce a reflex is known as a reflex arc. The basic components of an arc are: 1. Sensory receptor – respond to a stimulus 2. Sensory neuron – relay stimulus to gray matter 3. Integrating center – a region in the gray matter 4. Motor neuron – Impulse sent by the integrating

center to the part of the body that will respond 5. Effector – The part of the body that responds to

the motor neuron impulse usually a muscle or gland. Somatic effector is a skeletal muscle Autonomic (visceral) reflex – smooth or cardiac muscle, or

gland. Swallowing, urinating, and defecating are autonomic reflexes.

C. Damage or disease anywhere along a reflex arc can cause the reflex to be absent or abnormal which could indicate damage to the sensory or motor neurons or a spinal cord injury. The absence of the pupillary light reflex may indicate brain damage or injury.

CHECK POINT QUESTIONS 1-7 1. How is the spinal cord protected?

Vertebral ligaments, meninges, and cerebrospinal fluid along with the vertebral column.

2. What body regions are served by nerves from the cervical and lumbar enlargements? The Cervical Enlargement supplies the upper limbs

and the Lumbar Enlargement supplies the lower limbs

3. Distinguish between a horn and a column in the spinal cord. Horns are formed by gray matter and columns are

white matter. The horns are in the center of the spinal cord while the columns are to the outside of the spinal cord

4. How do spinal nerves connect to the spinal cord? Roots connect the spinal nerves to the spinal

cord

5. Which regions of the body are supplied by plexuses, and which are served by intercostal nerves? Plexuses supply Head, neck, shoulders, upper limbs,

abdominal wall, genitals, buttocks, lower limbs Intercostal supply muscles between ribs, abdominal

muscles and skin of the chest and back

6. What is the significance of the white matter tracts of the spinal cord? White matter tracts serve as highways for nerve

impulse conduction – sensory to the brain and motor toward muscles and other effectors.

7. How are somatic and autonomic reflexes similar and different. Both are reflexes and travel in an arc. Different in that somatic reflex has an effector that is a

skeletal muscle where the autonomic has an effector of smooth muscle, gland or cardiac muscle.

II. THE BRAIN A. Major Parts and Protective Coverings

1.The Brain is on of the largest organs of the body. It weighs almost 3 pounds.

2. The four major parts of the brain are:Brain stem – continuous with the spinal cord

Diencephalon – above the brain stemCerebrum – The main portion of the brainCerebellum – (little brain) posterior to the brain stem

3. The brain is protected by the cranium and the cranial meninges.Outermost - Dura materMiddle – arachnoid materInner-most – pia mater

4. Brain Blood Supply and the Blood-Brain Barrier a. If blood flow to the brain stops, even briefly,

unconsciousness may result. Brain neurons that are totally deprived of oxygen for four or more minutes may be permanently injured.

b. The main source of energy for brain cells is glucose. If blood entering the brain has a low level of glucose, mental confusion, dizziness, convulsions, and loss of consciousness may occur.

c. A Blood-brain barrier (BBB) protects brain cells from harmful substances and pathogens by preventing passage of many substances from blood into the brain tissue. Lipid-soluble substances such as oxygen, carbon dioxide, alcohol, and most anesthetic agents , easily cross the barrier. Trauma, some toxins, and inflammation can cause a breakdown of the blood-brain barrier.

5. Cerebrospinal Fluid a. Cerebrospinal Fluid (CSF) is a clear, colorless liquid that

carries oxygen, glucose, and other needed chemicals from the blood to neurons and neuroglia. It also removes wastes and toxic substances produced by brain and spinal cord cells.

b. CSF circulates through the subarachnoid space, around the brain and spinal cord, and through cavities called ventricles.

c. There are 4 ventricles: two lateral ventricles, one third ventricle, and one fourth ventricle. These are found around the midbrain area of the brain.

d. Choroid plexuses are the sites of CSF production. Normally the volume of CSF remains constant because it is absorbed as rapidly as it is produced. Hydrocephalus is a condition of elevated CSF pressure. In adults this may be caused by head injury, meningitis, or subarachnoid hemorrhage. Tumors, inflammation , or developmental malformation can also interfere with the drainage of CSF.

B. PARTS OF THE BRAIN 1. Brain Stem – located between the spinal

cord and the diencephalon. It consists of 3 regions a. Medulla oblongata – posterior part of the

brain stem. Many vital activities are controlled by the medulla: Cardiovascular center – regulates rate and force of the

heartbeat along with the diameter of the blood vessels.

Medullary rhythmicity area – adjusts the basic rhythm of breathing

Area associated with touch and the sensation of vibration

Reflex area for swallowing, vomiting, coughing, hiccupping, and sneezing

2. Pons – bridge or middle area of the brainstem. A bridge or connection for parts of the brain with

each other. 3. Midbrain – connects the pons to the

diencephalon Substantia nigra – area associated with Parkinson

disease Red nuclei – function with the cerebellum to

coordinate muscular movements Superior and Inferior colliculi – reflex areas that

are associated with the eyes, hearing, startle reflex and sudden head movements

4. Much of the brain stem consists of small clusters of neuronal cell bodies (gray matter) intermingled with small bundles of myelinated axons (white matter) . This region is known as the reticular formation which contains both ascending and descending tracts. The ascending tract is called the reticular

activating system (RAS). The RAS helps maintain consciousness and is active during awakening from sleep. Inactivation of the RAS produces sleep.

The descending tract is to help regulate muscle tone.

C. DIENCEPHALON 1. The Major regions of the diencephalon are

Thalamus – principal relay station for sensory impulses that reach the cerebral cortex from other parts of the brain and the spinal cord

Hypothalamus – small area of the diencephalon, but controls many important body activities that relate to homeostasis; Controls the autonomic nervous system- heart rate,

movement of food, urinary bladder Control the pituitary gland and production of hormones Regulates emotional and behavioral patterns Regulates thirst and hunger Controls body temperature Regulates circadian rhythms (sleep and wake cycle) and

consciousness

Pineal Gland – about the size of a small pea. The pineal gland secretes the hormone melatonin and is considered part of the endocrine system. Melatonin promotes sleepiness and contributes to the setting of the body’s biological clock

3. CEREBELLUM The cerebellum consists of two cerebellar

hemispheres. It is located at posterior to the medulla and pons and below the cerebrum. The arrangement of the white and gray matter of the cerebellum resembles the branches of a tree. The cerebellum helps to coordinate skeletal muscle contractions. It regulates posture and balance and is essential for all skilled motor activities, from catching a baseball to dancing. Damage to the cerebellum through trauma or disease disrupts muscle coordination. This condition is called ataxia.

4. CEREBRUM A. The cerebrum is the largest area of the

brain. It provides us with the ability to read, write, and speak; to make calculations and compose music; to remember the past and plan for the future; and to create. Parts of the cerebrum include: Cerebral cortex – outer rim of gray matter Gyri – folds of the cerebral cortex Fissures – deep grooves between gyri

Longitudinal fissure separates the left and right hemispheres of the cerebrum

Sulcus – shallow grooves between gyri Corpus callosum – white matter that connects

the two hemispheres.

B. Each hemisphere has four lobes which are named after the bones they cover:Frontal lobeParietal lobeTemporal lobeOccipital lobe

C. OTHER PARTS OF THE BRAIN 1. Limbic System – the limbic system is

sometimes called the “emotional brain” . It plays a primary role in emotions such as pain, pleasure, docility, affection, and anger. It also plays roles related to survival instinct and functions in memory.

2. Sensory areas Visual – located in the occipital lobe Auditory – located in the temporal lobe Gustatory – middle of cerebrum – taste Olfactory – Temporal area – smell

3. Motor areas Primary motor area – controls voluntary

contractions of specific muscles on the opposite side of the body. Found in the frontal lobe

Broca’s Speech area – Controls speaking and understanding language. It is also found in the frontal lobe

4. Association Areas Somatosensory association area – lets you recognize

objects by past sensual (taste, smell, touch) experiences

Visual association area – same as above, but for vision only

Auditory association area – same as above, but for sounds, music, or noise only.

5. Wernicke’s area – located in the left temporal and parietal lobes, interprets the meaning of speech by recognizing spoken words. It is active as you translate words into thoughts. Aphasia is the inability to use or comprehend

words. This may result to an injury to the Broca’s speech area or the Wernicke’s area.

D. MEMORY Memory is the process by which information

acquired through learning is stored and retrieved. For an experience to become part of memory, it must produce structural and functional changes in the brain.

E. Electroencephalogram (EEG) At any instant, brain neurons are generating millions

of nerve impulses. Taken together, these electrical signals are called brain waves. A record of such waves is called an electroencephalogram or EEG. These records are used to study normal brain functions such as sleep or diagnose a variety of brain disorders such as epilepsy, tumors, metabolic abnormalities, sites of trauma, and degenerative diseases.

III. CRANIAL NERVES There are 12 pair of cranial nerves which are

part of the PNS. They are designated with roman numerals and with names. The numbers indicate the order (anterior to posterior) in which they emerge from the brain. The name indicates the distribution or function. See Table 10.2 pg. 263 – 265 for names and functions.

IV. AGING AND THE NERVOUS SYSTEM The brain grows rapidly during the first few

years of life. From early adulthood onward, brain mass declines. By the time a person reaches age 80, the brain weighs about 7% less than it did in young adulthood. The number of synaptic contacts declines and the capacity for sending nerve impulses to and from the brain also declines. As a result, processing of information decrease, voluntary motor movements slow down, and reflex times slow.