Major Divisions in the Central Nervous System

SPINAL- large mixed nerve• White Matter- outer portion of the

mylenated fibers running up and down.

• Grey Matter- cell bodies of associative and motor neurons.

– 31 pairs of spinal nerves- sensory fibers enter dorsal root (surface)- than into gray matter.

– Destruction of dorsal root= loss of sensation in parts which supply sensory neurons.

– Destruction of ventral roots- muscular paralysis of body part supplied by motor neuron.

– Two main functions of spinal cord:

a) Coordinating center- reflexes

b) Connects peripheral nervous system to brain

BRAIN- meninges, cerebralspinal fluid (lymph)

• 4 Ventricles- cavities; fluid circulates• 12 pairs of cranial nerves• 2 large hemispheres (longitudinal

halves)• Divided into three region: Forebrain,

midbrain, hindbrain. • Meninges- protective membrane

a) 3 membranes- Durameter, Arachnoid layer, Piameter

b) Protect brain and spinal cord

STRUCURAL DIVISIONS OF THE HUMAN BRAIN

HINDBRAIN• MEDULLA- essential to life (vital center)

– Controls breathing rate (vagus, phrenic nerve)

– Controls heart rate (vagus-acetylcholin-slower rate; accelerator- adrenalin)

– Blood Pressure (diameter of arterioles)– ½ of the cranial nerves originate here– Swallowing, vomiting, coughing, salivation

center– Reticular activating system- nerves to

activate or awaken forebrain. Forebrain cannot react unless awakened by reticular formation. Destruction leads to death or coma. More extensive than inhibitory center

– Reticular inhibitory system- inhibition of voluntary movements (movements induced by cerebral cortex, reflexes, depressions of respiration and blood pressure.)

CEREBELUM- The Little Brain• Largest Segment of the hindbrain• Receives impulses from proprioreceptors in

muscles, tendons, joints- coordinates muscle activity )impules away from motor cortex of cerebrum)

• Coordinates impulses from the eyes, semicircular canals to maintain balance

*In birds- large, intricate, used for balance in flight

Pons- “bridge”

• *a) relay station to carry impulses from one side of the cerebellum to another

• b) coordinates left side of brain with right

• c) some cranial nerves originate here

Midbrain

• 1. In man - smaller and inconspicuous

• ( optic lobes of fish, frogs, reptiles, birds)

• 2. Contains all afferent and efferent nerve fibers that pass between spinal cord and higher brain centers

• 3. Contains all nuclei involved in many autonomic reflexes

Forebrain

• Consists of the hypothalamus, pituitary, thalamus, cerebrum.

Hypothalamus: Homeostatic Center

• homeostatic center - receives impulses from cerebral cortex• a) regulates body temperature ( dilate or constrict skin arterioles)• *b) regulates water balance - sensitive to osmotic pressure of blood -

stimulates ADH secretion • - regulates many autonomic functions• c) regulates food intake - feeding and satisfy center• d) regulates gastric secretion - if overactive leads to ulceration-hemorrhage• e) regulates adenohypophysis - releasing factor• ACTH - RF - hormonal (cortex) / neural adrenal medula• TSH - RF - fight-or-flight• Prolactin - RF• LH - RF• FSH - RF• STH - RF• *f) integration center for sex drive, anger, thirst, pleasure• *g) produces posterior pituitary hormone ( ADH and oxytocin)

Thalamus – Main Relay Center

• a) main relay center between brainstem (midbrain, pons, medulla, and cerebrum)

• b) thalamus and cerebral cortex act as a unit ( many functions cannot be separated)

• c) pain center (from free nerve endings)• *d) relay station between sense organs and cerebrum -

restricts many impulses (during sleep) that normally reach cortex

• Memory - may be stored by chemical and structural changes in neurons of sensory cortex

• Emotions - may originate from interactions between cerebral cortex and limbic system

CEREBRUM- sets man apart from other vertebrates. Volume is approx 1400g

• Part of learning, reasoning, memory, judgment (10-14 billion associative neurons)

• Speech and writing area- hearing center, visual center, associative area.

• Central Sulcus (fissure of Rolando)- bewtween frontal and parietal lobesa) Broca’s Area

Frontal- voluntary motor responses- motor control of speech

Parietal- Sensory cortex- recieves impulses from sensory neurons for touch, taste.

• Left Frontal- Broca’s Area (motor)• Left Parietal- Wernicke’s area (comprehension of spoken and

written language) 2 areas joined by nerve bundle.• Right Frontal and Parietal- associated with music and spatial

relations– Temporal Lobe- delineated by lateral sulcus (Fissure of

Sylvius). Auditory (taste, smell, hearing, speech).– Occipital Lobe- visual cortex. Neurons are specific to one

aspect of the shape of an object.– Left Brain- Speech, logic, writing, math.– Right Brain- discrimination of shape and form.

Autonomic Nervous System

• The autonomic nervous system is a functional division of the peripheral nervous system. It consists of motor neurons which control involuntary organs (heart, smooth muscles of viscera - digestive, respiratory, excretory, circulatory organs)

• Purpose: To maintain homeostasis - constant internal environment

DivisionsSYMPATHETIC DIVISION: neurons which enable the body

to react to stress of emergencies (“fight-or-flight”)PARASYMPATHETIC DIVISION: neurons which return the

body to basic “non-emergency” level

1. Heart: neurons secrete adrenalin at pacemaker.Increase heart rate

1. Heart: vagus nerve releases more *acetylcholine (main neurotransmitter of parasympathetic system).

Decreases heart rate.

2. Liver: glycogen is broken down to glucose and glucose is released into blood.

Increase blood glucose.

2. Does not affect the liver.

3. Lungs: dilation of bronchioles.Increases oxygen intake and carbon dioxide output.

3. Lungs: constriction of bronchioles.Decreases oxygen intake and carbon dioxide output.

4. Digestive Organs: Decreased peristalsis.Slows digestive process.

4. Digestive Organs: increased peristalsis.Increases digestive process.

5. Skeletal Muscles: dilation of arterioles.Increases blood supply.

5. Skeletal Muscles: constriction of arterioles. Decreases blood supply.

6. Breathing Rate: increases number of impulses to rib muscles and diaphragm.

6. Breathing Rate: decreases number of impulses to rib muscles and diaphragm.

Decreases breathing rate.

7. Kidney: decreases urine volume 7. Does not affect the kidney.

Summary: Visual

Peripheral Nervous System: PNS

• The peripheral nervous system is composed of the cranial and spinal nerves. (The autonomic fibers, sometimes classified as a separate cranial nerves connected with the brain. They are structurally different form the spinal nerves, of which there are 31 pairs. Some of the cranial nerves are entirely sensory (afferent), some of which there are 31 pairs. Some of the cranial nerves are entirely sensory (afferent), some are wholly motor (efferent); the remainder contain both sensory and motor fibers and are known as mixed nerves. Although the number differs, the cranial nerves are sufficiently similar in fish, amphibians, reptiles, birds, and man to have the same names. The names are based partly on the system innervated and partly on the structure and path of the nerves.

Cranial NervesI. Olfactory nerve:* Sensory; transmits impulses arising in receptor cells of the nasal

cavity.

II. Optic nerve:* Sensory; transmits impulses arising in the retina.

III. Oculomotor:* Motor; innervates 4 of the 6 muscles of the eye, and the ciliary muscle of the eye. Eye movement.

IV. Trochlear:* Motor; innervates 1 of 6 muscles in the eye. Eye movement.

V. Trigeminal: Mixed; both sensory and motor. Sensory fibers innervate front part of scalp, upper and lower jaw (including teeth), and tongue. Motor fibers innervate muscles used in chewing.

VI. Abducens:* Motor; innervates one of the 6 eye muscles. Eye movement.

VII. Facial nerve:* Mixed; innervates *muscles of the face and scalp, outer ear, and neck. Stimulates secretion of *parotid gland. Sense of *taste in first 2/3 of tongue. Facial expressions.

VIII. Auditory: (acoustic)* Sensory; transmits impulses from inner ear.

IX. Glossopharyngeal: Mixed; sense of taste for back 1/3 of tongue, sense of touch and temperature in palate, tonsils, and pharynx. Moves muscles in the pharynx.

X. Vagus:*-heart rate-breathing rate-receives impulses from alimentary canal

Mixed; movement of muscles in larynx, and pharynx. Movement of muscles of heart, bronchi, esophagus, stomach, pancreas, gall bladder, small intestine, fist 1/3 of colon. Stimulate gastric and pancreatic secretions.

XI. Accessory: Motor; accessory to vagus. Innervates breathing organs, pharynx, digestive tract.

XII. Hypoglossal:* Motor; innervates muscles of tongue.

Nerves Cells and Conduction• Neuron – nerve cell; basic unit of structure and function• Neuron Parts:

a. Dendrites – branched extensions that carry signal towards cell body

b. Cell body (soma or cyton) - contains the nucleus and other organelles

c. Axon - convey outgoing messages from the neuron to other cells (longer than dendrites)

• The functional kinds of neurons within the reflex arc:a. Sensory (afferent) – receptor neurons – carry impulses to the brain and spinal cord (CNS)b. Interneuron (associative) – connector neurons – found within the CNS; transmit signals between sensory, motor, and other interneuronsc. Motor (efferent) – effector neurons – carry impulses from CNS to effector

More Nerve Cells

Nerve: Organ composed of-• Hundreds or thousands of neurons• Other tissues – epithelium, connective, blood vesselsTypes of nerves (3)• Sensory Nerve – consists of sensory neurons (ex:

Auditory or optic nerve conducts to the CNS)• Motor nerve – consists of all motor neurons (Signal

from CNS conducts to effector muscle or gland)• Mixed nerve – consists of both sensory and motor

neuron bundles – neurons separated by connective tissues (ex: All 31 pairs of spinal nerves)

Look at Your Diagram• Motor Neuron – Longitudinal View (Numbers correspond with additional diagram)

1. dendrites – transmit impulse towards cell body

2. nucleus – essential to cell functions

3. cyton – synthesizes the neurohumor or neurotransmitter (ex: adrenaline or acetylcholine)

4. neurofibrils – protein tubules which carry impulses throughout cell

5. schwann cell – cell around axon – membrane (neurilemma) essential to regeneration of neuron

6. myelin sheath – lipid layer around axon; an insulator and increases rate of conduction

7. axis cylinder – composed of neurofibrils – carry impulse throughout neuron

8. nodes of ranvier – gaps between successive schwann cells

9. motor end plate – (axon terminals) site where neurotransmitters are stored and released through the synapse to an effector

10. axon – carry impulses away from the cell body to the synapse or to the effector

Stimulus - Impulses• Stimulus – environmental change which causes a response;

usually a form of energy (ex: radiant, electrical, pressure, sound, chemical)

• Impulse – electrochemical change along a neuron• Threshold level stimulus – minimum strength needed to initiate a

nerve impulse (different for different neurons and individuals)• Nerve impulse – “all or none”• Once the impulse is initiated by the threshold stimulus, the neuron

responds 100%; impulse rate is steady and doesn’t vary in strength

• Signal conduction stages:– Before impulse –

– nonconducting (polarized membrane) – “resting potential”– Sodium pump uses ATP to maintain high Na+ outside, and K+ inside, making

the inside more negative in charge

Continued

– Threshold Stimulus causes action potential – disrupts the semipermeability of the membrane

• Impulse – wave of reverse polarity along the neuron

• Na+ rushes in during conduction while Cl- and K+ leave

– After conduction – » the sodium-potassium pump shuttles sodium ions out and

potassium ions in, re-establishing the resting potential » refractory period – lapse of time required for neuron to

restore original charges (1/250sec to 1/2500sec in humans, depending on cell)

» cell is temporarily insensitive to stimulation

Conduction – Impulses• Rate of conduction – 2 factors

– diameter of neuron – larger cell diameter, the faster the rate of conduction– myelin sheath – enhances rate

– non-myelinated cells – slowest conduction rate (sensory neurons)– myelinated cells (motor) – faster rate

• At axon terminal of neuron – axon stores and releases neurohumors into adjacent tissues:– May be an effector (muscle or gland) or an adjacent nerve cell (dendrites or cell body)– To release of neurohumors across synapse:

• 1. Influx of Ca2+ on presynaptic neuron • 2. Causes synaptic vesicles to fuse with the membrane of the presynaptic neuron. • 3. The vesicles release neurotransmitter molecules into the synaptic cleft. These molecules diffuse across the cleft and bind to the

receptors of ion channels embedded in the postsynaptic membrane. • 4. Neurotransmitter opens specific ion channels, causing a Na+ influx that depolarizes the postsynaptic membrane. • 5. The neurotransmitter molecules are quickly degraded by enzymes or are taken up by another neuron

– Synapse – neurotransmitter either activates or inhibits adjacent neuron here– excitatory postsynaptic potential (EPSP) - electrical change that is caused by the binding of neurotransmitter to the

receptor on postsynaptic cell (helps conduct signal to next neuron)– inhibitory postsynaptic potential (IPSP) – binding of neurotransmitter hyperpolarizes the membrane --(more

negative than resting potential) --making it more difficult for an action potential to be generated in postsynaptic cell• Summation of postsynaptic potentials – • Summation – occurs when synaptic terminals acts simultaneously, or in rapid succession, on the same postsynaptic cell• Types:

– temporal summation - when transmitters from the same or multiple cell stimulate the postsynaptic cell in rapid succession, helping cause another action potential

– spatial summation – when transmitters from different cells act on a postsynaptic cell at the same time, helping cause another action potential

• Major types of neurotransmitters – – Acetylcholine – motor neurons– Adrenaline (epinephrine) – motor, sensory, associative– Noradrenalin (norepinephrine) – motor, sensory, associative– Serotonin – associative neurons

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