Unit IV: CoordinationReflex Arc

Chapter 11 – pp 363-371

Chapter 12 – pp 413-421

Internal Coordination

• Endocrine and nervous system maintain internal coordination

– endocrine =

– nervous =

• Reflex Arc:

1. sense organs receive information

2. brain and spinal cord determine responses

3. brain and spinal cord issue commands to glands and muscles

Subdivisions of Nervous System

Two major anatomical subdivisions

• Central nervous system (CNS)

• Peripheral nervous system (PNS)

– consists of nerves and ganglia

Brain

Nerves

Ganglia

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Spinalcord

Neural Tissue

A nerve is a bundle of nerve fibers (axons) wrapped in fibrous connective tissue.

• emerge from the CNS to carry signals between organs

• Neurons and neuroglial cells

Functions of Neurons

1. Excitability (irritability)

– stimuli

2. Conductivity

– produce traveling electrical signals

3. Secretion

– at end of nerve fiber, a neurotransmitter is secreted

Structure of a Neuron

• Soma (cell body)

• Dendrites

• Axon (nerve fiber)− trigger zone

• Myelin Sheath– some nerve fibers– insulating layer (mostly lipid) – formed by neuroglia

A Representative Neuron

Functional Classes of Neurons

Cell body

Dendrites

Dendriticprocess

Axon

Synapticterminals

DendritesInitialsegment

Axon

Axon

Synapticterminals

Dendrites

Axon

Synapticterminals

Structural Classes of Neurons

• Multipolar neuron

• Bipolar neuron

• Unipolar neuron

– Peripheral fiber carries impulses from source of sensation

– Central fiber carriers impulses to spinal cord

• Anaxonic neuron

Neuroglial Cells

• Also known as Glial cells

• Outnumber neurons

• General functions:

− Protect

− Support, maintain physical structure of neural tissue

− Repair

− Maintain nutrient supply to neurons

Types of Neuroglial Cells

Occur in the Central Nervous System

• Oligodendrocytes

– form myelin sheaths in CNS

• Ependymal cells

– line cavities of brain and spinal cord

– produce and circulate CSF

• Microglia

– macrophages (WBC)

– Phagocytosis

– in areas of infection, trauma or stroke

Types of Neuroglial Cells

• Astrocytes– most abundant glial cells– cover brain surfaces– formation of blood-brain barrier– regulate composition of cerebrospinal fluid – remove neurotransmitters and potassium ions from ICF– repairing damaged tissue with scar tissue

Occur in the Peripheral Nervous System• Schwann cells

– form myelin sheath in PNS• Satellite cells

– surround somas of neurons in ganglia

Neuroglial Cells of CNS

Section ofspinal cord

Ependymal cell

Microglial cell

Neurons

Gray matter White matter

Myelinatedaxons

Astrocytes

OligodendrocyteCapillary

Axon

Myelin(cut)

Nodes

Unmyelinated axon

Myelination

DemyelinationHallmark of some neurodegenerative autoimmune diseases:

• Multiple Sclerosis– Demyelination in CNS– Own immune system attacks and damages myelin– Scars form in white matter of CNS– Cause unknown, no cure

• Cerebral Palsy– Damage to developing oligodendrocytes usually during infancy– Mutations, lack of oxygen, interruption of blood flow– Treatment of symptoms, no cure

• Leukodystrophies– Results from defect in the gene that controls the production of only

one component molecule of myelin– Affects growth and maintenance of white matter– Inherited, no cure

Nerve Signal

Depends on two factors:

• Presence/absence of myelin

• Diameter of fiber

- large/thicker fibers have more surface area for signals

• Functions

– fast signals employed when speedy responses are needed

– slow signals used when quick responses are not important

Axon MyelinStep 1: the axon and myelin degenerate andfragment.

Step 2: The Schwanncells proliferate and macrophages remove the debris distal to the injury site.

Macrophage

Regeneration Tube

Step 3: The axon grows along the path created by theSchwann cells.

Step 4: As the axon elongates, the Schwanncells wrap around it.

Site of injury

Regeneration of Nerve Fiber in PNS

Reflexes

Properties:

– Require stimulation

– Quick

– Involuntary

– Stereotyped

Reflex Arc:

Somatic receptors → afferent nerves → integrating center →

efferent nerves → skeletal muscles

The Stretch Reflex

• When a muscle is stretched, it contracts and maintains increased muscle tone (stretch reflex)– helps maintain equilibrium and posture– balances tension in extensors and flexors– mediated by the brain

• Reciprocal inhibition prevents muscles from working against each other

• Very sudden muscle stretch causes tendon reflex– knee-jerk (patellar) reflex is monosynaptic reflex

The Tendon Reflex

To brain

4

3

5

8

7

6

2

1

Primary afferentneuron stimulates alpha motor neuron to extensor muscle

Primary afferentneuron excited

Muscle spindlestimulated

Extensor musclestretched

Flexor muscle(antagonist) relaxes

Alpha motor neuronstimulates extensormuscle to contract

Interneuron inhibitsalpha motor neuron to flexor muscle

Primary afferentneuron stimulatesinhibitory interneuron

Flexor (Withdrawal) Reflexes

• Occurs during withdrawal of a limb from pain

• Contract flexor muscles

• Polysynaptic reflex arc

• Not just one sudden response as in stretch reflex

• Ipsilateral reflex arc

Crossed Extensor Reflexes

• Maintains balance by extending other limb

• Contract extensor muscles

• Contralateral reflex arcs explained by pain at one foot causes muscle contraction in other leg

Golgi Tendon Reflex

• Golgi tendon organs in a tendon near its junction with a muscle

• Excessive tension on tendon inhibits motor neuron– muscle contraction

decreased

• Also functions when some parts of muscle contracts more than others