Transcript Unit 4 – Coordination Reflex Arc

Unit IV: Coordination
Reflex 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
Central nervous
system (CNS)
Peripheral nervous
system (PNS)
Brain
Two major anatomical subdivisions
• Central nervous system (CNS)
Spinal
cord
Nerves
• Peripheral nervous system (PNS)
– consists of nerves and ganglia
Ganglia
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
Dendrites
Structural Classes of Neurons
Dendrites
Dendritic
process
• Multipolar neuron
Cell body
Axon
• Bipolar neuron
• Unipolar neuron
Synaptic
terminals
Dendrites
Initial
segment
Axon
Axon
Synaptic
terminals
– Peripheral fiber carries impulses
from source of sensation
– Central fiber carriers impulses to
spinal cord
• Anaxonic neuron
Axon
Synaptic
terminals
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
Astrocytes
Section of
spinal cord
Capillary
Oligodendrocyte
Ependymal cell
Unmyelinated
axon
Myelin
(cut)
Microglial cell
Neurons
Gray matter
Myelinated
axons
White matter
Nodes
Myelination
Demyelination
Hallmark 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
Regeneration of Nerve Fiber in PNS
Site of injury
Step 1: the axon
and myelin
degenerate and
fragment.
Step 2: The Schwann
cells proliferate and
macrophages remove
the debris distal to the
injury site.
Step 3: The axon
grows along the
path created by the
Schwann cells.
Step 4: As the axon
elongates, the Schwann
cells wrap around it.
Axon
Myelin
Macrophage
Regeneration Tube
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
6 Primary afferent
neuron stimulates
inhibitory interneuron
4 Primary afferent
neuron stimulates
alpha motor neuron
to extensor muscle
7 Interneuron inhibits
alpha motor neuron
to flexor muscle
5
3 Primary afferent
neuron excited
Alpha motor neuron
stimulates extensor
muscle to contract
2 Muscle spindle
stimulated
1 Extensor muscle
stretched
8 Flexor muscle
(antagonist) relaxes
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