CH 13: 10-04-13 (BIO 181) Flashcards
- Rapid, automatic nerve responses triggered by specific stimuli
- Controlled by spinal cord alone, not the brain
• Spinal Reflexes
- Ends between vertebrae L1 and L2
- Bilaterally symmetrical and is divided into left and right by grooves
- Posterior median sulcus: on posterior (dorsal) side
- Anterior median fissure: on anterior (ventral) side
- Enlargements of the Spinal Cord
- Are seen at regions that are involved with the sensory/motor control of the limbs
- These expanded regions have more gray matter per segment than other regions
- Cervical enlargement
- Provides innervation to shoulders and upper limbs
- Lumbar enlargement
- Provides innervation to pelvis and lower limbs
- Total of 31 Spinal Cord Segments
- Nomenclature is based on vertebrae where spinal nerves originate
Spinal Cord
- Specialized membranes that isolate spinal cord from surroundings and is continuous with cranial meninges
- Functions of the spinal meninges include
- Protect spinal cord
- Carry blood supply
Spinal Meninges
• Viral or bacterial infection of meninges
• Meningitis:
• Outer layer of spinal cord
dura mater
middle meningeal layer
arachnoid mater
inner meningeal layer
pia mater
- Between spinal dura mater and walls of vertebral canal
- Contains loose connective and adipose tissue
- Represents the anesthetic injection site
• Epidural Space
- Between arachnoid mater and pia mater
- Contains a network of collagen/elastin fibers
- Filled with cerebrospinal fluid (CSF)
- Represents site of Spinal tap (withdrawal of CSF from lumbar region)
• Subarachnoid space
- posterior
- For sensory functions
- contain sensory nuclei that connect to peripheral receptors
dorsal spinal segment
- anterior
- For motor functions
- contain motor nuclei that connect to peripheral effectors
ventral spinal segment
- Carries information from place to place
* Contains primarily myelinated axons (and less unmyelinated axons)
• White Matter (outer region)
- Region of integration and motor command initiation
- Contains neuron cell bodies, neuroglia, unmyelinated axons
- Has projections (gray horns)
• Gray Matter (inner region)
- Posterior (dorsal) gray horns contain somatic and visceral sensory nuclei
- Anterior (ventral) gray horns contain somatic motor nuclei
- Lateral gray horns: are in thoracic and lumbar segments; contain visceral motor nuclei
• Organization of gray horns
• Axons that cross from one side of cord to the other before reaching a particular destination in the gray matter
• Gray commissures
- afferent fibers originate at sensory receptors in the skin and project to the dorsal horn (one on each side) of the gray matter
- Cell bodies of afferent fibers are located outside of the spinal cord – in the Dorsal Root Ganglia
gray matter• Dorsal: (posterior)sensory functions (afferent fibers)
- efferent fibers travel from the ventral horn (one on each side) of the gray matter to the target muscle
- Cell bodies of efferent fibers are located within the spinal cord
gray matter• Ventral: (anterior) motor functions (efferent fibers)
- Both the afferent and efferent fibers come together to form the spinal nerve (not too far away from the spinal cord)
- Thus, all spinal nerves are___
mixed
consist of bilateral fiber tracts that transmit information between the brain and spinal cord
white matter
from the spinal cord to the brain
ascending tracts
from the brain to the spinal cord
descending tracts
___ and ___Tracts are made up of axon bundles called Tracts or fasciculi
ascending and descending
• between posterior gray horns and posterior median sulcus
• Posterior white columns in white matter
- lie between anterior gray horns and anterior median fissure
- Anterior white commissure: area where axons cross from one side of spinal cord to the other
• Anterior white columns in white matter
• located on each side of spinal cord between anterior and posterior columns
• Lateral white columns in white matter
- Originate in sensory touch receptors in the skin
- Communicate sensory information to the thalamus
- Terminate in the somatosensory cortex
dorsal columns ascending tracts
- Originate in the primary motor cortex
* Terminate in the ventral horn of the spinal cord and communicate to motor neurons of skeletal muscle
Descending Tracts - Example
• Pyramidal Tracts
- Fibers originating in the dorsal and ventral horns are called dorsal and ventral roots, respectively.
- Dorsal and ventral roots join to form a spinal nerve
- Each spinal nerve divides into a dorsal and ventral ramus
spinal nerves
___rami innervate the skin and muscles of the back
dorsal
____rami innervate the limbs and the rest of the trunk
ventral
- From the spinal cord, axons of spinal nerves must enter Sympathetic Ganglion at the Sympathetic Chain (outside of spinal cord)
- At sympathetic ganglion, these preganglionic neurons synapse onto postganglionic neurons, which then go to effector organs
- As the spinal nerves extend into the periphery, they branch out to form the PERIPHERAL NERVES that ultimately innervate the body tissues and organs.
- Therefore, peripheral nerves are interconnecting branches of spinal nerves that form at the region where the dorsal (sensory) and ventral (motor) roots unite
- Thus, peripheral nerves are made up of motor AND sensory fibers
- Each pair of spinal nerves innervates a SPECIFIC region of skin on the body
- Therefore, it is possible to MAP OUT the body’s surface into different sensory regions called Dermatomes
Peripheral Distribution of Spinal Nerves
• Bilateral region of skin that is innervated or monitored by specific pair of spinal nerves
• Dermatomes
• Loss of sensation of a particular ____ (or skin region) indicates damage to the corresponding spinal nerves
dermatome
- The peripheral distribution of spinal nerves discussed so far apply mainly to the THORACIC nerves
- This distribution is different in nerves that control the skeletal muscles of the neck and upper/lower limbs (cervical, lumbar, and sacral nerves)
- Distribution of Cervical, sacral, and Lumbar spinal nerves
- The ventral rami of these regions form a complex interwoven network of nerve fibers called a Nerve Plexus
Spinal Nerves and Plexuses
- The intermixing or blending of fibers of ventral rami of one spinal nerve with adjacent spinal nerves
- Ultimately give rise to specific nerves that control skeletal muscles of the neck and limbs
• Nerve Plexuses
- Includes ventral rami of spinal nerves C1–C5
- Innervates neck, thoracic cavity, diaphragmatic muscles
- Major nerve (but not the only)
- Phrenic nerve (controls diaphragm)
• The Cervical Plexus
- Includes ventral rami of spinal nerves C5–T1
* Innervates pectoral muscles and upper limbs
• The Brachial Plexus
- Includes ventral rami of spinal nerves T12–L4
* Innervate pelvic area and lower limbs)
• The Lumbar Plexus
- Includes ventral rami of spinal nerves L4–S4
- Innervate lower limbs and feet
- Major nerves
- Sciatic nerve and some of its branches
- Fibular nerve
- Tibial nerve
• The Sacral Plexus
- About 10 million
* Deliver information to CNS
• Sensory neurons
- About 1/2 million
* Deliver commands to peripheral effectors
• Motor neurons
- Only in CNS
- About 20 billion
- Interpret, plan, and coordinate incoming and outgoing signals
• Interneurons
• Functional groups of interconnected neurons (interneurons)
• Can involve neurons in one or several brain or spinal cord regions
• Pattern of organization of neuronal pools =
» NEURAL CIRCUIT
• Interneurons are organized into Neuronal Pools
• An axon of one neuron has several collaterals that communicate with several neurons
• Divergence
• A neuron receives communication from many neurons
• Convergence
- Moves information in single line or a stepwise fashion
* Neurons or pools work sequentially
• Serial processing
• Moves same information along several paths simultaneously
• Parallel processing
- Positive feedback mechanism whereby axons go back to the source of the impulse to further stimulate neurons
- Functions (keeps going) until inhibited
• Reverberation
• The various interconnections that occur among neurons and between neuronal pools produce
simple AND complex reflexes
- Rapid, automatic responses to specific stimuli
- Coordinated in the spinal cord
- One neural reflex produces one motor response that typically does not vary
- Basic building blocks of neural function
Neural Reflexes
- The wiring of a single reflex
- Beginning at receptor
- Ending at peripheral effector
- Generally opposes original stimulus (negative feedback)
• Reflex arc
- Step 1: Arrival of stimulus, activation of receptor
- Physical or chemical changes
- Step 2: Activation of sensory neuron
- Graded depolarization (graded, not action, potential)
- Step 3: Information processing in the CNS
- Triggered by neurotransmitters
- Step 4: Activation of motor neuron
- Action potential
- Step 5: Response of peripheral effector
- Triggered by neurotransmitters
Five Steps in Neural Reflexes
- Sensory receptor
- Afferent neuron
- Integration center
- Efferent neuron
- Effector organ
Summary of the Neural Pathway for a Reflex
Consists of Five Components
• Reflex arc: 1) muscle spindle → 2) Afferent neurons
→ 3) Spinal cord → 4) Efferent neurons → 5) Quadriceps muscle
• Knee jerk reflex
• Reflex arc: 1) Photoreceptors → 2) Afferent neurons
→ 3) Midbrain nuclei → 4) Efferent neurons → 5) Pupils
• Pupillary reflex
- Integration occurs in the spinal cord
* Example: muscle spindle stretch reflex (knee-jerk reflex)
• Spinal (by site of information processing)
- Integration occurs in the brain
* Example: pupillary reflex (pupil constricts in response to light)
• Cranial (by site of information processing)
- basic neural reflexes formed before birth
- Inborn reflexes that all neurologically normal people are born with
- Example: pupillary reflex (pupil constricts in response to light)
• Innate reflexes (by early development)
- learned motor patterns
- Reflexes that develop as a result of experience – these differ according to each person’s life experiences
- Example: salivation of Pavlov’s dog
• Acquired reflexes(by early development) :
- Involve signals sent to skeletal muscles via somatic neurons
- Example: muscle spindle stretch reflex (knee-jerk reflex)
• Somatic (skeletal) (by motor response)
- Involve signals sent to smooth muscle, cardiac muscle, and glands via autonomic neurons
- Example: baroreceptor reflex to control blood pressure
• Automatic (Visceral) (by motor response)
• Reflexes which involve a neural pathway consisting of only two neurons and one synapse
• Example:
– muscle spindle stretch reflex (knee-jerk reflex)
– Postural reflex
• Monosynaptic (by complexity of neural circuit)
• Reflexes which involve more than two neurons and multiple synapses (and more complex)
• Examples are most other reflexes
– Tendon reflex
– Withdrawal reflex
• Polysynaptic (by complexity of neural circuit)
