Unit 4 Flashcards
*Upper Motor Neurons
- Neurons in the motor system that command lower motor neurons
- Originate (cell bodies in) the brain
*Lower Motor Neurons
- Directly command muscle contraction (innervate muscle)
- Last neuron in pathway
- Originate (cell bodies) in ventral horn of SC
*Skeletal Muscle
- Controlled by somatic motor system (voluntary control)
- Striated
*Cardiac Muscle, Smooth Muscle
- Controlled by autonomic motor system (involuntary control)
- Controls peristalsis, blood pressure and flow
- Cardiac= striated, smooth= not striated
*Anatomy
muscles–> muscle fiber/muscle cell–> myofibrils
- Motor neurons innervate muscle cells
- Muscle fibers contract and fire APs
- Depolarization causes release of calcium filled vesicles–> induce contraction of sarcomeres
*Muscle Innveration steps/ How a lower motor neuron causes a mucle fiber/cell to contract
- Lower motor neurons release ACh
- ACh produces large EPSP in muscle fiber
- EPSP evokes muscle action potential
- Action potential triggers Ca2+ release
- Fiber contracts
- Ca2+ reuptake
- Fiber relaxes
*Alpha Motor Neurons
- A type of lower motor neuron that innervates skeletal muscles directly
- One AMN per muscle fiber, but can innervate multiple fibers
- Smaller motor units= finer control
*Motor Unit
-Motor neuron and all the muscle fibers it innverates
*Motor Neuron Pool
-All the AMNs that innervate a single muscle
*Types of Motor Units
- Slow twitch (red muscle)
2. Fast twitch (white muscle)
Slow Twitch (Red Muscle)
- Large number of mitochondria and enzymes (myoglobin)
- Slow to contract and slow to fatigue
- Can sustain contraction
- Least force, longer time
*Fast Twitch (White Muscle)
-Fewer mitochondria
-Anaerobic metabolism
-Contract and fatigue rapidly
Types: fast fatiguable and fast-fatigue resistant
*Fast Fatiguable
- Highest force
- Smallest ammount of time
*Fast-Fatigue resistant
-Medium force and time
*Control of contraction by Alpha Motor Neurons
-Reserve maximal contraction for when needed
Control force by:
1. Varying firing rate of motor neurons
2. Recruiting additional symergistic motor units
-Recruiting more motor units that contract in that area
*Muscle Spindles
- Specialized skeletal muscle fibers contained in fiberous capsules ( stretch receptors, intrafusal fibers)
- Parallel w/ muscle
- Middle third= swollen, where group 1a sensory axons wrap around muscle fibers
- **Proprioceptors= body awareness
- **Detect changes in muscle length
- Contractile fibers
- *Shorten, but don’t contribute much force
- *Innvervated by gamma motor neurons (LMNs)
Group 1a Sensory Neurons
- Muscle spindle receptors
- Thickest myelinated axons in body
- Largest and fastest conductance
- Enter SC via dorsal roots, branch and innervate interneurons and AMNs in ventral horn
- One 1a axon synapses on every AMN in motor pool for same muscle it originated in
- **Fire less frequently when contracted
*Stretch Reflex
- Alpha motor neuron fires from 1a stimulation (extrafusal)
- Gamma Motor neuron fires from 1a stimulation (intrafusal)
- *Only in SC, not brain
- When a muscle is pulled, it tends to pull back (myotatic reflex= muscle stretch)
- **Protective
- Sensory feedback loop from muscle
- **As muscle is stretch, firing rate increases
*Gamma Motor Neurons
- -Bulk of muscle mass= Extrafusal muscle fibers, innervated by alpha motor neurons
- -Muscle spindles= intrafusal muscle fibers, innervated by gamma motor neurons
*Muscle movement
- Muscles move joints by pulling
- Flexors= bend/contract
- Extensors= extend muscle
*Reciprocal Inhibition
-Contraction of one muscle set accompanied by relaxation (inhibition) of antagonist muscle
*3 types of input to Lower Motor Neurons
- information from UMNs originating in brain (voluntary movement)
- Sensory info from muscle spindles (muscle length)
- Interneurons from SC (generate spinal programs)
*Spinal Interneurons
- Most input to AMNs mediated by spinal interneurons
- *Excitatory use glutamate
- *Inhibitory use glycine
- Synaptic input to spinal interneurons
1. Primary sensory axons
2. Descending axons from brain
3. Collaterals of LMN axons
4. Other interneurons - Curcuits= responsible for executinf stereotyped and reflexive movements of body
*Flexor Withdrawal Reflex
- Doesn’t require concious input
- Complex reflex arc used to withdraw limb from aversive stimulus
- Depends on how painful the stimulus is and where it occurs
- Triggered by delta A axons that branch to neighboring spinal segments (nociceptors)
- *Slower than strerch reflex bcuz delta A axons smaller than 1a axons
*Crossed-Extensor Reflex
-Flexor w/drawal reflex in lower extremity needs to be balanced on opposite side of body
-Activation of extensor muscle and inhibition of flexors on opposite sode
Breakdown:
1. On same side of painful stim: excite flexors, inhibit extensors
2. On opposite side: excite extensors and inhibit flexors
*Central Pattern generators
- Neuron or neurons that generate rhythmic patterns of activity
- Circuits of spinal interneurons
*SIMPLE Central Pattern Generator
- Reciprocal inhibition btwn 2 sets of neurons–> rhythmic output
- UMN–> excitation and inhibition–> extensory and flexor LMNs
- As one is excited, simultaneouslt inhibits other so limb is only flexed or extended
- Excitations of 2 are counterbalanced–> rhythmic pattern of flexing and extending
*Axons descending from brain (upper motor neurons
- Lateral pathways
- Corticospinal tract
- Rubrospinal tract - Ventromedial Pathways
- Tectospinal tract
- Vestibulospinal tract
- Pontine reticulospinal tract
- Medullary reticulospinal tract
*Lateral Pathways
-Voluntsary movement and fine coordination of distal muscles
*Ventromedial Pathways
- Originate in brainstem
- Voluntary movement of proximal muscles and trunk
- Involuntary movements
- Maintaining body position/posture
- Defensive reactions
- Balance
*Corticospinal Tract
- Originates in neocortex (brain)
- Decussates in medulla (midline)
- Largest and longest in CNS
*Rubrospinal Tract
- Originates in red nucleus of midbrain (brainstem)
- Decussates in Pons (midline)
- Vestigal, can help with recovery after injury to corticospinal
- Controls flexing of arms
*Vestibulospinal tracts
- Head balance, head turning
- Close connection w/ cochlea
- Originate in medulla (brainstem)
*Tectospinal Tract
- Orienting responses to sights and sounds
- Originates in colliculus in midbrain (Brainstem) and has close connection to retina
*Pontine Reticulospinal Tract
- Starts in pons (brainstem)
- Enhances antigravity muscles
- Maintains standing by extensor muscles in legs
- Different than the vestibulospinal tract
*Medullary Reticulospinal Tract
- Starts in medulla (brainstem)
- Liberates antigravity muscles from reflex control
- Opposite from pontine tract
*Pyramidal Cells/ Betz Cells
-Drive numerous motor pools and different muscles
*Axons in corticospinal tract
Two sources of input to Betz cells”
1. Cortical areas (area 6 anterior, areas 3, 1, and 2 posterior)
2. Thalamus (which recieves basal ganglia and cerebellar connections)
**Envolvement occurs b4 exciting corticospinal tract
*Primary motor cortex/ M1
- Brodmann Area 4/ precentral gyrus/ motor strip/M1
- Anterior to central sulcus
- Organization mirrors arrangement of primary somatosemsory cortex
*Wilder Penfield
- Stimulated primary motor cortex
- Causes contract of small group of muscles depending on where in the motor map u stimulate
- Direction of movement is encoded by the net result of groups of neurons firing (pop coding)
*Somatosensory Association Cortex
- A5
- Inputs from somatosensory cortex areas 3,1, and 2
*Posterior Parietal Cortex
- A7
- Inputs from higher orfer visual cortical areas
*Prefrontal Cortex
- Represents highest levels of motor control
- Decisions made about actions and outcome
- Planning
- Abstract thought
- Anticipating consequence of action
*Premotor Cortex “higher” motor area
- A6, skilled movement
- 2 parts w/ similar fxns, diff groups of muscles innervated
- Preparing and initiating ,ove,ent plans
- Highly connected to M1
1. Lateral region-> premotor area (pma)–> responds to cues to move
2. Medial region–> supplementary motor area (SMA)–? prepares sequences of voluntary movement and active 1-2 secs before movement
*Steps in preparation of movement
- Ready= parietal and frontal lobes
- Set- area 6; supplementary and premotor areas
- Go- area 4
- Mentally rehearsing movements only activates A6
*Basal Ganglia/ Frontostriatal System
- Selection and initiation of volitional movements
- Recieves input from neurons in frontal cortex in the striatum
- *Provides major input to area 6
- *Forms 2 main loops
1. Direct pathway
2. Indirect pathway - *At rest, thalamus is tonically inhibited, for motor cortx to be excited, talamus must be disinhibited
*Parkinson’s Disease
- Trouble initiating willed movements due to increased inhibition of thalamus by basal ganglia
- Sypt: hypokinesia, bradykinesia, akinsesia, rigidity, tremors
- Cellular basis: Loss of input to direct pathway
- L-dopa treatment
- Inability to propagate movement
*Huntington’s Disease
- Symp: chorea, hyperkinesia, dyskinesias, dimentia, personality disorder
- Loss of indirect pathway
- Cortical degeneration responsible for dementia and personality changes
*Cerebellum
- Fxn: sequencing of muscle contractions, monitoring and correcting both plans and actions
1. Copy of motor plan
2. Vestibular and Proprioceptive input - Lesions–> ataxia, dyssynergia, and dysmetria
What happens if upper motor neurons are damaged?
-We lose voluntary muscle control
Behavior
- Basic fxn of life
- Maintains homeostasis
- Motivated behavior= result of brain’s response to internal and external cues
Somatic and Autonomic NS
- SNS directs quick, specific behavior
- ANS input sets stage for general behaviors
- -Sympathetic= fight, flight
- -Parasympathetic= rest, digest
Homeostasis
- Physiological mechanism respond to changes to maintain regulated set range
- Changes identified through sensory systems
- Ex: rxn to cold
- -Physiological= constricted blood vessels, goose bumps
- -Behavioral= shivering, seeking warmth (due to physiological)
Cycle of Homeostasis
-Imbalance–> change detected by receptor–> input via afferent pathway sent to control center–> output sent via efferent pathway to effector–> response to change= imbalance corrected
Hypothalamus
- Essential for maintaining homeostasis
- Directly linked to pituitary gland (allows control/comm over periphery)
- Specific cells responsive to particular stimuli (direct or indirect)
- Produces wide variety of neuropeptides
- –Projects to forebrain, brainstem
- –Direct release into periphery via pituitary
Magnocellular neurons
- Express oxytocin, vasopressin
- Release to periphery
Parvocellular neurons
- Express CRH and other neuropeptides
- Produce effects in periphery
Neurohormones
- Hormones produced in the brain
- Can act at distant sites throughout the body
- Actions start and last on a relatively longer time scale than neurotransmitters
Vasopressin and Fluid regulation
-Hypothalamus gets input from blood volume and salt concentration
Low blood volume –> Angiotensin II detected by SFO —> vasopressin cells in hypothalamus.
-SFO–> lateral hypothslamus (promotes drinking behavior)
One of the results: kidneys retain water
Leptin
- Peptide
- Secreted from fat cells, activate lepin receptors in arcuate nucleus of the hypothalamus
- Decreased leptin stimulates feeding behavior
- Secretion response: increased metabolism
- ANS response: increase sympathetic activity and metabolism
- Somatic/ behavior response: inhibit feeding behavior