Neurobiology Exam 3

Chapters 8, 10, 15

Motor Systems

Performs so many different motor tasks with speed and accuracy: reflex, rhythmic, and voluntary

What processes give motor systems the ability to perform activities?

1. commands to motor neurons and muscles are distributed in hierarchically interconnected areas of the spinal cord, brain stem, and forebrain.

 

2. sensory information relating to movement is processed in different systems that operate in parallel.

Movement

A single relocation of a body part, usually resulting from a brief muscle contraction

Reflex

A simple, stereotyped, and unlearned response to particular stimulus

Acts

Complex, sequential behaviors

Motor Plan (Program)

A set of muscle commands that is established before the action occurs

Electromyography (EMG)

Records the electrical activity of muscles

Tendons

Connect muscle to bone in a reciprocal fashion

Antagonistic Muscle

When one muscle group contracts and it stretches the other group

Synergist Muscle

Muscles that act together to move a limb

Skeletal Muscles

Used for movement of the skeleton

What are Skeletal Muscles made of?

Striate muscle

Striate muscle

Overlapping layers of proteins myosin and actin give a striped appearance

Muscle Anatomy: Muscle Belly / Body is…

Whole muscle

Muscle Anatomy: The muscle is made up of…

Muscle Fibers

Muscle Anatomy: Muscle fibers contain…

Myofibrils

Muscle Anatomy: Myofibrils contain…

Sarcomeres (Actin and Myosin)

Action of the Muscle: Sliding Filament Model

• Contraction of the muscle increases the overlap of actin and myosin filaments with muscle fibers

• As they slide past each other, the muscle shortens

• Distance between the z-lines shorten

What makes actin and myosin interact?

ATP, which generates contractile muscle force

Neuromuscular Junction (NMJ) Process

• Acetylcholine (ACh) is released at the NMJ

• ACh reaches receptors on the membranes of muscle fibers

• Na++ channels open and sodium ions enter the cytoplasm of the muscle fiber

• The sodium influx also causes the release of stored calcium ions into the muscle 

• The calcium ions cause the actin and myosin to slide along one along another to produce a contraction

• For the muscle to relax the calcium lvls must decrease a bit

Tetanus

If motor action potentials occur very rapidly (high frequency over short time) there is no time to reduce calcium, which causes uncontrollable sustained muscle contractions.

Cause of Tetanus:

Caused by tetanus toxin (bacteria) which blocks GABA release from inhibitory neurons, so there is an overexcitation of motor neurons and powerful contractions 

Rigor Mortis

• 2-3 hours after death, all stored calcium is released and triggers muscle contractions

• Muscle stay contracted until they start to disintegrate 

Myasthenia Gravis

• Autonimmune disorder where the body produces antibodies that break down ACh receptors.

• Muscle contractions become weak if not treated, they may stop breathing

Treatment:

• Immunosuppressants to decrease antibodies production

• Administer an enzyme that breaks down acetylcholinesterase (which is a ACh inhibitor)

Myotonia

Neuromuscular condition in which the relaxation of a muscle is impaired

• Ex.) Myotonic “fainting” goats

Motor Unit

Motoneuron’s axon and all of its target fibers

Motor units very in what?

In hoe forcefully they twitch and how quickly they fatigue

When muscle contractions get stronger…

Larger motor units are recruited

What do spiraling sensory and γ-motor axons innervate?

The intrafusal fibers in muscle spindles

Alpha motor neurons function

Contract muscles

Gamma motor neurons function

Provide proprioceptive info to muscle stretch

Coactivation of Alpha-Gamma motor axons…

Allows muscle spindles to sense muscle stretch even during a contraction

Cardiac Muscle

• Cardiac muscle fibers are electrically coupled to one another (use gap junctions)

• Rhythmic (no neural input requested)

• Neural input regulates Heart Rate (HR)

• Norepinephrine (NE) increased HR

• Ach decrease HR

Cardiac Muscle: ACh Receptors…

• In the heart are metabotropic (muscarinic)

• At the NMJ are ionotropic (nicotinic)

Endocrine Glands

• Secrete hormones into the blood

• Magnocellular neurons in the hypothalamus release oxytocin or vasopressin posterior pituitary gland

• Oxytocin release is involved in the ejection of mother’s milk and uterine contractions

  • Vasopressin (ADH) helps the kidney retain water to regulate BP

• Cells in the anterior pituitary gland secrete hormones that control other glands

Portal Veins Function:

Carry releasing factors to the anterior pituitary, where they stimulate the release of pituitary hormones

What causes milk ejection from mammary glands?

Burst of action potentials in supraoptic neurons

• Eacj burst causes a pulse of oxytocin release

HPA Axis:

• Releasing Hormone: CRH (corticotropin releasing hormone)

• Pituitary Hormone: ACTH (adrenocorticotropic hormone, aka corticotropin)

• Principle End Organ: Cortex of the adrenal gland

• End Organ Hormones: Cortisol

HPG Axis:

• Releasing Hormone: GnRH (Gonadotropin releasing hormone)

• Pituitary Hormone: LH and FSH (luteinizing hormone and follicle stimulating hormone)

• Principle End Organ: Gonads (testes or ovaries)

• End Organ Hormones: Testosterone or Estradiol 

HPT Axis:

• Releasing Hormone: TRH (Thyrotropin releasing hormone)

• Pituitary Hormone: TSH (Thyroid stimulating hormone, aka Thyrotropin)

• Principle End Organ: Thyroid

• End Organ Hormones: Thyroxin and other thyroid hormones

What does hippocampus lesion affect?

Spatial memory recall in part through increased stress hormone release

What does early exposure to the estrogen mimic, DES, do?

Causes adult obesity

• DES: Diethylstilbestrol (

Reflex

An involuntary, stereotyped response to stimulus

Light-Induced Pupillary Constriction

A reflex that occurs against a background of sympathetic activation promoting pupillary dilation

• The default state of pupil size is Pupil Dilation

• Light is modulation stimulus

Is a reflex that antagonizes pupillary dilation

Pupillary Constriction Pathway (Response to light)

1. From Retina

2. Through Optic nerve

3. Integrated through Olivary Pretectal Nucleus to Edinger-Westphal Nucleus

4. Through Oculomotor nerve

5. Through Parasympathetic Ganglia

6. To Pupillary constrictor muscle

Pupillary Dilation Pathway (Tonically active)

1. From Hypothalamus 

2. Through IML

3. Through Parasympathetic ganglia

4. To Pupillary dilator muscle

Ways reflexes protect us from harm:

• Tears

• Blinking from corneal stim involves a circuit through spinal trigeminal and facial motor nuclei

• Withdraw from pain causes muscle flexors to contract and muscle extensors to relax. Goes through the spinal cord and includes a set of inhibitory interneurons 

The corneal eye blink reflex involves:

• Three sets of neurons

• Two cranial nerves: Trigeminal Nerve, Facial Nerve

  • The cornea is innervated by the trigeminal nerve that projects to the spinal trigeminal nucleus

  • Neurons in the spinal trigeminal nucleus project to neurons in the facial motor nucleus that innervates the eyelid closing muscles (Orbicularis oculi) 

How the leg withdrawal reflex moves the leg out of harm’s way:

• Reflex: Step on tack and withdraw your foot

• Activates the mechanosensitive neurons (foot) that projects into the spinal cord

• Synapse on excitatory interneurons that project to hamstring motor neurons

• Sensory neurons also synapse on the inhibitory interneurons (red) that project to motor neurons innervating the quadriceps 

Knee jerk response is a stabilizing stretch reflex…

Reflex: Tapping the patellar tendon results in your leg straightening 

• Stretched tendon —> stretches the quadriceps

• Activates quadriceps muscle spindle receptor —> spinal cord motor neurons —> quadricep contract

• Also inhibit interneurons that project to motor neurons that innervate hamstring

Reciprocal Innervation

Sending opposing commands to antagonistic muscles

When does the bicep stretch reflex stabilize the hand?

When it experiences a change in load

• Motor commands from the brain can adjust the “set point” for limb position

Eye Movements

• Oculomotor — 3rd cranial nerve (inward)

• Trochlear — 4th cranial nerve (downward)

• Abducens — 6th cranial nerve (outward)

Optokinetic Reflex

Slow tracking of moving objects. Eyes reset position to keep images stable on retina. Needs visual input (not in dark)

Eye Stabilization: Optokinetic Reflex Circuitry

Retina: Photoreceptors — bipolar — ganglion cells (optic nerve)

• Some ganglion cells send projections to the nucleus of the optic tract — then on to the vestibular complex for each eye

• Involves reciprocal innervation for both eyes to look in the same direction

Vestibular Ocular Reflex

Stabilizes gaze by countering head movements

• Head moves side to side while eyes stayed focused, therefore eyes are moving in the opposite direction

Turn head LEFT Semicircular canal input to vestibular complex to turn eyes to the RIGHT

So abducens nucleus to turn Right E (R) and oculomotor nucleus to turn Left E (R)