BI 315 Exam II Terms

Sensory neurons (sensory receptors/afferents)

contain the molecular machinery necessary to detect stimuli and convert them into nervous system activity

Effector neurons (motor neurons / motoneurons)

synapse onto non-neuronal cells (muscles or glands) that perform (effect) some physiological function

Interneurons

neurons that locally process information, involved in more complex circuits; more flexible and context dependent

Afferent pathways

Ascending pathways; transmit information from sensory receptors to brain/ CNS

Efferent Pathways

Descending pathways; transmit information from CNS/ brain to effectors

What does division of neurons depend on?

how neurons develop and where their cell bodies are

Central Nervous System

composed of brain and spinal cord; contains cells with their cell bodies in the brain/spinal cord (all interneurons, somatic motor neurons, and a few sensory neurons) and axons of afferent PNS cells

Peripheral Nervous System

composed of cranial nerves and spinal nerves; contains cells with their cell bodies outside the brain/spinal cord (most sensory neurons, autonomic motor neurons) and axons of efferent CNS cells

Somatic Nervous System

includes all extero-receptors and voluntary motor neurons; produce all voluntary behavior and somatic reflexes via regulating skeletal muscle contractions

Autonomic Nervous System

includes most interoreceptors and all involuntary effector neurons (regulate smooth and cardiac muscles and glands); produce most involuntary behaviors and regulate internal homeostasis; divided into sympathetic and parasympathetic

Sympathetic ANS

promotes “fight-or flight” responses

Parasympathetic ANS

promotes “rest-and-digest” responses

Acetylcholine (ACh)

the major efferent neurotransmitter in the somatic and parasympathetic NS

Norepinephrine

the major efferent neurotransmitter of the sympathetic NS; norepinephrine and epinephrine both activate the same family of adrenergic receptors

Enteric Division of ANS

Largely autonomous, localized to the walls of the gut and regulated gut contractions that move food through the gut during digestion

Forebrain

contains the somatosensory vortex, the motor cortex and the hypothalamus

Somatosensory vortex

a sensory region where somatosensory information from the body is received and processed

Motor Cortex

a motor region where the commands to voluntarily contract skeletal muscles are generated

Hypothalamus

contains multiple nuclei responsible for homeostatic regulation of other body systems and generation of important motivational states (i.e. hunger)

Cerebellum

important motor region involved in motor learning and real-time adjustments of motor output via sensory feedback

Medulla

one of the brainstem regions involved in critical homeostatic regulation of other body systems like cardiovascular and respiratory functioning

Transduction

the process of detecting and converting a signal/sensory stimulus into neuronal activity; the first step in processing any sensory information

(En)coding

the way information in a signal is represented

Spatial codes

tell us which neurons are active (e.g. labeled lines)

Rate coding

tell us how fast a neuron is firing

Temporal coding

tell us when exactly a neuron is firing or how firing changes over time

Labeled lines

anatomically separate circuits for different types of sensory information; information can be conveyed by which neuronal pathways are active

Receptive fields

Defined by sensory receptors that respond to stimuli with specific qualities

Somatosensation

the sensation of the body’s physical interactions with itself and the environment

Mechanoreceptors

Detection of mechanical pressure or distortion

Cutaneous mechanoreceptors

mechanoreceptors in the skin — your sense of “touch”

Proprioreceptors

mechanoreceptors in muscles/joints

Mechanotransduction

mechanoreceptor proteins open when the cell membrane is deformed, tend to be cation channels that cause depolarizing receptor potentials

Cutaneous mechanoreception

occurs in the skin; different receptor types have different properties and respond to different types of stimuli

Nociception

pain mechanoreceptors; generally have a higher firing threshold than other somatosensory cells; no amount of stimulation will ever elicit the sensation of pain in non-nociceptor thermoreceptor

Proprioception

the sensation of the position of, and load on, different body parts; involves somatosensory neurons in muscles and connective tissues; vision, inner ear and skeletal muscles also play a role

Extrafusal muscle fibers

make up skeletal muscles; shorten (contract) and pull on their attachment points to the bones, thus moving them around

Muscle spindles

composed of modified skeletal muscle fibers (intrafusal fibers) and innervated by mechanosensory neurons (spindle stretch receptors)

What does the spindle stretch receptor do?

Encodes the degree to which a muscle is stretched out; degree of muscle contraction is rate coded; synapses on alpha-motor neurons

Proprioreceptor fucntions

provide information about body part position to voluntary circuits; provide the sensory information that triggers involuntary skeletal muscle stretch reflexes; critical components of central feedback system that compare expected and actual motor output

Muscles

contractile organs that pull on their attachment points or squeeze whatever they are wrapped around

Skeletal muscles

muscles that are attached to your bones and under the direct control of the somatic NS, which cause your body movements; effect all the voluntary movements of our body

Smooth/cardiac muscles

muscles that cause involuntary internal movements (i.e. heartbeat, dilating blood vessels)

Motor control

the initiation and patterning of motor activity (when and why muscles contract), which is often generated by the nervous system

Alpha motor neurons

excite the extrasfusal muscle fibers, which generate the forces that actually shorten the whole muscle; synapses on extrafusal muscle fibers

Reflexive motor activity

a rapid, involuntary motor response to a stimulus; usually generated locally by simple neural circuits, but sometimes generated by the brain

Voluntary motor activity

actions accompanied by conscious awareness, planning and attention; generated centrally in the motor cortex

Reflexes serve two non-mutually exclusive functions

1) To respond to a sensory feedback

2) To cause rapid movements when speed is critical

Feedforward control

when initiating voluntary movements, the motor cortex sends commands to skeletal muscles to contract based on internal goals/motivational states; predictive regulation of posture and balance (i.e wind up toy)

Feedback control

motor output is refined based on sensory feedback from vision, muscle spindles, and other senses (i.e. the myotactic reflex)

myotactic stretch reflex steps

1) sensation of muscles stretch by muscle spindle receptors

2) excitation (leading to contraction) of the stretched muscle

3) inhibition of neurons synapsing on the antagonistic muscle(s)

Gamma motor neurons

tightens intrafusal fibers independently of extrafusal muscle contraction; adjust the sensitivity of the muscle spindle feedback; muscle spindles slacken as extrafusal fibers contract; synapses on intrafusal muscle fibers

Nervous system’s core functions

to encode, process, and store information, and to drive the activity of other physiological systems based on the activity

Decoding

attempts to “read out” information from nervous system using functional techniques

muscle tissue

specialized for generating movements via contraction

Myocyte

a skeletal muscle fiber; a single, long, multinucleate cell

Myofibrils

long, rod-like complexes of contractile units packed in skeletal muscle fibers

Sarcoplasmic reticulum (SR)

a modified endoplasmic reticulum; myofibrils are closely related with this

Transverse tubules (T-tubules)

deep invaginations of the muscle cell membrane; allow electrical signals to quickly reach the interior of a cell

Sarcomere

one complete contractile unit; within a myofibril

Sliding Filament Theory

sarcomere contraction; thin filaments slide past the thick filaments

Costameres

a protein complex that links myofibrils to the muscle fiber cell membrane and the ECM, allowing force transmission through the tissue and shortening of the whole muscle

Thin filaments

composed of long chains of actin proteins complexed with troponin and tropomyosin

Thick filaments

composed of chains of myosin proteins arranged with their heads (aka cross bridges) facing the distal part of the filament

Cross-bridge cycle

1) myosin head binds ATP, ATP is hydrolyzed and the myosin head cocks and binds to actin (setting up a mouse trap)

2) free P_i quickly releases and myosin undergoes another conformational change (power stroke) pulling thin filaments toward the center of the sarcomere (setting off mouse trap)

3) ADP is released and myosin remains bound to actin (rigor) until another ATP is bound to myosin

During a single skeletal muscle contraction, each myosin molecule goes through many binding/unbinding cycles

Excitation-contraction coupling

excitation by motor neurons triggers Ca²⁺ influx into the cell cytoplasm; Ca²⁺ influx into the muscle cell cytoplasm initiates sarcomere contraction

How does Ca²⁺ initiate sarcomere contraction?

End-plate potentials (EPPs)

caused by ACh binding to end plate receptors —> results in very large postsynaptic graded potentials; a single EPP is usually sufficient to depolarize muscle fiber above threshold

True or False: muscle cell APs propagate all across the cell membrane, except down the T-tubules

False; they propagate all across including down the T-tubules

What are the two major muscle performance parameters?

Force (tension) and speed (velocity) of the contraction

Force (tension)

proportional to the number of active myosin heads cycling in the muscle: max force is therefore proportional to muscle cross sectional area

Speed (velocity)

depends on mass of the load being moved, number of muscle fibers contracting (and their type), and the speed at which the myosin heads can cycle through one power stroke in the fiber