What’s the motor system?

1. Motor systems control different movements from simple reflexes (knee jerk) to voluntary movements (96mph fast ball)

2. Motor systems generate a motor output

3. Motor systems are made up of motor units

Motor Unit

• A single motor neuron innervates a group of muscle fibers - this arrangement is called a motor unit

•When a motor neuron fires, the muscles it innervates contract.

• For controlled movement, many motor units must be synchronized.

Cerebellum

• Cerebellar cortex

  • Cortex that covers surface of cerebellum

• Deep cerebellar nucleus

  • Nuclei located within cerebellar hemispheres

  • Receive projections from cerebellar cortex and send projections out of cerebellum to other parts of brain

• Cerebellar peduncle (pee dun kul)

  • One of three bundles of axons that attach each cerebellar

  • hemisphere to the dorsal pons

Damage to cerebellum impairs standing, walking, or performance of coordinated movements

• Cerebellum receives visual, auditory, vestibular, and somatosensory information, and it also receives information about individual muscle movements being directed by brain

• Cerebellum integrates this information and modifies the motor outflow, exerting a coordinating and smoothing effect on the movements

• Cerebellar damage results in jerky, poorly coordinated, exaggerated movements; extensive cerebellar damage makes it impossible even to stand

Basal Ganglia

• Basal ganglia: control of voluntary movements, procedural learning, cognition, and emotions

• regulates posture, counteracts tremor and maintains muscular contractions.

• regulates motor control

• Key Structures:

  • Caudate

  • Putamen

  • Globus pallidus

  • Substanitia nigra

  • Nucleus accumbens

  • Subthalamic nucleus

Spinal Cord

• Lateral Corticospinal Tract

• Ventral Corticospinal Tract
  *the ventral and lateral corticospinal tract make up the major pathway from the motor cortex to the spinal cord

• Rubrospinal Tract

• Reticulospinal Tract

• Tectospinal Tract

Lateral Corticospinal Tract -

• originates in the motor and premotor areas of the cortex.

• Fibers in pyramidal cells of the cortex pass through the internal capsule, cerebral peduncles to the

• medullary pyramids, cross midline and

• terminate on motoneurons and interneurons in the lateral gray matter

Ventral Corticospinal tract -

• originates in the motor and premotor areas of the cortex.

• Fibers in pyramidal cells of the cortex pass through the internal capsule, cerebral peduncles to the medullary pyramids,

**does not cross midline terminate in the cervical and upper thoracic levels.

• Control limb posture and position of the head.

• Controls trunk muscles on both sides of the body.

Rubrospinal tract -

• originate in the red nuclei cross midline at the level of the pons before descending in the spinal cord to terminate on interneurons.

Vestibulospinal tracts -

• originates with cells in the lateral vestibular nucleus

• Descends uncrossed in the spinal cord to terminate on medial motoneurons that control postural muscles and extensor and flexor muscles.

Reticulospinal tracts -

• Cells originating from the pons are excitatory
  

• descends ipsilaterally and ends on segmental interneurons that provide bilateral excitation to medial extensor motor neurons.

Tectospinal tract -

• Cells originating from the medulla are inhibitory.
  

• terminate in the cervical and upper thoracic levels.
  

• Control limb posture and position of the head.

Reflexes

• limb movement is produced by coordinated muscle action of extensors (muscles open or extend) and flexors (close or flex the joint).

• Myotatic reflex is activated by muscle stretch - example: Knee Jerk Reflex

Postural adjustments

• Context

  • Maintain balance--supported v/s unsupported

• Feedback

  • Error correction Response lags stimulus; sometimes too late

• Feed-forward

• Response anticipates stimulus More timely, but depends on internal models Practice, learning

Voluntary movements

• All the connections in the cerebellum allow actual or intended movements to be compared during execution to the plans of the movement in the cortex.

• Usually include multiple systems - vestibular, visual, premotor, motor, spinal tracts

• Organized around purposeful act

Voluntary movements are organized by motor programs

• Translate goal into action

  • Formation of a movement representation, or motor program

• Program

  • To produce the desired goal, which muscles should contract and when

• 2 Key movement characteristics

  • Spatial (hand path; joint angles) - Kinematic plan

  • Forces/loads - Dynamic plan

• All accomplished by contracting muscles

Central Pattern Generators

• coordinated, rhythmical movement.

• Sensory feedback is not necessary (and not ignored)

• Two key examples - respiration and walking

Overview of Muscle Tissue

• Nearly half of body’s mass

• Can transform chemical energy (ATP) into directed mechanical energy, which is capable of exerting force

• To investigate muscle, we look at:

  • –  Types of muscle tissue

  • –  Characteristics of muscle tissue

  • –  Muscle functions

Types of Muscle Tissue

• Terminologies: Myo, mys, and sarco are prefixes for muscle

  – Example: sarcoplasm: muscle cell cytoplasm

• Three types of muscle tissue

  – Skeletal – Cardiac – Smooth

• Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers

Characteristics of Muscle Tissue

All muscles share four main characteristics:

1. Excitability (responsiveness): ability to receive and respond to stimuli

2. Contractility: ability to shorten forcibly when stimulated

3. Extensibility: ability to be stretched

4. Elasticity:abilitytorecoilto resting length

Muscle Functions

• Four important functions

1. Produce movement: responsible for all locomotion and manipulation

   1. Example: walking, digesting, pumping blood

2. Maintain posture and body position

3. Stabilize joints

4. Generate heat as they contract

Skeletal Muscle Anatomy

• Skeletal muscle is an organ made up of different tissues with three features: nerve and blood supply, connective tissue sheaths, and attachments

Muscle Fiber Microanatomy and Sliding Filament Model

• Skeletal muscle fibers are long, cylindrical cells that contain multiple nuclei

• Sarcolemma: muscle fiber plasma membrane

• Sarcoplasm: muscle fiber cytoplasm

• Contains many glycosomes for glycogen storage, as well as myoglobin for O2 storage

• Modified organelles – Myofibrils

  • –  Sarcoplasmic reticulum

  • –  T tubules

Myofibrils

• Myofibrils are densely packed, rodlike elements

  • –  Single muscle fiber can contain 1000s

  • –  Accounts for ~80% of muscle cell volume

• Myofibril features

  • –  Striations

  • –  Sarcomeres

  • –  Myofilaments

  • –  Molecular composition of myofilaments

Myofibrils: Straitions

• Striations: stripes formed from repeating series of dark and light bands along length of each myofibril

– A bands: dark regions
▪ H zone: lighter region in middle

of dark A band

– M line: line of protein (myomesin) that bisects H zone vertically

– I bands: lighter regions

▪ Z disc (line): coin-shaped sheet of proteins on midline of light I band

Myofibrils: Sarcomere

• Sarcomere

• Smallest contractile unit (functional unit) of muscle fiber

• Contains A band with half of an I band at each end

• Consists of area between Z discs

• Individual sarcomeres align end to end along myofibril, like boxcars of train

Myofibrils: Myofilaments

• Myofilaments

• Orderly arrangement of actin and myosin myofilaments within sarcomere

• Actin myofilaments: thin filaments

• Extend across I band and partway in A band

• Anchored to Z discs

• Myosin myofilaments: thick filaments

  • Extend length of A band

  • Connected at M line

• Sarcomere cross section shows hexagonal arrangement of one thick filament surrounded by six thin filaments

Myofibrils: Molecular

• Molecular composition of myofilaments

– Thick filaments: composed of protein myosin that contains two heavy and four light polypeptide chains

• Heavy chains intertwine to form myosin tail

• Light chains form myosin globular head

  • During contraction, heads link thick and thin filaments together, forming cross bridges

▪ Myosins are offset from each other, resulting in staggered array of heads at different points along thick filament

Myofibrils: Molecular

• Molecular composition of myofilaments (cont.)

• Thin filaments: composed of fibrous protein actin

  • Actin is polypeptide made up of kidney-shaped G actin (globular) subunits

    • G actin subunits bears active sites for myosin head attachment during contraction

  • G actin subunits link together to form long, fibrous F actin (filamentous)

  • Two F actin strands twist together to form a thin filament

– Tropomyosin and troponin: regulatory proteins bound to actin

Myofibril: Molecular

• Molecular composition of myofilaments (cont.)

  • Other proteins help form the structure of the myofibril ▪

    • Elastic filament: composed of protein titin

      • Holds thick filaments in place; helps recoil after stretch; resists excessive stretching

    • Dystrophin

    • Links thin filaments to proteins of sarcolemma

    • Nebulin, myomesin, C proteins bind filaments or sarcomeres together

    • Maintain alignment of sarcomere

Sarcoplasmic Reticulum and T Tubules

Sarcoplasmic reticulum: network of smooth endoplasmic reticulum tubules surrounding each myofibril

• Most run longitudinally

• Terminal cisterns form perpendicular cross channels at the A–I band junction

• SR functions in regulation of intracellular Ca2+ levels

• Stores and releases Ca2+

Sarcoplasmic Reticulum and T Tubules

T tubules

• Tube formed by protrusion of sarcolemma deep into cell interior

  • Increase muscle fiber’s surface area greatly

  • Lumen continuous with extracellular space

  • Allow electrical nerve transmissions to reach deep into interior of each muscle fiber

• Tubules penetrate cell’s interior at each A–I band junction between terminal cisterns

  • Triad: area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere

• Triad relationship

  • T tubule contains integral membrane proteins that protrude into intermembrane space (space between tubule and muscle fiber sarcolemma)

  • Tubule proteins act as voltage sensors that change shape in response to an electrical current

  • SR cistern membranes also have integral membrane proteins that protrude into intermembrane space

  • SR integral proteins control opening of calcium channels in SR cisterns

• Triad relationships (cont.)

  • When an electrical impulse passes by, T tubule proteins change shape, causing SR proteins to change shape, causing release of calcium into cytoplasm

Sliding Filament Model of Contraction

• Contraction: the activation of cross bridges to generate force

• Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces

  opposing shortening

• Contraction ends when cross bridges become inactive

• In the relaxed state, thin and thick filaments overlap only slightly at ends of A band

• Sliding filament model of contraction states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more

  • Neither thick nor thin filaments change length, just overlap more

• When nervous system stimulates muscle fiber, myosin heads are allowed to bind to actin, forming cross bridges, which cause sliding (contraction) process to begin

• Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward center of sarcome in a ratcheting action

  • Causes shortening of muscle fiber

• Z discs are pulled toward M line

• I bands shorten

• Z discs become closer

• H zones disappear

• A bands move closer to each other

Muscle Fiber Contraction

Background and Overview

• Decision to move is activated by brain, signal is transmitted down spinal cord to motor neurons which then activate muscle fibers

• Neurons and muscle cells are excitable cells capable of action potentials

  • Excitable cells are capable of changing resting membrane potential voltages

• AP crosses from neuron to muscle cell via the neurotransmitter acetylcholine (ACh)

• Ion Channels

  • Play the major role in changing of membrane potentials

  • Two classes of ion channels:

    • Chemically gated ion channels – opened by chemical messengers such as neurotransmitters

    – Example: ACh receptors on muscle cells

    • Voltage-gated ion channels – open or close in response to voltage changes in membrane potential

The Motor Unit

• Motor unit consists of the motor neuron and all muscle fibers (four to several hundred) it supplies

  • Smaller the fiber number, the greater the fine control

• Muscle fibers from a motor unit are spread throughout the whole muscle, so stimulation of a single motor unit causes only weak contraction of entire muscle

Background and Overview

Anatomy of Motor Neurons and the Neuromuscular Junction

• Skeletal muscles are stimulated by somatic motor neurons

• Axons (long, threadlike extensions of motor neurons) travel from central nervous system to skeletal muscle

• Each axon divides into many branches as it enters muscle

• Axon branches end on muscle fiber, forming neuromuscular junction or motor end plate

  • Each muscle fiber has one neuromuscular junction with one motor neuron

• Axon terminal (end of axon) and muscle fiber are separated by gel- filled space called synaptic cleft

• Stored within axon terminals are membrane-bound synaptic vesicles

  • Synaptic vesicles contain neurotransmitter acetylcholine (ACh)

• Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors

• NMJ consists of axon terminals, synaptic cleft, and junctional folds

Generation of an Action Potential Across the Sarcolemma

• Resting sarcolemma is polarized, meaning a voltage exists across membrane

  • Inside of cell is negative compared to outside

• Action potential is caused by changes in electrical charges

• Occurs in three steps

  • 1. Generation of end plate potential (EPP)

  • 2. Depolarization

  • 3. Repolarization

Generation of an Action Potential Across the Sarcolemma

1.End plate potential

• ACh released from motor neuron binds to ACh receptors on sarcolemma

• Causes chemically gated ion channels (ligands) on sarcolemma to open

• Na+ diffuses into muscle fiber

• Some K+ diffuses outward,but not much

• Because Na+ diffuses in, interior of sarcolemma becomes less negative (more positive)

• Results in local depolarization called end plate potential

2.Depolarization: generation and propagation of an action potential (AP)

• If end plate potential causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na+ channels in membrane will open

• Large influx of Na+ through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction

• AP spreads across sarcolemma from one voltage-gated Na+ channel to next one in adjacent areas, causing that area to depolarize

3.Repolarization: restoration of resting conditions

• Na+ voltage-gated channels close, and voltage-gated K+ channels open

• K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage

• Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete

• Ionic conditions of resting state are restored by Na+-K+ pump

• Na+ that came into cell is pumped back out, and K+ that flowed outside is pumped back into cell

Excitation-Contraction (E-C) Coupling

• Excitation-contraction (E-C) coupling: events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction)

• AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR

  • Ca2+ release leads to contraction

• AP is brief and ends before contraction is seen

The Muscle Twitch

• Muscle twitch: simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron

  • Muscle fiber contracts quickly, then relaxes

• Twitch can be observed and recorded as a myogram

  • Tracing: line recording contraction activity

• Three phases of muscle twitch

  • Latent period: events of excitation-contraction

    coupling

    • No muscle tension seenng

  • Period of contraction: cross bridge formation

    • Tension increases

  • Period of relaxation: Ca2+ reentry into SR

    • Tension declines to zero

• Muscle contracts faster than it relaxes

Graded Muscle Responses

• Normal muscle contraction is relatively smooth, and strength varies with needs

  • A muscle twitch is seen only in lab setting or with neuromuscular problems, but not in normal muscle

• Graded muscle responses vary strength of contraction for different demands

  • Required for proper control of skeletal movement

• Responses are graded by:

  • Changing frequency of stimulation

  • Changing strength of stimulation

Wave (temporal) summation results if two stimuli are received by a muscle in rapid succession

• Muscle fibers do not have time to completely relax between stimuli, so twitches increase in force with each stimulus

• Additional Ca2+ that is released with second stimulus stimulates more shortening

If stimuli frequency increases, muscle tension reaches near maximum

• Produces smooth, continuous contractions that add up (summation)

• Further increase in stimulus frequency causes muscle to progress to sustained, quivering contraction referred to as unfused (incomplete) tetanus

If stimuli frequency further increase, muscle tension reaches maximum

• Referred to as fused (complete) tetanus because contractions “fuse” into one smooth sustained contraction plateau

• Prolonged muscle contractions lead to muscle fatigue

Isotonic and Isometric Contractions

• Isotonic contractions: muscle changes in length and moves load

  • Isotonic contractions can be either concentric or eccentric:

    • Concentric contractions: muscle shortens and does work

      • Example: biceps contract to pick up a book

    • Eccentric contractions: muscle lengthens and generates force

      • Example: laying a book down causes biceps to lengthen while generating a force

• Isometric contractions

  • Load is greater than the maximum tension muscle can generate, so muscle neither shortens nor lengthens

• Electrochemical and mechanical events are same in sotonic or isometric contractions, but results are different

  • In isotonic contractions, actin filaments shorten and cause movement

  • In isometric contractions, cross bridges generate force, but actin filaments do not shorten

    • Myosin heads “spin their wheels” on same actin- binding site

Differences between Smooth and Skeletal Muscle Fibers

• Smooth muscle fibers are spindle-shaped fibers

  • thin and short compared with

    skeletal muscle fibers which are wider and much longer

  • Only one nucleus, no striations

• Lacks connective tissue sheaths

  • Contains endomysium only

• Contain varicosities (bulbous swellings) of nerve fibers instead of neuromuscular junctions

  • Varicosities store and release neurotransmitters into a wide synaptic cleft referred to as a diffuse junction

  • Innervated by the autonomic nervous system

• Smooth muscle has less elaborate SR, and no T tubules

  • SR is less developed than in skeletal muscle

    • SR does store intracellular Ca2+, but most calcium used for contraction has extracellular origins

• Sarcolemma contains pouchlike infoldings called caveolae

  • Caveolae contain numerous Ca2+ channels that open to allow rapid influx of extracellular Ca2+

• Smooth muscle fibers are usually electrically connected via gap junctions whereas skeletal muscle fibers are electrically isolated

  • Gap junctions are specialized cell connections that allow depolarization to spread from cell to cell

• There are no striations and no sarcomeres, but they do contain overlapping thick and thin filaments

• Smooth muscle also differs from skeletal muscle in following ways:

  • Thick filaments are fewer and have myosin heads along entire length

    • Ratio of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)

    • Thick filaments have heads along entire length, making smooth muscle as powerful as skeletal muscle

  • No troponin complex

    • Does contain tropomyosin, but not troponin

    • Protein calmodulin binds Ca2+

  • Thick and thin filaments arranged diagonally

    • Myofilaments are spirally arranged, causing smooth muscle to contract in corkscrew manner

  • Intermediate filament–dense body network

    • Contain lattice-like arrangement of non contractile intermediate filaments that resist tension

    • Dense bodies: proteins that anchor filaments to sarcolemma at regular intervals

      • Correspond to Z discs of skeletal muscle

    • During contraction, areas of sarcolemma between dense bodies bulge outward

      • Make muscle cell look puffy

Contraction of Smooth Muscle

• Mechanism of contraction

  • Slow, synchronized contractions

  • Cells electrically coupled by gap junctions

    • Action potentials transmitted from fiber to fiber

  • Some cells are self-excitatory (depolarize without external stimuli)

    • Act as pacemakers for sheets of muscle

    • Rate and intensity of contraction may be modified by neural and chemical stimuli

  • Contraction in smooth muscle is similar to skeletal muscle contraction in following ways:

    • Actin and myosin interact by sliding filament mechanism

    • Final trigger is increased intracellular Ca2+ level

    • ATP energizes sliding process

    • Contraction stops when Ca2+ is no longer available

Contraction of Smooth Muscle

• Contraction in smooth muscle is different from skeletal muscle in following ways:

  • Some Ca2+ still obtained from SR, but mostly comes from extracellular space

  • Ca2+ binds to calmodulin, not troponin

  • Activated calmodulin then activates myosin kinase (myosin light chain kinase)

  • Activated myosin kinase phosphorylates myosin head, activating it

    • Leads to crossbridge formation with actin

• Stopping smooth muscle contraction requires more steps than skeletal muscle

  • Relaxation requires:

    • Ca2+ detachment from calmodulin

    • Active transport of Ca2+ into SR and intracellularly

    • Dephosphorylation of myosin to inactive myosin

Contraction of Smooth Muscle

• Energy efficiency of smooth muscle contraction

  • Slower to contract and relax but maintains contraction for prolonged periods with little energy cost

    • Slower ATPases

    • Myofilaments may latch together to save energy

  • Most smooth muscle maintain moderate degree of contraction constantly without fatiguing

    • Referred to as smooth muscle tone

  • Makes ATP via aerobic respiration pathways

Contraction of Smooth Muscle

• Regulation of contraction

  • Controlled by nerves, hormones, or local chemical changes

  • Neural regulation

    • Neurotransmitter binding causes either graded (local) potential or action potential

    • Results in increases in Ca2+ concentration in sarcoplasm

    • Response depends on neurotransmitter released and type of receptor molecules

      • One neurotransmitter can have a stimulatory effect on smooth muscle in one organ, but an inhibitory effect in a different organ

  • Hormones and local chemicals

    • Some smooth muscle cells have no nerve supply

      • Depolarize spontaneously or in response to chemical stimuli that bind to G protein–linked receptors

      • Chemical factors can include hormones, high CO2, pH, low oxygen

    • Some smooth muscles respond to both neural and chemical stimuli

Contraction of Smooth Muscle

• Special features of smooth muscle contraction

  • Response to stretch

    • Stress-relaxation response: responds to stretch only briefly, then adapts to new length

      • Retains ability to contract on demand

      • Enables organs such as stomach and bladder to temporarily store contents

    • Length and tension changes

      • Can contract when between half and twice its resting length

        • Allows organ to have huge volume changes without becoming flabby when relaxed

Spinal nerve

• Peripheral nerve attached to the spinal cord

Afferent axon

• Axon directed toward central nervous system, conveying sensory information

Dorsal root ganglion

• Nodule on a dorsal root that contains cell bodies of afferent spinal nerve neurons

Efferent axon (eff ur ent)

• Axon directed away from central nervous system, conveying motor commands to muscles and glands

Extrapyramidal System

• Part of the Motor System

• Named “extrapyramidal” to separate it from the tracts that originate in the cortex

• Originates in the brainstem

• Carries motor fibers to the spinal cord

• Responsible for involuntary movement

Amyotrophic Lateral Sclerosis (ALS)

• ALS is characterized for the degeneration of Lower motor neurons in the ventral horn of the spinal cord and brainstem (lower motor neurons).

• There is also degeneration of pyramidal neurons in the motor cortex (upper motor neurons).

• ALS has an incidence of 1-3 people per 100,000 population

• ALS has a prevalence of 5-9 per 100,000 population (20,000 people with ALS at any

  time in USA)

• The onset of symptoms is assumed to occur when approximately an 80% loss of

  motor neurons has been achieved

• There is not treatment for ALS, and the survival rate is 1-10 year with onset at 40-50

  years of age.

• The causes of ALS remain unknown, 10% of cases are familial

NMJ disorders: Genetic defects in myelin

• A. Myelin production and function in the Schwann cell are adversely affected by multiple genetic defects including abnormalities in transcription factors, ABC (ATP-binding cassette) transporters in peroxisomes, and multiple proteins implicated in organizing myelin. In compact myelin thin processes of Schwann cells are tightly wrapped around an axon. Viewed microscopically at high power, the site of apposition of the intracellular races of the Schwann cell membrane appears as a dense line, whereas the apposed extracellular faces are described as the intraperiod line (see definition in part C).

• B. Peripheral axons are wrapped in myelin, which is compact and tight except near the nodes of Ranvier and at focal sites described as "incisures"

• C. The rim of cytoplasm, in which myelin basic protein (MBP) is located, defines the major dense line, whereas the thin layer of residual extracellular space defines the intraperiod line. Three myelin-associated proteins are defective in three different demyelinating neuropathies

NMJ disorders: Morphological abnormalities in MG

• Morphological abnormalities of the neuromuscular junction in myasthenia gravis. At the neuromuscular junction ACh is released by exocytosis of synaptic vesicles at active zones in the nerve terminal. Acetylcholine flows across the synaptic cleft to reach receptors that are concentrated at the peaks of junctional folds. Acetylcholinesterase in the cleft rapidly terminates transmission by hydrolyzing ACh. The myasthenic neuromuscular junction has a reduced number of ACh receptors, simplified synaptic folds, and a widened synaptic space, but a normal nerve terminal.

• Turnover of ACh receptors increases in myasthenia.

• Normal turnover of randomly spaced ACh receptors takes places every 5 to 7 days.

• In myasthenia gravis and experimental myasthenia gravis, the cross-linking of ACh receptors by antibodies facilitates endocytosis and the phagocytic destruction of the receptors, which leads to a two- to threefold increase in the rate of receptor turnover. Binding of antireceptor antibody activates the complement cascade, which is involved in focal lysis of the postsynaptic membrane. This focal lysis is probably primarily responsible for the characteristic morphological alterations of postsynaptic membranes in myasthenia

The vestibular apparatus of the inner ear.

1. The orientations of the vestibular and cochlear divisions of the inner ear are shown with respect to the head.

2. The inner ear is divided into bony and membranous labyrinths. The bony labyrinth is bounded by the petrous portion of the temporal bone. Lying within this structure is the membranous labyrinth, which contains the receptor organs for hearing (the cochlea) and equilibrium (the utricle, saccule, and semicircular canals). The space between bone and membrane is filled with perilymph, whereas the membranous labyrinth is filled with endolymph. Sensory cells in the utricle, saccule, and ampullae of the semicircular canals respond to motion of the head. Adapted, with permission, from lurato 1967)

The left and right horizontal semicircular canals work together to signal head movement.

• Because of inertia, rotation of the head in a counterclockwise direction causes endolymph to move clockwise with respect to the canals. This deflects the stereocilia in the left canal in the excitatory direction, thereby exciting the afferent fibers on this side. In the right canal the afferent fibers are hyperpolarized so that firing decreases.