Locomotion and movement

Locomotion is the voluntary movements resulting in a change in location.

Types of movement in human being

· Amoeboid movement: By pseudopodia. E.g. Macrophages, leucocytes, cytoskeletal elements like microfilaments etc.

· Ciliary movement: By cilia. E.g. ciliary movements in trachea (to remove dust particles and foreign substances), ciliary movements in oviducts (for the passage of ova).

·  Muscular movement: By muscles. E.g. movements of limbs.

HUMAN MUSCULAR SYSTEM

-    Muscles are mesodermal in origin.

-    40-50% of the body weight.

-    Have excitability, contractility, extensibility &elasticity.

-    Based on location, muscles are 3 types:

Striated or voluntary muscles	Non-striated (Visceral) muscles	Cardiac muscles
Striations present	Absent	Present
Voluntary	Involuntary	Involuntary
Attached to skeleton	In visceral organs	In heart wall
Rich blood supply	Poor blood supply	Rich blood supply
Fatigue muscle	Non-fatigue	Non-fatigue
Multinucleate	Uninucleate	Uninucleate
More mitochondria	Less mitochondria	More mitochondria

STRUCTURE OF STRIATED (SKELETAL) MUSCLE

-    Skeletal muscle is made of muscle bundles (fascicles) held together by collagenous connective tissue layer (fascia).

-    Each fascicle contains number of muscle fibres (muscle cells). Muscle fibres are lined by plasma membrane

(sarcolemma) enclosing the sarcoplasm.

-    Each muscle fibre contains myofilaments (myofibrils).

-    Each myofibril has alternate dark (Anisotropic or A-band) and light striations (Isotropic or I-band). This is due to the presence of 2 fibrous contractile proteins- thin Actin filament and thick Myosin filament. I-bands contain actin. A-bands contain actin and myosin.  They are arranged parallel to each other.

-    A-band bears a lighter middle region (H band) formed of only myosin. A thin dark line (M-line) runs through the centre of H-zone.

-    I-band is bisected by a dense dark band called Z-line. Region between two Z-lines is called sarcomere. They are the structural and functional units of a muscle.

Structure of contractile proteins

-    Each actin filament is made of 2 filamentous (F) actins which form double helix.

-    F-actin is a polymer of monomeric Globular (G) actins.

-    Actin contains 2 other proteins (tropomyosin & troponin).

-    Two filaments of tropomyosin run along the grooves of the F-actin double helix.

-    Troponin (having 3 subunits) is seen at regular intervals on tropomyosin. In the resting state a subunit of troponin masks the binding sites for myosin on the actin filaments.

-    Each myosin filament is a polymer of many monomeric proteins called Meromyosins.

-    A meromyosin has 2 parts:  

o  Heavy meromyosin or HMM or cross arm (globular head + short arm): It projects outwards.

o  Light meromyosin or LMM (tail).

-    The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

Mechanism of muscle contraction

According to sliding filament theory, contraction of a muscle fibre takes place by the sliding of actin over the myosin.

The steps are given below:

-    An impulse from the CNS reaches the neuromuscular junction (Motor-end plate) via motor neuron. (A motor neuron + muscle fibres = a motor unit).

Neuromuscular junction is the synapse between a motor neuron and the sarcolemma of the muscle fibre. 

-    The synaptic vesicles release a neurotransmitter (Acetylcholine) that generates an action potential in the sarcolemma. It spreads through the muscle fibre and causes the release of Ca²⁺ ions into the sarcoplasm.

-    Ca binds with a subunit of troponin on actin filaments and removes the masking of active sites for myosin.

-    Using energy from ATP hydrolysis, myosin head binds to exposed active sites on the actin to form a cross bridge. This pulls actin filaments on both sides towards the centre of A-band. The actin filaments partially overlap so that H-zone disappears.

-    The Z- line attached to actins is also pulled inwards. It causes a shortening (contraction) of sarcomere. I-bands get shortened, whereas A-bands retain the length.

-    Myosin releases ADP and Pi and goes back to its relaxed state.  A new ATP binds and the cross-bridge is broken.

-    The ATP is again hydrolyzed by the myosin head and the above process is repeated causing further sliding.

-    When Ca²⁺ ions are pumped back to sarcoplasmic cisternae, actin filaments are again masked. This causes the return of Z-lines back to their original position, i.e., relaxation.

The reaction time of the fibres varies in different muscles. Repeated activation of the muscles leads to the accumulation of the lactic acid causing muscle fatigue.

This is due to anaerobic breakdown of glycogen in muscles.

Red muscle fibres and white muscle fibres

Red (Aerobic) muscles	White muscle
Red coloured due to myoglobin.	White coloured due to the lesser amount of myoglobin
More mitochondria	Less mitochondria
Aerobic metabolism	Anaerobic metabolism
Slow & sustained contraction	Fast contraction for short period

HUMAN SKELETAL SYSTEM

Consists of a framework of bones (206) and few cartilages.

Human skeletal system has 2 parts: axial & appendicular.

1.  Axial skeletal system (80 bones)

  Includes Skull, Vertebral column, Sternum and Ribs.

a.    Skull (29 bones): It includes

o Cranial bones (8):  Include Frontal (1), Parietals (2), Temporals (2), Occipital (1), Sphenoid (1) & Ethmoid (1).

o Facial bones (14): Include Nasals (2), Maxillae (2), Zygomatics (2), Lacrimals (2), Palatines (2), Inferior nasals (2), Mandible (1) and Vomer (1).

o Hyoid bone (1): U-shaped bone seen below buccal cavity.

o Ear ossicles (3 x 2 = 6): Malleus (2), Incus (2) & stapes (2).

b.   Vertebral column

-    Formed of 26 vertebrae. Includes Cervical vertebrae (7), Thoracic vertebrae (12), Lumbar vertebrae (5), Sacral vertebrae (1-fused) and Coccygeal vertebrae (1-fused).

-    Skull articulates with First vertebra (atlas) with the help of 2 occipital condyles (dicondylic skull).

-    Vertebra has a central hollow portion (neural canal) through which the spinal cord passes.

-    Number of cervical vertebrae are 7 in almost all mammals.

-    The vertebral column protects the spinal cord, supports the head and serves as the point of attachment for the ribs and musculature of the back.

c.    Sternum or Breast bone (1)

-    Flat bone on the ventral midline of thorax.

d.   Ribs (12 pairs)

o True ribs (first 7 pairs):  They are attached to thoracic vertebrae and ventrally connected to sternum with the help of Hyaline cartilage.

o Vertebrochondral (false) ribs (8^th, 9^th & 10^th pairs):  They do not articulate directly with the sternum but join the 7^th rib with the help of Hyaline cartilage.

o Floating ribs (11^th & 12^th pairs): They are not connected ventrally (no connection with sternum or other ribs).

-    Each rib has 2 articulation surfaces on its dorsal end and is hence called bicephalic.

2.  Appendicular skeletal system (126 bones)

a.    Bones of fore-limbs (30 x 2 = 60)

Include Humerus (1), Radius (1), Ulna (1), Carpals (wrist bones-8), Metacarpals (palm bones-5) & Phalanges (digits-14).

b.   Bones of hind-limbs (30 x 2 = 60)

Include Femur (thigh bone- 1), Patella (knee cap- 1), Tibia (1) & fibula (1), Tarsals (ankle bones-7), Metatarsals (5) & Phalanges (digits-14).

c.     Pectoral girdle (2x2=4)

-    Include clavicle (2) & scapula (2).

-    Scapula is a large triangular flat bone situated in the dorsal part of the thorax between the second and 7^th ribs.

-    Scapula (shoulder blade) has a slightly elevated ridge (spine) which projects as a flat, expanded process (acromion). The clavicle (collarbone) articulates with this.

-    Below the acromion is glenoid cavity which articulates with the head of humerus to form the shoulder joint.

d.    Pelvic girdle (2)

-    Formed of 2 coxal bones. Each coxal bone is formed by the fusion of 3 bones- Ilium, Ischium & pubis.

-    At the point of fusion of Ilium, Ischium and Pubis is a cavity (Acetabulum) to which the thigh bone articulates.

-    The 2 halves of the pelvic girdle meet ventrally to form pubic symphisis containing fibrous cartilage.

JOINTS

Joints are points of contact between bones, or between bones and cartilages. 3 types.

1.    Fibrous (Immovable) joints: E.g. sutures b/w skull bones.

2.    Cartilaginous joints (Slightly movable joints): Bones are joined together with the help of cartilages. E.g. Joints between the adjacent vertebrae.

3.    Synovial (movable) joints: They have a fluid filled synovial cavity between articulating surfaces of 2 bones.

Types of synovial joint

¨     Ball & socket joint: E.g. Shoulder joint & hip joints.

¨     Hinge joint: E.g. Knee joint, elbow joint etc.

¨     Pivot joint: E.g. Joints b/w atlas & axis.

¨     Gliding joint: E.g. Joints b/w carpals.

¨     Saddle joint: E.g. Joints b/w carpal & metacarpal of thumb.

DISORDERS OF MUSCULAR & SKELETAL SYSTEMS

·   Myasthenia gravis: Auto immune disorder. Affects neuromuscular junction leading to fatigue, weakening and paralysis of skeletal muscles.

·   Muscular dystrophy: Progressive degeneration of skeletal muscles. Mostly due to genetic disorder.

·   Tetany: Rapid spasm in muscle due to low Ca²⁺ in body fluid.

·   Arthritis: Inflammation of joints.

·   Osteoporosis: Age-related disorder characterized by decreased bone mass and increased chances of fractures. Decreased level of estrogen is a common cause.

·   Gout: Inflammation of joints due to accumulation of uric acid crystals.

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Neural control and co-ordination

NERVOUS (NEURAL) SYSTEM

-    Controls and coordinates the body activities. 

-    Conducts and integrates the information.

-    Responses to stimuli.

Neuron

Neuron is the structural and functional unit of neural system. It is composed of

-    Cell body (cyton): Contains cytoplasm, cell organelles and Nissl’s granules (granular bodies).

-    Dendron: Short fibres projecting from the cyton. Their sub branches (dendrites) transmit impulses towards the cyton.

-    Axon: A long fibre which transmit impulses away from the cell body. The branching of axon is called axonite.  Each axonite ends as a bulb-like structure called synaptic knob.

Types of Neurons

·   Unipolar: One axon only. Found usually in embryo.

·   Bipolar: one axon and one dendron. Found in the retina.

·   Multipolar: One axon and 2 or more dendrons. Most common type. Found in the cerebral cortex.

Types of axon

·   Myelinated axon: It is enveloped with Schwann cells that form a myelin sheath around the axon. Found in spinal and cranial nerves. The white coloured area, formed of myelinated nerve fibres is called white matter. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.

·   Non-myelinated axon: Schwann cells present but no myelin sheath. The gray coloured area without myelin sheath is called gray matter. Found in autonomous nerves.

HUMAN NERVOUS (NEURAL) SYSTEM

It Includes,

·   Central neural system: Includes brain & spinal cord.

·   Peripheral nervous system (PNS): Includes all nerves.

PERIPHERAL NEURAL SYSTEM (PNS)

It includes cranial nerves and spinal nerves. PNS has 2 divisions. They are

·   Somatic neural system: Relays impulses from the CNS to skeletal muscles.

·   Autonomic neural system: Transmits impulses from CNS to involuntary organs and smooth muscles. It includes sympathetic & parasympathetic nerves. Sympathetic system prepares body to cope with emergencies, stresses and dangers. It increases heartbeat, breathing rate, constricts arteries, and elevates BP. Parasympathetic system returns the body to a resting state after stressful situations and slows down heartbeat, dilates arteries, lowers BP etc.

Nerve fibres of PNS are 2 types:

·   Afferent (sensory) fibres: Carry impulses from tissues or organs (such as sense organs) to CNS.

·   Efferent (motor) fibres: Carry impulses from CNS to tissues or organs (such as muscles and glands).

GENERATION & CONDUCTION OF NERVE IMPULSES

Impulse transmission is electrochemical. It consists of 3 steps:

1.   Maintenance of resting membrane potential

-    Resting membrane potential is the potential difference existing in a resting neuron (unstimulated neuron).

-    The resting membrane is electrically polarized i.e. outside is +vely charged and inside is –vely charged.

-    The resting membrane has only a poor permeability for Na⁺ and has a higher permeability for K⁺. Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. So there will be an ionic concentration gradient across the resting membrane.

-    These ionic gradients are maintained by active transport of ions by sodium-potassium pump which transports 3 Na⁺ outwards for 2 K⁺ into the cell. So outer surface of axonal membrane possesses a +ve charge while its inner surface becomes –vely charged. This is called polarized state.

-    The resting membrane potential is -70 mV.

2.   Action potential

-    The minimum strength of a stimulus required to stimulate a neuron is called Threshold stimulus.

-    When a neuron is stimulated, Na⁺ pump stops temporarily in stimulated region. It leads to a heavy inflow of Na⁺, so that inner side of the membrane becomes +ve and outside –ve. This reversal of polarity is called depolarization.

-    Immediately Na⁺ pump becomes active and resting membrane potential is restored (repolarisation).

-    Depolarization and subsequent repolarization together constitute the action potential (i.e. a nerve impulse).

-    The action potential is about +30 mV.

3.   Propagation of action potential

-    Action potential formed at a point becomes a stimulus for the next region, i.e. a current flows on the inner surface from stimulated site to adjacent site. As a result, depolarization and repolarisation occur there.

-    This process is repeated and the action potential travels along the entire length of the neuron as nerve impulse.

Saltatory conduction

-    In myelinated fibre, ionic changes and depolarization occur only at the nodes of Ranvier.

-    Between the nodes the myelin sheath acts as insulator. So the action potential jumps from one node to the other. This is called saltatory propagation. Conduction is very fast in myelinated fibre than non-myelinated fibre.

Synaptic transmission of impulses

Synapse is a functional junction between two neurons.

It is 2 types: Electrical & Chemical.

1.  Electrical synapses

-    In this, the membranes of pre- and post-synaptic neurons are in very close proximity. So impulse transmission is similar to the transmission along a single axon.

-    Impulse transmission is faster than in chemical synapse.

-    Electrical synapses are very rare in human system.

2.  Chemical synapses

-    In this, there is a fluid filled space (synaptic cleft) between the presynaptic neuron and postsynaptic neuron.

-    The presynaptic regions have swellings called Synaptic knob (buttons). They contain synaptic vesicles filled with neurotransmitters like acetylcholine (Ach) or adrenaline.

-    When the impulse reaches the presynaptic region, the synaptic vesicles break and release the neurotransmitters. It diffuses across the synaptic gap to combine with the receptors found on the post synaptic neuron.

-    Based on neurotransmitter, chemical synapses are 2 types:

o Cholinergic synapse: Neurotransmitter is Acetylcholine.

o Adrenergic synapse: Neurotransmitter is Adrenalin.

REFLEX ACTION

It is the rapid, involuntary and unconscious actions of the body brought about by any part of the CNS through sudden stimulation from receptors. E.g.

¨  Sudden withdrawal of the hand when it touches a hot object.

¨  Touching lips of a nursing baby evokes a sucking reflex.

¨  Closing of the eyelids when a beam of light falls on them

¨  Knee jerk phenomenon.

¨  If a child sees or smells some food unknown to him he does not salivate. But if he sees or smells that food every time before tasting it, he begins to salivate (conditioned reflex).

Reflex arc (pathway of impulses in a reflex action) consists of 

§ A receptor organ receiving the stimulus.

§ A sensory (afferent) neuron, which transmits the impulses from sense organ to CNS.

§ An intermediate neuron (connector neuron) that connects the sensory and motor neuron.

§ A motor (efferent) neuron that conducts the impulse from the CNS to effector organ.

§ An effector organ (muscle/ gland): responds to impulse.

SENSE ORGANS (EYE, EAR, NOSE, TONGUE & SKIN)

1.  EYE

-    Visual organ present in the orbit of the skull and held in place by 6 muscles.

-    The lachrymal (tear) glands secrete tears which contain a bactericidal enzyme called Lysozyme.

-    The excess tear is passed out by nasolachrymal duct that opens into the nasal chamber.

-    Eyeball has 3 layers- sclera, choroid and retina.

a.    Sclera

-    Outermost layer that protects the inner structure and helps to maintain rigidity of the eyeball.

-    The transparent, anterior part of sclera is called cornea.

-    Conjunctiva (a transparent covering) protects cornea.

b.    Choroid

-    Highly vascular middle layer.

-    It contains melanocytes which produces melanin.

-    Just behind the cornea, choroid forms a circular opaque disc called Iris, which has a central opening (pupil).

-    Around the base of iris, ciliary body is present which contains ciliary muscles.

-    A biconvex lens is present just behind the iris, which is held in place by suspensory ligaments.

-    Lens can change its focal length to view near and distant objects. This is called power of accommodation. It is brought about by altering convexity of lens by ciliary body.

c.     Retina

-    Innermost layer formed of 3 layers of cells-from inside to outside- ganglion cells, bipolar cells & photoreceptors.

-    Photoreceptor cells are 2 types: cones and rods.

-    Cone cells:

§ For photopic (day light) vision & colour vision.

§ Contain iodopsin (violet coloured photopigment).

§ There are 3 types of cones for discriminating the primary colours (red, green and blue).

§ Defect in cone cells results in colour blindness.

-    Rod cells:

§ For scotopic vision (vision in dim light).

§ Contain rhodopsin (purple coloured photopigment synthesized from vitamin A).

§ Deficiency of vitamin A leads to lack of rhodopsin, which results in night blindness.

-    The spot at the back of eye, where optic nerve originates is called blind spot. Rods and cones are absent here.

-    Lateral to blind spot is a depressed area called Fovea centralis (yellow spot or macula lutea) that contains only cones and no rods. It is the area of keenest vision.

-    The aqueous chamber lies between the cornea and lens. It is filled with aqueous humor, secreted by the ciliary body. It nourishes the cornea and lens. The vitreous chamber lies behind the lens. It is filled with vitreous humor.

-    Both the fluids help in maintaining the form of eyeball and in focusing the light rays.

Mechanism of vision

-    Light reflected from an object enters the eye through cornea and lens. These are focused on retina and generate potentials (impulses) in rods and cones.

-    Eye contains photosensitive compounds (photopigments) formed of opsin (a protein) and retinal (an aldehyde of vitamin A). Light induces the dissociation of retinal from opsin. This changes membrane permeability.

-    As a result, potential differences are generated in the photoreceptor cells. It generates action potentials in the ganglion cells through the bipolar cells.

-    These are transmitted by optic nerves to visual cortex of brain. Impulses are analyzed and the image is recognized based on the earlier memory and experience.

-    Human being looks at objects with both eyes. So 2 retinal images are formed but only one image is seen. This is called binocular vision.

2.  EAR (STATO-ACOUSTIC ORGAN)

Organ for hearing and balancing. It has 3 divisions- External ear, middle ear and inner ear.

a.   External ear

-    Consists of pinna (ear lobe) & auditory meatus (ear canal).

-    The ear canal contains numerous ceruminous glands (modified sweat glands) which secrete wax (cerumen).

-    Hairs are present at the opening of ear canal.

-    Wax and hairs prevent entry of foreign objects like insects.

-    Ear canal ends in tympanic membrane (ear drum).

b.   Middle ear

-    Consists of tympanic cavity and ear ossicles.

-    Tympanic cavity is an air filled space that separates the external and inner ear portions.

-    Tympanum is a semi-transparent membrane covered by a thin layer of skin on its outer surface and by mucous membrane on the inside.

-    An auditory tube (Eustachian canal) connects middle ear to the pharynx. It maintains an equal pressure on either side of the eardrum and thus protects the tympanic cavity.

-    Ear ossicles include 3 small bones namely Malleus, Incus and stapes. Malleus is attached to tympanum.

-    Stapes is the smallest bone of the body. It is attached to membrane of oval window (fenestra ovalis) of inner ear.

c.    Inner ear

-    It consists of bony labyrinth and membranous labyrinth.

-    Bony labyrinth is a cavity filled with perilymph.

-    The membranous labyrinth consists of 3 semicircular canals, vestibule (utriculus + sacculus) and cochlea.

-    Of 3 semicircular canals, 2 are vertical and one is horizontal. The canals are attached to utriculus. One end of each canal has a bulging called ampulla. Inside it is a jelly-like lump called crista ampullaris. The long cilia from many cells of crista are grouped together in a bundle (cupula) that is covered with a gelatinous coating.

-    The inner lining of the ampulla and vestibule are formed of sensory hair cells.

-    Small masses of calcareous particles called otoliths (otoconia or ear stones) are located within the vestibule. The hairs of the sensory cells are in contact with otoliths.

-    Cristae (receptors on the ampulla) and macula (receptors on the utriculus & sacculus) are the organs of equilibrium and posture of body.

-    Cochlea (organ of hearing) is a coiled structure having 3 canals namely, upper scala vestibula, middle scala media and lower scala tympani.

-    A membrane called Reissner’s membrane separates the scala vestibula and scala media. The scala media and scala tympani are separated by basilar membrane.

-    The outer canals (S. vestibula and S. tympani) are filled with perilymph and scala media is filled with endolymph.

-    Terminally, Scala vestibula and S. tympani communicate with each other by a small opening called helicotrema.

-    Resting on the basilar membrane and projecting into scala media is complex receptor organ called Organ of Corti. It consists of row of sensory hair cells. The hairs of these cells project upwards and lie in close contact with tectorial membrane, which projects above them.

Mechanism of hearing

Pinna collects sound waves → waves pass through ear canal → reach the tympanic membrane → tympanic membrane vibrates → vibrations are transmitted to  ear ossicles & oval window → perilymph in the vestibular canal vibrates → vibrations reach the scala tympani and force the basilar membrane to vibrate → the hair endings of the sensory hair cells press against the tectorial membrane → sensory hair cells are excited → the impulses produced are carried by auditory nerve  to the auditory centre of the brain.

The sound pitch is determined by the frequency of sound. Human ear register sounds of frequencies b/w 20- 20,000 Hz.  

CHEMORECEPTORS (NOSE & TONGUE)

3.  NOSE (Olfactoreceptors): Organ of smell

-    The olfactory epithelium consists of Olfactoreceptors (confined to roof of the nasal cavity) and supporting cells.

-    The odorants dissolve in the mucous (secreted by supporting cells) and attach to receptor cells. The receptor cells generate olfactory impulses which are transmitted into brain by olfactory nerves.

4.  TONGUE (Gustatoreceptors): Organ of taste

-    Taste buds (Gustatoreceptors + supporting cells) are seen around the bases of taste papillae.

-    The food mixed with saliva enters the taste pores of the buds and stimulate the sensory cells. These impulses are carried to the brain, which gives the sense of the taste.

-    4 primary tastes are sweet, salt, sour and bitter.

5.  SKIN (Cutaneous receptors)

-    Largest sense organ.

It contains receptors for heat, cold, touch, pain & pressure.

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