Plasma Membrane

Plasma membrane is a thin outer membrane, It is a double layered measuring about 4.5 nm and made of phospholipids, cholesterol, glyco-lipid, & carbohydrate.

Functions:

1. Separate the cytoplasm inside a cell from extra cellular fluid.

2. Separate cell from one another

3. Provide an abundant surface on which chemical reaction can occur.

4. Regulate the passage of materials in to and out of cells. It also let some things in and keeps others out.

Movement across-cell membrane

•Movements a cross membrane takes place in two ways.

•These are passive and active movements.

•Passive movement(被动转运) uses its own kinetic energy(动能） (energy of motion）

•Active movement(主动转运) consumes energy in the form of ATP(Adenosine triphosphate).

Passive movement

a. Simple diffusion (单纯扩散), the random movements of molecules from area of high concentration to the area of low concentration.

–Example oxygen, carbon dioxide, and nitrogen gases; fatty acids; steroids; and fat-soluble vitamins (A, D, E, and K). Small, uncharged polar molecules such as water, urea, and small alcohols also pass through the lipid bilayer by simple diffusion.

b. Facilitated diffusion(易化扩散), larger molecules, which are not soluble in lipid need protein channel to pass through the plasma membrane. No direct energy needed. 

–Example: - Ions (K (potassium ions),Cl (chloride ions), Na (sodium ions) Ca (calcium ions), glucose, some vitamins, Amino acid passes through the cell membrane.

Active movements

•Substances move through a selectively permeable membrane from areas of low concentration on side of a membrane to an area of higher concentration on the other side.

•This is against concentration gradient. Therefore, it requires energy.

Phagocytosis（吞噬作用）

•Phagocytosis (吞噬作用）(Cell Eating) is a form of endocytosis(内吞作用)in which the cell engulfs large solid particles, such as worn-out cells, whole bacteria, or viruses.

•Only a few body cells, termed phagocytes(吞噬细胞), are able to carry out phagocytosis.

Cytoplasm(细胞质)

•Cytoplasm(细胞质) is located between the nucleus and plasma membrane

  –Cytosol (细胞液), the fluid portion of cytoplasm, contains water, dissolved solutes, and suspended particles. cytoplasm is the substance that surrounds

  –Organelles(细胞器)(little organs). Each type of organelle has a characteristic shape and specific functions.

Cytosol

•The cytosol (intracellular fluid) is the fluid portion of the cytoplasm that surrounds organelles and constitutes about 55% of total cell volume.

  Although it varies in composition and consistency from one part of a cell to another, cytosol is 75–90% water plus various dissolved and suspended components.

•Among these are different types of ions, glucose, amino acids, fatty acids, proteins, lipids, ATP, and waste products.

Organelles

Organelles are specialized portion of the cell with a characteristic shape that assume specific role in growth, maintenance, repair and control.

•Golgi Complex(高尔基氏体), near to the nucleus. It consist 4-8 membranous sacs. It process, sort, pack & deliver protein to various parts of the cell.

•Mitochondria(线粒体), a small, spherical, rod shaped or filamentous structure. It generates energy.

•Lysosomes (溶酶体)appear as membrane enclosed spheres. They contain powerful digestive (hydrolytic enzyme capable of breaking down many kinds of molecules.

Nucleus

•The nucleus is a oval-shaped structure. It is a large organelle that houses most of a cell’s DNA.

•Within the nucleus, Human somatic (body) cells have 46 chromosomes, 23 inherited from each parent.

• a single molecule of DNA (脱氧核糖核酸) (deoxyribonucleic acid); associated with several proteins, contains thousands of hereditary units called genes(基因) that control most aspects of cellular structure and function.

•Most cells have a single nucleus, although some, such as mature red blood cells, have none. In contrast, skeletal muscle cells and a few other types of cells have multiple nuclei.

CELL DIVISION

•Most cells of the human body undergo cell division, the process by which cells reproduce themselves.

The two types of cell division—somatic cell division (Mitosis有丝分裂)and reproductive cell division(meiosis减数分裂).

Mitosis有丝分裂

•In somatic cell division, a cell undergoes a nuclear division to produce two identical cells, each with the same number and kind of chromosomes as the original cell.

• Somatic cell division replaces dead or injured cells and adds new ones during tissue growth.

•In somatic cell division, a cell undergoes a nuclear division to produce two identical cells, each with the same number and kind of chromosomes as the original cell.

• Somatic cell division replaces dead or injured cells and adds new ones during tissue growth.

Meiosis (减数分裂)

•Reproductive cell division is the mechanism that produces gametes, the cells needed to form the next generation of sexually reproducing organisms.

•This process consists of a special two step division called meiosis (减数分裂), in which the number of chromosomes in the nucleus is reduced by half.

•An important aspect of homeostasis is maintaining the volume and composition of body fluids, watery solutions containing dissolved chemicals that are found inside cells as well as surrounding them.

•The fluid within cells is intracellular fluid, (ICF)(细胞内液).

The fluid outside body cells is extracellular fluid (ECF)(细胞外液).

Control of Homeostasis

The nervous system and the endocrine system, working together or independently, can usually bring the internal environment back into balance.
The nervous system regulates homeostasis by sending electrical signals known as nerve impulses (action potentials) to organs.
The endocrine system includes many glands that secrete hormones into the blood.
Nerve impulses typically cause rapid changes, but hormones usually work more slowly.
Both means of regulation, work toward the same end, usually through negative feedback systems.

Feedback Systems(反馈系统)

The body can regulate its internal environment through many feedback systems.
A feedback system includes three basic components — a receptor(感受器), a control center(中枢), and an effector(效应器).
A feedback system is a cycle of events in which the status of a body condition is monitored, evaluated, changed, remonitored, reevaluated, and so on.
Each monitored variable, such as body temperature, blood pressure, or blood glucose level, is termed a controlled condition.

Negative feedback system

A negative feedback(负反馈) system reverses a change in a controlled condition.

Consider the regulation of blood pressure.

Positive feedback system

A positive feedback(正反馈) system tends to strengthen or reinforce a change in one of the body’s-controlled conditions.

Normal childbirth provides a good example of a positive feedback system.

Four groups of Tissue

•Epithelial tissue:

covers body surfaces and lines hollow organs, body cavities, and ducts. It also forms glands.

•Connective tissue:

is one of the most abundant and widely distributed tissues in the body.

•Muscle tissue:

produces movement through its ability to contract. This constitutes skeletal, smooth and cardiac muscles.

•Nerve tissue:

found in the brain, spinal cord and nerves. It responds to various types of stimuli and transmits nerve impulses.

Epithelial tissue:

•They are subdivided in to:

- Covering & lining epithelium: it forms the outer covering of external body surface and outer covering of some internal organs. It lines body cavity, interior of respiratory & gastro intestinal tracts, blood vessels & ducts and make up along with the nervous tissue (the parts of sense organs for smell, hearing, vision and touch).

- Glandular epithelium: it forms the glands.

Basement membrane

•The basement membrane is a thin extracellular layer that commonly consists of two layers, the basal lamina and reticular lamina.

•The basement membrane functions as a point of attachment and support for the overlying epithelial tissue.

•Epithelial tissue has its own nerve supply, but is it lacks its own blood supply. The blood vessels that bring in nutrients and remove wastes are located in the adjacent connective tissue.

•Exchange of substances between epithelium and connective tissue occurs by diffusion.

Connective tissue:

•Connective tissue is one of the most abundant and widely distributed tissues in the body.

•In its various forms, connective tissue has a variety of functions.

•It binds together, supports, and strengthens other body tissues; protects and insulates internal organs; compartmentalizes structures such as skeletal muscles; serves as the major transport system within the body (blood, a fluid connective tissue); is the primary location of stored energy reserves (adipose(动物脂肪), or fat tissue)

Five Types of Connective Tissue are:

(1) loose connective tissue:

•The fibers of loose connective tissue are loosely intertwined between cells.

•The types of loose connective tissue are areolar(细隙的)connective tissue, adipose(脂肪多的)tissue, and reticular (网状的)connective tissue.

(2) dense connective tissue:

•contains more numerous, thicker, and denser fibers (packed more closely) but considerably fewer cells than loose connective tissue.

•There are three types: dense regular connective tissue, dense irregular connective tissue, and elastic connective tissue.

(3) cartilage: 

•Unlike other connective tissue, cartilages have no blood vessels and nerves. It consists of a dense network of collagenous fibers(胶原纤维) and elastic fibers(弹性纤维).

•Cartilages are classified in to hyaline, fibro & elastic cartilage 

•Cartilage provides strength and elasticity and maintains the shape of certain organs like epiglottis(会厌), larynx, external part of the ear and Eustachian tube(耳咽管).

(4) bone tissue:

•Bones store calcium and phosphorus; house red bone marrow, which produces blood cells; and contain yellow bone marrow, a storage site for triglycerides.

•Bones are organs composed of several different connective tissues, including bone, the periosteum骨膜, red and yellow bone marrow, and the endosteum骨内膜

•Bone tissue is classified as either compact or spongy, depending on how its extracellular matrix 模型 and cells are organized.

(5) liquid connective tissue (blood tissue and lymph):

1.Blood tissue

–Blood tissue is a connective tissue with a liquid extracellular called blood plasma, a yellow fluid that consists mostly of water with a wide variety of dissolved substances—nutrients, wastes, enzymes, plasma proteins, hormones, respiratory gases, and ions. And blood cells.

2. Lymph

–Lymph is the extracellular fluid that flows in lymphatic vessels. It is a connective tissue that consists of several types of cells in a clear liquid extracellular that is similar to blood plasma.

Muscle tissue:

•Muscle tissues are grouped in to skeletal, cardiac and smooth muscle tissue.

•- Skeletal muscle tissue are attached to bones, it is voluntary, cylindrical, multinucleated & striated

•- Cardiac muscle tissue: It forms the wall of the heart; it is involuntary, uni-nucleated and striated.

•- Smooth muscle tissue: located in the wall of hallow internal structure like Blood vessels, stomach, intestine, and urinary bladder. It is involuntary and non-striated.

Nervous tissue:

•Nervous tissue contains two principal cell types. These are the neurons and the neuroglia.

•Neurons are nerve cells, sensitive to various stimuli. It converts stimuli to nerve impulse. Neurons are the structural and functional unit of the nervous system. It contains 3 basic portions. These are cell body, axons and dendrites.

•Neuroglias are cells that protect, nourish and support neurons. Clinically they are important because they are potential to replicate and produce cancerous growths.

Skeletal and Muscular System

Functions of the Bone

•1. Support: it forms the internal framework

•2. Protection: bones protect soft body organs.

•3. Movement: muscles attached to the bone to move the body and its part.

•4. Storage: fat is stored in the internal cavities of bones. Bone it self-serves as a storehouse of minerals. The most important being calcium and phosphors.

•5. Blood cell formation: it occurs with in the marrow cavities of certain bones.

Functions of the Muscles

•The function of most muscles is to produce movements of body parts.

•A few muscles function mainly to stabilize bones.

•Muscles of the Head That Produce Facial Expressions.

Functions of the Joints

•Direct connect joints--Connected bone and bone.

•Indirect connect(articulation)—perform  movement

•Direct connect

1.fibrous joint

Spine of vertebra, head

2.cartilaginous joint

Cartilago costalis, Vertebrae

3.Synostosis—Hip bone, sacrum

•Indirect connect(articulation)

    ---Joints on the limbs

structure of Articulation

1.Articular surface--articular cartilage

2.Articular capsule

1.Fibrous membrane

2.Synovial membrane– produce synovial fluid

3.Articular cavity

The Muscles

Muscles contracting – relaxing

Muscles contracting – relaxing and Joint movement

The Muscles

•Origin:

–One end of the muscle is attached by a tendon to a bone that remains relatively stable, usually closer to the midline of the body.

•Insertion:

–The tendon on the other end of the muscle is attached to the bone moved by the muscle.

TYPES OF MOVEMENTS AT SYNOVIAL JOINTS

••Flexion弯曲

••Extension伸展

••Hyperextension过度伸展

••lateral flexion侧屈

••Abduction外展

••Adduction内收

••Circumduction环行（运动）

Flexion弯曲

•Flexion is a bending movement around a joint in a limb (as the knee or elbow) that decreases the angle between the bones of the limb at the joint.

Extension伸展

•Extension is the opposite of flexion, describing a straightening movement that increases the angle between body parts.

Hyperextension过度伸展

•Hyperextension is the joint movement extension of a body part to a maximum level or past the position of normal extension.

lateral flexion侧屈

•Lateral flexion describes the active or passive bending movement of a body part in the lateral direction, that is to say sidewards.

Abduction外展

•Abduction is movement away from the mid-line of the body.

Adduction内收

•Adduction is the movement of a body part toward the body's midline.

Circumduction环行（运动）

•Circumduction is a conical 圆锥形的movement of a limb extending from the joint (e.g. shoulder or hip) at which the movement is controlled.

•True circumduction allows for 360 degrees of movement.

Muscles move the eyeballs

Muscles that move the eyeballs are called extrinsic eye muscles because they originate outside the eyeballs (in the orbit) and insert on the outer surface of the sclera (“white of the eye”). The extrinsic eye muscles are some of the fastest contracting and most precisely controlled skeletal muscles in the body.

••Three pairs of extrinsic eye muscles control movements of the eyeballs:

••(1) superior and inferior recti (上、下直肌)

••(2) lateral and medial recti (外、内直肌)

••(3) superior and inferior obliques. (上、下斜肌)

Central nervous system (CNS)

•Brain
•Cerebrum
   •Left hemispheres
   •Right hemispheres
   (Connected by corpus callosum)
•Cerebellum
•The brain Stem
   –Thalamus （丘脑）
       »Hypothalamus （下丘脑）
            •pituitary gland（脑垂体）
   –Midbrain (中脑)
   –Pons (脑桥）
   –Medulla oblongata（延髓）

•Sensory nerves
   –Transmit messages from the body’s receptors to the CNS
   –Nerve impulses pass into the spinal cord and brain 

•Motor nerves
   –Transmit messages from CNS to the muscles or other structures that respond to stimulate.
   –Nerve impulses from the spinal cord and brain to muscles and glands

Function:
   •Sensory function
     –internal ---Maintain homeostasis
     –External---felling
   •Integrative function整合功能.
     –Analyzing
     –Storing (Memories)
    –making decisions
    –Perceptions知觉
    –Behaviors
  •Motor function
    –Muscle movements
    –Gland secrete

Peripheral nervous system (PNS)
  12 pairs of cranial nerves
  31 pairs of spinal nerves

PNS includes all nervous tissue outside the CNS. Components of the PNS include cranial nerves and their branches, spinal nerves and their branches, ganglia, and sensory receptors.

Sensory ---- that convey information from somatic receptors in the head, body wall, and limbs and from receptors for the special senses of vision, hearing, taste, and smell to the CNS .

Motor
   Somatic nervous system (SNS)---conduct impulses from the CNS to skeletal muscles.
   Autonomic nervous system (ANS) ---conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary.

Sympathetic nerves division
Parasympathetic nerves division.

•Neurons (nerve cells) possess electrical excitability, the ability to respond to a stimulus and convert it into an action potential(nerve impulse)

Parts of a Neuron
  (1) a cell body - a nucleus, cytoplasm, cellular organelles
  (2) Dendrites (树突)- receiving or input portions of a neuron
  (3) an axon- propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell.

Myelination (髓鞘 )
Axons surrounded by a multilayered lipid and protein covering, called the myelin sheath, are said to be myelinated. The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction.

Axons without such a covering are said to be unmyelinated

Central nervous system (CNS)
The CNS processes many different kinds of incoming sensory information.
It is also the source of thoughts, emotions, and memories.
Most nerve impulses that stimulate muscles to contract and glands to secrete originate in the CNS.

NTEGRATIVE FUNCTIONS OF THE CEREBRUM
Wakefulness and Sleep
    –hypothalamus
Learning and Memory
    –cerebral cortex
    – cerebellum

Hypothalamus
  The hypothalamus is the major control and integration center of the ANS.
The hypothalamus receives sensory input related to visceral functions, smell, and taste, changes in temperature, and levels of various substances in blood.
Output from the hypothalamus influences autonomic centers in both the brain stem (such as the cardiovascular, salivation, swallowing, and vomiting centers) and the spinal cord (such as the defecation and urination reflex centers in the sacral spinal cord). 

The corticospinal 皮质脊髓的pathways.
  The corticospinal pathways conduct nerve impulses for the control of muscles of the limbs and trunk.

Neural circuits 神经回路 :
  The CNS contains billions of neurons organized into complicated networks called neural circuits

•Cranial nerve : any of the nerves arising from the brainstem and exiting to the periphery of the head through skull openings.

In the human body there are 12 pairs of cranial nerves.

1. Olfactory nerve: Sense of smell.
2. Optic nerve: Ability to see.
3. Oculomotor nerve: Ability to move and blink your eyes.
4. Trochlear nerve: Ability to move your eyes up and down or back and forth.
5. Trigeminal nerve: Sensations in your face and cheeks, taste and jaw movements.
6. Abducens nerve: Ability to move your eyes.
7. Facial nerve: Facial expressions and sense of taste.
8. Auditory/vestibular nerve: Sense of hearing and balance.
9. Glossopharyngeal nerve: Ability to taste and swallow.
10. Vagus nerve: Digestion and heart rate.
11. Accessory nerve (or spinal accessory nerve): Shoulder and neck muscle movement.
12. Hypoglossal nerve: Ability to move your tongue.
    
    •Spinal nerve: A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.
    •In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column.
    
    •The somatic nervous system (SNS or voluntary nervous system) is the part of the peripheral nervous system associated with the voluntary control of body movements via skeletal muscles.
    
    •The somatic nervous system consists of afferent nerves or sensory nerves, and efferent nerves or motor nerves.
    
    Ganglia (神经节) are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside the brain and spinal cord.
    Ganglia are closely associated with cranial and spinal nerves.
    
    Sympathetic trunk ganglia
    sympathetic trunk ganglia are arranged anterior and lateral to the vertebral column, one on either side. Typically, there are 3 cervical, 11 or 12 thoracic, 4 or 5 lumbar, 4 or 5 sacral sympathetic trunk ganglia, and 1 coccygeal ganglion尾神经节.
    The right and left coccygeal ganglia are fused together and usually lie at the midline.
    
    Preganglionic neuron
    The first of the two motor neurons in any autonomic motor pathway is called a preganglionic neuron.
    Its cell body is in the brain or spinal cord, and its axon exits the CNS as part of a cranial or spinal nerve.
    
    Postganglionic Neurons
    the postganglionic neuron lies entirely outside the CNS.
    Its cell body and dendrites are located in an autonomic ganglion, where it forms synapses with one or more preganglionic axons.
    
    • Parasympathetic ganglia are the autonomic ganglia of the parasympathetic nervous system.
    
    Nerve impulse
    •A nerve impulse is the way nerve cells (neurons) communicate with one another. Nerve impulses are mostly electrical signals along the dendrites to produce a nerve impulse or action potential动作电位.
    •The action potential is the result of ions moving in and out of the cell. Specifically, it involves potassium (K+) and sodium (Na+) ions.
    •The ions are moved in and out of the cell by potassium channels, sodium channels and the sodium-potassium pump.
    
    Ion Channels
    •When ion channels are open, they allow specific ions to move across the plasma membrane.
    
    A synapse is the functional junction between one neuron and another, or between a neuron and an effector such as a muscle or a gland.
    
    Electrical Synapses
    action potentials (impulses) conduct directly between adjacent cells through structures called gap junctions.
    
    Chemical Synapses
    --- NEUROTRANSMITTERS(神经递体)
    acetylcholine (ACh)乙酰胆碱
    amino acids氨基酸
    Gamma aminobutyric acid(GABA) γ-氨基丁酸
    Norepinephrine (NE)去甲肾上腺素
    5-hydroxytryptamine (5-HT) 5-羟色胺
    
    The autonomic nervous system contributes to homeostasis by responding to subconscious visceral sensations and exciting or inhibiting smooth muscle, cardiac muscle, and glands.
    
    ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons.
    
    The ANS usually operates without conscious control.
    A continual flow of nerve impulses from
     (1) autonomic sensory neurons in visceral organs and blood vessels propagate into
    (2) integrating centers in the central nervous system (CNS). Then, impulses in
     (3) autonomic motor neurons propagate to various effector tissues, thereby regulating the activity of smooth muscle, cardiac muscle, and many glands.
    

Sympathetic nerves division

The sympathetic nervous system controls “fight-or-flight” response. Danger or stress activates the sympathetic nervous system:

• Eyes: Enlarge  pupils to let more light in and improve  vision.
• Heart: Increase  heart rate to improve the delivery of oxygen to other parts of  body.
• Lungs: Relax  airway muscles to improve oxygen delivery to  lungs.
• Digestive tract: Slow down digestion so its energy is diverted to other areas of the body.
• Liver: Activate energy stores in  liver to an energy that can be used quickly.

Parasympathetic nerves division.

parasympathetic nervous system’s job is usually to relax or reduce the body’s activities. 

• Eyes: It constricts pupils to limit how much light enters the eyes. It also makes changes that can help improve close-up vision, and causes tear production in the eyes.
• Nose and mouth: It makes glands in mouth produce saliva, and glands in nose produce mucus. This can be helpful with digestion and breathing during times of rest.
• Lungs: It tightens airway muscles and ultimately reduces the amount of work the lungs do during times of rest.
• Heart: It lowers your heart rate and the pumping force of your heart.
• Digestive tract: It increases the rate of digestion and diverts energy to help  digest food. It also make pancreas to release insulin, helping body break down sugars.
• Waste removal: It relaxes the muscles that help the control when urinate or defecate.
• Reproductive system: It manages some of the body’s sexual functions.

The ears are intricate sensory organs responsible for the sense of hearing and, in the case of the inner ear, also for balance. 

Anatomy of the Ear: The ear is divided into three main parts:

1. Outer Ear: This includes the visible portion of the ear (pinna or auricle), the ear canal (external auditory meatus) and the eardrum (tympanic membrane). The pinna captures sound waves from the environment and directs them into the ear canal.
2. Middle Ear: The ear canal connects to the middle ear, where three small bones called the ossicles are located: the hammer (malleus), anvil (incus), and stirrup (stapes). These bones transmit and amplify sound vibrations from the eardrum (tympanic membrane) to the inner ear. The middle ear is also connected to the back of the throat through the Eustachian tube, which helps equalize air pressure. The oval window is a small membrane-covered in the middle ear. The oval window is crucial in transmitting sound vibrations from the middle ear to the fluid-filled cochlea, where the actual process of hearing begins. It serves as a connection point between the mechanical vibrations of the ossicles and the fluid-filled environment of the inner ear, allowing for the efficient transfer of sound energy. 
3. Inner Ear: The inner ear contains the cochlea, a spiral-shaped, fluid-filled structure responsible for converting sound vibrations into electrical signals that the brain can interpret. It also houses the vestibular system, which is responsible for maintaining balance. The semicircular canals are a set of three fluid-filled, tubular structures located within the inner ear. They play a crucial role in the body's sense of balance and spatial orientation, allowing us to maintain our balance and coordinate movements.

Hearing Process: The process of hearing involves several steps:

1. Sound Waves: Sound waves from the environment enter the ear canal and travel to the eardrum.
2. Eardrum Vibrations: When sound waves reach the eardrum, they cause it to vibrate. The eardrum separates the outer ear from the middle ear.
3. Ossicle Movement: The vibrations from the eardrum are transmitted to the ossicles in the middle ear. These bones amplify the vibrations.
4. Cochlear Fluid: The stirrup bone in the middle ear transmits vibrations to the oval window, a membrane-covered opening in the cochlea. This creates pressure waves within the cochlear fluid.
5. Hair Cell Activation: Within the cochlea, specialized hair cells are responsible for translating the pressure waves into electrical signals. The movement of the fluid causes the hair cells to bend. This bending action triggers the release of neurotransmitters, creating electrical impulses in the auditory nerve.
6. Auditory Nerve: The electrical impulses generated by the hair cells travel along the auditory nerve to the brain's auditory cortex, where they are processed and interpreted as sound.

Balance (Vestibular) System:The inner ear's vestibular system plays a crucial role in maintaining balance and spatial orientation. It consists of semicircular canals filled with fluid and tiny hair cells that detect head movements and changes in position. This information is sent to the brain, which helps us maintain our balance and coordination.