CSEC Biology

CSEC ® BIOLOGY

Anne Tindale

Reviewers: Usha Chickoree, Shaun deSouza

Locomotion in humans

Learning objectives

• Identify the major bones of the human skeleton

• Draw, label and annotate a diagram of a long bone of a forelimb

• Relate the structure of the human skeleton to its functions

• Identify the different types of joints found in the human skeleton.

• Explain the action at movable joints.

• Describe the mechanism of movement in a human forelimb.

Structure of the human skeleton

The human skeleton serves as a framework for the body, and it can be divided into the axial skeleton and the appendicular skeleton. The skeleton of an adult human is made of 206 bones of different shapes and sizes that are held together at joints by ligaments. Movement in humans is brought about by skeletal muscles working across these joints. The major bones of the skeleton are shown in Figure 12.3.

cranium skull

clavicle (collar bone)

scapula (shoulder blade)

rib cage

vertebral column

hip bone of the pelvic girdle

thumb

pectoral girdle

sternum (breast bone)

humerus

ulna radius forelimb (arm)

carpals metacarpals

patella (knee cap)

big toe

femur

tibia

phalanges hindlimb (leg)

bula

tarsals metatarsals

phalanges

lower jaw

upper jaw

Figure 12.3 The human skeleton

The skeleton is made mainly from two types of connective tissue: bone and cartilage

• Bone is a hard tissue that makes up the bulk of the skeleton. It is composed of living cells embedded in a hard, non-living matrix made up of calcium salts, mainly calcium hydroxyapatite (Ca10(PO4)6(OH)2), together with some tough, rubbery collagen (protein) fibres. The cells in their matrix are arranged in rings around central canals that contain blood vessels to supply the cells with oxygen and nutrients, and to remove carbon dioxide and other waste.

• Cartilage is an elastic, rubbery and flexible tissue. It is composed of living cells surrounded by a non-living matrix made up mainly of collagen fibres. Cartilage does not have blood vessels running through it.

The axial skeleton

The axial skeleton forms the central axis of the body and consists of the skull, the vertebral column, the ribs and the sternum

Skull

The skull is made up of the cranium and upper jaw, which are fused, and the lower jaw, which articulates with the upper jaw. The cranium consists of eight fused bones, and it encloses and protects the brain and sense organs of the head. Movement of the lower jaw enables a person to chew and talk.

Vertebral column

The vertebral column or spine is composed of 33 bones known as vertebrae. Each vertebra has a cylindrical body, several projections for muscle attachment and a hole in the centre through which the spinal cord runs. Intervertebral discs of cartilage lie between the bodies of adjacent vertebrae. These discs allow for some movement of the vertebral column and act as a cushion to absorb shock. The column supports the head and the body, provides points of attachment for the pelvic girdle and for many muscles, and protects the spinal cord which runs through it. It also allows some movement.

Ribs and sternum

Twelve pairs of ribs are attached to the vertebral column; together with the sternum or breast bone they form a cage, known as the rib cage. The rib cage encloses and protects the heart, lungs and major blood vessels, and provides an attachment point for the pectoral girdle and many muscles of the upper body. Movements of the ribs and sternum are essential for breathing.

Figure 12.4 Compact bone tissue under the microscope

spinal cord

body of the vertebra

intervertebral disc

Figure 12.5 Side view of the vertebral column and the spinal cord running through two vertebrae

The appendicular skeleton

The appendicular skeleton consists of the pectoral and pelvic girdles, the arms (forelimbs) and the legs (hindlimbs). The girdles connect the limbs to the axial skeleton.

Girdles

The pectoral girdle consists of two scapulae or shoulder blades and two clavicles or collar bones. The scapulae are flat, triangular-shaped bones that are attached to the back of the rib cage. Each scapula has a socket for the ball of the humerus to fit into and articulate with, and their shape provides a large surface area for attachment of muscles that move the arms. The clavicles are long, thin bones that support and hold the arms away from the trunk, which allows them to move freely.

The pelvic girdle is a basin-shaped structure composed of two broad, flat hip bones that provide a large surface area for the attachment of muscles that move the legs. Each hip bone has a socket for the ball of the femur to fit into and articulate with. The hip bones are fused to the bottom of the vertebral column to provide support for the lower body and to transmit the thrust from the legs to the vertebral column, which moves the body forwards. The pelvic girdle also protects the internal reproductive organs, bladder and lower part of the digestive system.

Limbs

The limbs are composed of long bones that have joints between them to allow for easy movement. Being long, the bones provide a large surface area for the attachment of muscles. The main function of the arms is to grasp and manipulate objects, and the long bones allow the arms to have a long reach The main functions of the legs are for support and movement, and the long bones permit long strides to be taken.

Both the arms and the legs are built on the same basic pattern, each bone in the arm having a corresponding bone in the leg. This pattern is known as the pentadactyl limb because the limbs have five digits. The bones making up the limbs are shown in Figure 12.6.

Structure of a typical long bone

forelimb

pectoral girdle

hindlimb

humerus

ball and socket joint

femur

radius

ulna

carpals –

wrist bones

metacarpals –hand bones

phalanges –finger bones

hinge joint

tibia

fibula

tarsals –

ankle bones

metatarsals –

foot bones

phalanges –

toe bones

thumb or big toe

A typical long bone is composed of a shaft running down the centre. The walls of the shaft are made of compact bone which is dense and strong, and it has a hollow cavity, known as the marrow cavity, down its centre. The cavity is filled with yellow bone marrow, which serves as a fat store for the body. Being hollow and cylindrical with walls of compact bone provides the shaft with strength and reduces the chances of breakage occurring across the bone.

The two ends of the bone are wider than the shaft and have compact bone around the outside. The ends are filled with spongy bone, which consists of a network of bony bars. This makes the ends light and strong and able to withstand stress in all directions. The spongy bone has red bone marrow in the spaces between the bars which produces blood cells. Articular cartilage covers both ends where they articulate with other bones, to reduce friction. In the case of the humerus and femur, the top end forms a ball-shaped head that fits into a socket in a girdle. The external features and internal structure of the humerus are shown in Figures 12.7 and 12.8 on page 204.

pelvic girdle

Figure 12.6 The pentadactyl limb

bean seedlings

petri dish moist cotton wool support

clinostat – rotates slowly to eliminate the effect of gravity

2 After 2 days, all the roots of the seedlings in dish X have grown downwards with gravity and all the shoots have grown upwards against gravity. In the absence of gravity, the roots and shoots of the seedlings in dish Y continued to grow in the direction in which they were growing at the start of the experiment.

Figure 13.3 Investigating the effect of gravity on seedlings

Responses of invertebrates

Invertebrates such as millipedes, earthworms, woodlice and termites move their whole bodies towards or away from stimuli. These responses are known as taxic responses or taxes, and they help the organisms to survive. The responses of invertebrates to different stimuli is summarised in Table 13.1.

Table 13.1 Responses of invertebrates

Stimulus Response

Light Most move away from the light into darkness.

Moisture Most move away from dry areas into areas with moisture.

How the response aids survival

Makes the organisms harder to be seen by predators.

Prevents desiccation (drying out), especially if the organisms do not have waterproof body coverings.

1Two petri dishes are lined with moist cotton wool. Three bean seedlings are placed inside each dish as shown and covered with the lid to hold the seedlings in position. Dish X is held vertically by a support and dish Y is attached to a clinostat. Both dishes are placed in the dark

Stimulus Response

Temperature Move away from very low or very high temperatures.

Chemicals Move towards chemicals given off by food and potential mates, and away from harmful chemicals.

How the response aids survival

Prevents extremes of temperature affecting enzyme activity.

Enables organisms to find food, which is essential for survival, and a mate for reproduction, and to avoid being harmed by chemicals such as insecticides.

Touch Move away or curl up if touched. Helps the organisms escape from predators, or gives protection against predators.

Using a choice chamber to investigate responses of invertebrates

Taxes can be investigated using a choice chamber. This is a piece of apparatus in which the organisms are provided with different environmental conditions in areas that are adjacent to each other; for example, moist and dry, or dark and light. The organisms are placed in the centre of the chamber between the two environments and allowed to move around freely. The number of organisms in each environment is then recorded after a fixed length of time.

To create moist and dry conditions, wet cotton wool can be placed below a gauze platform in one chamber to give off water vapour, and calcium chloride can be placed below a platform in the other to absorb any water vapour from the air, as shown in Figure 13.4. To create dark and light conditions, one chamber can be covered completely in black paper or fabric and the other left uncovered.

Recalling facts

Distinguish between the following pairs of terms:

a stimulus and response b receptor and effector.

Construct a table that names the receptors and effectors in plants and in animals.

a Name the THREE main stimuli to which plants respond.

b Outline how plants respond to EACH of the stimuli named in a

Identify THREE stimuli to which invertebrates respond, and explain why it is important for them to respond to EACH of these stimuli.

Figure 13.4 A simple choice chamber

Applying facts

Explain the importance of EACH of the following responses to the organism.

a A bud of a night-flowering cactus opening after dark.

b The leaves of a Mimosa plant folding when touched.

a Identify the stimulus to which shoot 3 is responding in the investigation illustrated in Figure 13.2 on page 211. Explain how you arrived at your answer.

b Suggest why shoot 2 in the same investigation grew very little, if at all, after its tip was removed.

Suggest why the experiment illustrated in Figure 13.3 on page 212 is carried out in the dark.

Give TWO differences between responses in plants and responses in invertebrates.

Windel turns his compost heap regularly and notices it contains a large number of earthworms, millipedes, woodlice and many other small invertebrates that he does not recognise. Explain why so many of these organisms are in Windel’s compost heap.

Analysing data

A choice chamber was set up as illustrated in Figure 13.4 on page 213, with wet cotton wool under the platform in the left chamber and calcium chloride under the platform in the right chamber. Ten woodlice were then released into the connecting passageway and the number in each chamber was counted every minute for the next 10 minutes. The results are given in Table 13.2.

Table 13.2 Table showing the number of woodlice in each chamber every minute

a Describe the behaviour of the woodlice over the 10-minute period.

b Explain why the woodlice behaved in the way described in a

c What conclusion can be drawn from the results?

d Suggest how a control could be set up.

e Identify TWO other environmental factors that may affect the behaviour of woodlice.

f Design an experiment to test ONE of the factors named in e. Include your hypothesis, the aim of your experiment and your expected results.

The human nervous system Learning objectives

• Describe the main divisions of the human nervous system.

• Describe the structure and functions of motor, sensory and relay neurones

• Explain the relationship between receptors, the central nervous system and effectors.

• Explain the coordinating function of the brain and spinal cord

• Explain a simple reflex action

• Describe the functions of the main regions of the brain.

The structure of the nervous system

The nervous system is responsible for coordinating and controlling all the activities of the body. It is made up of neurones or nerve cells which transmit messages as electrical impulses throughout the system, linking receptors, which are present in the sense organs, to effectors, which are muscles and glands. The system is divided into two parts: the central nervous system and the peripheral nervous system

• The central nervous system (CNS) consists of the brain and the spinal cord

• The peripheral nervous system (PNS) consists of cranial and spinal nerves that connect the central nervous system to all parts of the body. The cranial nerves emerge from the brain and the spinal nerves emerge from the spinal cord.

Neurones

Neurones are specialised cells that conduct nerve impulses throughout the nervous system.

Neurones make up both the CNS and PNS. All neurones have a cell body with thin fibres of cytoplasm extending from it called nerve fibres. Nerve fibres that carry impulses towards the cell body are called dendrites. Nerve fibres that carry impulses away from the cell body are called axons; each neurone has only one axon, but most have many dendrites. Axons branch at their ends and each branch terminates in a bulb-like structure called a synaptic knob or synaptic bulb. The nerve fibres in certain types of neurones are covered with a fatty sheath known as a myelin sheath. There are three types of neurones: sensory neurones, motor neurones and relay or intermediate neurones

• Sensory neurones transmit impulses from receptors to the CNS. The cell bodies of sensory neurones lie just outside the CNS. Each sensory neurone has a short axon, as shown in Figure 13.6 on page 216.

• Motor neurones transmit impulses from the CNS to effectors. The cell bodies of motor neurones lie inside the CNS. Each motor neurone has a long axon, as shown in Figure 13.7.

• Relay or intermediate neurones transmit impulses throughout the CNS. They link sensory and motor neurones and their nerve fibres lack myelin sheaths.

Nerves are made up of bundles of nerve fibres of sensory and/or motor neurones surrounded by connective tissue. The brain and spinal cord are made up mainly of relay neurones and the cell bodies of motor neurones.

Central nervous system (CNS) brain

spinal cord

Peripheral nervous system (PNS)

cranial nerves

spinal nerves

Figure 13.5 The human nervous system

myelin sheath – fatty sheath to insulate and protect the nerve

fibre and speed up transmission of impulses

receptor

Figure

dendrites

node of Ranvier –constriction in the myelin sheath

cell body

synaptic knobs

axon – short

dendrites

direction of nerve impulse

effector – a muscle or gland

myelin sheath

node of Ranvier

Figure

cell body

axon – long direction of nerve impulse

synaptic knob

When a receptor is stimulated, impulses pass from the receptor along sensory neurones into the CNS, where they pass into relay neurones. The impulses then pass into motor neurones which carry them out of the CNS to effectors. The connection between the three types of neurones is summarised in Figure 13.8.

receptor

effector

CNS sensory neurone

PNS

synapse relay neurone –does not have a myelin sheath

boundary of the CNS motor neurone

13.6 Structure of a sensory neurone

13.7 Structure of a motor neurone

Figure 13.8 The connection between a receptor and an effector

Transmitting impulses between neurones

The site of transmission of nerve impulses between adjacent neurones or between a neurone and an effector cell is known as a synapse. Adjacent neurones do not touch. There are tiny gaps called synaptic clefts between the synaptic knobs at the ends of axons and the dendrites or cell bodies of adjacent neurones. When a nerve impulse arrives at a synaptic knob, it causes chemicals, known as neurotransmitters, to be released into the gap. These neurotransmitter molecules diffuse rapidly across the gap and cause an impulse to be set up in the adjacent neurone. This ensures that impulses travel in one direction only and allows many neurones to interconnect.

Synapses are also found between synaptic knobs of motor neurones and effector cells Neurotransmitters released into the gaps initiate a response in the effector cells. For example, they cause muscle cells to contract and glands to release secretions.

Coordinating function of the central nervous system

The function of the central nervous system is to coordinate the activities of all parts of the body. Receptor cells in sense organs detect stimuli and send impulses along sensory neurones to the CNS. The CNS processes these impulses and sends new impulses out along motor neurones to specific effectors so that the most appropriate action is taken. Relay neurones, running throughout the brain and spinal cord, pass the impulses from the sensory neurones to the motor neurones.

For example, during a game of tennis, as the ball comes towards a player, receptor cells in the player’s eyes are stimulated as she watches the ball approach, and impulses travel along sensory neurones to her brain. The impulses pass into relay neurones which carry them from her brain to her spinal cord. They then pass into motor neurones leading to the muscles in her arm and into other motor neurones leading to the muscles in her legs. These impulses coordinate the contraction of her arm and leg muscles, enabling her to move and hit the ball.

Simple reflex actions

A simple reflex action is a rapid, automatic, involuntary response to a stimulus by a muscle or gland. Simple reflexes are also known as inborn reflexes. Most are present from birth and they are fixed, meaning that when a particular receptor is stimulated, it always results in the same response. They happen without conscious thought and they are not learned. They are important because they help protect the body from danger or damage, and they aid in survival. The pathway between the receptor and the effector in a simple reflex is known as a reflex arc. A reflex arc involves the following.

• A receptor that detects the stimulus.

• A sensory neurone that carries the impulse to the central nervous system.

• A relay neurone in the central nervous system that carries the impulse to a motor neurone.

• A motor neurone that carries the impulse away from the central nervous system.

• An effector that responds to the stimulus.

Simple reflex actions are classified into two types: cranial reflexes and spinal reflexes

Figure 13.9 A network of interconnected neurones in the brain

Cranial reflexes

In cranial reflexes, impulses pass through cranial nerves and the brain. Examples of cranial reflexes include the pupil reflex, blinking, sneezing, coughing and saliva production. The pupil reflex is summarised in Figure 13.10.

Spinal reflexes

In spinal reflexes, impulses pass through spinal nerves and the spinal cord. Spinal reflexes include the withdrawal reflex in response to pain; for example, when the finger is pricked, pain receptors are stimulated and the hand is rapidly withdrawn from the source of the pain, and the knee jerk reflex, which lacks a relay neurone. Figure 13.11 shows the use of a simple flow diagram to illustrate the pathway along which impulses travel in the withdrawal reflex and Figure 13.12 on page 219 illustrates the knee jerk reflex.

stimulus: a painful prick to the nger

receptor: pain endings in the skin

impulses in a sensory neurone

impulses in a relay neurone in the spinal cord

response: muscle contracts pulling the nger from the source of the pain

effector: biceps muscle in the arm

impulses in a motor neurone

Figure 13.10 The pupil reflex

Figure 13.11 A simple flow diagram to illustrate the withdrawal reflex