4.5.2 The human nervous system

4.5.2.1 Structure and function

The Nervous System

The Nervous System

• The nervous system is pivotal in homeostasis
• It helps humans react to surroundings and coordinate their behaviour
  • E.g. if it is too hot, the receptors on the skin pick this up and send impulses through the nervous system to the brain

Endocrine System

• The nervous system can work with the endocrine system
• Therefore there needs to be teamwork between these systems to control the body
• Nervous impulses may activate an effector gland to release hormones

Two Parts

• The central nervous system consists of the brain and the spinal cord
• The peripheral nervous system is made up of the nerves that send impulses to and away from the CNS

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Neurones, Nerves and Nerve Impulses

• The nervous system sends information in the form of electrical signals, called nerve impulses
• These pass along cells called neurones
• Nerves are made up of bundles of neurones

Types of Neurones

• Sensory neurones are first in the circuit, and send impulses from the receptor to the CNS
• Relay neurones forms a link (a relay) between the sensory and motor neurones
• Motor neurones are the final neurone in the circuit and carry the impulses from the CNS to the effectors (glands or muscles)

Properties of Neurones

• Axons carry information across the body.
• Myelin Sheath are insulating layers of fat to prevent short circuits
• Cell body has the nucleus and lots of dendrons and dendrites extending from it
• Dendrites are shorter ends of dendrons, which receive information
• Synapses between each neurone are where electrical impulses cross

Synapses

Axons use neurotransmitters to conduct impulses across the synapse:

1. An impulse arrived at the first axon and synapse
2. Neurotransmitters are released into the synaptic cleft
3. Neurotransmitters travel and bind to the second axon
4. Impulse is created in the second axon

Reflexes

• The reflex arc controls reflexes, which are quick responses by the body
• Reflexes do not involve the conscious part of the brain
• Reflexes are therefore rapid and automatic
• Reflexes are the same in everyone

Reflex Arc Pathway

• Receptor: the stimulus is picked up
• Sensory neurone: an impulse is transmitted down this to the CNS
• Relay neurone: runs through the sensory and motor neurones
• Motor neurone: the impulse moves from the CNS to the motor neurone
• Effector: causes a protective response against the stimulus

Examples

• Knee-jerk reflex – if you tap the patellar tendon in the knee (just under the knee-cap), the knee will jerk
• Hot object – if you touch a hot object your hand will quickly move away to protect you from a burn

Examples

• Knee-jerk reflex – if you tap the patellar tendon in the knee (just under the knee-cap), the knee will jerk
• Hot object – if you touch a hot object your hand will quickly move away to protect you from a burn

4.5.2.2 The brain (biology only)

Role of the Brain

• The brain is part of the CNS that works to control complex behaviour
• This can include emotions, memory and conscious responses
• The brain is complex

Cerebral Cortex

• The cerebral cortex is involved in higher-level functions
  • Emotions, thoughts, personality, intelligence
• The cerebral cortex is divided into two hemispheres and split in 50 different areas

Cerebellum

• The cerebellum controls movement, balance and muscle coordination
• Without the cerebellum, our movements would be erratic

Medulla

• The medulla is involved in unconscious control, such as heart rate and breathing rate
  • E.g. the medulla produces adrenaline during exercise to help increase our heart rate

Mapping the Brain

• When research is done into the brain, the aim is to ‘map’ the regions of the brain to particular functions
• Much of the research has focused on making a map of the cerebral cortex

MRI Scans

• MRI scans use magnetic fields and electromagnetic waves
• It investigates activity and the structure of the brain
• We can look at the parts of the brains which are most active during different activities

Electrical Stimulation

• Electrical stimulation is used to map areas of the brain
• Often the patient is asked what they experienced after stimulation
• Electroencepthalograms (EEGs) are studied to observe this electrical activity

Analysing Brain Damage

• We look at patients with brain damage to understand the importance of specific parts of the brain
  • If a patient with poor speech had an injury in a certain part of the cerebral cortex, it is likely that this area is involved in speech

Dangers of Brain Research and Surgery

• The intricacy of the brain makes surgery very difficult
• The side effects of surgery and its complications can sometimes be worse than the condition
• Both research and treatment of the brain is risky
• Research into the brain may often be slower than other organs

4.5.2.3 The eye (biology only)

Key Functions of Eye

• Refracting light to make them converge together onto one point on the retina
• Responding to light intensity by changing the pupil size
• Maintenance and protection

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Components

• Retina: Contains all of the light receptors (rods and cones) in the back of the eye
  • The retina absorbs light energy and send an impulse to the brain
• Optic Nerve: Send electrical impulses from the retina to the CNS
• Sclera: white coating of the eye, provides protection
• Pupil: hole in the eye where light enters

Components

• Iris: controls the diameter and size of the pupil, to control how much light enters the eye
• Cornea: transparent layer acts as a window to focus light on the retina
• Lens: transparent and biconcave to refract light on retina
• Ciliary Muscles: control shape of lens
• Suspensory Ligaments: attach ciliary muscles to the lens

Accommodation

• The lens refracts the light entering the eye so that it can converge on the retina
• Accommodation changes the shape of the lens to adjust how much refraction is taking place
• It is a reflex action

Nearby Objects

• The suspensory ligaments relax
• The ciliary muscles contract
• The lens becomes fatter and more curved
• Increased refractive effect of the lens is required

Far Objects

• The suspensory ligaments contract
• The ciliary muscles relax
• The less becomes thinner and less curved
• Reduced refractive effect of the lens is required

Defects of the Eye

Myopia

• Myopia is short sightedness
• Myopia could be due to a thin lens, causing lots of refraction
• Corrective concave lenses can be used
• Lenses are found in glasses or contact lenses

Defects of the Eye

Hyperopia

• Hyperopia is long sightedness
• Hyperopia could be due to a thick lens, causing little refraction
• Corrective convex lenses can be used
• Lenses are found in glasses or contact lenses

Treatments for Hyperopia and Myopia

• Glasses: have corrective concave or convex lenses
• Contact lenses: contact lenses are more user-friendly substitutes
• Laser surgery: involves using a laser to change the shape of the cornea, more commonly used for myopia
• Replacement lenses: involves surgery which requires a cut into the cornea and insert a lens of the correct shape

4.5.2.4 Control of body temperature (biology only)

Temperature

Responses to Increases in Temperature

Body Temperature Response

PRACTICE QUESTION

Responses to Decreases in Temperature

Body Temperature Response

Temperature Graphs

• You could be given a cooling curve and asked to describe the changes happening in the body at different stages

Temperature Experiments

• Temperatures can be measured at different places of the body
• The most common places are the ear, mouth, nose, armpit and forehead
• Forehead thermometers and skin temperature sensors are often used