Biology B10: The human nervous system

What do enzymes and cells need in order to work properly?

V stable conditions

• Can’t work well if conditions around them change too much

Homeostasis

The regulation of the internal conditions of a cell / organism to maintain optimum conditions for function in response to internal + external changes

What does homeostasis do?

Maintains optimal conditions for enzyme action + cell functions

What does homeostasis consist of?

Automatic control systems that ensure internal body conditions stay as constant as possible

Internal conditions

The conditions inside your body

In the human body, what internal conditions does homeostasis control?

1. Body temperature

2. The water content of the body

3. Blood glucose concentration

What do automatic control systems in humans involve?

• Nervous responses in nervous system

• Chemical responses in hormone system

What do all automatic control systems need to function?

1. Receptor cells

2. Coordination centres

3. Effectors

Receptors cells

Detect changes in the environment (stimuli)

• Pass info to coordination centre

Environment

Refers to the bodies

• Internal conditions (eg blood glucose concentration)

• External conditions (eg temp of skin)

Stimulus

A change to the environment

Coordination centres

Receives + processes info from receptor cells

• Sends instructions to the effector

Examples of coordination centres

* Brain
* Spinal cord
* Pancreas

Effectors

• Muscles or glands

• Bring about responses to stimulus that restore body conditions to optimum levels

What does the human nervous system consist of?

* Central nervous system (CNS)
* Peripheral nervous system (PNS)

What makes up the central nervous system (CNS)?

• The brain

• Spinal cord

What makes up the peripheral nervous system (PNS)?

All of the nerves in the body

Role of nervous system

Enables humans to react to surroundings + coordinate behaviour

• Done thru reflec arc

Neurones

Nerve cells

How is information sent through the nervous system as?

Electrical impulses

* Electrical signals that pass along neurones

Nerves

A bundle of neurones

How does our nervous system work?

1. Receptors detect stimulus

2. Sends EI down neurones to the CNS

3. CNS = coordination centre

4. CNS sends EI down the motor neurones to effectors

5. Effectors bring about a response

   • Eg muscles contract or glands secrete hormones

Sensory neurones

Nerve cells that carry impulses from sense organs to CNS

Motor neurones

Carry impulses from the CNS to the rest of the body

* To make effectors respond

How do effectors like muscles and glands respond to the arrival of impulses?

* Muscles: contracts
* Glands: secretes hormones

What makes reflexes automatic and rapid?

No decision making by conscious part of the brain

Since reflexes are automatic and rapid how does it help us?

Protect us from danger

• So minimise damage to the body

Why are reflexes important?

• Protect us from danger

• They take care of basic body functions

  • Eg breathing

What types of neurones do reflexes involve?

• Sensory neurones

• Motor neurones

• Relay neurones

Nerve cell labelled

• Axon = Long, EI travel along

• Axon surrounded by myelin sheath (fatty cells)

  • Insulates EI

• Dendrites = branched endings, connect neurons tog

Relay neurones

Connect a sensory + motor neurone

• In CNS

Reflex pathway

1. Stimulus
2. Receptor
3. Sensory neurone
4. Relay neurone
5. Motor neurone
6. Effector
7. Response

Synapses

Junction, gap where the ends of two neurons meet

• Ensures EI travel only in 1 direction

In the reflex arc, where are synapses found?

Between

• Sensory + relay neurones

• Relay + motor neurones

How do synapses work?

1. EI arrives at the end of the axon on the neuron

2. Chemicals released from vesicles

3. Diffuse across synaptic gap

4. Chemicals attatch to surface of next neurone + set up new EI

Which is the only place in the nervous system where drugs can affect the nervous system and why?

Synapses

• Only part of the NS where messages are chemical, not electrical

How do neurotransmitter chemicals cross the synapse?

Diffusion

• Down a CG

• Passive process

How can drugs trigger impulses in different regions of the brain?

When they bind to neurotransmitter receptors

What can the frequent use of drugs lead to?

Overstimulation of neurons → loss of function

Brain

• Controls complex behaviours eg language

• Made up of interconnected neurons

• Organ part of the CNS

Cerebral cortex

• Highly folded, outer layer of the brain

• Controls consciousness, memory + language

Cerebellum

Controls balance + coordinates movements

Medulla

• Controls unconscious activities

  • Heart + breathing rate

Hypothalamus

Controls body temperature

Pituitary gland

Produces different hormones

Why is the understanding of the brain limited?

• V complex

• Diff regions can’t be studied in isolation

Why is studying the brain difficult + difficult to treat brain damage/diseases?

• Protected by skull → tricky to access

• Structures in brain are v complex

  • So difficult to work out which parts of the brain carry out specific functions

• Delicate + easy to damage

Why can brain surgery be performed on conscious patients?

No sensory nerve endings in the brain

Benefits of procedures carried out on the brain and nervous system

Improving quality of someone’s life

Risks of procedures carried out on the brain and nervous system

• More permanent damage

  • As we don’t fully understand how brain + NS work

Eye

• Sense organ

• Contains receptors sensitive to light intensity + colour of light

Cornea

* Transparent lens
* It refracts light rays as it enters the eye and focuses it onto the retina

Iris

Colored part of the eye

• Controls size of the pupil (+ so how much light reaches the retina)

• Made of muscles that contract or relax to change the size

Lens

• Transparent lens

• Starts focusing of light rays onto the retina

What can the lens do?

Change shape, allowing us the focus on distant or near objects

Retina

* Back of the eye
* Contains receptor cells for light

What do the receptor cells in the retina allow us to detect?

• Light intensity

• Light colour

Purpose of the eye?

To receive light + focus it onto the retina at the back of the eye

How does the eye detect light?

1. Light rays pass thru the cornea → pupil in the centre of the iris → lens

2. Lens focuses LR onto the retina (back of eye)

3. Receptor cells in retina send the EI down the optic nerve to the brain

4. When brain receives these messages + interprets them as a visual image

Sclera

* White part of the eye
* Tough outer structure that protects the eye

Optic nerve

Sensory neurone that carries impulses betw the eye + the brain

How do the ciliary muscles and suspensory ligaments work with the lens?

Allow us to focus on distant or near objects

Pupil

The space in the centre of the iris which light passes through

What two groups of muscle is the pupil reflex controlled by?

• Radial muscle

• Circular muscles

Blind spot

Point at which the optic nerve leaves the eye that has no retina

• But the brain fills in the gap

How does focusing light differ in the cornea and the lens?

• Cornea: fixed focus

• Lens: allows us to focus on near and distant objects (shape of lens can change)

Accomodation

The ability to change the shape of the lens to focus on near or distant objects

How does light from distant and near objects need to be focused and why?

• Distant: small amount

  • Light from distant objects travels in almost parallel rays

• Near: a large amount

  • Light from near objects spreads out v strongly

How do we focus on distant objects?

1. Ciliary muscle relaxes

2. So suspensory ligaments pulled tight

3. Lens pulled thin

4. So LR only slightly refracted

   • LR will be focused on a point on the retina

How do we focus on near objects?

1. Ciliary muscle contracts

2. So the suspensory ligaments loosen

3. Lens is thicker

4. It refracts the LR more strongly

   • The LR will be focused on a point in the retina

Hyperopia

Long-sightedness

• Can focus on distant objects

• Can’t focus on nearby objects

Myopia

Short-sightedness

* Can focus on near objects
* Can’t focus on distant objects

How is an image formed?

Upside down

• But the brain interprets it the right way up

General causes of eye defects

Light rays don’t focus correctly on the retina

• So they are short or long-sighted

• Image on retina = out of focus

What is the brain made of?

Billions of interconnected neurones

What does the brain have?

Diff regions that carry out diff functions