Topic 9: Nervous System

what is a stimuli

change in environment causing organism to respond

main parts of nervous system

1. central nervous system (CNS)– brain + spinal cord

2. peripheral nervous system (PNS) – cranial nerves, spinal nerve and sense organs

how nervous system responds to a stimulus

1. receptors detect stimulus, transmits nerve impulse

2. nerve impulse is transmitted across sensory neurone to CNS

3. CNS integrates and interpret information. Initiates an appropriate response

4. In CNS, nerve impulse is transmitted across a synpase from sensory neurone to relay neurone thenacross another synapse from relay neurone to motor neurone. (by release of neurotransmitters)

5. nerve impulse leaves CNS along motor neurone to effector to carry out the response

what are neurones and what do they contain

• specialised cells adapted for transmission of nerve impulses

• consists of a cell body, synaptic knobs, myelin sheath (schwann cell) and nerve fibers (axons and dendrons)

types of neurones

1. sensory neurones: transmit nerve impulses from the receptors to the CNS

2. motor neutones: transmit nerve impulses from the CNS to the effectors

3. relay neurones: transmit nerve impulses from the sensory neurone to motor neurone INSIDE THE CNS

cell body in neurone

has nucleus and organelles

nerve fibres in a neurone

• cytoplasmic extensions from the cell body

dendrons:

• transmit nerve impulses towards the cell body

• receive impulses

• branch into many smaller, numerous dendrites

axons:

• transmits impulses away from the cell body

schwann cell (and myelin sheath) in a neurone

• wrap around nerve fibers at regular intervals

• membranes of the schwann cells form the myelin sheath.

• neurones which are enclosed by schwann cells are called myelinated neurones

nodes of ranvier: 1mm gaps between schwann cells where myelin sheath is absent

functions of myelin sheath

• provides protection and insulation

• prevents the loss of current

• speeds up transmission of nerve impulse

synaptic knobs in a neurone

• bulbous structures found at the end of axons

• relay nerve impulses from 1 neurone to another through the release of chemicals called neurotransmitters in synaptic vesicles

what is a nerve

bundle of nerve fibre covered by connective tissue

nerves may contain

• sensory nerve fibres only

• motor nerve fibres only

• both sensory and motor nerve fibres

gray matter vs white matter

GRAY MATTER:
* consist mainly of cell bodies
* forms outer layer of brain and central part of spinal cord

WHITE MATTER:
* consist mainly of nerve fibres (axon + dendron)
* forms outer layer of spinal cord and central part of brain

what are spinal nerves

consist of dorsal roots and ventral roots

dorsal roots: axons from sensory neurones into spinal cord

ventral roots: axons from motor neurones into spinal cord

definition of voluntary action

action under the control of a will

definition of reflex action

immediate response to a specific stimuli without conscious control

reflex action can be classified as

1- spinal reflexes:
* controlled by the spinal cord
* occurs below neck

2 - cranial reflexes:
* controlled by the brain (NOT OF CONSCIOUS WILL)
* occurs in head region

reflex arc definition and components of the reflex arc

shortest pathway by which the nerve impulses travel from the receptor to the effector in a reflex action.

components:
1. receptor
2. sensory neurone
3. relay neurone in the CNS
4. motor neurone
5. effector

spinal reflex: withdrawal of hand from hot pan

1. heat from the pan stimulates temperature receptors in the skin, generates nerve impulses

2. nerve impulse is transmitted across sensory neurone to spinal cord

3. In spinal cord, nerve impulse is transmitted across a synpase from sensory neurone to relay neurone then across another synapse from relay neurone to motor neurone. (by release of neurotransmitters)

4. nerve impulse leaves spinal cord along motor neurone to bicep muscles. bicep muscles contract to bring out the sudden and immediate withdrawal of hand

5. at spinal cord, relay neurone also transmits the nerve impulses to the brain (Not part of the reflex arc)
   this makes the brain aware of what is happening

reflex vs voluntary action

reflex
- does not involve conscious control
- stimulus is always involved
- faster
- the same stimulus will always result in the same response

voluntary
- involves conscious control
- stimulus may not always be involed
- slower
- the same stimulus may not always result in the same response

nervous vs endocrine system

nervous system
- involves nerve and chemical impulses
- impulses are transmitted by neurones
- quick responses
- responses are short- lived
- may be voluntary or involuntary
- usually localised

endocrine system
- involved hormones
- hormones are transmitted by blood
- usually slower responses
- responses may be short or long lived
- always involuntary
- tend to affect one or more target organs