Chapter 13 - Nervous System In Mammals I

Nervous System in Mammals I

What you will learn… • state and explain the nervous system. • outline the functions of sensory, relay and motor neurones. • discuss the function of the brain and spinal cord in producing a coordinated response as a result of a specific stimulus.

What you will learn today.. • • • • • • •

define sensitivity. explain what are receptors. state the overview of the pathway of nervous system and its functions. state the parts of the nervous system. explain the structural characteristics of neurons. differentiate an axon and a dendron. relate the structural characteristics to the functions of the nerve fibres.

Sensitivity • Living things are able to react to changes in the external environment. • Sensitivity is the ability to respond to a stimulus. • The response is usually beneficial to the organism.

Sensitivity • Euglena, a singlecelled organism can make use of light to carry out photosynthesis. • Thus, it will swim towards light.

Receptors • A specialized cell that can sense something about its environment. • Information like light, sound, touch, temperature and chemicals can stimulate the receptors. • These type of information are called stimuli (singular: stimulus). • Receptors are often part of the sense organ.

Nervous System • Humans are made up of many different type of cells. • It is important to coordinate and control their functions so that the whole body can survive and function efficiently. • This vital role is carried out by a system of nerves. • This is known as the nervous system.

Nervous System (Process)

1.Danger is detected by sense organs.

5.Muscles contract and body moves away.

2.Message is sent to the brain along a sensory nerve.

4.Message is sent to the muscles along a motor nerve.

3.Brain interprets the message. Decides to run away.

The nervous system coordinates the entire process in a fraction of a second.

Nervous System The nervous system performs three functions: • Collects information on the internal and external environment. • Processes the information in relation to past experiences. • Acts upon the information by coordinating the responses.

Parts of Nervous System brain

cranial nerve

spinal cord spinal nerves

Central Nervous System

Peripheral Nervous System

•Brain

•Cranial nerves

•Spinal cord

•Spinal nerves •Sense organs

Neurones • The basic unit of the nervous system is the neurone (nerve cell). • Like all animal cells, the neurone has a cell membrane, cytoplasm and a nucleus.

Neurones • Neurones are highly specialised for their function of receiving and conducting impulses.

Cell body

• Each neurone consists of a cell body which contains the nucleus. • The most striking feature of a neurone is its cytoplasmic processes (nerve fibre).

Nerve fibres

Nerve Fibres There are two main type of nerve fibres: • Axon  carries impulses away from the cell body. • Dendron  carries impulses towards the cell body. nerve impulses

sense organ

axon

dendron cell body

branches of axon with synaptic ends

Nerve Fibres • Each axon or dendron is surrounded by an insulating fatty sheath called the myelin sheath. • The myelin sheath is surrounded by a thin membrane called the neurilemma. • The myelin sheath is not continuous but separated at intervals by nodes of Ranvier. nerve impulses

motor end plates

Myelin sheath

axon muscles fibres

Neurilemma

Nodes of Ranvier

cell body

dendrites

Neurones • Myelin sheath  • Neurilemma  • Nodes of  Ranvier

Insulating layer Provides nourishment Speeds up transmission

What you will learn today… •

describe the structural characteristics and functions of the 3 types of neurones.

•

describe the structural characteristics and functions of the synapse.

•

state the pathway which nervous impulse is transmitted.

•

state the two distinct regions of the central nervous system.

Neurones There are 3 main types of neurones: •

Sensory/Receptor neurones

•

Motor/Effector neurones

•

Intermediate/Relay neurones

CNS

Sensory Neurone • Transmit impulses from the sense organs/receptors (ears, eyes, touch receptors, temperature receptors etc.) to CNS. • Has a long dendron and a short axon.

CNS

Motor Neurone • Transmit impulses from the CNS to the effectors (muscles, or glands etc.) • Has a long axon and a short dendron which branches out to form dendrites.

Relay Neurone • Transmit impulses from sensory neurone to the motor neurone. • Found in CNS.

Synapse • Impulses are transmitted from the axon of one neurone to the dendron of another. • The impulse will move across a junction between the two neurones. • This junction is called a synapse. • Electrical impulses are transmitted across the synapse by chemical means.

Neurotransmitter Receptors

O v e r v i e w

Pathway of Impulses Receptor

Sensory Neurone Synapse

Relay Neurone (CNS) Synapse

Effector

Motor Neurone

Summary Receptor

Sensory Neurone Synapse

Relay Neurone (CNS) Synapse

Effector

Motor Neurone

consist of

Grey Matter

White Matter

Information processing

Information transmission

Brain of a Mammal A mammalian brain can be divided into three parts: • The forebrain consists:  cerebrum, hypothalamus and pituitary gland. • The midbrain • The hindbrain consists:  cerebellum and medulla oblongata.

Human Brain

Forebrain •Cerebrum is made up of two cerebral hemispheres. •Concerned with emotions, intelligence, memory, learning and voluntary actions.

Cerebral hemisphere

Forebrain • The floor of the cerebral hemisphere is called hypothalamus. • Concerned with regulation of body temperature, blood osmotic pressure, appetite, sleep and emotion.

Hypothalamus

Forebrain • Attached to the hypothalamus is the pituitary gland. • Concerned with the secretion of many hormones. Pituitary Gland

Midbrain • Consists of the optic lobes which are represented in mammals by four small bodies. • Concerned with visual reflexes, e.g. movement of the eyeballs.

Midbrain

Hindbrain • Cerebellum lies behind the optic lobes. • Large and many folds. • Concerns with muscular and body coordination.

Cerebellum

Hindbrain • Medulla oblongata lies below the cerebellum. • Concerns with involuntary actions, e.g. heartbeat, peristalsis and rate of respiratory movements.

Medulla Oblongata

Spinal Cord and Spinal Nerves • From medulla oblongata to the end of the vertebral column. • Spinal nerves emerge at intervals along the length of the spinal cord. 31 pairs of spinal nerves in Man. • Central canal contains cerebrospinal fluids runs through the middle of the spinal cord bring nutrients to the spinal cord.

Portion of spinal cord Each spinal nerve divides into 2 roots: • Dorsal roots • Ventral roots

Dorsal Root • Dorsal part of the spinal cord to the CNS. • Contains only receptor neurons. • Cell bodies aggregate to form dorsal root ganglion. • Receptor neurone transmits impulses from the receptor to the spinal cord.

Ventral Root • Ventral part of the CNS to the spinal cord. • Contains only motor neurones. • No cell bodies.

Central Nervous System CNS consists of 2 distinct regions: Grey matter • consists mainly of cell bodies of neurones. • Forms the outer layers of the brain and the central portions of the spinal cord.

White matter • Consists of mainly nerve fibres. • Forms the central parts of the brain and the outer layers of the spinal cord.

Nervous System 3 types of reactions VOLUNTARY

CONDITIONED

Control by the WILL

Acquired from past experience

REFLEX NOT control by the will

Voluntary Actions • Action that is under the control of the will. • Impulses transmitted from brain to spinal cord along the relay neurone to the appropriate effector neurones and then to the muscles. • Does not involve sensory neurone.

Reflex Actions • An immediate response to a specific stimulus without conscious control. • An automatic response. • An involuntary action. • Does not pass through brain.

REFLEX ACTIONS Spinal Reflex Action • reflex actions that are controlled by the spinal cord

Cranial Reflexes • reflex actions that are controlled by the brain • usually occur in the head region

Reflex Actions • A reflex action is a rapid, automatic response to a stimulus. • A reflex arc is the route an impulse travels during a reflex action. • In a reflex arc, impulses are sent along the spinal cord into the brain. • The spinal cord controls initial response. • The brain gives an awareness of a particular pain.

The Reflex Arc A reflex arc consists of: 1. a receptor or sense organ 2. a receptor neurone 3. a reflex center (spinal cord or brain) 4. an effector neurone 6. an effector (muscle or gland)

The Reflex Arc

An Example

Withdrawal of hand • Heat stimulates the nerve endings in the skin. • Impulses produced will travel to the spinal cord. • Impulses are transmitted across a synapse to a relay neurone to another synapse then to an effector neurone. • Impulses then leave the spinal cord to the effector (bicep muscle will contract hand withdrawal)

Knee-Jerk Reflex Arc sensory neurone

dorsal root ganglion

relay neurone

motor neurone

ventral root effector is a muscle receptor responds to stretch

A knee-jerk reflex arc involves three neurones.

Conditioned Reflex • A reflex action acquired from past experience or learning with a stimulus which is originally ineffective in producing the response. • A conditioned reflex differs from a normal spinal or cranial reflex because the conditioned reflex has been modified by past experience.

Pavlov’s Experiment • Rings a bell just before giving the dog food to eat. • Repeat this procedure for several times. • Dog will salivate whenever a bell is rung.