Chpt. 34 The Nervous System

Nervous System and Endocrine System are responsible for coordination of activities in the body A Nervous System allows an organism to detect and to respond to stimuli in its internal or external environment The nervous system consists of: - CNS (central nervous system) – BRAIN & SPINAL CORD - PNS (peripheral nervous system) – SPINAL NERVES

Responding to Stimuli Four processes involved: Reception – stimulus detected by neurons and sense organs Transmission – neurons in the PNS carry messages from receptors to the CNS and from the CNS to effectors (e.g. muscles) Integration – incoming messages sorted, processed and a response decided upon occurs in CNS, especially in brain.

Response – carried out by effectors (muscles, glands) when stimulated by neurons

Neurons Functional unit of the nervous system is the NEURON ( nerve) It connects with many other nerves receiving and transmitting information A nerve is bundle of neurons that connect one part of the nervous system with another 3 types of neurons: - Sensory neuron - Motor neuron - Interneuron

Sensory (afferent) Neuron: Takes messages FROM sense organs TO the CNS Motor (efferent) Neuron: Bring messages FROM the CNS TO the effectors of the body e.g. muscles Interneuron: Connect the sensory and motor neurons and are only found in the CNS

Structure of Neurons

Nerve Endings: connect sensory neurons to receptor cells or sense organs Cell Body: contains nucleus and cell organelles. Control centre, specific neurotransmitter receptors, synthesis of neurotransmitters. Note: a group of cell bodies located outside CNS is called a ganglion Dendrites: initiates an impulse in a neuron sending it towards the cell body Axon: - carries impulses away from the cell body - many axons combine to form nerve fibre

Axon: - end of each axon forms many axon terminals each of which ends in a neurotransmitter swelling. Synaptic Knob (neurotransmitter swelling): secretes a chemical transmitter (neurotransmitter) that passes an impulse from one neuron to the next Myelin Sheath: fat rich membrane which insulates electrical impulses. Schwann Cells: - located along length of neuron - secretes the myelin sheath Note: MS is caused by degeneration of the myelin sheath in the CNS Nodes of Ranvier: gaps in the myelin sheet which help to speed up the passage of the electrical impulse

Note: An interneuron is a short neuron found in CNS not enclosed in myelin sheath!!!!!

Nerve Impulses and their Transmission Resting Neuron: in its resting state inside of axon is negatively charged and outside positively charged Transmission: when neuron receives stimulus of sufficient strength an electrical impulse travels along the dendrite and axon to the neurotransmitter swellings electrical impulse involves movement of ions the neuron will only carry messages once there is a stimulus of sufficient strength at the dendrites. If the stimulus is not strong enough, no message is transmitted

Threshold: minimum stimulus needed to cause an impulse to be carried once the threshold (minimum intensity of stimulus needed to generate an impulse) level is reached, a message is transmitted All or Nothing Law: if threshold is reached an impulse is carried, but if threshold is not reached no impulse is carried. the strength of an impulse is always the same. A strong stimulus does not cause a stronger impulse but can cause the receptors to send lots more impulses at increased occurence.

Movement of Impulse: Once threshold reached: - at site of stimulation the inside of axon becomes positively charged and the outside negatively - this change in charge causes next section of axon to react in a similar way - a chain rxn occurs and a movement of positive charge runs along the inside of the axon. Note: ATP is required for this chain rxn to occur - once impulse has moved along, the area it has passed through returns to its resting state

Refractory Period: Each section of neuron can only carry one impulse at a time thus there is a delay of approx 5ms between any two impulses. This delay is called the REFRACTORY PERIOD. Speed of Impulse: dependent on whether myelin sheath is present or absent around the neuron If myelin absent speed of impulse reduced to approx 2 m/s if myelin present impulse can jump from one Ranvier node to the next – speed approx 120 m/s speed also dependent on width of axon/dendrite: - the larger the diameter the faster impulse travels

Synapse Synapses are specialised junctions (regions) between a neuron and it’s target cell. The target cell may be another neuron, muscle or gland cell Synaptic Cleft: tiny gap between two neurons at synapse When the impulse reaches the end synaptic knob, the electrical impulse cannot be transmitted to the next neuron as there is a tiny gap called the SYNAPTIC CLEFT between each neuron In the synaptic knob are vesicles containing chemicals called NEUROTRANSMITTERS e.g acetylcholine, dopamine These chemicals are released when the impulse arrives at the synapse. They diffuse across the synaptic cleft causing an impulse to start in the second neuron

Enzymes are released into or are present in the gap which break down the neurotransmitters. As a result only one impulse is sent each time a neurotransmitter is released The broken down neurotransmitters are reabsorbed by the neuron and used to make more neurotransmitters The next neuron can be excited or inhibited --- when neurotransmitters ceases due to over stimulation, the impulse is inhibited and the effector ceases to be stimulated Summary of events at synapse: electricalchemicalelectrical impulse impulse impulse

SYNAPSE & NEUROTRANSMITTERS

Functions of Synapses Transmit impulses from one neuron to another or to an effector. Control direction of impulse i.e. only allow a one way flow. Only one part of the body reacts to stimulus Nerve pathways in the brain can become established for fast reactions Impulse can be blocked by certain chemicals – anaesthetics, depressants etc.

The Central Nervous System Consists of: - the BRAIN - the SPINAL CORD

The BRAIN:

Acts as an interpreting centre to sort and process incoming impulses and decide on response. Brain and spinal cord are protected by bone and covered by 3 membranes called MENINGES Meningitis is an inflammation of the meninges. There are two causes of meningitis – a virus and a bacterium Viral: less severe infection causes headache, fever, neck ache no specific treatment Bacterial: dangerous form causes skin rash, vomiting, convulsions, coma, death treated with antibiotics

THE BRAIN CEREBRUM: Carries out conscious thought processes and voluntary actions CEREBELLUM: Controls balance and muscle co-ordination MEDULLA OBLONGATA: Connects the spinal cord with rest of brain Controls breathing and heartbeat HYPOTHALMUS mainly concerned with homeostasis (maintaining a constant internal environment) it monitors levels of sugars & hormones in the blood

THALAMUS: Acts as a sorting centre, relaying different messages to relevant parts of the brain PITUITARY GLAND: Controls the secretions of most endocrine glands in the body

Nervous System Disorder Parkinson’s Disease Is a neurodegenerative disease that affects voluntary control of muscles in the body Description: Persistent tremor, especially the limbs Later muscles and body become stiff and rigid Thought processes affected late in the course of disease Cause: Degeneration of a specific group of brain cells that secrete dopamine which is responsible for the stimulation of motor neurons. When dopamine production is depleted, motor neurons are unable to control movement & co-ordination

Prevention: No known preventative measures Treatment: No cure available Drugs that mimic the action of dopamine or that can be converted to dopamine in the brain are used in treatment. Dopamine secreting cells can be implanted into the brain

The SPINAL CORD: Composed of nerve tissue and surrounded by vertebrae (protection) Transmits impulses to and from brain and also controls many reflex actions The spinal cord extends down through the vertebral column (neural canal - lined by meninges) of the back Parts of peripheral nerves are attached to the spinal cord along its length

Parts of the SPINAL CORD: GREY MATTER Contains a lot of cell bodies and dendrites of interneurons & motor neurons that are not myelinated WHITE MATTER (axons only) This contains a lot of nerve fibres that are myelinated ie. axons connecting with neurons in the brain CENTRAL CANAL (centre of grey matter) Contains cerebrospinal fluid to supply food and oxygen to the spinal cord

DORSAL ROOT: Contains fibres of the sensory neurons bringing the nerve impulses INTO the spinal cord VENTRAL ROOT: Contains fibres of motor neurons bringing the nerve impulses OUT of the spinal cord MENINGES: 3 layers of membranes that surround the spinal cord and the brain with fluid to nourish them & cushion them from hitting against the backbone & skull

Spinal Cord & Transmission of Impulse

Automatic, involuntary, unthinking response to a stimulus i.e. it is the automatic response to a stimulus which is not controlled by will The function is to protect the body against harm A REFLEX ARC is a specific neural pathway involved in bringing about a reflex action Examples of reflex actions: blinking, breathing, protective actions taken when falling etc. Reflex Action

1.Receptors to pressure & pain are stimulated 2.Sensory neurons carry the impulses to the spinal cord by way of the dorsal root 3.The sensory neuron synapses with many neurons in the spinal cord of the CNS: - an interneuron may carry the signal to the brain to ’advise it’ about the situation. - another interneuron carries the impulse to a motor neuron 4. A motor neuron carries the impulse from the spinal cord to the effector 5.The effector responds to the impulse in a predictable way by contracting or secreting while at the same time the impulse reaches the brain allowing us to feel pain Reflex Arc Pathway:

REFLEX ACTION --- The Reflex Arc

Route Along a Reflex Arc Receptor Sensory neuron Spinal cord Interneuron Motor neuron Effector