Nervous Coordination: Neurons, Reflex and Voluntary Actions

Nervous Coordination

The nervous system is made up of nerve tissue whose cells, called neurons, are capable of transmitting information through nerve impulses.NEURONS

Neuron Structure

Neurons are divided into three parts:

• the cell body (nucleus and organelles)
• dendrites (short, branch-like projections)
• the axon (longer projection that ends in the axon terminals)

Dendrite Axon Erminal Node of Ranvier Cell body 0 Axon Schwann cell Nucleus Myelin sheath In general, axons are protected by a white substance called myelin.NERVES

Nerve Formation

The axons of various neurons sometimes group together to form nerve fibres, and these in turn can group together to form nerves.

A nerve impulse is an electrical signal which is transmitted along the neuron membrane and then on to other neurons through synapses.

Nerve Axon Blood vessels Nerve fibresNERVE IMPULSE

Synapse and Impulse Transmission

What is synapse The microscopic gap between two neuron Action potential SYNAPSE Pre-Synaptic Post-Synaptic www.syedgilanis.com The neuron that transmits the message is called the transmitter neuron and the neuron that receives the message is called the receptor neuron. The nerve impulse is transmitted from the axon of the transmitter neuron to a dendrite (this image), the cell body or the axon of the receptor neuron.SYNAPSE

Synaptic Transmission Process

1. The nerve impulse which has been transmitted along the membrane of the transmitter neuron reaches the end of its axon.
2. The arrival of the nerve impulse causes neurotransmitters (stored in the axon terminals) to be released into the synaptic cleft (small gap between neurons).
3. Neurotransmitters bind to the membrane of the receptor neuron and generate a new nerve impulse.

Synapse Neurotransmitters Presynaptic Terminal O O Receptors Synaptic Cleft Postsynaptic TerminalTHE NERVOUS SYSTEM

Functions of the Nervous System

The nervous system:

• receives information from the receptors
• processes and coordinates it
• and generates orders for the effectors.

The central nervous system or CNS coordinates all the body's functions. It is formed by the encephalon and the spinal cord.

The peripheral nervous system or PNS links the CNS to the body's organs.

Brain Spinal cord Peripheral nervous system Central nervous system Ganglion NerveTHE CENTRAL NERVOUS SYSTEM (CNS)

The Encephalon

It is protected by the skull or cranium and 3 membranes, the meninges (the cerebrospinal fluid circulates between them). It is divided into 3 parts:

• the brain: houses advanced functions, such as memory, intelligence and will.
• the cerebellum: controls balance and voluntary movements.
• the brainstem: links the encephalon to the spinal cord and controls involuntary functions (breathing or heart rate- medulla oblongata)

Cerebrum Thalamus Hypothalamus Depalo Pituitary gland Midbrain Pons Brainstem Medulla Cerebellum Spinal cordTHE CENTRAL NERVOUS SYSTEM (CNS)

The Spinal Cord

It is a long, thin bundle of nerves protected by the spine, which links the encephalon to the rest of the body.

It serves as a conduit for nerve impulses from the receptors to the encephalon, and from the encephalon to the effectors.

It also coordinates simple responses called reflex actions.

Spinal cord Vertebra Spinal nerveTHE PERIPHERAL NERVOUS SYSTEM (PNS)

PNS Components and Divisions

Formed by: - 31 pairs of nerves that stem from the spinal cord (spinal nerves) - 12 pairs of nerves that stem from the encephalon (cranial nerves) Nerves are divided into:

• sensory nerves: carry information from the receptors to the CNS.
• motor nerves: carry information from the CNS to the effectors.

The PNS is divided into:

• The somatic PNS: consists of the sensory and motor nerves. It controls voluntary movements.
• The autonomic PNS: consists of motor nerves that regulate the activity of the internal organs. It controls involuntary movements. It is divided into:
• The sympathetic nervous system: stimulates the body to react to stress, fear, etc.
• The parasympathetic nervous system: causes the body to relax.

They act in opposite ways, they are antagonistic.THE PERIPHERAL NERVOUS SYSTEM (PNS)

Parasympathetic Nervous System Functions

The parasympathetic nervous system

1. Contracts the pupil
2. Stimulates salivation (very active)
3. Decreases the heart rate
4. Contracts the bronchi
5. Stimulates the activity of the stomach
6. Stimulates activity of the gallbladder
7. Stimulates activity of the pancreas
8. Inhibits bowel sphincters
9. Contracts the bladder
10. Stimulates the reproductive organs

Sympathetic Nervous System Functions

The sympathetic nervous system

1. Dilates the pupil
2. Stimulates salivation (gently)
3. Accelerates the heart rate
4. Relaxes the bronchi
5. Inhibits activity of the stomach
6. Stimulates the adrenal glands
7. Stimulates the release of glucose in the liver
8. Inhibits activity of the pancreas
9. Stimulates the bowel sphincters
10. Relaxes the bladder
11. Stimulates ejaculation in males

How does the nervous system work? Responses do not always occur in the same way. Some may be involuntary, that is, they are carried out unconsciously, as in the case of reflex actions, while others are conscious reactions, as in the case of voluntary actions.

Reflex Actions

They are fast, involuntary, automatic responses that are controlled by the spinal cord and are triggered in emergency situations.

Ex. We snatch our hand away when we touch a hot surface.

Voluntary Actions

They are conscious, deliberate responses that are coordinated by the encephalon.

Ex. Moving the computer mouse.REFLEX ACTIONS

Reflex Arc Elements

A series of nervous elements are involved in reflex actions. These are Known as the reflex arc:

1. A receptor perceives the stimulus; for ex. a sting.
2. A sensory nerve transmits the impulse to the spinal cord.
3. An association neuron or interneuron generates an immediate response order.
4. The order travels along a motor nerve to the effector organ.
5. An effector organ; for ex. a muscle, executes the response (in this case it contracts and we pull our hand away).REFLEX ACTIONS

Reflex Action Pathway

1. The receptor perceives the stimulus ..
2. The sensory nerve carries the signal to the spinal cord.
3. The interneuron generates the order.
4. The effector (a muscle) responds (contracts).
5. The motor nerve carries the order to the effector.VOLUNTARY ACTIONS

Voluntary Action Pathway

1. A receptor perceives the stimulus.
2. A sensory nerve transmits the impulse to the spinal cord and from there to the encephalon (or specifically to the cerebral cortex)
3. The cerebral cortex (surface of the brain rough and full of circumvolutions) turns the information into a conscious sensation (an image, shape, feeling of cold, etc.) and uses both this and prior experiences to generate a response order that is sent through the spinal cord to a motor nerve.
4. The order travels along a motor nerve to the effector organ.
5. An effector organ; for ex. a muscle, contracts voluntarily to execute the response.

The right-hand part of the encephalon controls the left-hand side of the body and vice versa. This is because the nerve fibres of the spinal cord cross over when they enter the encephalon and end up in the opposite hemisphere of the brain.VOLUNTARY ACTIONS

Voluntary Action Steps

1. The receptor perceives the stimulus.
2. The sensory nerve carries the impulse to the spinal cord and from there to the encephalon.
3. The encephalon generates an order which is sent through the spinal cord to a motor nerve.
4. The motor nerve carries the order to the effector.
5. The effector (a muscle) responds (contracts).