THE NERVOUS SYSTEM

9.1 Introduction

A. Neurons = masses of nerve cells that transmit information to other nerves, tissues or cells (nerve impulses)

1. Cell Body - contains the nucleus and two extensions
2. Dendrites - shorter, more numerous, receive information
3. Axons - single, long fiver which conducts impulses away from the cell body

B. The nervous system is divided into two parts:

1. Central Nervous System (CNS) - brain and spinal cord
2. Peripheral Nervous System (PNS) - peripheral nerves through the body
- includes 31 pairs of spinal nerves
- includes 12 pairs of cranial nerves

9.2 Three Basic Functions

1. Sensory Function - gathers info about changes occurring within and around the body - Sensory receptors, at ends of peripheral nerves send signals (nerve impulses) to the CNS. Sensory Neuron

2. Integrative Function - information is "brought together," interpreted, to create sensations, create thoughts, add to memory, make decisions, etc. Association neuron or interneuron

3. Motor Function - responses to signals (impulses). Signals sent from the CNS to effectors (muscles or glands). The goal is usually to maintain stable conditions (especially internal) - Homeostasis. Motor neurons.

- Somatic Nervous System (skeletal muscles)
- Autonomic Nervous System (smooth muscles, glands)

C. Neurons: Nerve cells. Unique structure - cell body with many extensions or processes (nerve "fibers") which conduct impulses. There are two types of processes:

1. Dendrites - shorter, more numerous. These, along with the cell body, form the receptive surfaces of neurons.

2. Axons - single, long "fiber" which conducts impulse away from the cell body. Sometimes it is "branched" (collaterals). End has many fine branches.

9.3 Neuroglial Cells (neuroglia)

- supportive tissue of the nervous system (more numerous than neurons). Five types

1. Microglial Cells
2. Oligodendrocytes
3. Astrocytes
4. Ependymal Cells
5. Schwann cells

*Myelin Sheaths

neuroglia

neuron

9.4 Neurons

Axon

Dendrite

Neurofibril

Chromatophilic substance

Myelin

Nodes of Ranvier

Myelinated (white matter) vs Unmyelinated (grey matter)

 

Classification of Neurons

1. Functional (sensory neurons, motor neurons, interneurons)
2. Structural (multipolar, bipolar, unipolar)

9.5 Cell Membrane Potential

Resting Potential / Threshold Potential / Action Potential

1. Neuron membrane maintains resting potential
2. Threshold stimulus is received
3. Sodium channels open
4. Sodium ions diffuse inward, depolarizing the membrane
5. Potassium channels open
6. Potassium ions diffuse outward, repolarizing the membrane
7. The resulting action potential causes a local bioelectric current that stimulates adjacent portions of the membrane.
8. Wave of action potentials travel the length of the axon as a nerve impulse

9.6 Nerve Impulse

*Propagation of action potentials along a nerve axon

Impulse Conduction – speed of an impulse proportionate to diameter of axon (greater diameter = faster impulse)

Myelinated axons conduct faster than unmyelinated axons

Example: Motor neuron associated with skeletal muscle 120 m/2
Sensory neuron on skin (unmyelinated) travels at .5 m/s

A: Neuron (Presynaptic)
B: Neuron (Postsynaptic)

  1. Mitochondria
  2. Synaptic vesicle full of neurotransmitter
  3. Autoreceptor
  4. Synaptic cleft
  5. Neurotransmitter receptor
  6. Calcium Channel
  7. Fused vesicle releasing neurotransmitter
  8. Neurotransmitter re-uptake pump

synapse

 

9.7 The Synapse

Synapse – junction between two communicating neurons
Nerve pathway – nerve impulse travels from neuron to neuron

Synaptic Transmission

Dendrite  cell body  along axon  synapse (gap)

To complete the signal, a NEUROTRANSMITTER is released at the gap to signal the next neuron

Excitatory – increase membrane permeability, increases chance for threshold to be achieved
Inhibitory – decrease membrane permeability, decrease chance for threshold to be achieved

Types of Neurotransmitters

Acetylcholine – stimulates muscle contraction
Monoamines – Norepinephrine & Dopamine (sense of feeling good, low levels = depression)
Serotonin (sleepiness)
Endorphins (reduce pain, inhibit receptors)
Synapses are highly susceptible to drugs and fatigue

Curare (poison used by S. American Indians) and atropine stops Acetylcholine from depolarising the post-synaptic membrane, i.e. become paralysed.

Strychnine and some nerve gases inhibit or destroy acetylcholinesterase formation. Prolongs and enhances any stimulus, i.e. leads to convulsions, contraction of muscles upon the slightest stimulus.

Cocaine, morphine, alcohol, ether and chloroform anaesthetise nerve fibers. (inhibitory)

Mescaline and LSD produce their hallucinatory effect by interfering with nor-adrenaline & serotonin

9.8 Impulse Processing

Neuronal pool – groups of neurons that make hundreds of synaptic connections and work together to perform a common function

9.9 Types of Nerves

Sensory Nerves – conduct impulses into the brain or spinal cord
Motor Nerves – carry impulses to muscles of glands
Mixed Nerves - contain both sensory and motor nerve

9.10 Nerve Pathways

Reflex arc – simple pathway, includes only a few neurons (reflexes)

Reflex Behavior – automatic, subconscious responses to stimulu

Knee-jerk reflex (patellar tendon reflex

stimulus knee  sensory nerve  spinal cord  motor nerve

Withdrawal reflex – occurs when you touch something painful

reflex